Arbuscular Mycorrhizal Symbiosis: A Strategy for Mitigating the Impacts of Climate Change on Tropical Legume Crops

Climate change is likely to have severe impacts on food security in the topics as these regions of the world have both the highest human populations and narrower climatic niches, which reduce the diversity of suitable crops. Legume crops are of particular importance to food security, supplying dietary protein for humans both directly and in their use for feed and forage. Other than the rhizobia associated with legumes, soil microbes, in particular arbuscular mycorrhizal fungi (AMF), can mitigate the effects of biotic and abiotic stresses, offering an important complementary measure to protect crop yields. This review presents current knowledge on AMF, highlights their beneficial role, and explores the potential for application of AMF in mitigating abiotic and biotic challenges for tropical legumes. Due to the relatively little study on tropical legume species compared to their temperate growing counterparts, much further research is needed to determine how similar AMF–plant interactions are in tropical legumes, which AMF species are optimal for agricultural deployment and especially to identify anaerobic AMF species that could be used to mitigate flood stress in tropical legume crop farming. These opportunities for research also require international cooperation and support, to realize the promise of tropical legume crops to contribute to future food security.


Introduction
In a climate-stressed world, threats to food security will be exacerbated, resulting in widespread hunger and malnutrition, particularly in the tropics, which include parts of Asia, Africa, Central America, and the Caribbean [1]. The tropical latitudes are home to the most highly concentrated human populations [2] and yet tropical species have a higher sensitivity to changes in climate than temperate species, meaning that tropical crops, including tropical legumes, are most vulnerable to climate change [3]. Major crops that sustain global food security, such as soybean (Glycine max) and rice (Oryza sativa), have been reported to face enormous biotic and abiotic challenges as the globe accelerates toward a warmer, more unstable future [4]. Crop pests that have expanded their range due to climate change have had a significant impact on agriculture, with one notable example being the rapid spread of a nature pest host, Johnsongrass (Sorghum halepense), a highly invasive weed that affects the production of a variety of crops, including soybean and sorghum (Sorghum bicolor) in America [5]. Furthermore, the effects of rising temperatures are projected to become more complex and difficult to manage, with multiple hazards occurring concurrently, ranging from droughts and heatwaves to sea-level rise and floods. Ironically, agriculture has contributed significantly to the climate crisis in a vicious loop, accounting for roughly a quarter of global greenhouse gas emissions [6,7]. While atmospheric carbon dioxide levels have surpassed 420 parts per million (ppm) [8], the highest in based protein substitutes. Apart from highlighting the beneficial role of AMF in enhancing leguminous crops under abiotic stresses, this review explores the potential application of AMF in fending off biotic challenges such as pests and diseases. Wherever possible examples from topical legumes have been used; however, for comprehensive coverage, examples from temperate legumes and from non-legume plants have been used where there is no information for any tropical legume species. The research gaps, challenges, and strategies for maximizing the potential of these bio-fertilizers are also discussed. There is a definite need for research on the AMF-mediated promotion of legume growth and productivity, but determining where to begin can be challenging.

Molecular Insight into the Establishment of Symbiosis between AMF and Tropical Legumes
Most studies on legume-AMF colonization and signaling have been conducted on the model legumes, birdsfoot trefoil (Lotus japonicus) and barrel medic (Medicago truncatula), while tropical legumes such as cowpea (Vigna unguiculata) and pea (Pisum sativum) have received relatively less attention [26]. The process of establishment of symbiosis between AMF and legume plants can be categorized into three stages: precolonization, root colonization, and arbuscule formation and nutrient exchange ( Figure  1).

Pre-Colonization
Chemical communication between AMF in the rhizosphere and host plant roots initiates the symbiosis process. Under phosphate deficiency, the roots of host plants secrete strigolactones (SLs), a class of phytohormones [27]. Plants produce significantly less strigolactones under high inorganic phosphate (Pi) conditions than under low Pi conditions [28]. SLs are mainly carotenoid derivatives synthesized from all-trans-βcarotene via a four-step enzymatic pathway [29]. The all-trans-β-carotene is isomerized into cis-β-carotene by carotene isomerase before undergoing cleavage by carotenoid cleavage dioxygenases (CCD). The cis-β-carotene is cleaved and modified by CCD7

Pre-Colonization
Chemical communication between AMF in the rhizosphere and host plant roots initiates the symbiosis process. Under phosphate deficiency, the roots of host plants secrete strigolactones (SLs), a class of phytohormones [27]. Plants produce significantly less strigolactones under high inorganic phosphate (Pi) conditions than under low Pi conditions [28]. SLs are mainly carotenoid derivatives synthesized from all-trans-β-carotene via a four-step enzymatic pathway [29]. The all-trans-β-carotene is isomerized into cisβ-carotene by carotene isomerase before undergoing cleavage by carotenoid cleavage dioxygenases (CCD). The cis-β-carotene is cleaved and modified by CCD7 (reported as RAMOSUS5 (RMS5) in pea, More Axillary Braching3 (MAX3) in soybean) into 9-cis-β-apo-10 -carotenal, which is subsequently cleaved into carlactone by CCD8 (reported as RMS1 in pea, More Axillary Braching4 (MAX4) in soybean) [30,31]. Carlactone is oxidized into SLs by members of the cytochrome P450 (CYP) family [32,33]. SLs are then released into the rhizosphere by ATP-binding cassette transporters in root hypodermal cells [34]. SLs are important for stimulating AMF hyphal branching and elongation to support root colonization between plant and AMF [27]. However, SLs are not the sole compound responsible for the establishment of root colonization. Plants with impaired SL synthesis have shown reduced AMF colonization but do not completely impede it [35].

