The Utilization of Seed Priming as a Tool to Overcome Salt and Drought Stresses: Is Still a Long Way to Go?

Abstract

Abiotic stresses, sometimes due to dramatic environmental changes, such as sudden and heavy rainfalls, or drought, increasing temperatures or soil salinization, deeply affect the growth and yield of plants. Progress in terms of improving the abiotic stress tolerance of plants can be obtained via classical breeding and genetic engineering, which can be slow, or by practice, such as acclimation and seed priming. The latter can improve seedling performance, and it can be considered a short-term approach. Seed priming with different agents and biopriming may offer the possibility to improve stress tolerance, even though its beneficial effect depends on crop species, dose, and time of application. The aim of this review is to highlight some of the current research trends that may ultimately lead to strategies for stress-proofing crop species. The focus is on those abiotic stresses, e.g., drought and soil salinity, that are most often associated with climate change and poor agricultural practices and those crops that are most important for human nutrition. Comments are provided on the challenges and pros and cons of this methodology.

1. Introduction

The increasing impact of climate change is becoming a serious threat to plant biodiversity and agriculture. Plants must face sudden changes in temperature, flooding, drought, and increasing soil salinity due to poor agricultural practice. Such experiences of an ever-changing environment and adverse conditions are predominant and induce stress in plants. Their survival depends on strong countermeasures based on the onset of distinct sensing and signalling mechanisms that perceive and respond to stresses. When undergoing abiotic stress, a plethora of complex events are activated involving different levels, from the cellular to the whole-organism level. In these complicated mechanisms, major changes include transcriptional modifications and translational and post-translational changes [1,2,3,4]. Both genotypes and plant developmental stages play an important role in the effectiveness of stress responses [2], which is widely determined in terms of the genetic plasticity of a tolerant species [3]. Detailed information on how plants respond to salt and drought stress is available in the literature [1,2,3,4]. Plant responses to stress may depend on their intrinsic capability to cope with it, environmental conditions, and the availability of symbionts, as pointed out by Mason et al. [5].

The risk of a future lower food supply has raised the interest of researchers toward more sustainable agricultural methods to improve plant’s tolerance against abiotic stresses, including salt and drought. The latter are among the most studied because of their heavy impact on plant growth and productivity; their deleterious effects on plant development and physiology have been reported in several papers [2,6,7]. The increasing risk for crop production means that researchers are looking for protocols and methods to set up in order to improve plant tolerance toward abiotic stresses, starting from the application of breeding and genetic engineering to acclimation and seed priming.

2. Overview and Conventional Priming Methods

Seed priming is dealing with seed treatments with the aim of improving seed germination and plant tolerance to stress. Seed germination is a critical parameter for the successful development of crops, and it is considered the test to determine cultivar tolerance to abiotic stress [7,8]. Seed priming involves prior exposure to abiotic stress, which makes a seed more resistant to future exposure to stressful conditions. The protocols are designed to improve germination rates by applying a series of different parameters during the initial stages of the process [3,8]. It has been applied successfully in counteracting salt stress in several glycophytes, such as tomato (Solanum lycopersicum L.) [9,10], hot pepper (Capsicum annuum var. acuminatum L.) [11,12], lettuce (Lactuca sativa L.) [13], maize (Zea mays L.) [14,15], okra (Abelmoschus esculentus L.) [16], pea (Pisum sativum L.) [17], pepper (Capsicum annuum L.) [18], milk thistle (Silybum marianum L.) [19], soybean (Glycine max L.) [20] and Brassica napus [21].

The basic steps of the protocols, carried out under controlled conditions, include the embedding of the seeds in solutions containing different priming agents (Figure 1), avoiding radicle protrusion; a desiccation step follows the first one [22,23]. A summary of the different methodologies and priming agents is reported in Figure 1.

The treatment acts on the seed’s pre-germinative metabolism, enhancing the performance of the following germination phase, i.e., an equal stage of germination and faster and uniform emergence with respect to unprimed seeds [8], and boosting seedling stress resistance at the same time [21,22,24] by responding quicker and better to stress exposure. The phenomenon is reported as “the primed state” [25]. Since the rapid germination of seeds and stand establishment are fundamental for growth and crop production in stressful environments, seed priming has proven to be beneficial for crops grown under such conditions (

Table 1. A literature survey of more recent papers concerning different priming protocols. The table cited only papers published in the last 10 years (2013–2023).

