Seed priming technology as a key strategy to increase crop plant production under adverse environmental conditions

Farmers and seed companies constantly require high-quality seeds with excellent agronomic performance. However, faced with environmental adversity, limited natural resources and increasing food demand around the globe, more attention has turned to improving crop plant production by implementing efficient strategies. Seed priming technology has shown promising biological improvements leading to suitable agronomic performance in crop plants under adverse environmental conditions. Seeds are subjected to controlled conditions that are conducive to complex physiological, biochemical, and molecular changes, conferring specific stress tolerance to subsequent germination and growth conditions. In this review paper, we aimed to study the recent approaches in the efficiency of hydropriming, osmopriming, chemopriming, hormopriming, nanopriming, matrix priming, biopriming, physical priming and hybrid priming procedures in the production of crop plants under environmental adversity, as well as their biological mechanism changes. All priming methods demonstrated relevant changes in the biological mechanism related to crop plant production by mitigating salinity effects, heavy metals, and flooding stress and enhancing chilling, heat, drought and phytopathogen tolerance. We strongly recommend that researchers combine multiple priming methods, known as hybrid priming, in their investigations to provide novel technologies and additional biological approaches to enhance the knowledge of crop plant science. Thus, the findings shed light on the use of seed priming technology as a key strategy to increase crop plant production under environmental adversity by acquiring stress tolerance and enhancing agronomic traits to meet the global food demand.


Introduction
Faced with the increasing population and limited natural resources, climate change has imposed extra adverse conditions on crop plant production. This situation has worsened to the extent that food consumption has increased disproportionately to the population increasing in the last few years 1 . Moreover, it is predicted that by 2050, salinization consequences will affect 50% of arable lands in the world 2 . In terms of economic losses, environmental stressors have decreased crop production in developing countries, with USD 9.5 billion lost to diseases and pest infestation, USD 29 billion to drought, and USD 47 billion to other causes between 2005 and 2015, according to the FAO 3 . At the field level, during their lifespans, crop plants are exposed to several biotic and abiotic stressors such as temperature, sunlight, soil moisture, dissolved solids, atmospheric composition, phytopathogens and pests. Consequently, these factors reduce crop production, affect economic stability and threaten global food security (Fig. 1). Among the several biological changes in crop plants that experience unsuitable environmental conditions, seed dormancy is considered one of the most common physiological consequences that significantly decrease crop production 4 . Dormancy is characterized by the inhibition of germination while waiting for favorable conditions 5 . In terms of phytohormones, germination and dormancy are controlled by the balance of hormone ratios, mainly ABA (abscisic acid) and GAs (gibberellins) 4,5 . These hormones are stimulated by specific growthrelated genes, which in turn are down/upregulated mainly by environmental factors 6,7 .
In an attempt to mitigate the negative impacts of biotic/abiotic stressors on crop plant production and increase agronomic traits, numerous studies have focused on static farming management, such as watering volume and frequency 8 , fertilizer and pesticide amounts 9 , and other techniques, including the use of resistant varieties 10 . Thus, among the several technologies available to increase crop production, one of the most feasible, low-risk and cost-effective is seed priming [11][12][13][14][15][16] . Seed priming is defined as a 'pregermination' metabolism inducing several physiological, biochemical, and molecular changes to activate stress-responsive genes associated with germination 17 , in which the seed prepares for imminent environmental stress. Stress tolerance acquired through priming treatments has been suggested to possibly be associated with "priming memory". According to Chen and Arora 11 , "priming memory" invokes stress tolerance in seeds depending on the conditions that were previously imposed on the seeds. In other words, seeds retain the preceding stress memory after the priming procedures, which may aid in the attainment of tolerance to subsequent stresses (Fig. 2). Figure 2. Schematic representation summarizing the mechanism underlying environmental stress tolerance acquired by primed seeds.