Root Colonization
When AMF encounters plant-derived SLs, in response, it secretes mycorrhizal (Myc) factors. It has been proposed that these Myc factors are recognized by Lysin motif receptorlike kinases (LysM-RLK), which further activates Symbiosis receptor kinase (SYMRK in L. japonica, Does not make infections2 (DMI2) in M. truncatula) [36]. SYMRK/DMI2 is reported to be associated with 3-hydroxy-3-methylglutaryl-CoA reductase (HMGR), an enzyme which is involved in mevalonate production [37]. Mevalonate promotes nuclear calcium oscillations in legume root cortical cells [38]. CASTOR and POLLUX (reported in L. japonicus and soybean), two cation channels located on the nuclear membrane, and DMI1 (reported in M. truncatula), a single inner-membrane-localized channel, were also reported to play crucial roles in inducing calcium oscillations [39,40]. Calcium oscillations activate the calcium-and calmodulin-dependent protein kinase (CCaMK) [41], which phosphorylates CYCLOPS. Following that, CYCLOPS binds to a cis-element in the Reduced arbuscular mycorrhizal1 (RAM1 in L. japonicus, M. truncatula and soybean) promoter and induces its transcription [41,42]. RAM1 encodes a GRAS-domain transcription factor that regulates the expression of RAM2. RAM2 encodes a glycerol-3-phosphate acyl transferase (GPAT) that is involved in the production of cutin monomers [43]. Cutin monomers are essential for hyphopodia formation in AMF [44]. Hyphopodia are specialized hyphal branches composed of lobbed cells which help in the adhesion of AMF to plant roots [43]. Mutation in RAM1 and RAM2 showed a reduction in AMF colonization and impaired hyphopodia formation [45]. The nucleus of the root epidermal cell in contact with the hyphopodium migrates and positions itself underneath the hyphopodium to initiate the formation of the pre-penetration apparatus, a subcellular tube-like structure [46].

Arbuscule Formation and Nutrient Exchange
The pre-penetration apparatus of the host plant allows AMF hypha to grow intracellularly [46]. When AMF hypha reaches the root cortical cells, it forms an arbuscule (intercellular branching hyphal structure). A plant protein, Vapyrin, plays an important role in epidermal penetration and arbuscule formation [47]. Silencing of the Vapyrin gene in M. truncatula showed impaired epidermal penetration and abolishment of arbuscule formation [48]. During the arbuscule formation, the root cortical cell undergoes a major transient reorganization and envelopes the arbuscule in a membrane known as the periarbuscular membrane (PAM) [49]. The PAM, which is continuous with the plant plasma membrane of the cortical cell, is the main interface for nutrient exchange. The area between the PAM and the AMF plasma membrane is called the periarbuscular space (PAS). HA1, a proton (H + )-ATPase pump, generates a proton gradient across the PAM to energize two PAM-localized proteins such as Phosphate transporter4 (PT4) and Ammonium transporter2 (AMT2) [50]. Following that, the energized PT4 imports phosphate, and AMT2 imports ammonia from the arbuscules to the plant cell. In return, plants provide carbon sources to the AMF in the form of sugars and lipids [19]. Sugars, mainly in the form of hexose, get transported to the PAS by sugar transporters such as Sugars Will Eventually be Exported Transporters1b (SWEET1b) [51]. Sugars in the PAS are transported by Monosaccharide Transporter2 (MST2) into the arbuscules [52]. AMF lack genes required to synthesize long chain fatty acids [53]. Therefore, AMF depend on host plants for important fatty acids. The involvement of plant fatty acid in arbuscule development has been demonstrated in M. truncatula. Medicago truncatula mutant lines defective in two lipid biosynthesis genes FatM and RAM2 showed impaired arbuscule branching [46]. FatM encodes an ACP-thioesterase, which terminates fatty acid chain elongation and releases 16-carbon fatty acid. The 16-carbon fatty acid, along with CoA used as a substrate by GPAT (encoded by RAM2) to produce 16-carbon β-monoacylglycerol [49]. This fatty acid compound is diffused across the PAM by two half-ABC transporters, Stunted Arbuscule1 (STR1) and STR2 located at the PAM [54]. The expression of STR1 and STR2 in AMF is dependent on Pi concentration. High Pi represses the expression of STR1 and STR2. Hence, depending on Pi supply, the AMF may starve for plant lipids.

Alleviation of Abiotic Stress
Numerous studies report that climate change-related abiotic stresses including heat, drought, salinity, and waterlogging impair the uptake of soil nutrients and water balance [55][56][57]. These phenomena cause crop yield to reduce significantly, resulting in increased pressure for food production. Although the use of chemical fertilizers can increase soil fertility and crop productivity, excessive use of agro-chemicals reduces the soil organic matter and quality, and the residues contribute to pollution of adjacent water bodies [58].
To circumvent these problems, farming communities are considering soil microorganism such as AMF for sustainable agricultural practice (Table 1). Reduced AMF root colonization 3.
The salinity effects on pigeon pea were more serious in salt-sensitive than salt-tolerant genotype Increased sodium ion and proline content 1.
Multispecies-based consortium AMF performed better than single species AMF and non-inoculated in salt-stressed plants [77]  Increased antioxidant enzyme activity, osmolyte concentration 4.
Regulation of genes for stress response, oxidation-reduction, proline catabolism, cell wall biogenesis and so on 1.
Enhanced growth, higher photosynthetic rate, leaf relative water content, osmolyte accumulation but lower leaf relative electrolyte conductivity 2.
Increased antioxidant enzyme activities but reduced MDA concentration 3.
Increased peanut yield, protein content in kernel 4.
AMF inoculation helped in regulation of genes responsible for oxidation-reduction process, pyruvate transport, carbohydrate metabolic process, and cell wall biogenesis and cell growth [22] Heat

