Plant Hormone Regulation of Competitive Growth: Implications for Agriculture and Inclusive Fitness

Abstract

While “survival of the fittest” implies that competition is the main driver of evolution, cooperation and altruism are also widespread in nature, even among plants. This suggests that natural selection favors regulatory systems that balance competitive growth with restraint, depending on context. We propose that plant hormones are key mediators of this balance, acting along a spectrum from competition to cooperation. Based on evidence from developmental, ecological, and evolutionary studies, we classify major plant hormones by their roles in competitive behavior: auxin, gibberellins, and brassinosteroids drive competitive foraging and resource acquisition, while cytokinins, abscisic acid, strigolactones, ethylene, salicylic acid, and jasmonate are linked to growth restraint, resource conservation, and communal defense. This functional partitioning reflects a modular hormonal architecture that allows plants to adapt flexibly to their environment and social context. We explore how this classification could inform the use of plant hormones in agriculture and advance research in plant kin selection and inclusive fitness.

1. Introduction

Understanding plant social behavior begins with recognizing how evolutionary incentives, ecological conditions, and molecular mechanisms interact to shape growth and resource use. One of the clearest entry points into this interplay is the evolutionary logic of altruism. Altruism is fundamental because it reveals how cooperation, not just competition, shapes ecosystems, allowing organisms to optimize resource use and enhance group survival under ecological constraints. Altruism—defined as an organism’s voluntary reduction of its own reproductive success, growth, or resource access for the benefit of others—is a widespread and evolutionarily significant phenomenon observed across diverse life forms [1]. According to Hamilton’s theory of inclusive fitness, altruistic traits can be favored by natural selection when the fitness costs to an individual are offset by benefits to genetically related recipients, thereby increasing the transmission of shared genes [2]. This competitive restraint is not unconditional: altruism evolves under specific selective pressures, particularly in ecological contexts involving shared, limited resources [3].

Under such conditions, the classic Tragedy of the Commons can arise, where unchecked self-interest leads to resource overexploitation and reduced group survival [4]. Importantly, while altruism can preserve communal resources, competition remains a crucial evolutionary force, enhancing individual fitness, particularly within kin groups, and driving adaptation to environmental challenges [5]. In plants, preventing overexploitation of shared resources likely requires internal regulatory mechanisms, raising the question of how such restraint is enforced at the physiological level.

The balance between altruism and competition is not a fixed dichotomy but a fluid, context-dependent strategy space shaped by two primary forces: external environmental conditions and internal species-specific traits. Resource availability—particularly the degree to which a resource is exhaustible and unequally distributed—emerges as a dominant ecological driver [6]. In parallel, species differ in their intrinsic capacity to acquire, allocate, and conserve resources through physiological, behavioral, and architectural traits [7]. These dynamics have long-term evolutionary consequences, contributing to differences among major plant lineages.

Rather than considering ecological conditions as discrete states (e.g., abundance versus scarcity), they should be considered along a continuum of environmental pressure. Under conditions of high resource availability, altruistic strategies are often favored because their individual costs are low while their benefits to related neighbors are substantial. As resources become more limited, competitive strategies tend to prevail—particularly in species lacking effective internal mechanisms for resource conservation. However, when scarcity becomes severe enough that unchecked exploitation threatens survival, competition can become counterproductive. Under these conditions, altruistic restraint may re-emerge as an adaptive strategy. Traits such as growth suppression, coordinated resource use, or communal signaling can enhance collective persistence. Thus, cooperation and competition are best understood not as fixed traits but as flexible, context-dependent responses shaped jointly by environmental demand and intrinsic biological capacity [6,8,9,10].

Pathogen pressure is an additional and powerful selective force favoring altruistic strategies. Effective defense against pathogens—through immune signaling, early warning cues, or growth suppression that limits pathogen replication—often requires coordinated responses among neighboring individuals [11,12]. Such communal defenses are especially advantageous among genetically related plants, where reducing individual growth or accelerating tissue sacrifice can limit pathogen spread and protect shared genetic interests [13,14]. In this way, pathogen pressure mirrors the logic of resource-based altruism: shared vulnerability favors shared restraint.

At the molecular level, the balance between altruistic and competitive strategies in plants is likely governed by regulatory mechanisms that adjust physiological responses to both environmental and social cues. Hormones are widely recognized as central regulators of behavioral and physiological trade-offs in animals. In plants, direct evidence linking hormonal signaling to inclusive fitness has yet to be found; however, studies of kin selection have already identified competitive growth traits that are known to be under hormonal control. Building on this foundation, we categorize plant hormones according to their influence on competitive growth and their capacity to modulate cooperative dynamics. We examine how major hormones, including cytokinins (CKs), auxin, gibberellins (GAs), abscisic acid (ABA), brassinosteroids (BRs), strigolactones (SLs), ethylene, salicylic acid (SA), and jasmonate (JA), modulate growth, defense, and resource allocation in ways relevant to inclusive fitness theory. We further consider how these hormone-mediated strategies are reflected in large-scale evolutionary patterns, particularly the divergence between dicots and monocots, and discuss the implications of this framework for crop biology and agricultural practice.

2. Plant Competitive Traits Identified by Kin Selection Studies

A growing body of ecological and evolutionary research demonstrates that plants exhibit social behaviors shaped by genetic relatedness and environmental context. These studies have identified specific growth traits that plants modulate to navigate between competitive and altruistic interactions with neighboring individuals.

2.1. Empirical Evidence for Kin Selection in Plants

Although Hamilton’s foundational work in the 1960s suggested that kin selection could, in principle, also apply to plants—particularly in relation to reproductive investment [2]—direct empirical support has emerged only in recent decades. This growing body of work has substantially expanded our understanding of plant ecological and evolutionary strategies. Experimental and observational studies show that plants adjust growth, defense, and reproductive traits according to the genetic relatedness of neighboring individuals. These kin-dependent responses include both growth restraint and competitive escalation, indicating that plants can discriminate among neighbors and modulate their phenotype accordingly. These findings demonstrate that mechanisms of kin recognition and kin-modulated growth regulation are active and ecologically relevant in the plant kingdom. However, the physiological and molecular bases of these responses remain an active area of investigation. The remainder of this section synthesizes this literature and examines the mechanisms through which such kin-dependent strategies arise. Here, we use altruism in the strict Hamiltonian sense to denote heritable traits that reduce an individual’s direct fitness while increasing the fitness of genetically related individuals. By contrast, we use restraint or cooperative allocation to describe phenotypic strategies that limit competitive escalation or enhance group-level performance but whose direct fitness costs have not been explicitly demonstrated.

2.2. Kin-Dependent Growth Strategies Above- and Belowground

In terms of growth dynamics, kin-selected altruism in plants often manifests as suppression of shade-avoidance responses when individuals grow near genetically close relatives [15,16,17]. In contrast, interactions with genetically distant neighbors can elicit spiteful traits—such as exaggerated shoot growth—that reduce neighbor fitness with little or no direct benefit to the plant itself [15,18]. Plant kin selection effects are, however, most pronounced belowground, where reduced lateral root proliferation in the presence of kin reflects altruistic restraint, whereas enhanced root growth, including increased lateral branching, toward non-kin reflects competitive foraging [17,19,20]. Together, these root-level interactions suggest that belowground competition for soil resources is especially prone to Tragedy-of-the-Commons dynamics [21,22,23].