Seed priming technologies are emerging as a potential and promising method to increase crop production efficiently under unsuitable environmental conditions 13,16,[18][19][20][21] . Seed priming methods are capable of improving the morphophysiological pattern, regulating phytohormones, reprogramming gene expression, and inducing the metabolism of important enzymes 13,22,23 . Germination occurs in three phases after the dry seeds are sown: (I) imbibition, (II) 'pregermination', and (III) emergence 14,24 . The procedure of seed priming is known to trigger 'pregermination' without radicle emergence. Different antioxidants, such as catalase (CAT), ascorbate peroxidase (APX), superoxide dismutase (SOD), and peroxidase (POD), are commonly triggered during seed priming procedures. These antioxidants protect cellular membranes against the harmful effects of reactive oxygen species (ROS) and help mitigate environmental stressors and improve seed germination and seedling growth 24,25 (Fig. 3). Figure 3. Schematic illustration of the seed priming process and comparison to the subsequent germination between unprimed seeds and primed seeds 13,14 . During phase I (imbibition), controlled water uptake allows the synthesis of protein and induces respiratory activities through messenger ribonucleic acid (mRNA). Phase II (pregermination) is related to several physiological, biochemical and molecular activities related to germination, such as protein synthesis, metabolic processes, mitochondrial synthesis, alterations in soluble sugars, and repair processes, but the emergence of radicles is prevented. Re-dehydration is necessary when the final sowing is postponed, in the case of seed companies. In primed seeds, phase III (emergence) is identified by memory priming activation, which results in better seed emergence performance, while unprimed seeds spend time performing phases I and II longer 14 . Priming technology has been appreciated by farmers and seed companies due to its great agronomic performance with a wide range of crop plants 13,16,24,26 . Several investigations have demonstrated the advantages of seed priming procedures in crop plant production 13,16,[18][19][20][21] , and reports of the negative effects of seed priming on agriculture remain scarce 27 . Considering the increasing food demand, limited natural resources and climatic change, the development of agricultural strategies is urgent to produce food efficiently 16,19,[28][29][30] . In this context, the aim of this study is to summarize some of the main seed priming technologies: hydropriming, osmopriming, chemopriming, hormopriming, nanopriming, matrix priming, biopriming, physical priming and hybrid priming, referencing efficiency in the production of crop plants under adverse environmental conditions. The study also provides updated progress on seed priming technology as well as the agronomical potential of physiological, biochemical, and molecular approaches.

Seed priming technologies
The current study presents a wide range of evidence on the use of seed priming methods in crop plant production under adverse environmental conditions. In addition, a synopsis of several investigations on the biological effects of seed priming technology subjected to stress conditions and the impact on crop plant production is listed in Table 1.

Hydropriming
Hydropriming, an ecofriendly, feasible, agronomically efficient, and cost-effective procedure to overcome many environmental stress conditions, allows suitable germination and seedling growth in Lupinus angustifolius L. 31 , Oryza sativa 32 , and Helianthus annuus L. 33 . Hydropriming is a simple method that involves soaking the seeds in pure water for a particular period at a controlled temperature under dark or light conditions 14 . A previous investigation reported the potential of hydropriming to enhance tolerance to low-temperature conditions in L. angustifolius 31 . The study investigated the impact of low temperature (7 °C) on the physiological and biochemical changes during germination under the influence of the hydropriming method (3 h at 20 °C). The effectiveness of hydropriming in protecting seeds against cold damage during germination was confirmed due to reduced cell membrane permeability, amylolysis activity, and regulation of ABA content.
Hydropriming is widely used by seed companies to overcome irregular seed germination and stand establishment caused by unfavorable environmental conditions, such as drought, saline soils and heavy metal accumulation 19,34 . These stressors affect cell division and elongation, reduce nutrient uptake and translocation, and decrease tissue water status and photosynthesis, which consequently causes a reduction in enzymatic activity and overproduction of ROS 35 . To protect the cells from damage, plants activate a self-defense mechanism that controls ROS activities, such as H2O2 36 . Forti et al. 34 promoted better seed germination and seedling establishment of Medicago truncatula in soil contaminated with heavy metals via the seed hydropriming method. The study successfully upregulated genes involved in DNA repair, such as OGG1 (8-oxoguanine glycosylase) and FPG (formamidopopyrimidine-DNA glycosylase) genes, and antioxidant activities (SOD and APX).
Anaerobic conditions during germination and seedling growth normally lead to poor establishment and low crop yield. Mondal et al. 32 investigated the responses of hydropriming to the growth index and physiological processes of rice genotypes during germination and seedling growth under anaerobic conditions. Hydropriming significantly enhanced the emergence and seedling growth of rice in flooded soils. According to the study, hydropriming treatment was able to improve the breakdown of stored carbohydrates by enhancing the enzymatic activities of starch catabolic enzymes and maintaining lower malondialdehyde (MDA) concentrations, as supported by other investigations 37 .