Heat Stress
Extreme heat stress affects legume growth and metabolism. Under heat stress, heatsensitive enzymes involved in physiological processes are inhibited, causing a disruption in the cellular homeostasis; this is associated with the accumulation of harmful reactive oxygen species (ROS) [86]. This was reflected in elevated electrolyte leakage, proline, malondialdehyde (MDA), and hydrogen peroxide (H 2 O 2 ) contents in heat-stressed faba bean [87] and mung bean (Vigna radiata) [88] plants (Table 1). Heat stress also affects the reproductive stages and yield of legumes. Heat-stressed green bean (Phaseolus vulgaris) plants showed delayed flowering and decreased pollen viability and pod and seed yields as compared to non-stressed plants [89]. Similarly, a 10-day exposure to 35/16 • C (day/night) during the flowering and pod development stage resulted in decreased pod yield in chickpea (Cicer arietinum) plants [90] (Table 1).
Studies have shown that AMF help in alleviating the negative effects of heat stress in many plants, although relevant information on AMF-legume interaction is limited. Under heat stress, the ability of plant roots to absorb water and nutrients reduces. AMF improve plant tolerance to heat mainly through enhancement of water and nutrient uptake, which in turn improves plant growth and yield under heat stress [91]. This is evident from higher water use efficiency, water holding capacity, and relative water content in AMF-inoculated maize (Zea mays) grown at 40 • C [92]. Meanwhile, AMF-inoculated asparagus (Asparagus officinalis) accumulated more of the macronutrients nitrogen (N), phosphorous (P), and potassium (K), and micronutrients such as calcium (Ca), magnesium (Mg), and iron (Fe) than non-AMF plants under heat stress [93]. In the model legume plant M. truncatula, a night temperature elevated by 1.53 • C negatively affected growth. Inoculation with the AMF Rhizophagus irregularis mitigated the effects of heat stress and enhanced M. truncatula growth in terms of biomass, flower and seed number, leaf sugar, shoot zinc, and root phosphorus contents [83] (Table 1). While the mechanisms have not been reported for any legume species, AMF have been associated with improved photosynthetic capacity, stomatal conductance, and transpiration rate in heat-stressed maize inoculated with a mixed AMF culture of R. irregularis, Funneliformis mosseae, and F. geosporum [94]. The improved photosynthesis in AMF-inoculated plants could be attributed to protective effects from the symbiotic fungi against oxidative damage caused by high temperature. Under heat stress, mycorrhizal plants often exhibit enhanced activities of various antioxidant enzymes [93,95]. AMF Septoglomus deserticola and S. constrictum ameliorated heat stress-associated oxidative damage in tomato (Solanaceae lycopersicum) by reducing the levels of lipid peroxidation and H 2 O 2 , while elevating the antioxidant enzyme activities in root and leaves [96].

Drought Stress
The impacts of drought on legume growth and yield involve a series of complex processes. Legumes respond directly to drought stress by triggering stomatal closure to prevent water loss through transpiration. This is evident from studies on soybean and green bean that reported a decrease in stomatal conductance and transpiration rate during a drought stress treatment [21,62]. Stomatal closure limits the CO 2 intake and subsequently inhibits photosynthesis. In a drought treatment, a drought-sensitive soybean cultivar Anta82 experienced reduced photosynthesis rate after 3 days [21]. As a result of reduced photosynthesis under drought conditions, legume growth and yield are strongly inhibited. Reduction in various growth and yield parameters were also reported in drought-stressed green bean [62], soybean [21,60,61], and chickpea [65] plants (Table 1).
Studies have shown that AMF symbiosis aids in physiological regulation of legumes, which enhances their tolerance to drought. AMF colonize plant roots and develop an extensive network of extraradical hyphae in the soil surrounding the root, which help to absorb water from the soil [97] and enhance water uptake under drought conditions. Higher stomatal conductance, transpiration, and photosynthesis rates occurred in AMFassociated legumes such as soybean and green bean [21,62] (Table 1). AMF treatment on legumes is also beneficial for the uptake of nutrients: When administrated with 45% to 75% water holding capacity, mycorrhizal green bean plants had increased macro-and micronutrients including N, P, K, Mg, Fe, zinc (Zn), manganese (Mn), and copper (Cu) [63]. AMF treatment also improved the green bean yield under drought stress as seeds with higher nutrient contents including N, K, Mg, Ca, vitamin B1, folic acid, crude fiber, and protein were produced compared to non-mycorrhizal plants [62].
Under long term water deficit, stomatal closure and low CO 2 supply alter the cellular homeostasis and disrupt the electron transport and carbon-reduction cycle [98]. Low intercellular CO 2 leads to over-reduction of electron transport components and subsequent leakage of electron to oxygen molecules which generates ROS. A high ROS accumulation causes oxidative damage to the nucleic acids, proteins, and lipids [99]. Drought stress resulted in high superoxide radical (O 2 − ), H 2 O 2 , and MDA accumulation in black locust which was ameliorated by AMF treatment (Table 1). Mycorrhizal black locust exhibited higher activities of the antioxidant enzymes superoxide dismutase (SOD), peroxidase (POD), catalase (CAT), ascorbate peroxidase (APX), and glutathione reductase (GR) [68]. The ROS generated during drought stress also damage the photosynthetic apparatus of plants and this was evident from the reduced photosynthetic chlorophyll pigments in chickpea under water stress. This condition was ameliorated in mycorrhizal chickpea plants reflected by enhanced chlorophylls and carotenoid content [65].
Several studies have shown that interaction with AMF altered gene expression in the host legumes in response to drought stress. To combat the ROS generated during a drought treatment, mycorrhizal black locust plants exhibited higher gene expression for antioxidant enzymes such as Cu/Zn-superoxide dismutase (Cu/Zn-SOD), ascorbate peroxidase (APX), and glutathione reductase (GR) in the roots, stems, and leaves or at least one of the organs [68]. This shows that AMF-legume interaction provides protection against drought by inducing the expression of genes responsible for antioxidant enzyme activities. Aroca, et al. [100] reported that AMF treatment enhanced the tolerance of green bean plants to drought through regulation of various aquaporin genes that were found to be differentially expressed in the presence of AMF. A transcriptome analysis also revealed upand downregulation of aquaporin-related genes in the leaves and roots of drought-stressed mycorrhizal green bean plants, together with altered transcription levels in pathways for osmoregulation, DNA repair, and response to oxidative stress during drought stress [64].