2.3. Resource Availability Shapes Altruism: Competition Trade-Offs

Resource availability is central to whether plants adopt competitive escalation or context-dependent restraint, which, under high relatedness, may function as altruism. In animals, even mild resource limitation can suppress altruistic behavior, as individuals increasingly prioritize self-maintenance when resources decline [24]. A comparable pattern appears in plants, expressed primarily belowground: altruistic responses decline, and resource allocation shifts toward competitive root foraging [19]. By contrast, when mineral nutrients are abundant, plants more readily express competitive behaviors both above- and belowground, although belowground competition is generally less intense in nutrient-rich soils than under nutrient limitation [25,26]. Together, these patterns suggest that soil nutrients—particularly nitrogen—often represent the more limiting resource in natural systems compared to carbon acquisition via photosynthesis. This is consistent with the fact that nitrogen is highly variable and often suboptimal in natural soils (outside fertilized agricultural systems) and can be depleted by root uptake, whereas carbon acquisition via photosynthesis is constrained primarily by light availability rather than by resource depletion [27,28]. Plants may also mitigate shading stress collectively through the cooperative suppression of strong shade-avoidance responses [15,18].

Additional evidence that soils constitute a more competitive environment for plants comes from studies showing that solitary plants often develop disproportionately large root systems that exceed what is required to support shoot growth and reproduction [29]. In the absence of kin-based community contexts, plants thus default to a competitive root-foraging strategy that is energetically costly and can ultimately reduce reproductive fitness. Consistent with this interpretation, solitary plants often produce fewer offspring than plants grown with kin, which invest less in root expansion and more in shoot growth, resulting in larger leaf area and enhanced photosynthetic capacity [29].

2.4. Kin Selection Effects Across Growth, Reproduction and Defense

Kin-dependent strategies influence multiple aspects of plant form and function, shaping how plants allocate resources to growth, defense, and reproduction in the presence of related or unrelated neighbors. Much of this plasticity is expressed through modulation of competitive behaviors, particularly in root and shoot architecture.

Table 1. Major Competitive Plant Traits Identified in Kin Selection Studies.

Trait	Description of Competitive Expression
Shoot elongation	Increased shoot apical dominance leading to vertical elongation for light competition [33].
Shoot branching angle	Increased branching angle (prostrate/lateral growth) to occupy horizontal space [15,17,34].
Leaf angle	Increased leaf angle to maximize light interception and shading of neighbors [35].
Root system size	Increased total root biomass to enhance access to soil resources [21].
Lateral root growth	Increased initiation and expansion of lateral roots to outcompete neighbors for soil resources [18,36,37].
Seed size	Increased seed size, conferring an early growth advantage over competitors [29,38].
Seed germination	Accelerated germination (reduced dormancy) to preempt resource acquisition by neighboring seedlings [39].

highlights major phenotypic traits associated with competitive escalation that have been identified in kin selection studies. Although these responses are often most visible in vegetative growth, kin-dependent effects can also extend to reproductive investment. In several species, aspects of floral development—including flower size, coloration, and nectar production—vary with the genetic relatedness of neighboring plants [30,31,32]. Larger, more conspicuous, and nectar-rich flowers in kin groups have been interpreted as a form of cooperative investment, enhancing collective pollinator attraction and thereby increasing inclusive fitness.

In addition to resource-mediated interactions, plants display context-dependent communal or competitive responses to biotic stressors such as herbivory and microbial attack. These responses involve strategic trade-offs between emitting volatile signals that alert nearby conspecifics and allocating resources to localized defense [40]. Together, these patterns underscore that plant responses to stress are not fixed but are flexibly tuned by genetic context and environmental conditions.

2.5. Summary

Collectively, the growing body of evidence indicates the following:

• Plants exhibit kin-modulated behaviors consistent with Hamilton’s inclusive fitness theory, demonstrating that altruistic traits in plants can evolve through natural selection when benefits accrue to genetically related individuals.
• Kin-dependent modulation spans multiple trait classes, including growth, defense, and reproductive investment.
• The magnitude and direction of these responses are context dependent, varying with environmental conditions, population genetic structure, and resource availability.

Together, these findings point to an emerging framework of plant social dynamics that parallels—while remaining mechanistically distinct from—kin selection processes described in animals. What remains unresolved, however, is how plants implement these kin-responsive strategies at the physiological level.

3. Hormonal Modulation of Competitive Behavior

Plants express competitive behavior primarily through growth. Shifts in root proliferation, shoot elongation, leaf and shoot branching angles, and defense allocation repeatedly emerge in studies of plant–plant interactions, revealing that competition is enacted through changes in architecture and resource investment. Yet most research has focused on these phenotypic outcomes rather than on the underlying regulatory mechanisms.

Unlike animals, whose social interactions are mediated by neural and behavioral plasticity, plants navigate social environments through adjustments in metabolism and development [41]. Competitive behavior in plants therefore manifests as coordinated changes in growth patterns and allocation strategies. Importantly, the very traits that define competitive escalation are well established to be under hormonal control.

This convergence naturally directs attention to plant hormone systems. As integrators of developmental, metabolic, and stress-related processes, hormones are ideally positioned to translate social and environmental cues into coordinated growth responses. We therefore propose that the modular organization of plant hormone pathways provides a mechanistic framework for understanding how competitive growth strategies are structured along the competition–cooperation spectrum revealed by kin selection studies. Although direct measurements of hormonal dynamics under these competitive contexts remain limited, the established hormonal control of the underlying traits makes this a plausible and testable hypothesis.

3.1. Hormonal Modularity as an Evolutionary Framework for Growth Trade-Offs

Plant hormone functions are organized modularly, reflecting their central role in regulating biological trade-offs, situations in which competing physiological processes cannot be simultaneously optimized [42]. This modular organization enables plants to prioritize specific growth or survival strategies in response to external conditions. A defining feature of this architecture is extensive cross-talk among hormone modules, which supports coordinated and flexible responses.

The existence of such modular specialization raises an important evolutionary question: how did distinct hormones arise and become associated with particular functional domains of plant growth and physiology? One plausible explanation is that plant hormones originated as metabolic byproducts of biochemical pathways that support core physiological processes [43]. Over evolutionary time, the stable association of metabolic byproducts with these processes facilitated their co-option as regulatory signals, ultimately positioning them as central regulatory nodes responsible for maintaining and coordinating essential functions as plant complexity increased.

From this perspective, hormones did not initially arise as pleiotropically acting signaling molecules but rather as evolutionary focal points that helped maintain optimal regulation of fundamental physiological capacities, such as reproduction, stress tolerance, and resource acquisition [43]. Their subsequent modular deployment thus reflects evolutionary pressure to preserve these core functions while allowing flexible reallocation of resources in response to environmental variability. This logic provides a foundation for understanding how hormone modules could later be repurposed to mediate higher-order strategies, including the balance between competitive and cooperative growth.

One fundamental strategy shaped by these evolutionary pressures is reproductive maximization. CKs appear unique among plant hormones in their capacity to maximally promote reproductive growth under optimal resource conditions [42]. At their evolutionary origin, CKs are composed of compounds that are stably associated with cell division (i.e., reproduction) and photosynthesis, and this stable association likely underpinned their evolution towards pleiotropic promoters of these core plant functions [43]. CK signaling functions as a default developmental program that favors shoot growth, photosynthesis, and reproductive investment, yet it is readily downregulated when environmental stress renders maximal reproduction maladaptive for survival. Consistent with this role, the CK signaling pathway relies primarily on direct activation cascades that efficiently promote gene expression linked to growth and reproduction.

In contrast, hormone pathways associated with stress adaptation or physiological “well-being” typically employ double-negative regulatory architectures that keep baseline activity low until specific adverse conditions occur [42]. This organizational asymmetry reflects fundamental fitness principles: reproduction is favored by default, whereas survival-oriented responses are engaged selectively to minimize unnecessary resource expenditure [44,45,46]. Once activated, these “well-being”, stress-associated modules suppress CK action as part of a coordinated shift from reproduction toward survival.