Osmopriming
Routinely used by seed companies to enhance the vigor of seeds, osmopriming techniques have shown promising germination and plant growth performance 38,39 under several adverse environmental conditions, such as chilling 40 , salinity 41,42 and drought 12 . In this method, seeds are immersed in an osmotic solution with low water potential (ψ) through the use of polyethylene glycol (PEG), mannitol, sorbitol, glycerol, or inorganic substances, such as NaCl, KCl, K2SiO3, KNO3, MgSO4, and CaCl2. This immersion allows the seeds to absorb water slowly, thereby culminating in less cellular damage. Tabassum et al. 43 studied osmopriming (with 1.5% CaCl2 solution) in the production of wheat (Triticum aestivum L.) under drought stress. The results revealed promising crop plant enhancement in comparison with hydropriming treatments regarding osmolyte accumulation, tissue water, leaf area, and yield. The positive plant responses were better with osmopriming due to a considerable decrease in lipid peroxidation and acquired drought tolerance. These results are in agreement with Chen and Arora 44 , who discussed that the stress tolerance acquired by osmopriming is, in part, related to the gradual accumulation of proteins, such as dehydrins (DHNs), usually reported to protect against cellular dehydration, and a more robust antioxidant system in relation to the activation of pregerminative metabolism. DHNs (group 2 LEA proteins) are watersoluble lipid vesicle-associated proteins involved in the adaptive responses of plants to environmental stress tolerance 45 . Later, Chen et al. 46 successfully improved chilling and desiccation stress tolerance in Spinacia oleracea L. cv. Bloomsdale by osmopriming seeds. In this study, the authors associated stress tolerance with the accumulation of DHN-like proteins in spinach seeds, since they exclusively accumulated during the early phase of osmopriming in response to environmental stressors.
Soil salinity is another adverse condition that causes losses for crop plant production, especially in arid and semiarid regions 42 . Soil salinity becomes more extensive yearly, particularly as a result of inappropriate agronomic management 47,48 . Salinity stress conditions cause increasing osmotic pressure, ion uptake imbalanced, and oxidative stress in sorghum, hence affecting the early growth stages and decreasing crop production 49 . Recently, wheat seeds osmoprimed with potassium silicate (K2SiO3) was reported to be the most effective agent to relieve the negative impact of salinity stress during germination and plant growth 50 . Kubala et al. 41 detected improvements in the germination and seedling growth of Brassica napus primed with polyethylene glycol (-1.2 MPa) under salinity stress (NaCl: 100 mM). According to the study, the improvement of germination performance and seedling establishment in osmoprimed treatments was due to increased P5CSA gene expression and decreased PDH gene expression associated with proline accumulation and H2O2 concentrations. Further understanding of molecular markers is of interest for providing suitable protocols for seed priming programs for each plant species and local environmental constraints. Thus, this aspect is rather important because such molecular indicators linked with biological mechanism changes allow the prediction of seed quality and may provide further knowledge to support future studies and provide assistance to seed companies.
The consequences of low temperature in wheat are reported to be increased ROS concentrations in the seeds, which disturb several biological mechanisms and cause an imbalance between the ability of leaves to absorb light and release energy to the cells to perform essential metabolic activities 51,52 . Considering that there is an estimated high demand for wheat in the future due to the rising population 53 and climate change, Li et al. 40 improved the cold tolerance of wheat plants with osmopriming treatment in seeds (30 mM NaCl). The priming procedure successfully enhanced the photochemical efficiency in seedlings by decreasing MDA accumulation and alleviating cell death. Lower MDA activity indicates a decrease in lipid peroxidation, which maintains the integrity of the membrane and leads primed seeds to a better germination performance 54 . In another study, Zhang et al. 55 investigated the physiological and biochemical effects of osmopriming (PEG 8000 solution) on the germination performance and seedling establishment of sorghum [Sorghum bicolor (L.) Moench] under various soil moisture conditions. The experiment provided promising results concerning uniform emergence and decreased drought stress for suitable seedling establishment. The priming procedure strengthened the antioxidant activities of POD, CAT, SOD, and APX, which led to the enhancement of drought tolerance in sorghum plants.

Chemopriming
Chemopriming involves inorganic substances, such as hydrochloric acid (HCl), selenium (Se), fungicides and pesticides, or organic substances, such as essential oils, dairy products and crude plant extracts. Biochemical changes, such as antioxidant activity, are one of the most common improvements in seeds treated with chemopriming, leading to a reduction in heavy metal uptake 56 , tolerance to chilling 57 , salinity 58 , and drought stress 59 in several crop plants 60 .