Salinity Stress
Salinity impedes the growth and decreases the yield of many legume species. Salinity tests (ranging from 50 to 250 mM NaCl) resulted in lower plant height and shoot and root biomass in green bean [70], soybean [71], pigeon pea [73], cowpea [78], and chickpea [80]. Salt stress also decreased pod yield and dry weight in faba bean and green bean [69,70] ( Table 1). These negative impacts are attributed to the osmotic, ionic and oxidative stresses that are induced under high salinity [101]. Studies have shown that AMF symbiosis reduces the negative impacts of salt stress in legumes. AMF-inoculated pea [77] and green bean [70] plants exhibited greater plant height, root length, and shoot and root biomass under salinity treatment, while mycorrhizal faba bean plants had higher numbers of pods per plant and higher pod dry weight [70,77]. Other than model AMF species such as Glomus irradicans [70], R. irregularis [73], and F. mosseae [79], the use of native AMF inoculum from saline soil also improved the growth and development of pigeon pea (Cajanus cajan) plants under salt stress [72,73].
High salinity levels induce osmotic stress that impairs the root system by reducing the water availability for plant metabolic processes [102]. AMF root colonization and extension of the fungal extraradical hyphae into the soil significantly enhance the water uptake in various legumes. This was reported for legume species such as cowpea, green bean, and fenugreek, where the AMF-inoculated plants exhibited significantly higher relative water content in their leaves when compared to non-inoculated plants [70,75,78] (Table 1).
Salinity also causes an ionic stress attributed to sodium (Na + ) and chloride (Cl − ) ion build-up in the plant cytosol, which is toxic to the plants [103]. At the same time, other mineral ions such as K, Ca, P, and Mg are lower in legume plant cells [69,70,77,78], which can result in reduced photosynthetic pigments [70,75]. Mycorrhization of legumes alleviates the deleterious effects of salinity-induced ionic stress by reducing uptake of toxic Na + and Cl − , which is crucial in maintaining the ionic homeostasis in legume plant cells. As a result of AMF-inoculation, faba bean [69] and pea [77] plants retained higher P, K, Mg, and Ca contents and showed improved growth compared to non-mycorrhizal plants under high salinity. Mycorrhizal legumes also retained higher pigment contents compared to non-mycorrhizal legumes in a salinity condition, indicating the role of AMF in modulating plant ion contents such as Mg, an essential component of photosynthetic pigment [69,71,72].
Salinity also causes oxidative stress attributed to the build-up of ROS such as H 2 O 2 and O 2 − which damage cellular lipid, proteins, and nucleic acids [104]. In response to salt stress, legumes produce antioxidant enzymes or antioxidant molecules to scavenge the aggressive ROS [73,105] and increase levels of proline that could reduce the oxidation of lipid [106]. AMF inoculation can enhance the salt tolerance of legumes such as green bean [70], faba bean [69], and pigeon pea [73] by elevating antioxidant enzyme levels ( Table 1). Compared to non-inoculated plants, these mycorrhizal legumes showed higher levels of SOD, CAT, GR, and POD. Mycorrhization of fenugreek and grass pea plants was also associated with enhanced proline content [75,76] although levels were reduced in mycorrhizal pea plants [77]. In addition, MDA, an end-product of lipid peroxidation, was reduced in various mycorrhizal legumes such as peanut [22], pigeon pea [73], and soybean [71] ( Table 1).
The complexity of AMF-induced salt tolerance in legumes has been alluded to from transcriptome studies: Transcriptome analysis identified differentially expressed genes (DEGs) that were responsible for regulation of biological processes pertinent to oxidation and reduction, oxidative stress response, cell wall, and cellular component organization in the roots of mycorrhizal peanut plants that were salt-stressed, as compared to non-mycorrhizal plants. The study also reported higher expression of peroxidase and glutathione S-transferase genes in mycorrhizal peanut plants [22]. In another study, AMFmediated salt tolerance in Sesbania cannabina was associated with DEGs related to oxidationreduction processes, photosynthesis, and several transcription factor groups. Elevated expression of genes related to SOD, CAT, POD, and GR was also observed in mycorrhizal S. cannabina plants, indicating the AMF role in enhancing the plant ROS-scavenging capability under salinity stress [81].
A majority of legume species form symbioses with rhizobia, which are beneficial for growth and productivity. However, the number of root nodules, nodule biomass, and leghemoglobin content in various legume plants decreases under high salinity [69,72,102,107] ( Table 1). This indicates reduced nitrogen fixation as leghemoglobin is responsible for supplying oxygen to nitrogen-fixing bacteria and protecting nitrogenase against oxygen damage in the nodules [108]. AMF inoculation alleviated these salt stress impacts in the legumes faba bean and soybean, which demonstrated enhanced nodulation and higher nitrogenase activity in their nodules [69,71,72]. Mycorrhization also led to higher accumulation of trehalose that can act as osmoprotectant in the nodules of salt-stressed pigeon pea [72] plants. Nonetheless, high salinization reduces AMF root colonization and spore counts in the rhizosphere, as reported in faba bean [69], grass pea [76], and pigeon pea [74] plants.