In light of this evolutionary framework, auxins, BRs, SLs, ABA, and SA can also be traced to the specific core physiological functions with which they were originally associated as metabolic byproducts and later evolved to regulate in a more integrated manner [43]. These foundational associations provide a mechanistic basis for their contrasting roles in promoting competitive versus altruistic growth strategies, examined in detail in Section 3.3. and Section 3.4. In contrast, the evolutionary origins of GAs, ethylene, and JA are less clearly understood. Although their roles in growth regulation and stress responses are well established, the pathways through which they became incorporated into modern hormonal signaling networks remain unclear. Nevertheless, it is plausible that these hormones also arose through analogous processes of metabolic co-option and subsequent functional specialization during plant evolution [43].

3.2. Conceptual Features Linking Hormones to Competition and Cooperation

The competitive traits identified in kin selection studies (Table 1)—particularly those related to root and shoot foraging behaviors—are strongly regulated by hormones. This convergence suggests that hormonal control may lie at the core of how plants adjust growth strategies under inclusive fitness scenarios. Before examining individual hormones, it is therefore useful to outline the general regulatory features that make hormone systems especially well suited to mediating the balance between competitive and cooperative behavior. Three conceptual features are especially important.

3.2.1. Hormonal Mediation of Growth: Reproduction Trade-Offs

At a fundamental level, competitive behavior in plants is expressed primarily through growth and is therefore most effectively regulated by hormone modules that control biomass allocation and organ expansion. A critical distinction in this context is between growth directed toward resource acquisition (foraging) and growth that directly supports reproduction. Investment in foraging enhances access to limiting resources but often comes at the expense of reproductive output, whereas restrained growth and resource conservation tend to favor reproductive efficiency and, by extension, inclusive fitness. Hormones that bias allocation toward foraging at the cost of reproduction thus align with competitive strategies, whereas hormones that restrain foraging or prioritize reproductive efficiency align more closely with altruistic outcomes.

3.2.2. Resource Depletability and Growth Architecture

The type of resource under competition further shapes how hormonal regulation translates into competitive behavior. Although plants compete for carbon via access to light, atmospheric CO₂ itself is not depleted by plant use and does not fluctuate on short ecological timescales [44]. In contrast, soil resources—particularly water and mineral nutrients—are spatially heterogeneous, rainfall-dependent, and readily exhausted by plant uptake. Consequently, belowground growth represents the primary arena in which competitive escalation gives rise to Tragedy-of-the-Commons dynamics [21,47]. Hormonal control of the shoot-to-root growth ratio therefore becomes a key determinant of competitive intensity under conditions of resource limitation. Under nutrient or water stress, plants typically increase root growth to enhance foraging, often at the expense of shoot growth, indicating that strong root-directed hormone activity both amplifies competition for soil resources and constrains competitive responses aboveground [48].

3.2.3. Signal Mobility and Evolutionary Constraint

The physicochemical properties of hormones impose additional evolutionary constraints on their potential roles in competition. Hormones that are volatile or mobile beyond the emitting individual inherently influence neighboring plants. From an evolutionary standpoint, traits that reliably enhance the competitive capacity of neighboring individuals are unlikely to be favored by selection [2,49,50]. Accordingly, hormones capable of acting beyond the individual plant are more plausibly associated with growth restraint, stress signaling, or communal defense than with competitive escalation [51,52]. Consistent with this expectation, volatile or semi-volatile plant hormones typically suppress aggressive growth and promote collective responses to environmental or biotic stressors.

3.2.4. Conclusions

Building on the preceding discussion of hormonal influences, we classify plant hormones into two broad functional groups based on three criteria: (1) whether they bias growth toward resource foraging versus restraint, (2) their typical induction context (resource limitation vs. stress or abundance), and (3) signal mobility. Auxins, GAs, and BRs are classified as predominantly competition-promoting, whereas CKs, ABA, SLs, ethylene, SA, and JA are classified as restraint- or cooperation-associated.

Throughout the remainder of this section, we distinguish empirically established hormone–trait relationships from hypothesis-driven interpretations linking these traits to kin selection and cooperative outcomes that have not yet been uncovered. Importantly, the hormone classifications used here are not categorical but probabilistic. Each hormone participates in multiple physiological processes and can contribute to either competitive or cooperative outcomes depending on developmental timing, tissue specificity, environmental context, and interaction with other signaling pathways. Our framework therefore describes dominant biases in growth regulation rather than fixed ecological roles.

3.3. Hormones That Promote Increased Foraging: Pathways of Competitive Growth

Among the major growth-promoting hormones, auxins, GAs, and BRs are most consistently associated with competitive growth responses and are characteristically upregulated under conditions of limited light, water, or mineral nutrients, indicating a shared role in promoting foraging-oriented growth at the expense of reproductive investment (Figure 1). In contrast, CK activity is typically reduced under these same conditions, reinforcing a functional distinction between hormones that promote competitive resource acquisition (growth for foraging) and those that favor reproductive efficiency under favorable environments (growth for reproduction).

Auxins, GAs, and BRs thus form a coherent group of hormone modules that enhance competitive traits by increasing investment in root and/or shoot expansion when resources become limiting. In the following sections, we examine these three hormones individually, focusing on their physiological targets, regulatory logic, and specific contributions to competition-promoting traits across the plant life cycle.

3.3.1. Auxins: The Primary Foraging Hormone

Auxins regulate plant growth in ways that closely align with competitive growth behavior. Auxin biosynthesis and signaling are upregulated under mild resource limitation, including mineral nutrient deficiency and moderate water stress, resulting in enhanced foraging growth [48]. Because auxin metabolism is closely linked to amino acid biosynthesis—pathways that encode information about cellular nutritional state—it is plausible that auxin was integrated early into regulatory networks coordinating growth with nutrient availability [42,43].

Under mineral nutrient limitation, auxin promotes a shift in the shoot-to-root growth ratio toward root expansion at the expense of reproductive investment [48]. This response is driven primarily by increased lateral root initiation and growth, a trait strongly associated with competitive acquisition of soil resources [29]. Following the evolution of vascular systems, auxin-mediated root growth became essential for water acquisition, and auxin signaling is correspondingly enhanced under mild dehydration stress [43,48].

Auxin-driven competition is not restricted to the root system. Auxin promotes increased seed size [53], larger shoot branching angles and leaf angles [54], and shade-avoidance responses [55]. In aboveground competition, auxin acts in concert with GAs: reduced red:far-red light ratios stimulate auxin synthesis and transport, reinforcing shoot apical dominance and facilitating elongation responses mediated largely by GA activity [55].

Auxin is well established as a central regulator of root foraging, shoot architecture, and reproductive development. Because these traits frequently underlie competitive growth strategies, auxin can be interpreted as a major hormonal module associated with competitive escalation across developmental stages and resource domains. We therefore hypothesize that attenuation or dysregulation of auxin action represents a key regulatory step when plants shift toward cooperative or altruistic strategies (Figure 1). Importantly, auxin also plays essential roles in symbiosis, developmental coordination, and collective resource acquisition [42], indicating that its apparent competitive bias reflects dominant outcomes under resource limitation rather than an exclusive function.