Considered one of the most aggressive abiotic stresses, chilling conditions significantly decrease the germination index, leading to poor seedling growth of crop plants by reducing starch metabolism and lowering the respiration rate. Many crop plants are very sensitive to chilling stress during germination 61 . Hussain et al. 57 studied the physiological and biochemical mechanisms of rice cultivars primed with several methods and then subjected to chilling stress (18 °C). The application of selenium (50 μM) was shown to be one of the most effective treatments compared to osmopriming (calcium chloride: 100 mg L −1 ), redox priming (hydrogen peroxide: 50 μM), and hormopriming (salicylic acid: 100 mg L −1 ), allowing rice to thrive under chilling stress. The investigation also found that chemopriming treatment induced several physiological activities, such as peroxidase, catalase, and superoxide dismutase, and enhanced the accumulation of glutathione and free proline at the seedling stage, which provided a strong antioxidative defense system under chilling stress.
Another constraint experienced by farmers around the world, especially in Asian countries, is the high levels of arsenic-contaminated groundwater in rice cultivation, which is considered a limitation for normal rice growth, reducing germination by 70% in some cases. A study with chemopriming (selenium: 0.8 mg L −1 ) treatment in rice under arsenic stress conditions demonstrated an enhancement of germination, shoot length, and seedling biomass 56 . In this case, the plant responses to chemopriming were reflected in the enhancement of biological mechanisms due to the reduction in arsenic uptake, hence suppressing oxidative damage by increasing antioxidant accumulation in rice seedlings. In another case with heavy metals in the soil, nickel stress resistance [at 50 ppm Ni(NO3)2] was detected in zucchini seedlings (Cucurbita pepo L. cv. Courgette d'Italie) by chemopriming with H2S and CaCl2 62 . In this study, chemopriming induced postgerminative crossadaptation by improving photosynthetic pigments and seedling biomass, as well as increasing the content of ascorbate, total thiols, and glutathione reductase activity in leaves, while ascorbate peroxidase activity decreased significantly.

Hormopriming
Phytohormones naturally mediate the regulation of biological mechanisms in plant species 4,63 . Abscisic acid, auxin, brassinosteroids, cytokinins, ethylene, gibberellins, jasmonates, salicylic acid, and strigolactones are phytohormones involved in regulating seed dormancy, germination and plant development, as well as defense responses to environmental stressors 4,64,65 . These substances have been evaluated in experimental studies to detect plant responses to unsuitable environmental conditions 66,67 , which may help the development of tools and specific protocols for enhancing crop plant production.
Hormopriming is considered one of the most effective methods and is widely applied by seed companies to improve stress tolerance in crop plants, such as drought in maize 68 , salinity in wheat 69 and chilling in rice 70 . ABA and GAs are recognized to control physiological, biochemical, and molecular mechanisms in tomato, such as germination, seedling growth, transportation and partitioning of specific nutrients, and reprogramming of gene expression, as reported by Nakaune et al. 6 . Knowledge about the dynamic changes in phytohormone and gene expression during seed priming and during germination may facilitate understanding of the biological mechanism to develop new concepts and specific technologies to improve agronomic traits. For instance, Yang et al. 22 shed light on the biological mechanism underlying rapid germination in tomato seeds treated with hormopriming, discussing the dynamic changes in the transcript levels involved in the ABA and GA pathways. The study detected higher expression levels of SlCYP707A2, which is considered an important catabolic enzyme in the ABA pathway and maintains the low concentration of ABA, in seeds with rapid germination rates.
The deleterious effect of chilling and drought conditions on the germination and development of plants is, in part, because it induces the accumulation of a large amount of ROS [71][72][73] . Moreover, chilling and drought conditions cause reductions in carbohydrates, lipids, and proteins, resulting in cell damage. Most rice varieties are sensitive to lowtemperature conditions during germination and seedling development, leading to severe economic losses 74,75 . Wang et al. 70 investigated the effects of seed hormopriming (salicylic acid) against chilling stress on rice germination and seedling growth. The results showed increasing germination performance and enhanced morphological attributes, such as length of shoots and weight of shoots, and weight of roots. In this case, the agronomic improvements were correlated with higher α-amylase activity and total soluble sugar content. In accordance with these results, Pál et al. 76 suggested a similar response to hormopriming in the enhancement of plant tolerance to chilling stress by modifying the antioxidant activity system. As a natural response to stress, priming seed methods induce higher α-amylase and/or β-amylase activity, which results in an increased breakdown of starch and subsequent buildup of sugar levels. These enzymes play pivotal roles in mitigating environmental stress by increasing the rate of respiration, improving germination speed, and promoting suitable seedling emergence and establishment in plants 77 .