Waterlogging Stress
Legumes are very susceptible to waterlogging stress and do not thrive under inundated conditions. Under waterlogged conditions, soybean [109], cowpea [110], faba bean, grass pea, and lupins [111] suffered significant growth reduction and yield loss. Waterlogging causes air in the soil to escape and available oxygen is greatly reduced, resulting in a hypoxic condition in the rhizosphere that inhibits plant root respiration. Although plants can maintain energy production through anaerobic respiration, the process also accumulates toxic metabolites such as lactic acid, ethanol, aldehydes, and various ROS species [112,113]. High ROS levels cause oxidative damage to the cell membrane [114] and photosynthetic apparatus [110]. This is evident from the high cell membrane injury in green bean plants waterlogged for 7 days, and the significant decrease in stomatal conductance, chlorophyll contents, transpiration, and photosynthesis rates, as well as final seed yield in waterlogged mung bean [115] and cowpea [110]. Waterlogging also prevents root uptake of essential nutrients from the soil such as nitrogen and minerals. This leads to a nutrition imbalance which also contributes to reduced growth and yield loss in legumes. Although faba bean plants survived a 20-day waterlogging treatment, the plants showed severe reduction in total nitrogen uptake, in addition to decreased seed and biomass production [116].
Studies have shown AMF symbiosis to help plants to survive waterlogging stress. However, waterlogged soil creates an anaerobic condition which is unfavorable for AMF which are obligate aerobes [117] and decreases root colonization in various plants [117,118]. Nonetheless, some AMF such as Gigaspora species in rice ecosystems were found better adapted to semi-aerobic and anaerobic soils [119]. These AMF were shown to provide protective effects and enhance the growth of various plants under waterlogging stress. Although there is limited study on the role of AMF in enhancing legume growth and survival in waterlogged conditions, it was shown that the AMF root colonization of green bean plants was not affected by repeated short term flooding [85]. When AMF-colonized green bean plants were treated with short term flooding, they exhibited enhanced growth in terms of root dry weight [85] (Table 1). This indicates the potential of AMF in alleviating the negative effects of waterlogging in legumes. As climate change is associated with more extremes in weather, including flooding, as well as predicted rises in sea levels in the tropical regions, it will be valuable to explore AMF that are able to mitigate waterlogging stress in tropical legumes.

Alleviation of Biotic Stress
The role of AMF in the alleviation of biotic stresses in tropical legumes has received much less attention compared to abiotic stresses [120]. The following are examples of pests and diseases that affect legumes and for which AMF have been reported to relieve symptoms (also summarized in Table 2). From the limited number of available reports, it can be suggested that much further research is needed in this area. Reduced disease incidence and severity [125] Soybean (Glycine max) Rhizophagus irregularis

1.
Increased plant height, fresh and dry weight, pod number 2.
Increased chlorophyll and NPK content 3.

1.
Increased shoot dry weight, number of pods and root nodules 2.
Increased chlorophyll and NPK content 3.
Reduced chlorophyll and NPK content 3.

1.
Increased plant height, fresh and dry weight, pod number 2.
Increased chlorophyll and NPK content 3.

1.
Increased shoot dry weight, number of pods and root nodules 2.
Increased chlorophyll and NPK content 3.
Reduced root galling and nematode multiplication [128] Plants 2022, 11, 2875 22 of 33 Increased total plant biomass [136] 3.2.1. Bacterial Pathogens of Legumes AMF colonization triggers mycorrhizal-induced resistance (MIR) in host plants, and activates immune responses such as callose deposition, cell wall thickening, and production of ethylene, ROS, and antimicrobial compounds [137]. Studies have shown that AMF colonization reduces disease symptoms of several bacterial pathogens in tropical legumes. Pseudomonas syringae pv. Glycinea (Psg) is a pathovar that causes bacterial blight in soybean plants, producing effector proteins that suppress the host plant's immunity and resulting in leaf chlorosis and necrosis, lesions on soybean pods, and discoloration of the stem [122]. Inoculation of soybean with AMF species Entrophospora infrequens reduced Psg colonization of plants seven-fold compared to non-AMF plants [121]. The higher uptake and transfer of N and enhanced biomass production induced by E. infrequens was suggested to have an important role in improving the immunity of soybean plants. Another bacterial pathogen of legumes, Xanthomonas campestris pv alfalfae, is a pathovar that is responsible for leaf spots in several legume plants, including alfalfa and M. truncatula [124]. Inoculations of AMF species Glomus intraradices (reclassified as R. intraradices), Glomus versiforme (reclassified as Diversispora versiformis), and Gigaspora gigantea in M. truncatula infected with X. campestris reduced disease symptoms in the leaves and upregulated defense-related genes compared to non-AMF plants [123].