This figure summarizes the three major hormone classes identified in this review as primarily promoting competitive growth behaviors: auxins, GAs, and BRs. For each class, the upper panels indicate environmental triggers commonly associated with increased biosynthesis or signaling activity, including mineral nutrient deficiency, water limitation, and carbon/light limitation. The middle panels summarize dominant phenotypic effects, with emphasis on increased shoot and/or root foraging. The lower comparison table provides a qualitative synthesis of their relative influence on competitive growth. The “Competition Index” reflects the inferred strength of each hormone’s association with traits linked to resource acquisition and spatial expansion, based on recurring phenotypic patterns described in the literature. “Foraging Focus” indicates whether the hormone consistently promotes resource acquisition structures (roots and/or shoots), whereas “Reproduction Focus” indicates a dominant role in direct reproductive allocation. All indices are qualitative and intended as conceptual guides rather than quantitative rankings. The figure highlights dominant allocation tendencies and does not imply that these hormones function exclusively in competitive contexts or that their effects are independent of broader signaling crosstalk.

3.3.2. GAs and Carbon Acquisition Focus

Like auxins, GAs promote competitive behavior, but their effects are largely restricted to the shoot. GA biosynthesis and signaling are upregulated under suboptimal light conditions but suppressed under soil nutrient deficiency, reflecting GA’s limited role in belowground competition [56].

GAs play an essential role in maintaining photosynthetic performance under light limitation by supporting chloroplast development and function [57,58,59]. Accordingly, GAs promote adaptive shoot responses during neighbor-induced shading, including stem elongation, increased leaf angles, and reduced branching [60,61]. These responses raise the shoot-to-root growth ratio and reinforce GA’s specialization in carbon foraging. This strategy carries trade-offs, as enhanced elongation typically coincides with reduced reproductive investment [42,62]. In addition to these vegetative effects, GAs also contribute to competition by increasing seed size and reducing dormancy, thereby providing competitive advantages during early establishment [56].

GAs are well established as regulators of stem elongation and carbon-acquisition strategies. In contrast to auxin-mediated root proliferation, which directly exploits spatially limited and exhaustible soil resources, GA-driven elongation primarily enhances access to light—a resource that is not depleted locally at the ecosystem scale. On this basis, GA may exert a comparatively weaker influence on competitive dynamics than auxin. We therefore propose that GA occupies a more restricted position within the hormonal architecture of competitive growth (Figure 1).

3.3.3. BRs: Competitive Reproduction Maximization

BRs promote growth responses under mild deficiencies in both carbon and soil resources, with biosynthesis and signaling enhanced under such conditions [63]. Unlike auxins and GAs, BRs stimulate growth in both shoots and roots, increasing allocation toward vegetative expansion and thus increasing the foraging-to-reproduction ratio across multiple organs [64]. This dual action represents a distinct competitive strategy within the hormonal network.

BR function is closely associated with two core physiological processes conserved across plant evolution: cell division (and ultimately reproduction) and dehydration tolerance [43]. In this regard BRs resemble CKs in their capacity to support reproductive growth, but their reproductive promotion is more limited. Under resource shortage, BRs readily shift toward foraging-oriented growth, diverting resources from reproduction when light, water, or nutrients become limiting [42,43,64]. Consistent with this pattern, BRs promote many of the same competitive traits observed in auxin- and GA-mediated responses, including enhanced shoot elongation and leaf angle expansion under shade, as well as increased lateral root growth under mild drought or nutrient limitation [42,63,65]. Their consistent stimulation of increased seed size may further contribute to competitive advantage [42].

Compared with auxin, however, BRs may exert a more moderate influence on competitive escalation. Because BR function remains strongly linked to reproductive development, their capacity to drive sustained root-biased foraging—often central to aggressive competition for soil resources—may be more constrained (Figure 1). We therefore interpret BRs as contributing to competitive growth, but within a narrower regulatory scope than auxin.

3.4. Hormones That Limit Foraging: Growth Restraint and Cooperative Allocation

Competitive growth is not always advantageous to individual plants. Under many environmental and social conditions, limiting resource acquisition can increase long-term survival and reproductive success [8]. Several plant hormone systems function primarily to limit aggressive foraging, conserve shared resources, and stabilize growth under stress. In this section, we examine three such hormone classes—CKs, ABA, and SLs—that consistently suppress competition-promoting traits and bias plants toward restrained, potentially cooperative growth strategies. Figure 2 provides a schematic overview of some of the key relevant features of these three hormones, which are presented in more detail below.

3.4.1. CKs: Reproduction Maximization Under Favorable Conditions

Although CKs are classified as growth-promoting hormones, they differ fundamentally from auxins, GAs, and BRs in that they do not promote competitive foraging. CK biosynthesis and signaling peak under favorable environmental conditions and are downregulated by shading (low red: far-red light ratios), water limitation, and mineral nutrient deficiency [48,66]. Consequently, CK activity appears to be most strongly associated with environments in which competition for resources is minimal.

This figure summarizes three hormone classes—CKs, ABA, and SLs—that are associated with growth restraint, resource conservation, or reduced competitive escalation under distinct environmental conditions. The upper panels indicate primary environmental triggers. CK activity is typically elevated under favorable nutrient and water availability, whereas ABA and SL signaling increase under dehydration and soil nutrient limitation. The middle panels summarize dominant physiological effects. CKs promote reproductive allocation and inhibit excessive root proliferation, ABA enforces generalized growth inhibition and resource conservation under severe stress, and SLs redirect growth under moderate stress by suppressing lateral proliferation while preserving selective elongation. The lower bar represents a qualitative “Cooperation Index,” indicating the inferred extent to which each hormone’s dominant allocation pattern reduces competitive escalation. This index reflects recurring phenotypic associations rather than quantitative measurement. “High” indicates strong restraint of foraging traits (e.g., ABA under severe stress), “Moderate” indicates selective or conditional restraint (e.g., SLs), and “Low” indicates minimal suppression of competitive expansion (e.g., CKs under resource abundance). Importantly, the restraint depicted here does not necessarily imply Hamiltonian altruism. In many cases—particularly for ABA and SLs—growth suppression represents individually adaptive stress tolerance rather than cooperative signaling. The figure highlights dominant allocation tendencies and does not depict full signaling crosstalk networks.

Functionally, CKs promote reproductive investment rather than resource acquisition [42,48]. They favor shoot development over root growth but do not stimulate shoot elongation or increased leaf angles—traits central to light competition [66,67]. Belowground, CKs suppress overall root system expansion, particularly lateral root initiation and growth. This restriction limits root proliferation to levels sufficient to support shoot and reproductive development and may thereby enhance reproductive output under resource-abundant conditions [42,48].

This allocation strategy is consistent with inclusive fitness logic: by limiting costly root expansion and prioritizing reproduction, CKs reduce unnecessary resource expenditure while enhancing reproductive efficiency—potentially benefiting genetically related neighbors as well. In contrast, BRs, which combine reproductive promotion with competitive foraging responses, may promote biological fitness primarily at the individual rather than the community level.

In reproductive traits, CKs increase the total seed number at the expense of individual seed size [42]. Increasing seed number at the cost of seed size enhances reproductive output but reduces the competitive capacity of individual offspring, making this pattern consistent with reproductive maximization rather than strong competitive escalation. Nonetheless, CK-mediated altruism is not without constraints. As a hormone class that promotes shoot development, CKs inevitably increase aboveground spatial occupation—particularly through enhanced shoot branching—which can still impose competitive pressure on neighboring plants, especially since this increased canopy expansion can under some conditions also increase competitive shading, underscoring the context dependence of their ecological effects.

3.4.2. ABA: Growth Restraint Under Severe Stress

As a general inhibitor of plant growth, ABA is not typically associated with competitive growth behavior. ABA biosynthesis and signaling are strongly induced under severe soil resource limitation, particularly drought, placing its primary function in environmental contexts where restraint and conservation are favored [68,69].