Plant	Stress	Priming	Priming Agent	Growth Conditions	Limits	Reference
Wheat	Drought	Osmopriming	Ascorbic acid	Greenhouse	Lack of enzymatic analysis. No information about fruit and yield.	[27]
Wheat	Salt	Hormopriming	Gibberellin acid	Botanical Garden	Lack of enzymatic analysis.	[28]
Mountain Rye	Drought	Hormopriming	Gibberellic acid Salicylic acid	Laboratory	No information about fruit and yield. No metabolic analysis.	[29]
Rice	Salt	Hormopriming	Polyamines Gibberellin acid	Greenhouse	No enzymatic analysis. No information about fruit and yield.	[30]
Rapeseed	Salt	Osmopriming	PEG	Laboratory	No information about fruit and yield. No metabolic analysis.	[31]
Faba bean	Salt	Osmopriming	Melatonin	Laboratory	No information on yield and fruits.	[32]
Alfalfa	Drought	Osmopriming	PEG	Greenhouse	No biochemical analysis. No information about fruit and yield.	[33]
Tomato	Salt	Osmopriming	PEG	Laboratory	Lack of enzymatic analysis. No information about fruit and yield.	[34]
Wheat	Salt	Hormopriming	Abscisic acid	Greenhouse	Lack of metabolic analysis. No information about fruit and yield.	[35]
Wheat	Drought	Hormopriming	Gibberellic acid Salicylic acid	Field	No enzymatic analysis.	[36]
Wheat	Drought	Osmopriming	PEG	Experimental station	No metabolic analysis.	[37]
Tomato	Salt	Hormopriming	Salicylic acid	Growth chamber	No enzymatic analysis. No information about fruit and yield.	[38]
Maize	Salt	Halopriming Osmopriming	NaCl Sugar	Net house	Lack of metabolic and enzymatic analyses. No information about fruit and yield.	[39]
Chili pepper	Salt	Halopriming	NaCl KNO3	Laboratory	Growth limited only to 2 weeks.	[40]
Rapeseed	Salt	Hormopriming	Polyamines	Growth chamber	No information about fruit and yield.	[21]
Soybean	Salt	Hormopriming	Jasmonic acid	Greenhouse	No information about fruit and yield.	[41]
Maize	Drought	Halopriming	NaCl	Growth chamber	No biochemical analysis.	[42]
Rice	Salt	Halopriming	NaCl KNO3 CaCl2 KCl	Greenhouse	No information about fruit and yield. Lack of enzymatic analysis.	[43]
Hibiscus tea	Light Temperature Salinity	Halopriming	NaCl	Laboratory	Tests performed exclusively on seeds.	[44]
Winter savory	Drought	Osmopriming	PEG	Laboratory	Tests performed exclusively on seeds.	[45]
Trigonella foenum-graecum	Drought	Nanopriming	Magnetite nanoparticles from leaves of Pulmonaria longifolia	Growth chamber	Lack of metabolic and enzymatic analyses. No information about fruit and yield.	[46]
Quinoa	Salt	Halopriming	NaCl	Greenhouse	No information about fruit and yield. No enzymatic analysis.	[47]
Tomato	Salt	Hormopriming	Polyamines	Greenhouse	No information about fruit and yield.	[10]

). In fact, priming protocols allow the primed seeds to develop and counteract salt and drought stress by activating antioxidant response, both non-enzymatic and enzymatic, increase the synthesis of proline and other osmolytes, and decrease the negative effects on chlorophylls and photosynthesis [10,21,24]. The methodology is quite simple and low-cost; thus, it can be used by farmers to increase germination, the uniformity of plants in the field, and crop performance [26].