Fungal Pathogens of Legumes
Soil-borne fungal pathogens compete with AMF for infection sites and the presence of arbuscules in plant host cells can prevent invasion by hyphae of fungal pathogens [138]. Charcoal root rot is a soilborne disease caused by the broad-range fungus Macrophomina phaseolina. This pathogen infects the roots and lower stem of over 500 plant species including legumes such as peanut, soybean, and chickpea [139]. Multiple AMF species have shown to reduce the symptoms caused by M. phaseolina in legumes. In chickpea plants, inoculation with individual AMF species Glomus fasciculatum (reclassified as Rhizophagus fasciculatus), Glomus constrictum (reclassified as S. constrictum), G. intraradices (reclassified as R. intraradices), Gigaspora margarita, or Acaulospora sp. were able to reduce root-rot severity and increase plant growth, chlorophyll content, and the number of pods compared to non-AMF plants [127,128]. AMF species R. irregularis has been reported to upregulate pathogenesis/disease-resistant genes and increase lignin production in soybean plants under M. phaseolina infection compared to non-AMF plants [126]. This response reduces disease incidence and severity, leading to an improvement in plant growth and yield [125]. Various wilt diseases in legumes are caused by fungi, including Fusarium wilt in pigeon pea due to infection with Fusarium udum. Symptoms of infected plants include chlorosis, leaf and stem drooping, and wilting [130]. Inoculation of pigeon pea with AMF species G. fasciculatum (reclassified as R. fasciculatus) was able to significantly reduce wilting severity, and increased plant height, shoot dry weight, and phosphorous content compared to non-AMF plants [129]. Another wilt infection of pea that also leads to severe root rot and seedling damping off is caused by Aphanomyces euteiches [133]. This pathogen also affects a large range of other legume plants. Inoculation of pea with AMF species G. fasciculatum (reclassified as R. fasciculatus) and G. intraradices (reclassified as R. intraradices) significantly reduced spore production and root-rot severity compared to non-AMF plants [133,135,140]. Inoculation with G. intraradices (reclassified as R. intraradices) in pea also delayed and reduced the activity of A. euteiches enzymes such as glucose-6-phosphate dehydrogenase, phosphoglucomutase, and peptidase, which are essential for pathogenesis of A. euteiches [132,134]. Phytophthora sojae is a host-specific pathogen and only infects soybean. The disease causes damping off in seedlings, and root rot and stem lesions in mature plants [141]. Inoculation of AMF species G. intraradices (reclassified as R. intraradices) reduced oxidative damage in soybean plants infected with P. sojae by decreasing H 2 O 2 content and increasing jasmonic acid content, glutathione reductase activity, and the metabolism of nitrogen and carbon compared to non-inoculated controls [131].

Nematode Infections of Legumes
AMF colonization leads to altered composition of exudates from the roots of host plants, which may affect nematode motility and infection [142]. The effect of root exudates on nematodes is species-dependent, so the degree of protection will be variable [143]. Heterodera cajani is a nematode species that mainly infects pigeon pea but it is also able to infect other legumes such as cowpea and mung bean, causing root galling, stunting, and leaf chlorosis [144]. Inoculation with AMF species F. mosseae reduced the H. cajani population by over 40% in infected pigeon pea plants and increased plant length, shoot dry weight, and phosphorous content compared to non-inoculated plants [129]. Another nematode species that can infect more than 3000 plant species including legumes such as cowpea and chickpea is Meloidogyne incognita. Found in tropical and subtropical regions, M. incognita causes root galling, reduced growth and leaf chlorosis [145]. Inoculation with individual species of AMF species G. fasciculatum (reclassified as R. fasciculatus), G. constrictum (reclassified as S. constrictum), G. intraradices (reclassified as R. intraradices), G. margarita or Acaulospora sp. in chickpea infected with M. incognita increased plant height, fresh and dry weight, yield, and chlorophyll content together with a reduced nematode population and less root galling than non-AMF plants [127,128].

Insect Pests of Legumes
In addition to bacterial, fungal, and nematode pathogens, inoculation of some legume plants with AMF species has been shown to reduce damage caused by insects. Well established AMF colonization increases biosynthesis of jasmonates in host plants [146]. Jasmonates regulate production and emission of volatile organic compounds such as terpenoids which repel pests or attract predators of pests [147]. Inoculation of M. truncatula with AMF species R. irregularis reduced phloem ingestion by the pea aphid Acyrthosiphon pisum, resulting in higher carbon content in the plants compared to non-inoculated plants [135]. Another example is inoculation with AMF species G. intraradices (reclassified as R. intraradices) in Vigna mungo (black gram) exposed to tobacco cutworm (Spodoptera litura). AMF-inoculated plants showed higher lignin content and plant biomass compared to non-inoculated plants [136].