Exogenous application of methyl jasmonate (20 μM) and/or salicylic acid (2 mM) in maize (Zea mays L.) seeds showed the ability to improve physiological and biochemical attributes under drought stress in comparison to hydropriming 68 . Likewise, Samota et al. 78 primed droughttolerant and drought-sensitive rice seeds with methyl jasmonate or salicylic acid under drought conditions. The experiment detected effective growth and development of plants because of the mitigating of damaging effects of drought stress on the plant by increasing antioxidant activities in the shoot, lowered lipid peroxidation, reduced protein oxidation, and upregulated expression of drought-responsive genes. In another study, exogenous hormone application (5 mM GA3) improved the germination and establishment of alfalfa (Medicago sativa) seedlings under saline conditions (200 mM NaCl) by enhancing the activities of antioxidant enzymes (CAT, SOD, and APX) and reducing membrane damage 79,80 .

Nanopriming
Nanotechnology is an advanced method for agriculture because it has shown promising agronomic responses for a wide range of crop plants 81,82 . Nanoparticles have demonstrated enhanced biological activity in plants via nanofertilizers 83 and reduced toxicity of nanoherbicides 84 and nanopesticides 85 in recent decades 86 .
Nanopriming agents, such as silver and zinc oxide nanoparticles [87][88][89][90] , have been used to enhance germination indexes and seedling establishment in several plant species: O. sativa 91 , Carthamus tinctorius 92 , Citrullus lanatus 93 , and Thymus kotschyanus 94 . Moreover, nanopriming is one of the most efficient methods to induce salt tolerance capacity in plants by enhancing physiological and biochemical responses 95 . In this context, Shafiq et al. 96 detected improvements in agronomic traits of wheat plants treated with fullerenol nanopriming (0, 10, 40, 80 and 120 nM concentration) under salt stress (150 mM NaCl). The study showed that fullerenol induced better K + , Ca 2+ and P uptake, which was reflected in better ionic and ROS homeostasis and conferred grain yield recovery by plant stress resilience. Another study evaluated the germination indexes and seedling enhancement of sorghum [S. bicolor (L.) Moench] treated with nanoiron oxide (n-Fe2O3) under salt stress (150 mmol NaCl solution) 97 . The results demonstrated significant salt tolerance in plants treated with nanopriming through physiological improvements, such as photosynthetic rate, chlorophyll index, photosystem II efficiency, and relative water content, with the aim of decreasing membrane damage.
Further investigations at the molecular point of view, although scarce in nanopriming studies, would allow researchers to develop agronomic strategies to enhance crop production under stress conditions and to utilize natural resources more efficiently.

Matrix priming (MP)
In MP, seeds are mixed with organic/inorganic solid or semisolid water carriers (charcoal, clay, peat moss, sand, sawdust, vermiculite) during imbibition and incubated for a predetermined photoperiod with controlled temperature and oxygen availability 23,98,99 . In this procedure, the matrix potential of the priming solution with high water-holding capacity induces a slowdown of solute uptake by seeds, similar to the water soaking phenomenon experienced by seeds in a natural environment. Then, seeds are separated from the matrix and dried to near the initial moisture level. The procedure is flexible (by mixing with other materials), cost-effective, and able to treat a large number of seeds 100 . Important enhancement in crop production has been noted in many reports for the use of MP under environmental stress, such as drought 23 , salinity 101 , and low temperature 102 .
In an attempt to establish a protocol for MP, a classic investigation used artificial soil media in flats under supra-optimal temperature in comparison to polyethylene glycol (8000), inorganic salts, and nontreated seeds 98 . The study detected that tomato (L. esculentum), carrot (Daucus carota) and onion (Allium cepa) treated with MP improved seedling emergence by 50% and increased the dry weight by acquiring thermotolerance compared to nontreated seeds. In another study, broccoli and cauliflower seeds were subjected to MP (vermiculite and water) for two days of incubation 101 . Salt stress (50,100,150 or 200 mM NaCl) was mitigated with MP treatment by increasing the physiological attributes and biochemical activity, such as peroxidase and catalase, and the contents of proline, soluble sugar, and protein in both plant species. In this case, the accumulation of proline and soluble sugar in cells, as well as the high activities of protective enzymes, aided in enhancing salinity tolerance in broccoli and cauliflower. Moreover, the great availability of O2 to the seeds during the MP procedure may help the respiratory system, since it directly affects seed vigor 103 . In another study, Sen et al. 104 investigated the physiological and biochemical responses in mung beans (Vigna radiata) through MP with chitosan to overcome the adverse effects of salinity stress. Chitosan is recognized due to its biodegradability, bioactivity, biocompatibility, and nontoxicity to crop plant production. MP treatment significantly reduced the H2O2 and MDA content and increased the accumulation of protein, antioxidant activity, and phenolic compounds, leading to better plant growth-promoting traits.