According to the literature, through seed priming, abiotic stress tolerance may be obtained through the use of two strategies: (1) stimulation of the metabolic processes in the pre-germination phase related to the early mobilization of the seed storage reserve, elongation of embryo cell and endosperm weakening, i.e., conversion from quiescent seed in germinating state, and thus, increased germination [48,49,50,51,52,53,54]; (2) by imposing biotic stresses on seeds, which represses the protrusion of the radicle but supports stress responses, inducing cross-tolerance to abiotic stresses, the activation of enzymes, and osmotic adjustment. These stress tolerance strategies form a “priming memory” in germinating primed seeds that can play an important role in subsequent stress exposure and mediate more stress tolerance [24].

Several priming agents have been used to treat seeds [26], and, depending on the agents, different protocols are reported in the literature (Figure 1). The efficacy of the treatment is influenced by the osmotic potential of the priming solution, treatment duration, ambient temperature, seed lot and vigor, plant species, and storage conditions of the treated seeds. Thus, the effectiveness in improving stress response is variable. So far, the methods empirically applied by seed technologists require time-consuming experiments to detect and set up the best protocol [21,55].

3. Advanced Methods: Biopriming and Nanopriming

Besides chemical priming, the primed state can also be obtained through the colonization of plant roots by beneficial micro-organisms that can become endophytes [2,56,57]. Different species of Plant-Growth-Promoting Bacteria (PGPB), also reported as Plant-Growth-Promoting Rhizobacteria (PGPR), are often utilized as biofertilizers. Studies have reported improved plant fitness against environmental stress when seeds are inoculated with PGPB [2,58,59] (

Table 2. Biopriming application to different crop species: positive effects induced by Plant-Growth-Promoting Bacteria (PGPB) on plant salt/drought responses. The entries in this table are grouped according to the oldest paper listed first. Papers published in the last 10 years (2013–2023) have been considered.