AMF and Tropical Legumes for Sustainable Agriculture: Challenges and Prospects
From global warming to soil degradation and biodiversity loss, the world is facing enormous environmental challenges. Fortunately, there is growing recognition and awareness that urgent action is required to reduce anthropogenic impacts on the earth at all levels-research, governmental, business, and individual [148]. When it comes to how climate change affects people equally, it can be profoundly unfair because those who have contributed the least to the problem, such as the poor, underprivileged, ethnic minorities, and indigenous peoples, suffer disproportionately from its effects and are less resilient to extreme change [149]. It is worth emphasizing that crop yields are not solely determined by climate; global demand, agricultural practises, and political shifts will all have an impact on how farms fare in the future [150]. Farmers, for example, who adopt sustainable farming practises and diversify their fields with the assistance of researchers from a variety of fields ranging from genetic engineering to climate modeling, would be able to contribute to maintaining food security with the proper government support. All parties, including the producers and policymakers, must play a role in inspiring a fundamental shift in the way intensive farming is carried out [151].
Intriguingly, rising temperatures have been observed to allow some tropical crops to thrive in previously untapped growing regions further north-avocado, for example, is already grown in Sicily [152]. This may be true for some tropical legumes, but more research is needed to fully realize their potential. Numerous recent studies have shown that soil microorganisms, in particular AMF, can improve soil fertility and help plants grow more resiliently under climatic stresses [19]. Hence, researchers are now seeking to either employ effective microorganisms and/or genetically modify crops to contain their genes in order to increase crop yields, which might alter the makeup of microbial communities in the soils and enhance plant growth. For instance, AMF inoculation was found to positively alter mycorrhizal composition, resulting in increased growth of the tropical legume cowpea [153]. Nonetheless, the difficult first step is to locate elusive AMF in soil microbial communities, followed by complex biological engineering, which entails identifying the genes in AMF that stimulate efficient nitrogen fixation and then engineering them into plants in a way that the plants can use to produce the same traits [154]. Additionally, modern farming, which was introduced in the 1960s during the Green Revolution that saved billions of lives, is having a negative impact on soils and microorganisms (Figure 2), particularly through the use of high inputs such as chemical fertilizers [155,156]. The relationships between AMF and plants, particularly legumes that naturally interact with soil bacteria, require significant effort to progress (Figure 2). already grown in Sicily [152]. This may be true for some tropical legumes, but more research is needed to fully realize their potential. Numerous recent studies have shown that soil microorganisms, in particular AMF, can improve soil fertility and help plants grow more resiliently under climatic stresses [19]. Hence, researchers are now seeking to either employ effective microorganisms and/or genetically modify crops to contain their genes in order to increase crop yields, which might alter the makeup of microbial communities in the soils and enhance plant growth. For instance, AMF inoculation was found to positively alter mycorrhizal composition, resulting in increased growth of the tropical legume cowpea [153]. Nonetheless, the difficult first step is to locate elusive AMF in soil microbial communities, followed by complex biological engineering, which entails identifying the genes in AMF that stimulate efficient nitrogen fixation and then engineering them into plants in a way that the plants can use to produce the same traits [154]. Additionally, modern farming, which was introduced in the 1960s during the Green Revolution that saved billions of lives, is having a negative impact on soils and microorganisms (Figure 2), particularly through the use of high inputs such as chemical fertilizers [155,156]. The relationships between AMF and plants, particularly legumes that naturally interact with soil bacteria, require significant effort to progress (Figure 2). AMF and other recently developed biologicals come in a variety of forms, including biological sprays that are comparable to chemical pesticides and herbicides, or seed coatings made from naturally occurring microorganisms that function as catalysts to stimulate growth and nutrient intake in crops [19,120]. Examples include the use of AMF to increase drought tolerance of several legumes, including soybean and chickpea [21,71] and the management of nematodes on soybeans using bacterial seed coatings that are damaging to the parasite [157]. These advancements are not necessarily developed to replace conventional breeding or genetic engineering, but rather to provide extra means to feed an expanding world. There is a huge opportunity to explore AMF for improving tropical legumes, especially since only approximately 10% of pesticides come from nature [158]. Soil microorganisms help crops obtain nutrients from the soil more efficiently, while also increasing disease resistance and building healthy soil structures that support livelihoods and communities. Significant reforms and efforts, however, are required at all levels, whether locally or internationally.

Conclusions
Tropical regions will be most severely impacted by climate change and the legume crops that can be grown in the tropics are of particular importance to food security as a AMF and other recently developed biologicals come in a variety of forms, including biological sprays that are comparable to chemical pesticides and herbicides, or seed coatings made from naturally occurring microorganisms that function as catalysts to stimulate growth and nutrient intake in crops [19,120]. Examples include the use of AMF to increase drought tolerance of several legumes, including soybean and chickpea [21,71] and the management of nematodes on soybeans using bacterial seed coatings that are damaging to the parasite [157]. These advancements are not necessarily developed to replace conventional breeding or genetic engineering, but rather to provide extra means to feed an expanding world. There is a huge opportunity to explore AMF for improving tropical legumes, especially since only approximately 10% of pesticides come from nature [158]. Soil microorganisms help crops obtain nutrients from the soil more efficiently, while also increasing disease resistance and building healthy soil structures that support livelihoods and communities. Significant reforms and efforts, however, are required at all levels, whether locally or internationally.