Biopriming
Although it is not widely used in crop plant production, biopriming is an emerging, ecofriendly and promising method in which strains of Bacillus spp., Enterobacter spp., Pseudomonas spp., and Trichoderma spp., among others, are applied to seeds to improve germination indexes and uniformity, as well as seedling vigor and growth parameters 24,105 . In this method, the inoculation of beneficial microorganisms in seeds is able to colonize the rhizosphere, reducing seed and soilborne pathogens and hence improving the endophytic relationships with the plant 34,105-108 . Despite few investigations of agronomic performance 109 , biopriming has shown great synergistic potential between microorganisms and plants in inducing biotic and abiotic resistance 12,[110][111][112][113] .
Mycorrhizal fungi have the natural potential to activate the aggregation of several important proteins and transcripts on the roots, which improves the plant defense mechanism system 114 . Rozier et al. 108 used plant growth-promoting rhizobacteria (Azospirillum lipoferum) in maize cultivars, which improved the germination rate and seedling defense system by stimulating biochemical and physiological activity. In another experiment with biopriming, Trichoderma harzianum promoted drought tolerance in wheat through physiological protective mechanisms and increased phytopathogen resistance 115 . Additionally, working with Trichoderma as a biopriming agent in wheat, Meena et al. 116 reported enhancements in height, root length, yield, and chlorophyll content in different soil conditions. The study detected the improvement of nitrogen use efficiency, which is considered a relevant agronomical trait, since approximately 50% of the N applied to the field in intensive agricultural production systems is lost through leaching, surface runoff, volatilization, denitrification, and microbial consumption 117,118 .
Biopriming has been investigated as a disease management method because endophytic microorganisms can reduce biotic stress, which helps the biological system defend against phytopathogens 119 . In this way, Singh et al. 120 reported phytopathology control (Rhizoctonia solani) in maize treated with biopriming (Pseudomonas aeruginosa) via enhancement of antioxidative defense enzymes. A significant enhancement in physiological and biochemical responses was detected in maize plants treated with biopriming, such as activation of the phenylpropanoid pathway and enhanced accumulation of proline. The study also detected a significant regulation of stress-responsive genes (PR-1 and PR-10). In pearl millet [Pennisetum glaucum (L.) R. Br], biopriming with Pseudomonas fluorescens improved the germination and growth indexes and promoted resistance against downy mildew disease (Sclerospora graminicola) by physiological changes 121 .
2.8. Physical priming 2.8.1. Heat/Cold priming Temperature stress limits crop production and threatens global food security 122,123 . Crop plants that experience unsuitable environmental temperatures at the seed germination, seedling growth, and/or vegetative stage may impact negative effects on yield productivity through a cascade of physiological, biochemical, and molecular changes [124][125][126] . Efficient photosynthesis and photosynthetic partitioning are required for normal plant development. Considering that photosynthesis is highly sensitive to temperature, heat stress may disrupt chloroplast structures and their specific functions, decreasing the chlorophyll amount and stimulating the loss of crop production 127 .
In heat/cold priming, seeds are subjected to different temperatures for a predetermined period with minimal physiological impact. The seeds treated with heat/cold priming techniques allow activating biological mechanisms such as osmolytes and antioxidative defense, which are responsible for improving germination and plant development by reducing thermoinhibition. Heat priming is able to induce stressresponsive proteins (heat-shock proteins and late embryogenesis abundant proteins) and reprogram metabolic homeostasis and, which confers significant thermotolerance, allowing plants to withstand subsequent thermal stresses 13,128,129 . In this context, heat stress was conducive to a significant grain yield reduction in winter wheat, while heat primed seeds (40 °C for four hours) did not show such a loss of yield 130 . The study detected improvements in photosynthesis and antioxidant activity by gene expression modifications that lead to the thermotolerance of winter wheat plants. Although heat/cold priming in seeds has shown satisfactory results in crop production, most heat/cold priming investigations are made in vegetative tissues and rarely in seeds 131 .