Plant	Bacteria	Stress	Conditions	Limits	Reference
Wheat	Hallobacillus sp., Bacillus halodenitrificans	Salt	Laboratory	Lack of metabolic and enzymatic analysis. No information about fruit and yield.	[60]
Chickpea	Mesorhizobium ciceri	Salt	Growth chamber	Lack of metabolic and enzymatic analysis. No information about fruit and yield.	[61]
Mung bean Bean Peanut	Bradyrhizobium sp., Enterobacter sp., Chryseobacterium sp.	Salt Drought	Growth chamber	Lack of metabolic and enzymatic analysis. No information about fruit and yield.	[62]
Mung bean	Pseudomonas, Rhizobium	Salt	Growth chamber	Lack of metabolic and enzymatic analysis. No information about fruit and yield.	[63]
Cotton	Klebsiella oxytoca	Salt	Greenhouse	Lack of enzymatic analysis. No information about fruit and yield.	[64]
Pepper	Bacillus licheniformis	Drought	Growth chamber	Lack of enzymatic analysis. No information about fruit and yield.	[65]
Wheat	Serratia spp., Aerococcus spp.	Drought	Jars with soil	Lack of metabolic and enzymatic analysis. No information about fruit and yield.	[66]
Mung bean	Rhizobium sp., PGPR	Salt	Field	Lack of enzymatic analysis.	[67]
Rice	Bacillus pumilus, Pseudomonas pseudoalcaligenes	Salt	Greenhouse	No information about fruit and yield.	[68]
Wheat	Azosprillium lipoferum, Pseudomonas fluorescens	Salt	Greenhouse	Lack of enzymatic analysis. No information about fruit and yield.	[69]
Rice	Bacillus amyloliquefaciens	Salt	Greenhouse	Lack of enzymatic analysis. No information about fruit and yield.	[70]
Rice	Serratia sp.	Salt	Greenhouse	Lack of metabolic and enzymatic analysis. No information about fruit and yield.	[71]
Sunflower	Pseudomonasaeruginosa	Salt	Growth chamber	Lack of metabolic and enzymatic analysis. No information about fruit and yield.	[72]
Wheat	Bacillus thuringiensis	Drought	Growth chamber	No information about fruit and yield.	[73]
Maize	Pseudomonas syringae, Pseudomonas fluorescens	Drought, salt	Field	Lack of metabolic and enzymatic analysis. No information about fruit and yield.	[74]
Limonium sinense (Girard) Kuntze	Bacillus Arthrobacter Streptomyces Isoptericola	Salt	Greenhouse	Lack of enzymatic analysis. No information about fruit and yield.	[75]
Tomato Arabidopsis	Enterobacter sp. EJ01	Salt	Growth chamber	Lack of metabolic analysis. No information about fruit and yield.	[76]
Tomato	Pseudomonasputida	Salt	Greenhouse	Lack of enzymatic analysis. No information about fruit and yield.	[77]
Tomato	Pseudomonas fluorescens Pseudomonas migulae	Salt	Greenhouse	Lack of enzymatic analysis.	[56]
Cotton	Pseudomonas putida Pseudomonas chlororaphis	Salt	Growth chamber	Lack of enzymatic and metabolic analysis. No information about fruit and yield.	[78]
Arabidopsis thaliana	Micrococcus yunnanensis, Paenibacillus barengoltzii	Salt Drought	Growth chamber	Lack of enzymatic and metabolic analysis.	[79]
Groundnut	Bacillus cereus SVSCD1	Salt	Controlled conditions	Lack of enzymatic analysis. No information about fruit and yield.	[80]
Cucumber Lattuce	Aspergillus niger MJ1 Pseudomonas stutzeri DSM4166 Pseudomonas fluorescens CHA0-nif	Salt	Field	Lack of metabolic and enzymatic analysis.	[81]
Rice	Pseudomonasstutzeri	Salt	Growth chamber	Lack enzymatic analysis. No information about fruit and yield.	[82]
Velvet bean	Rhizobacteria	Drought	Growth chamber	Lack of metabolic and enzymatic analysis. No information about fruit and yield.	[83]
Sunflower	PGPB	Salt	Greenhouse	Lack of metabolic and enzymatic analysis.	[84]
Canola	Brevibacterium epidermidis Bacillus aryabhattai	Salt	Laboratory	Tests performed on seeds.	[85]
Chinese cabbage	Herbaspirillum sp.	Salt	Growth chamber	Lack of enzymatic analysis. No information about fruit and yield.	[86]
Chickpea	Bacillus amyloliquefaciens Pseudomonas putida	Drought	Greenhouse	Lack of metabolic analysis. No information about fruit and yield.	[87]
Rice	Bacillus amyloliquefaciens	Salt	Growth chamber	Lack of enzymatic analysis. No information about fruit and yield.	[88]
Paddy plants	Curtobacterium albidum	Salt	Greenhouse	No information about fruit and yield.	[89]
Arabidopsis	Flavobacterium crocinum HYN0056T	Salt Drought	Controlled-environment chamber	Lack of metabolic and enzymatic analysis.	[90]
Triticum aestivum	Paenibacillus pabuli Pseudomonas nitroreducens Bacillus megaterium	Salt	Growth chamber	Lack of metabolic and enzymatic analysis. No information about fruit and yield.	[91]
Brassica napus	Azospirillum brasilense Arthrobacter globiformis Burkholderia ambifaria Herbaspirillum seropedicae Pseudomonas sp.	Salt	Growth chamber	No information about fruit and yield.	[58]
Brassica napus	Arthrobacter globiformis	Salt	Growth chamber	No information about fruit and yield.	[59]
Maize	Novel Bacterial Strain SH-6	Drought	Laboratory	Lack of metabolic analysis and enzymatic analysis. No information about fruit and yield.	[92]

).

The phenomenon of ameliorating plant growth through the use of PGPB has been known for many years, and more recent evidence has proved that enhanced defense responses, induced by biopriming as a result of plant–plant communication in nature, can also boost crop resistance to both biotic and abiotic stresses in the field [2,93]. Researchers evidenced and appreciated the significant role played by the different bacterial species on both plant health and disease management based on their metabolic characteristics, such as the secretion of metabolites and enzymatic activity acting on plant development and response to biotic and abiotic stress [2,94,95]. Similar to rhizospheric bacteria, endophytic PGPB employ mechanisms enabling plant growth and imparting stress tolerance [57]. Thus, PGPB can help plants cope/overcome the deleterious effects of salt and drought through different interconnected mechanisms.