Evolutionarily associated with a core mechanism that alleviates cellular dehydration stress, ABA controls a suite of responses that mitigate severe water deficiency [43]. These responses include stomatal closure, inhibition of cell division and meristem activity in both roots and shoots, induction of protective metabolites, reduced leaf angles, suppressed seed growth, and enhanced seed dormancy [68,70]. Together, these responses sharply reduce growth and resource expenditure. ABA also contributes to mineral nutrient conservation: severe nitrogen starvation upregulates ABA synthesis and action, promoting increased nitrogen use efficiency through senescence-based nutrient reallocation [71,72].

A key distinction relevant to ABA’s potential role in cooperative behavior lies in the nature of water as a resource. Unlike carbon or mineral nutrients, a large fraction of absorbed water is lost through transpiration. During warm, dry periods, this loss yields diminishing physiological returns while rapidly depleting shared soil water reserves. Under such conditions, coordinated restraint—such as reduced transpiration—can confer community-level benefits. Through its regulation of stomatal aperture, ABA may be well positioned to mediate such resource-conserving responses. Consistent with this interpretation, such traits have been recognized as components of a breeding ideotype aimed at improving group performance, echoing classic proposals in crop science that features like reduced transpiration conserve shared water resources rather than enhance the advantage of any single individual [73].

3.4.3. SLs: Allocation Control Under Moderate Stress

Like ABA, SLs are not typically associated with competitive growth behavior in plants. Their synthesis and signaling increase under soil resource limitation, particularly water and nutrient scarcity [74]. Under these conditions, SLs promote resource conservation and channel growth into forms that appear distinct from competitive foraging.

SLs likely share evolutionary roots with ABA, emerging in association with early dehydration tolerance mechanisms [43]. Functionally, the SL module appears to operate under moderate stress, conditions insufficient to trigger full ABA-mediated growth arrest [43]. Unlike ABA, SLs do not generally halt growth; instead, they redirect it.

Belowground, SLs suppress lateral root formation while promoting elongation of the primary root, thus potentially enabling deeper soil exploration without incurring the metabolic cost of extensive root proliferation [43]. Aboveground, SLs enhance apical dominance by inhibiting lateral shoot formation [74]. Unlike auxin- and GA-mediated shade responses, SL-induced apical dominance is coupled with accelerated senescence, resulting in overall shoot growth restraint rather than strong competitive expansion [75].

Additional evidence consistent with the restraint-oriented role of SLs comes from their suppressive effects on leaf angle, shoot branching angle, and seed size [65,76,77]. Beyond dehydration responses, SLs also facilitate nutrient acquisition and conservation, in part by promoting symbiosis with arbuscular mycorrhizal fungi (AMF). Through these associations, plants gain access to phosphorus, nitrogen, and water without investing in expansive root systems [74,78].

Like ABA, SLs also promote stomatal closure and improve nitrogen use efficiency through altered metabolism and reallocation, supporting their proposed role in minimizing resource depletion under limiting conditions [78,79].

3.4.4. Comparative Roles of CKs, ABA, and SLs in Growth Restraint

Taken together, CKs, ABA, and SLs can be interpreted as forming a continuum of restraint-oriented hormonal strategies that differ in timing, intensity, and ecological context. Although all three are associated with suppression of competition-associated foraging traits, they do so through distinct regulatory logics (Figure 2).

CKs appear to function primarily under favorable conditions, where resources are abundant and competitive escalation is unnecessary. Their effects may therefore be characterized as preventive rather than reactive: by limiting root proliferation and promoting reproductive allocation, CKs may reduce overinvestment in competitive structures before resource limitation arises.

ABA can be viewed as operating at the opposite extreme. Strongly induced under severe environmental stress—particularly drought—it enforces system-wide growth arrest and resource conservation. ABA is therefore associated with prioritization of survival over both competition and reproduction when continued growth would rapidly exhaust shared resources.

SLs occupy an intermediate position. Activated under moderate resource limitation, they impose selective rather than global restraint. By suppressing lateral proliferation while allowing targeted elongation and coupling apical dominance with senescence, SLs may reduce competitive escalation while maintaining limited foraging capacity.

Together, these modules can be conceptualized as enabling a graded suppression of competitive growth as environmental stress intensifies, from CK-mediated reproductive efficiency under abundance to SL-mediated allocation control under moderate limitation and ABA-driven survival-oriented arrest under extreme scarcity. In this framework, restraint is directed toward traits most likely to intensify Tragedy-of-the-Commons dynamics, particularly excessive root and shoot expansion.

3.5. Volatile Hormones: Growth Restraint, Communication, and Collective Defense

Ethylene, together with volatile derivatives of SA and JA (e.g., methyl salicylate and methyl jasmonate), forms a distinct class of mobile defense signals capable of influencing neighboring plants [80,81]. From an evolutionary perspective, it would be expected to be maladaptive for volatile signals to enhance the competitive capacity of surrounding individuals, as this would reduce the relative fitness of the emitting plant and be selected against [40,82]. Consistent with this expectation, functional analyses indicate that all three hormones predominantly promote growth restraint, resource conservation, and communal defense responses [83,84]. Their volatility therefore suggests alignment with cooperative or altruistic strategies rather than competitive escalation. Figure 3 provides a schematic overview of some of the key relevant features of these three hormones, which are presented in more detail below.

This figure illustrates the defense-associated hormone modules ethylene, SA, and JA, highlighting both their common and hormone-specific effects. The upper panels indicate primary environmental triggers. Ethylene responds to both abiotic and biotic stress, SA is primarily activated by pathogen challenge, and JA is induced by herbivory and necrotrophic pathogens. The central “Common Effects” panel summarizes shared physiological outcomes across these hormones, including general growth inhibition (affecting root, shoot, and seed development), reduced foraging intensity, and mineral nutrient conservation. These shared effects reflect the growth–defense trade-off, in which allocation shifts from competitive expansion toward stress tolerance and immune activation. The lower panels depict hormone-specific functions. Ethylene mediates growth inhibition in response to mechanical impedance and modulates nutrient-use efficiency. SA promotes pathogen detection, systemic resistance signaling, and self-/non-self-recognition. JA activates anti-herbivory and anti-pathogen metabolic defenses. Although some of these hormones possess volatile derivatives capable of inter-plant signaling (e.g., ethylene, methyl salicylate, methyl jasmonate), the growth restraint depicted here does not necessarily imply Hamiltonian altruism. In most contexts, reduced growth represents an individually adaptive response to stress. Any potential group-level consequences should therefore be interpreted cautiously and not as direct evidence of cooperative signaling. The schematic emphasizes dominant functional tendencies rather than full signaling crosstalk networks.

3.5.1. Ethylene: Growth Restraint Under Stress

Ethylene is a central regulator of plant responses to both abiotic and biotic stress and acts as a general inhibitor of shoot and root growth in most species [85,86]. Large-scale studies in Arabidopsis show that ethylene-overproducing plants exert minimal competitive pressure on neighbors, supporting the interpretation that ethylene acts as a repressor of competitive behavior [87]. Beyond growth inhibition in response to physical obstruction, ethylene also redirects development in non-competitive ways. It suppresses lateral root formation, decreases leaf angles, and limits seed size [61,86,88].

Ethylene’s effects on nitrogen homeostasis further support its restraint-oriented role [89]. Severe nitrogen deficiency induces ethylene biosynthesis, yet ethylene does not stimulate competitive root foraging. It inhibits lateral root proliferation but enhances root hair formation, increasing the efficiency of nutrient absorption per unit root biomass. At the same time, ethylene downregulates total nitrogen uptake, reducing the metabolic costs associated with acquiring a severely limited resource. Thus, ethylene appears to improve uptake efficiency while restraining overall nitrogen flux, consistent with a strategy of resource conservation under extreme nutrient limitation [89].