Previous reports have studied the biological mechanisms of heat/cold priming to overcome the stress induced by temperature changes 125,132,133 . Moderate temperature as a priming treatment has shown physiological improvement in relation to stress tolerance under high temperatures in Agrostis stolonifera L. 134 . The study suggested that heat tolerance is a natural response to the higher concentration of saturated fatty acids in the leaf membranes. In another investigation, cold priming ameliorated cold stress in chickpeas 135 . Seeds were primed at 5 °C for 30 days, and plants were raised in a controlled environment. Cold stress negatively affected the biological mechanisms of chickpea plants, such as photosynthetic ability and photoassimilation capacity, and decreased the redox status of the cells and the production of osmolytes. Cold tolerance ability was detected in primed plants at the reproductive stage, according to the authors because of the improved leaf function, such as hydration status and photosynthetic and carbon fixation ability, in comparison to plants without priming treatment. Thus, although less information is available on the biological mechanism and molecular changes in seeds subjected to heat/cold priming, this method may contribute to inducing thermotolerance in crop plants for hotspot regions of warming levels of temperature changes 136 .

Cold plasma priming
Cold plasma is an ecofriendly and cost-effective priming method to efficiently improve crop plant production 133,[137][138][139] . Cold plasma priming involves the application of a mixture with ionized gas, positively charged particles, electrons, and neutral gas to seeds, which stimulates biological mechanism changes, such as the density of reactive oxygen species, phytohormone catabolism, reactive nitrogen species, and electrical conductivity 139,140 . According to several authors, in addition to eliminating phytopathogen contamination, this priming process also modifies the seed surface and facilitates the seed water uptake capacity, breaking dormancy and thus triggering modifications in hormones 133 , the proteome 141 , secondary metabolites 142 , and tissue differentiation 143 , leading to fast germination and better seedling growth and improving tolerance to environmental stress 133,137,139,140,144 .
In previous studies, physiological and biochemical responses were enhanced by cold plasma exposure in many crop plants: T. aestivum and A. sativa 145 , O. sativa L. 18 , Gossypium hirsutum L. 146 , Pisum sativum L. and Cucurbita pepo L. 147 . Seed treated with cold plasma has shown long-term effects at a later stage, such as the seedling stage 133,139 , to cope with biotic and abiotic stressors, such as drought stress and disease stress. For instance, Jiang et al. 137 reported that the exposure of tomato seeds to cold plasma (80 W) efficiently increased germination and growth response and regulated the defense mechanism system in the resistance to bacterial wilt (Ralstonia solanacearum). Similarly, Li et al. 144 reported that cold plasma treatment (120 W) in peanuts improved the germination rate, increased the shoot and root dry weights, and improved yield in comparison to the nonprimed treatment. Such improvements are related to the leaf area, nitrogen concentrations, and chlorophyll content increasing in response to cold plasma treatment. In another study, tomato seeds primed with cold plasma improved the germination potential and seedling growth rate under drought stress 133 . Moreover, the study detected improvements in antioxidant activity, phytohormone synthesis, and defense gene expression (β-1,3-glucanase) of tomato seedlings. Although cold plasma seed priming has shown promising results in crop plant production, the biological changes and their regulation in several crop plants to mitigate biotic/abiotic stress remain unclear. With the current advances in plasma technologies, future studies could focus on plasma-triggered modifications in the cellular transcription program of genes, concentrations of hormones, and proteome issues to improve the knowledge about the complex biological mechanism changes in crop plants under stress conditions.

Hybrid priming
The physiological, biochemical, and molecular responses in seeds subjected to a single priming method are considered difficult to understand in the biological system that controls biotic/abiotic stress resistance. However, understanding the plant response becomes more complex when hybrid priming (combined priming procedures) is applied to seeds 14 , plants 148 , or both 149 because different priming methods promote different effectiveness in plants. In this way, considering that crop plants during their lifespan are commonly exposed to several environmental conditions, such as drought, salinity, heat/cold/freezing, and/or phytopathogens, hybrid priming treatments of seeds may be effective and a desirable method to increase multiple stress tolerances in crop plants.