Some of the bacterial mechanisms involved in plant interactions have been elucidated, e.g., possible alteration of plant gene expression, thus the plant would less succumb to stress, or the bacteria ACC deaminase activity that decreases the level of stress ethylene synthesis in plant [96], or the production of different molecules, such as hormones, trehalose, proline, and betaine [2]. Moreover, the synthesis of biopolymers by bacteria as extracellular polymeric substances (EPS), like polysaccharides, may contribute to alleviating salt stress by linking Na⁺ cation, thus decreasing its bioavailability [57]. Furthermore, bacteria inoculation increases the antioxidant activity of the plant and the repair of membranes. In fact, in inoculated plants, the enhanced synthesis of phenolic compounds and/or the activity of antioxidant enzymes, e.g., SOD, CAT or APX, that can detoxify reactive oxygen species, have been reported [58,59,70]. Thus, many researchers have focused their attention on strain selection and utilization as bioinoculants on plants to ameliorate their tolerance to drought and salinity stresses [2,58,97]. For this purpose, halotolerant strains have been isolated, and their plant-growth-promoting activities have been characterized [98]. The principal effects of seed priming on plants are summarized in Figure 2.

More recently, other biotechnological tools have been applied to seed priming, such as nanopriming, which uses nanoparticles (NPs) to increase plant tolerance to various types of stress [99]. Some of them can act as stimulants, i.e., can break seed dormancy and improve seed metabolism, seedling vigor and plant growth [100,101] and, in the meantime, can activate enzymatic and non-enzymatic antioxidant responses, thus stimulating plant defence systems to environmental stresses [99]. The physical–chemical properties of NPs (i.e., size, zeta potential, and concentration) are the key factors influencing their uptake and translocation within the plant, thus determining the biological responses [100,102,103]. The most common applications of NPs include, besides nanopriming of seeds, foliar application and mixing of NPs in the soil. NPs, absorbed by the seed, promote water uptake and increase the germination rate. They can also induce oxidative respiration, producing H₂O₂, a signalling molecule that activates metabolic pathways associated with seed germination, including increased α-amylase activity and gibberellin biosynthesis. Elevated H₂O₂ production activates the antioxidant defence system. Foliar and soil applications of NPs also enhance plant growth through the same signalling mechanism involving H₂O₂, improving plant productivity and tolerance under stress conditions [99].

These studies are still in their infancy; therefore, further research is needed to better understand the effects of nanotechnology and to explore the full potential of nanopriming. Moreover, in agreement with Raza et al. [99], the risks and constraints associated with the use of NPs in agriculture, should be carefully assessed by considering several factors, such as the environmental influence of their application, economic sustainability, and functional performance.

4. Salt and Drought Stress Memory

In plants, the establishment of a “memory” of a past stress event has been reported by different authors; this would provide a more rapid reaction to recurrent stress in plants being previously exposed to stress [104]. An example is provided by Sadhukhan et al. [105] in their paper: plants exposed to drought develop both short-term and transgenerational memory in terms of stress, leading to a better response to subsequent exposure to drought. Several dehydration/rehydration cycles improved the water retention of Arabidopsis thaliana and Zea mays plants with respect to plants experiencing a first stress [106,107,108]. According to these results, we may hypothesize that once physiological, biochemical, and molecular mechanisms have been switched on, this fact ensures enhanced protection against new stress without “costs” for the plants, being associated with constitutive expression of stress-related genes [109]. Even though there is a general claim about a positive relationship between memory and better performance, this is not always the case. In fact, we need to point out that it has been reported a negative effect of memory that may enable disruptive effects on plant performance [110] since repeated stresses provoked an increase in sensitivity toward their deleterious effects [111,112]. Based on such controversial reports, it is not possible to draw an unambiguous conclusion, and thus further studies are needed.

Another important cue concerns the duration of the memory, which can vary noticeably. It may contribute to environmental acclimation, which is considered plastic and reversible and plays a role in improving stress response [113]. Memory duration can last from days to weeks or months, and when, stable and based on epigenetic mechanisms, even extended to the offspring, which is reported as transgenerational memory [110,114]. It is noteworthy to point out that following a recurrent stimulus, opposite behaviors can be observed, either a negative lowering response, known by the term “familiarization” or habituation [115] or a positive reinforcing response, named “sensitization” [116].