Although ethylene is induced under shading, it does not contribute to classical shade-avoidance responses. Rather, it counteracts auxin- and GA-mediated elongation, reinforcing its broader characterization as a hormone that constrains competitive growth rather than promoting it [90].

3.5.2. SA: Growth Restraint in Defense

SA and its derivatives are closely associated with the phenylpropanoid pathway, which produces antimicrobials and UV-protective metabolites [43]. Within this pathway, SA functions primarily as an intracellular defense hormone, whereas its methylated derivative, methyl salicylate (MeSA), acts as a mobile or semi-volatile signal [91,92].

SA activates immune responses locally and systemically, promoting growth restraint and defense-related metabolism. MeSA, by contrast, is a transport form that can move through the phloem or diffuse as an airborne cue, enabling long-distance or inter-plant communication. This biochemical partitioning allows plants to coordinate defense across tissues and potentially among neighboring individuals. Because neither SA nor MeSA promotes competitive growth, their volatility would not be expected to impose evolutionary costs associated with strengthening neighboring competitors.

Activation of SA signaling typically suppresses growth to support defense metabolism, placing SA centrally within the growth–defense trade-off [80]. This growth repression may also function as a form of communal restraint: smaller, slow-growing plants support lower pathogen loads, reducing the likelihood of pathogen amplification and spread to neighboring plants [82]. Such pathogen-driven restraint may parallel altruistic responses observed in other biological systems, where disease pressure favors traits that enhance inclusive fitness [93].

In addition to general growth inhibition, SA suppresses shoot elongation and apical dominance and limits lateral root development [80]. SA also promotes nitrogen conservation by enhancing nitrogen use efficiency [94,95]. Unlike ethylene, however, SA simultaneously stimulates nitrogen uptake, likely reflecting the elevated nutrient demand associated with sustained immune activation.

3.5.3. JA: Defense-Centered Growth Modulation

JA plays a dominant role in defense against herbivores and necrotrophic pathogens, stressors that cause rapid, localized tissue destruction and require strong, immediate defense responses [96]. JA exists in both nonvolatile forms (e.g., JA and JA-Ile, the primary bioactive conjugate) and volatile derivatives, most notably methyl jasmonate (MeJA).

Whereas JA and JA-Ile operate locally within tissues to activate defense signaling, MeJA functions as an airborne or phloem-mobile signal that can prime distal tissues or neighboring plants for herbivore defense. This division of labor resembles the SA/MeSA system: nonvolatile JA mediates intracellular defense activation, while volatile MeJA supports inter-plant or long-distance communication without directly promoting competitive growth in recipient plants.

Like SA, JA is a general inhibitor of plant growth and contributes to balancing growth and defense. Beyond its role in defense activation, JA influences several traits associated with restrained, non-competitive growth. These include reduced leaf angles [65] as well as reduced seed size and germination [97,98]. JA may therefore limit investment in traits that would otherwise enhance competitive foraging or early-establishment advantage.

JA’s influence on nitrogen metabolism supports a restraint-oriented interpretation of its function and parallels the influence of ethylene. Its synthesis increases under nitrogen deficiency; it suppresses nitrogen uptake and promotes nitrogen conservation through internal reallocation, particularly via chloroplast disassembly [99]. Together, these effects may limit resource depletion while supporting defense under conditions of biotic stress.

3.6. Hormonal Strategies Revealed by Conflict and Manipulation: Lessons from Allelopathy and Microbial Interactions

The preceding sections classify plant hormones according to their roles in promoting competitive foraging or enforcing restraint and potential cooperation. A useful test of this framework is whether it holds under ecological contexts where plants interact antagonistically with neighbors or are manipulated by parasites. Allelopathy and plant–microbial interactions provide such tests, as both involve targeted interference with growth, resource allocation, and defense [100].

Importantly, these interactions impose external selective forces rather than internally regulated developmental programs. As such, they provide independent lines of support for the functional partitioning of hormone modules described above.

3.6.1. Negative Allelopathy: Forcing Neighbors into Restraint

Allelopathy refers to the release of compounds by plants that alter the growth and physiology of neighboring individuals. In negative allelopathy, these compounds suppress the growth and fitness of neighbors, thereby reducing competition for space and resources [101,102,103,104]. A striking feature of many allelochemicals is that their effects converge on plant hormone regulation in the target plant, most prominently through auxin and ABA pathways.

Allelopathic interference with auxin signaling occurs through two primary mechanisms, both leading to reduced foraging capacity. In many cases, auxin biosynthesis is suppressed or degradation is enhanced, directly limiting root growth [105,106,107,108]. In other cases, auxin transport is disrupted, causing localized accumulation of inhibitory auxin concentrations in root meristems [108,109,110,111,112]. Despite these mechanistic differences, the functional outcome is consistent with reduced root expansion and diminished competitive foraging.

In contrast, allelochemical effects on ABA signaling tend to be more uniform, typically resulting in increased hormone activity [106,110,113,114]. Beyond suppressing seed germination, elevated ABA forces put plants into a low-growth, resource-conserving state analogous to endogenous stress-induced restraint. These hormone-mediated shifts not only suppress competitive growth but also promote water and nutrient conservation, potentially reallocating shared resources toward the allelopathic plant.

3.6.2. Plant–Microbial Interactions: Hijacking Competitive Modules

Plant-interacting microbes are currently classified as pathogens that negatively impact plant growth, symbionts that have a beneficial impact on physiology (especially nutrition), and plant-growth-promoting microbes (PGPMs). While pathogens are unambiguously parasitic, the other two groups involve interactions in which mutual benefit is tempered by the potential for exploitation by one partner. For example, symbioses with AMF and rhizobial bacteria enhance nutrient and water availability, yet these associations are actively restrained by CKs, such that the plant supplies photosynthates only in amounts compatible with its own reproductive interests [115]. Likewise, many PGPM effects can amount to functional parasitism, either by intensifying competitive dynamics or by promoting growth at times when inhibition is necessary for survival and inclusive fitness [116].

Because plant-interacting microbes often rely on some degree of parasitism—redirecting host resources for their own reproduction—it would be adaptive for them to manipulate host hormone systems to enhance resource acquisition. In contrast to allelopathy, which suppresses neighbor competitiveness, microbially-induced hormonal reprogramming often increases host foraging and growth, thereby creating a larger and more resource-rich substrate for exploitation [117].

Consistent with this logic, auxin, ethylene, JA and SA appear to be major targets of microbial manipulation. Among rhizosphere bacteria, a large proportion produce auxin, suggesting that this competition-promoting hormone may serve as a primary lever for altering host growth [118]. Many soil microbes also produce the enzyme 1-aminocycloporpane-1-carboxylate deaminase, which suppresses ethylene accumulation in the host, thereby further promoting growth-oriented behavior [118,119]. Manipulation of GA is less common than modulation of auxin and ethylene and appears most frequently in monocots, where it increases shoot foraging and vegetative expansion [120]. In addition, plant pathogens frequently suppress host immune mechanisms—including ethylene-, JA-, and SA-mediated defenses—thereby uncoupling immune activation from the growth inhibition that normally defines the growth–defense trade-off [100,121].

Some microbes also increase ABA signaling. Although this can reduce host competitiveness, its primary adaptive value for the microbe likely lies in immune suppression, as ABA antagonizes SA-mediated defense pathways and thereby facilitates pathogen success [122,123,124].