Hybrid priming, a method of multiple priming combined in a specific procedure, commonly acts synergistically with priming agents, promoting agronomical attribute improvements 14 through phytohormone regulation 141,150 , reprogramming of gene expression 21 , and changes in biological mechanisms 151 . For instance, single electrostatic field is a priming method 152 used to recover seed vigor 14 and to induce fast germination and plant growth in crop plants 16,153,154 . In this procedure, seeds are exposed to electrical current (kilovolts/centimeters) for a predetermined time, which promotes some biological changes, such as superoxide dismutase activities in onion seeds 14 and antioxidant metabolism in wheatgrass 152 . In the same way, single hydropriming is considered a relevant technique to overcome environmental stressors 33,155 , as mentioned in section 2.1 (Hydropriming). Thus, while single electrostatic field and single hydropriming improve limited biological changes, hybrid priming technology allows more biological changes to crop plants. Zhao et al. 14 developed a novel hydro-electro hybrid priming (HEHP) method to recover the potential vigor of onion seeds. They subjected the seeds to hydropriming (5 h) followed by electrostatic field irradiation (10 kv/cm for 40 s), incubation and desiccation (Fig. 4). The combined priming method successfully recovered the potential vigor index of onion seeds (612.38) via biological mechanism changes in comparison to single hydropriming (490.26), single electrostatic field irradiation (454. 85), and no priming (212.87). In the same way, our previous investigation adopted a similar HEHP method to achieve the rapid germination of tomato (S. lycopersicum var. HaoMei) with low vigor 156 . Potential synergism was detected between the priming procedures (hybrid priming) in phytohormone regulation (ABA/GA) and reprogramming of gene expression (SlNCED2 and SlDELLA), which reflected the enhancement of germination indexes and vigor responses (5.09; 127) in comparison to single hydropriming (3.19; 57.42), single electrostatic field irradiation (2.47; 14,82) and no priming (2.78; 13.85). Moreover, the study detected vigor improvement of tomato seeds stored for 60 days (446.59) with HEHP samples in comparison to primed seeds without storage (127). Thus, HEHP was demonstrated to be a promising and flexible method for adaptation in other crop plants.
Compared with the efforts made to study the single priming effect to increase stress tolerance in crop plants, the biological response of plants subjected to multiple priming procedures has drawn less attention until recently, as observed by previous reports 14,157 . Hassini et al. 158 improved broccoli (B. oleracea L. var. Italica) sprout growth and quality under salinity stress (150 mM NaCl) by combined priming with KCl (50 mM) and methyl jasmonate. Górnik and Lahuta 151 combined 24epibrassinolide (10 -6 , 10 -8 and 10 -10 M), salicylic acid or jasmonic acid (10 -2 , 10 -3 and 10 -4 M) with hydropriming (15% moisture content) followed by heat shock treatment (2 h at 45 °C) on sunflower seeds and then subjected them to chilling conditions (21 days at 0 °C) and a recovery period (72 h at 25 °C). The combined priming method increased the resistance of seedlings to chilling stress conditions mainly by promoting the activity of catalase and sugar metabolism, which alleviated the decrease in Fv/Fm. In another experiment, Li et al. 21 reported the ability of hybrid priming with exogenous salicylic acid and H2O2 to enhance the chilling tolerance (13 °C) of maize. Seed vigor and seedling establishment under chilling stress were improved in hybrid priming seed treatment. The synergistic effects of combined priming induced positive changes in the antioxidant system and hormone activity and increased the metabolites and energy supply, thereby providing biological conditions to enhance chilling tolerance in maize. In this case, the hybrid priming method induced the upregulation of gene expression related to GA biosynthesis, ZmGA20ox1 and ZmGA30ox2, and induced the downregulation of gene expression related to GA catabolism, ZmGA2ox1, while ABA catabolism gene expression, ZmCYP707A2, and the expression of ZmCPK11 and ZmSnRK2.1, encoding response receptors in the ABA signaling pathway, were all upregulated. The gene ZmRGL2, responsible for germination inhibition 159 , was decreased in the hybrid priming treatment.
Thus, optimization and standardization of priming agents, as well as specific procedures, are required for each crop plants in the hybrid priming method according to local environmental adversity conditions to improve crop plant productivity.

Research Gaps and Future Perspectives
Faced with climate change, limited natural resources and the everincreasing population around the globe, improved crop plant production with the development of new technologies that are agronomically efficient, feasible, cost-effective and, if possible, ecofriendly is urgent. Accumulated evidence has shown promising crop productivity when seeds are exposed to single or multiple priming procedures to enhance stress tolerance. Future studies with seed priming procedures may focus on the molecular level, in accordance with a proteomic and/or metabolomics approach, to identify and track the stress-responsive genes during and after priming procedures, as well as during plant development. Such studies may generate valuable results to determine the priming procedure for each plant species to enhance crop production under local adverse environmental conditions. Moreover, this work strongly encourages researchers to combine two or more priming procedures, and the use of this priming method may be valuable to synergistically activate biological mechanisms and enhance tolerance towards multiple biotic/abiotic stresses. Finally, seed companies may widely adopt priming technology as a key strategy to increase crop plant production under adverse environmental conditions.