The principle of priming is based on endowing “memory” of past stress events. How can priming treatment establish a memory? According to Galviz et al. [110], plant memory can be established by three possible pathways: one straightforward, which develops almost immediately without dependency on previous stimulus, and the other two based on learning and storing/calling. The three pathways can be independent or share certain connections [110]. The duration of a memory induced by priming is still a matter of debate.

So far, the known mechanisms underpinning the processes of plant memory are related to alterations in the concentration of many signalling molecules and transcription factors; those changes perfectly explain the alteration of the metabolism and the maintenance of such changes, even when stress exposure is ended and the response is obtained with recurrent exposure to stresses [114]. In their work, Conrath et al. [116] described the mechanism behind the first stress trigger accumulation or post-translational modification of single or multiple signalling proteins, which, after this event, remain inactive. Nevertheless, their presence allows an enhanced response when the plant is triggered by a second exposure to stress through hyperactivation of the signalling protein. This enhanced response is reported as cross-stress tolerance [117]. In such reversible post-translational modifications of inactive proteins that become active and vice-versa, a key role is played by phosphorilation and dephosphorilation due to the activity of members of mitogen-activated protein kinases (MAPK) [118]. It is noteworthy to mention that according to the data of several authors, the cross-stress tolerance gained towards a single stressor can lead to the tolerance of multiple stresses [119,120,121,122,123].

Transcriptomic research has led to the identification of changes in gene expression, levels of proteins and metabolites important in memory response [116]. From further experimental works, where the transcriptional responses to stress factors have been investigated, we have the availability of molecular data concerning memory. Several researchers have reported data on the mechanisms at the basis of the phenomenon, describing epigenetic regulation, transcriptional priming, primed conformation of proteins and other specific changes [105]. Chromatin seems to be a potential major component of the “memory” in responses to recurrent stresses, i.e., when transcription of genes, which are induced by dehydration stress, are super-induced by subsequent stress, we can observe a model for “positive memory” [104]. However, controversy still exists in the literature since the role of chromatin in transcriptional response to stress appears to be quite complex and associated with genes and stress signals and species-specific [8]. Even though we know that, for example, in drought stress, epigenetic mechanisms (e.g., DNA methylations, histone modifications and chromatin structure alterations) are important in the regulation of gene expression, contributing to epigenetic inheritance in plants transmitted to the next generation [123].

More recent data describe the effects of recurrent periods of droughts, more common than a single drought in the environment, with intermittent moist recovery intervals. While, during the first exposure to drought, plants cannot avoid the detrimental effects of the stress on plant morphology and physiology, surviving plants would memorize the first drought and respond better to the following droughts. The better response is based on a partial stomatal opening in the watered recovery interval, higher contents of osmoprotectants and ABA, and attenuation of photosynthesis in the subsequent exposure [105]. The study has demonstrated the regulating role of ABA and other hormone signalling related to transcriptional memory behavior at gene levels; high levels of histone methylation have also been reported. Interestingly, during drought, stalled RNA polymerase is detected and activated by a pause, breaking factors in a subsequent drought episode. It has been reported that during drought, DNA demethylation occurs near drought-responsive genes, with genetic control of the process. The progenies of such exposed plants show a better adaptation to this stress owing to the inheritance of such methylation patterns. Nevertheless, when the water recovery period is prolonged, it is possible to detect the loss of drought memory. These studies deserve more future research to shed more light on the genetic control of drought memory and the genetic and epigenetic controls of inheritance.

5. Application of Seed Priming Protocols to Crop Species and the Application in Agriculture

Plant response to stress depends on several factors, such as intrinsic capability to cope with a specific stress and habitat conditions. In recent years, priming-inducing activity in plants has been reported for many synthetic and natural molecules (e.g., proline, L-tryptophan, glutathione, and citric acid) and secondary metabolites (e.g., polyols, ascorbic acid, lipoic acid, glycine betaine, α-tocopherol, and melatonin) [124,125,126]. Based on these, we have at our disposal a plethora of different protocols to mainly apply to herbaceous crops.