Conclusions
Tropical regions will be most severely impacted by climate change and the legume crops that can be grown in the tropics are of particular importance to food security as a supply of dietary protein for humans, and as a source of protein in feed and forage for fish and meat production. In conjunction with the right soil microbiota, leguminous crops are able to replenish the organic carbon content of the soil under suitable environments and this adds to their importance in a climate-changing world. It is notable that AMF, by consuming carbon-rich exudates from plant roots, are able to assist plants to maintain a source-sink balance under elevated atmospheric CO 2 . Thus, the exacerbated challenges from abiotic and biotic stresses to tropical legumes in a climate-changing world require looking to soil health and the important microbiota, not least including AMF.
Despite the large-scale production and demand for the protein-rich soybean, currently only a few other legumes are widely utilized globally. A few lesser-known tropical legumes, such as winged bean and lentil are recognized as valuable protein alternatives to soybean and meat that could be grown more widely as sustainable dietary protein sources in the future, so there is a need for more research on these potential species and their symbiotic microbes. As the community composition, comprising thousands of rhizosphere species, varies depending on crop species and soil types, there is a need to identify the most beneficial species and strains for use with the various tropical legumes, and with this, better understand the interactions between plants and soil microorganisms, for optimal mitigation of anthropogenic as well as climate change-associated soil degradation. While three stages of establishment of symbiosis between AMF and legume plants have been categorized for model legume species (Figure 1), and can reasonably be expected to be broadly the same for all legumes, it is notable that the same genes or homologs involved in these symbiotic processes may be differently named in different plant species, leading to a lack of clarity, and this is an area where the wider scientific community needs stronger coordination to facilitate further advancements in knowledge.
AMF are natural root symbionts and bio-fertilizers with the capacity to boost crop resistance to abiotic stresses such as drought, heat, salinity, and extreme temperatures. Yet the mechanisms for upregulation of tolerance responses in host plants while also preventing the downregulation of essential metabolic pathways are far from fully understood, and it is not known how much these are conserved among different plant species, most especially for the majority of previously overlooked leguminous species. Climate change will pose a host of challenges to legume crop productivity, but it is notable that several of the component stresses, namely extreme temperatures, osmotic stresses due to salinity, drought, and flooding impact many of the same fundamental metabolic pathways, and that the symbiosis between plants and AMF is able to mitigate some of the damage to plants from these multiple factors, by a few common mechanisms.
AMF mitigation of heat stress, mitigation of water stress, and mitigation of salinity stress have each been associated with enhancement of water and nutrient uptake which support photosynthetic activity and in turn plant growth and yields (Table 1). There is also an associated activation of protective metabolism to scavenge ROS, maintain osmotic balance, and reduce membrane damage, which is similarly seen with exposure to biotic stresses. However, much of this has only been demonstrated in model plant species and a very limited number of legume crop species, and there is much space for research on tropical legumes and AMF. Water stresses and salinity stress, which impact water availability, require plants to regulate transpiration via stomatal opening and closing. This will affect the movement of CO 2 in and out of aerial parts of the plant and in turn affect photosynthesis. AMF symbiosis can mitigate water stress by increasing access to water through extraradical hyphae and it is notable that, at least in model legumes, AMF have been associated with higher stomatal conductance, transpiration, and photosynthesis rates as well as increased macro-and micronutrients including N, P, K, Mg, Fe, Zn, Mn, and Cu under water-stressed conditions. Under waterlogged conditions, however, only a few AMF species which are able to tolerate anoxic conditions are able to provide protection to their symbiotic plant partners, and there are no reported examples for legume hosts. Given the likelihood of increased flooding along with sea level rises due to climate change, this is another area where research is urgently required.
Turning to biotic stresses, AMF have been shown to reduce damage from plant pathogens and pests by various mechanisms that support plant health and immunity (Table 2), and a few studies have reported elevated gene expression of pathogenesis-and disease-resistance-related genes associated with AMF symbioses. However, as with abiotic stresses, there is a need for much further study of the underlying mechanisms and especially so for tropical legumes and their AMF.
While data and some suggestions of molecular mechanisms have been reported in model species and a few other legume species such as green bean, black locust, and chickpea, there is still much to learn concerning the mechanisms of signaling between AMF and plants that could in the future be used to breed legume crops and select AMF species that can be paired for high tolerance to abiotic and biotic stresses. Ongoing transcriptome studies to characterize genes expression show the picture to be complex, and while many of the fundamental underlying plant pathways are now quite well known, the genes involved in interaction with AMF are much less well characterized, even for model species and not at all for any tropical legume species.
AMF as a plant health promoting biofertilizer can mitigate many of the anticipated abiotic and biotic stresses associated with climate change. Their judicial application can help to reduce the amounts of chemical fertilizers required for more sustainable farming, and as the body of knowledge on the AMF of tropical legumes grows, there is much promise for their role in improving the production and availability of plant proteins in the tropics. It is noted, however, that such research requires a recognition of this need, and the will to provide support to those in less well-developed countries of the tropics, both to encourage more sustainability and self-sufficiency in food supply, and also to reduce pressures on shrinking biodiversity and environmental degradation. Supportive policies from international bodies, and not just national governments, are needed to encourage the implementation of sustainable practices, as well as to support urgently needed research. International cooperation and coordination will be needed to optimize the crops planted in the future, in which tropical legume species should strongly feature. Moreover, there is a need to recognize that the use of modern biotechnology approaches, such as gene edited crops, can be even more impactful when supported by effective microbes and this too is an area ripe for future study.