3.7. Synthesis: Hormone Modules Along the Competition–Cooperation Spectrum

Taken together, the preceding analyses indicate that plant hormone pathways can be interpreted as modular regulatory systems that differentially promote competitive foraging or enforce growth restraint under distinct environmental conditions. Evidence from stress physiology, allelopathy, and plant–microbial interactions further suggests that these modules are not only internally regulated but are also frequent targets of external manipulation, underscoring their central role in structuring growth allocation and resource strategy. This modular perspective provides a mechanistic foundation for examining how competitive and restraint-oriented strategies scale beyond individual interactions to broader evolutionary patterns, including those explored in the dicot–monocot transition in the following section.

4. Monocot Evolution: A Hormonal Shift Toward Communal Efficiency

Building on the preceding analysis of hormone-mediated competition and cooperation, this section examines how these strategies manifest at a broader evolutionary scale. The divergence between dicots and monocots suggests that hormonal regulation is not only context-dependent but also phylogenetically embedded. The evolutionary patterns discussed in this section represent synthesized trends derived from comparative physiology and ecology and are intended to be hypothesis-generating rather than definitive.

4.1. Evolutionary Origins of Reduced Competition in Monocots

A major transition in land plant evolution was the emergence of vascular systems, which enabled vertical shoot growth and the development of true roots that, beyond anchoring (a role also fulfilled by rhizoids in non-vascular plants), became essential for accessing soil resources. While this innovation greatly improved the acquisition of light, water, and mineral nutrients, it also created new arenas for competition—aboveground for light and belowground for soil resources—hereby increasing the potential of Tragedy-of-the-Commons dynamics, in which unchecked individual foraging reduces overall community fitness.

Under these conditions, natural selection is expected to favor traits that restrain excessive competition, particularly when individuals grow in close proximity to genetic relatives. Within this framework, the evolutionary contrast between dicots and monocots becomes especially informative. Although monocots are widely believed to have arisen from dicot ancestors [125], they exhibit a suite of architectural and developmental traits consistent with reduced competitive escalation.

Compared to dicots, monocots typically exhibit narrower leaf and shoot branching angles (especially the grasses), differences in vascular organization that constrain branching and elongation plasticity, and smaller root systems relative to shoot size [126,127,128]. Collectively, these traits may limit both aggressive aboveground shading of neighbors and excessive belowground resource monopolization.

4.2. Density, Compatibility, and Group-Level Fitness

Globally, monocots dominate agriculture, comprising the majority of staple crops such as wheat, rice, and maize [129]. One factor likely contributing to this dominance is their ability to maintain high performance at dense planting, conditions under which excessive competition would otherwise lead to declining yields. Experimental studies further support this interpretation: when monocots and dicots are grown together, monocots are often more negatively affected, indicating that dicots exert stronger individual-level competitive pressure [87].

Yet despite this disadvantage in direct pairwise competition, monocots achieve high biological fitness at the population scale, particularly under dense growth conditions. Dense monocot communities can rapidly form closed canopies that suppress the establishment of competing species by reducing light availability at the soil surface, a pattern that can be interpreted as a group-beneficial strategy rather than one based on aggressive individual competition [130]. The ecological success of this pattern is illustrated by the grasses, a monocot lineage that now covers approximately 40% of Earth’s terrestrial surface [131,132].

4.3. Auxin and Monocot Restraint

Auxin, a primary driver of competitive foraging, appears to operate under constrained regulation in monocots. Early agricultural observations revealed that synthetic auxin herbicides such as 2,4-D are far less effective in monocots than in dicots, suggesting reduced auxin sensitivity, enhanced degradation, altered regulatory control, or some combination of these factors [133]. Although both monocots and dicots possess the core components of auxin biosynthesis, transport, and signaling [54], studies with auxin-producing microbes demonstrate that monocots exhibit reduced responsiveness to IAA, indirectly supporting the hypothesis of diminished auxin sensitivity [134].

Beyond potential sensitivity differences, monocot developmental architecture places inherent limits on auxin’s ability to generate the traits most strongly associated with competitive escalation, such as prostrate shoot architecture and aggressive lateral root proliferation [135,136]. Some of these constraints arise from fundamental differences in root system organization [137]. Dicots typically possess a dominant taproot capable of producing large numbers of lateral roots in response to resource deficiency, enabling highly plastic and responsive foraging behavior. In contrast, monocots form fibrous, relatively shallow root systems with a more limited capacity for auxin-driven lateral root expansion. Monocots in general also have a higher shoot-to-root growth ratio, another feature of diminished auxin action [48]. These inherent architectural features may dampen the competitive amplification that auxin would otherwise promote.

Further evidence linking auxin sensitivity to competitive potential comes from rare monocot exceptions. Crabgrass, a highly competitive monocot weed, displays wide leaf and branching angles (i.e., prostrate growth) that allow it to effectively suppress neighboring vegetation [138]. Strikingly, crabgrass is also highly sensitive to auxinic herbicides, consistent with a more dicot-like auxin responsiveness and supporting a link between auxin sensitivity and competitive growth [139,140].

4.4. A Shift from Nitrogen Foraging to Carbon Efficiency

Auxin regulatory constraints in monocots are accompanied by a broader shift in resource strategy. Compared to dicots, monocots appear to place less emphasis on nitrogen foraging (an auxin-driven behavior) and greater emphasis on carbon acquisition and efficiency.

Relative to dicots, monocots respond less strongly to fluctuations in nitrogen availability [141]. They do not substantially increase nitrogen uptake when nitrogen is abundant, nor do they exhibit aggressive root expansion under deficiency. Instead, they appear to rely more on resource conservation than on the opportunistic foraging responses typical of dicots [141]. This approach is reflected in relatively stable internal nitrogen-to-phosphorus ratios across environmental conditions, in contrast to the greater stoichiometric plasticity observed in dicots [141]. In addition, the less aggressive foraging behavior of monocots towards soil resources is associated with their generally lower nitrogen and phosphorus content compared to dicots [142].

This reduced nitrogen focus is also evident in monocot biochemistry. Monocots typically allocate more biomass to carbohydrates than to proteins, particularly in seeds, which are starch-rich and comparatively protein-poor [143]. This shift is consistent with a more shoot-focused growth strategy and reduced nitrogen demand.

Further support for a carbon-centered strategy comes from the evolutionary history of photosynthesis. C₄ photosynthesis—which improves nitrogen use efficiency by reducing Rubisco requirements—originated earlier and more frequently in monocots than in dicots, despite the older evolutionary age of dicots [144]. Additionally, unlike many dicots, monocots do not form highly specialized nitrogen-fixing root nodules. Rather, they depend on more general endophytic associations that supply only modest nitrogen inputs, far below the self-fertilizing capacity of rhizobial nodulation [145].

4.5. Synthesis: Competitive Differences Between Dicots and Monocots

Table 2. Comparative Trends in Competitive Architecture and Resource Strategy Between Dicots and Monocots.