We have reviewed the more recent literature reporting data on seed priming applied to improve crop species tolerance to drought and salt stresses (Table 1). Recent papers on biopriming have also been reviewed (Table 2). Both methodologies have proven to be effective, and primed plants show better performance than unprimed ones when exposed to stress conditions. The choice of priming agents depends on the species and even on the genotype; thus, careful experimental trials are required before choosing not only the best agent but also the concentration and timing of the treatment. This may be a hindrance to the wide application of the methodology, but, on the other hand, it is relatively easy to implement in the short term with respect to the required time of the other approaches. Most of the papers, beside the effects on the threshold of tolerance toward salt and drought stresses have also considered the antioxidant activities and the presence of osmolytes in the primed plants, providing an insight into the mechanisms involved in enhancing tolerance. So far, most of the studies reported results obtained under laboratory or greenhouse conditions and on the first stages of plant development and no information on fruits and yield. The field performance of primed plants is mostly lacking since only few papers report field experiments. Moreover, experiments on priming duration should be performed to ensure the durability of the treatment.

6. Conclusions

Seed priming provides a useful tool in terms of ameliorating plant response to stress. The priming methodology is easy, and the cost–benefit ratio is in favor of broad application in agriculture. Literature surveys allow us to detect an increasing number of publications dealing with this methodology and stress response in crop plants. However, the potential of seed priming has still not been fully exploited.

So far, we know that plants share common physiological and biochemical responses to stress based on the induction of gene expression involving a profound metabolic remodeling. The majority of genes are those related to defense response, involving antioxidants, osmoprotectants and polyamines, as reported in several papers.

The elucidation of the molecular mechanisms involved in priming events is an active area of research, which pinpoints common and specific biochemical traits characterizing the primed state across species. Based on the results obtained so far, it is important to underline that the technologies based on plant stress memory, cross-stress tolerance and seed priming can be important and promising areas for future research and application in agricultural science. However, our limited knowledge of the molecular dynamics of pre-germinative seed metabolism is the main hindrance to more effective new-generation techniques [127]. Successful priming protocols result from the ability to properly act on the pre-germinative seed metabolism and stimulate events that are crucial for seed quality. Nevertheless, the technique requires constant optimization, and researchers are committed to addressing some open key questions to overcome the drawbacks still present. Primed seeds show faster and more uniform germination and an improved antioxidant defence system than non-primed seeds, but the priming remains strongly influenced by various factors, such as temperature, aeration, light, priming duration and seed characteristics. As an example, Zulfiqar and co-authors [126] reported 20 chemical priming agents enabling plant tolerance to salt stress; they were categorized into different groups, such as plant growth regulators, reactive agents, osmoprotectants, vitamins, mineral elements, and polymers. These different categories explain in part the difficulties in finding a unique mechanism of action and the high variability of stress response discussed above. The application of NPs at different stages of plant development may represent a further tool in the fight against abiotic stress, but their feasibility still needs more research under the perspectives of practical usableness, environmental impact, and cost-efficiency.

Anyway, the disadvantages of priming are very limited, with very few reports on the demerits of seed priming. The disadvantages do not often concern the method itself but are related to the subsequent agricultural practice: direct sowing of primed seeds is impossible, as wet seeds get caught in the seed drill, unsafe due to rain that delays sowing, or inaccessible for small farms. The benefits obtained by priming seeds may also be lost during the drying phase. Rapid drying can affect the sugar content, reducing drought tolerance and seed longevity. Sugars and their derivatives increase the desiccation tolerance and longevity of seeds, as they are involved in the maintenance and stability of glycolipids and glycoproteins of membranes during desiccation. However, during hydration, sugars are the first macromolecules to be metabolized, and the lack of subsequent re-accumulation is responsible for an accelerated deterioration.

Furthermore, most of the studies have been performed under controlled conditions, detecting the changes during the first stage of growth; to improve the effectiveness of priming and to increase the spread of this application, researchers need to make the leap from greenhouse and growth chamber to field, where the environmental conditions may influence plant responses. As reported in Table 1 and Table 2, data on yield of primed plants are still lacking. In addition, among the different questions to be answered, is a question concerning the duration of the priming memory. So far, we have no or few data about the epigenetic effects of priming on the next generation. The latter will likely provide a firm basis for developing reliable tools to enhance plant health and growth in varying field conditions, such as drought and soil salinity.