Trait Domain	Typical Pattern (Dicots vs. Monocots)	Key References and Evidence Strength
Leaf angle and canopy architecture	Dicots often display broader leaf-angle diversity and prostrate growth forms associated with horizontal space occupation; erectophile architecture is more common in grasses and many other monocots.	[35,61]—Moderate
Shoot branching plasticity	Dicots frequently exhibit greater lateral branching plasticity; many monocots (especially grasses) show more developmentally constrained branching linked to vascular organization.	[126,127]—Moderate
Root system type	Dicots commonly form dominant taproots with extensive lateral branching and strong plastic proliferation; monocots typically develop fibrous root systems with limited expansion capacity.	[47,126,128]—Strong
Nitrogen-responsive root proliferation	Many dicots show pronounced lateral root expansion under N limitation; monocots often exhibit more conservative morphological responses.	[141]—Moderate
Tissue N:P stoichiometry	Dicots tend to exhibit greater N:P plasticity across environments; monocots often show more stable internal N:P ratios.	[141,142]—Moderate
Seed biochemical allocation (C:N balance)	Many dicots (e.g., legumes) produce relatively protein-rich seeds; cereal monocots commonly produce starch-rich seeds with lower relative protein content.	[143]—Strong (crop systems)
Nitrogen fixation capacity	Nodulation-based symbiotic N fixation occurs in several dicot lineages but is absent in monocots.	[145]—Strong
C4 photosynthesis frequency	C4 photosynthesis evolved earlier and more frequently in monocots (especially grasses).	[132,144]—Strong
PGPM responsiveness (auxin/ethylene modulation)	Many PGPMs enhance auxin signaling and suppress ethylene, often increasing root proliferation; effects appear stronger and more plastic in dicot model systems, though comparative evidence remains limited.	[134]—Speculative/Indirect

summarizes the evidence pointing at decreased competitiveness in monocots compared to dicots, with the associated changes in development and physiology providing an increased capacity for monocot cooperative behavior.

5. Agricultural Implications

5.1. Domestication and the Erosion of Kin-Based Altruism

High-density planting—a defining feature of modern agriculture—has favored crop traits that suppress overt competition, including semi-dwarfism, reduced lateral root growth, and increased shade tolerance [146,147,148]. Importantly, these traits are often structural rather than regulatory: domesticated plants do not actively restrain growth in response to neighbors but are instead developmentally constrained from expressing strong competitive responses, regardless of neighbor identity.

Accumulating evidence suggests that domestication may have weakened kin-selected altruistic behaviors. Compared to wild relatives, many crops exhibit reduced performance when grown among genetically similar individuals, consistent with the loss of kin recognition or kin-responsive plasticity following domestication bottlenecks [29,149,150]. This erosion of genetic diversity likely disrupted inclusive fitness mechanisms that operate in natural plant populations.

These patterns align with Donald’s influential argument that crop improvement should prioritize group-level performance rather than individual vigor [73]. Breeding for communal compatibility—rather than maximizing the competitive potential of individual plants—may represent an underexploited strategy for improving yield stability in dense cropping systems.

5.2. Hormone-Guided Crop Design: Matching Growth Strategy to Yield Type

Recognition of kin selection and growth restraint in plants supports the importance of ideotype breeding, in which crops are optimized for collective performance. The same principle may apply to hormone-guided crop design, whether implemented through breeding, genetic engineering, or targeted hormone treatments. For example, modern wheat and rice breeding programs have explicitly selected semi-dwarf genotypes optimized for high-density planting, where reduced stem elongation improves yield stability in dense stands (e.g., Rht and sd1 lines) [73,146,147,148]. These density-dependent architectural shifts, caused by decreased GA action, illustrate how hormone-regulated traits are already evaluated under agronomically realistic competition regimes.

Our hormone classification framework highlights a key divide between hormones that predominantly promote competitive foraging (auxin, GAs, BRs) and those that bias growth toward restraint and cooperative outcomes (CKs, ABA, SLs, ethylene, SA, JA). This distinction may inform potential strategies for matching hormone action to crop yield type. For grain and fruit crops, where yield depends on reproductive output, enhancing competition-promoting hormones may be counterproductive, as the increased foraging can divert resources away from reproduction and reduce harvest index. In contrast, leafy and root crops may benefit from competition-enhancing hormones, as increased vegetative growth directly increases yield.

Among all hormone classes, CKs stand out as the most promising target for boosting grain yields because they enhance reproductive allocation while minimizing unnecessary competitive investment. However, elevated CK activity increases the shoot-to-root ratio, heightening sensitivity to drought and nutrient limitation [48]. Consequently, CK-based strategies are currently most effective in high-input systems, where ample water, fertilizer, and other inputs prevent stress-induced growth limitation. Future advances may decouple CK-driven yield gains from high resource inputs. Promising directions include engineering nitrogen fixation into crops [151], improvements in photosynthetic efficiency (e.g., carbon-concentrating mechanisms [152]) leading to lower Rubisco synthesis needs, and the development of drought-tolerance strategies that conserve water without growth arrest. Together, these innovations would allow CK-mediated, restraint-oriented growth strategies to be deployed more broadly and sustainably.

5.3. PGPMs: Benefits, Constraints, and Implications for Competitive Dynamics

PGPMs are often framed as sustainable tools for boosting crop productivity. However, when viewed through the lens of hormone-induced competition, their effects appear more nuanced and warrant re-evaluation.

Most PGPMs stimulate growth by increasing auxin signaling and suppressing ethylene, leading to enlarged root systems and increased shoot biomass [134,153]. While these changes can enhance performance in isolated plants, they also potentially intensify competitive behavior in dense plantings, an especially problematic outcome in grain and fruit crops, where reproductive efficiency rather than vegetative size determines yield. At the community level, such microbially induced growth escalation may reduce collective performance, producing a classic Tragedy-of-the-Commons dynamic. For example, field and greenhouse studies show that PGPM-induced increases in root proliferation or shoot biomass can enhance individual plant performance [134,153]. However, reported yield benefits are often context-dependent and vary across agronomic conditions, underscoring that microbially mediated growth promotion does not uniformly translate into improved performance under dense planting systems [153].

From an ecological perspective, many PGPMs function analogously to mild parasitic agents: by increasing host resource acquisition and body size, they improve microbial fitness, yet may reduce plant fitness at the population level. This does not negate their value but highlights the importance of context-dependent application. PGPMs may be most effective in vegetative crops, where increased individual growth directly translates into yield. In contrast, their use in reproductive crops must be carefully aligned with planting density and yield objectives, as growth promotion that shifts allocation away from reproduction can lower harvest index.

Ultimately, as with hormone treatments, the utility of PGPMs depends not on growth stimulation itself but on whether the induced hormonal state supports the cooperative or competitive allocation strategy best suited to a given cropping system.

5.4. General Recommendations

Based on the above framework, the following tentative recommendations emerge:

• Avoid strong Auxin/GA/BR stimulation in dense grain stands
• Consider foraging-promoting hormones for vegetative crops grown at low density
• Favor CK-based strategies for grain and fruit crops
• Differentiate between reproductive and vegetative crops when using PGPMs
• Use PGPMs cautiously at high density

6. Concluding Remarks

In this review, we broadly classified plant hormones based on their role in promoting competitive growth traits. Future research is expected to refine this classification, positioning these hormones along a more precise competition–cooperation gradient. Beyond its agricultural relevance, this framework aids in interpreting major developmental transitions. For instance, the rise and ecological success of monocots are linked to architectural and regulatory constraints on auxin-driven competition, alongside a shift toward carbon-use efficiency and compatibility in dense growth conditions.

We propose that this functional partitioning of plant hormones offers a conceptual framework for understanding inclusive fitness dynamics—such as kin selection and context-dependent cooperation—in plants. Empirical studies show that kin recognition alters traits under strong hormonal control, including root proliferation, shoot elongation, shade-avoidance responses, and defense allocation. For example, suppression of lateral root growth among kin reflects reduced auxin-dependent foraging, while changes in shade-avoidance responses align with modulation of auxin–GA signaling. Similarly, kin-dependent variations in defense investment correspond to pathways regulated by jasmonate, salicylic acid, and ethylene.

However, most current studies infer hormonal involvement indirectly through phenotypic outcomes rather than direct hormone measurements or pathway manipulation. Thus, while hormone-mediated trade-offs provide a compelling mechanistic basis for kin selection, direct experimental links between kin perception and specific hormone signaling events remain a critical gap for future research, as previously noted [154].