Revisiting the Potential of Seed Nutri-Priming to Improve Stress Resilience and Nutritive Value of Cereals in the Context of Current Global Challenges

Abstract

Most crop species are cultivated in nutrient-deficient soils, in combination with other challenging constraints that are exacerbated by the current climate changes. The significance of micronutrient shortage in stress management is often underappreciated, although their deficiency restricts both plant growth and resistance to abiotic stresses and diseases. While the application of nutrients to growing plants is a potential strategy to improve plant resistance to abiotic stresses, seed nutrient status may also play a role in crop stress tolerance as a storage and accumulation site of nutrients. To avoid hidden hunger problems, developing countries need to increase domestic cereal production, enhance their resilience to extreme weather events, and improve their nutritional status and quality. Here, we analyze the accumulated knowledge about the effects of nutri-priming in cereal crop species with a focus on mechanisms of application and stress tolerance, keeping in mind the risk of crop damage mostly caused by global climate change, which is driving an alarming increase in the frequency and intensity of abiotic stresses. We also propose new approaches to food production, which may be promising solutions for global warming, emerging diseases, and geopolitical conflicts recognized as major drivers of food insecurity.

1. Introduction

The early 21st century has been associated with climate fluctuations and food insecurity [1,2,3], which are intensified by a growing global population, anthropogenic pollution, and, more recently, pandemics and armed conflicts [4]. Despite continued efforts of scientists to tackle ongoing environmental issues by developing new strategies and practices, human behaviors and ameliorated agricultural practices are the main categories at present to cope with encroaching environmental stresses that limit plant production in agricultural and natural ecosystems. As a result of micronutrient deficiencies, malnutrition and hidden hunger are of growing concern worldwide. These issues are likely to be aggravated by global climate change. Presently, every third person in the world is suffering from malnutrition, mainly caused by micronutrient deficiency in the crops. Hence, the production of biofortified crops with increased micronutrient contents as an innovative and sustainable agricultural practice might contribute to meeting the increasing needs of a world population expected to surpass 10 billion by 2050 [5,6].

Micronutrients include boron (B), chloride (Cl), copper (Cu), iron (Fe), manganese (Mn), molybdenum (Mo), nickel (Ni), zinc (Zn), cobalt (Co), iodine (I), and selenium (Se). Not only their availability but also their proper balance in the soils is essential for normal growth, development, and optimum yield of plants [7]. Micronutrients are also essential dietary components since their supply is critical for human health. Consequently, their deficiencies can cause severe and even life-threatening symptoms in humans, notably in developing countries [8]. For example, a deficiency in Fe or Zn results in anemia and immunodeficiency. This is partly because humans are unable to produce microelements endogenously and therefore need adequate and safe micronutrients in their daily diet. Plant food is the main source of dietary intake for humans and represents a major source of micronutrients. However, most crops are grown in soils challenged by low nutrient bioavailability [9,10]. The ongoing global climate changes aggravate this issue, besides impacting water resource quality and/or availability and increasing the prevalence of soil salinization [11].

The need to enhance micronutrient availability to crop plants has led to the development of several strategies to avert negative impacts on food quality. Prominent among these strategies is breeding for stress tolerance, yet this process is costly, time-consuming, and could be stymied by public rejection of genetically modified crops [12]. Another strategy consists of further augmenting fertilizer input, but the latter is clearly at odds with sustainability [13]. Considering these challenges, alternative approaches that would be affordable and economically feasible clearly need to be implemented.

Priming, or hardening, consists of pre-soaking seeds or exposing plants to a given level of stress, triggering a stress memory response and allowing plants to better cope when later exposed to environmental stresses [14]. Nutrient priming of whole plants and seeds is a well-established, cost-effective method to enhance endogenous nutrients in crop plants, thus improving their nutrient status and enabling them to acquire stress resistance [15,16,17,18]. Osmotic seed priming (e.g., hydropriming) consists on soaking seeds in low osmotic potential solution [19], while nutri-priming involves pretreating (priming) seeds in nutrient-rich solutions [20]. Nutri-priming is an innovative, simple, efficient, environment friendly, and economically affordable method. The aim of this review is to evaluate the potential of nutri-priming in the mitigation of the negative impact of abiotic and biotic stresses on food security.

2. The 21st Century, an Era of Global Climate Change Threatening Plant Productivity and Consequently Food Security

“Food security is achieved when every human being has regular access to a sufficient quantity of affordable, nutritious food enabling an active and healthy life” [21]. Based on this definition, there is a strong connection between food security and agriculture. However, the dynamics of ecosystems and global agricultural production are presently facing tremendous challenges due to global climatic changes, alterations in the global water and carbon cycles, and extreme weather phenomena (such as unpleasant temperatures). Climate change enhances the intensity of multifactorial stress combination [22,23], leading, for example, to the mutual influence of high temperature, drought, and salinity [24]. The related changes of the physico-chemical soil characteristics and soil composition result in soil degradation, thus posing a serious threat to crop yield and food supply [25]. According to Soares et al. [26], climate change may significantly impact the accumulation of minerals, proteins, carbohydrates, and antioxidant components in almost all crop species.

The bioavailability of nutrients in soils highly depends on water supply because water is the main driver of macro- and micronutrients, facilitating their intake and transport in plants [27]. Severe drought was shown to disturb nutrient accumulation in food crops [27]. A similar effect is triggered by the accumulation of Na⁺ and Cl⁻, leading to a restriction in plant uptake of essential nutrients (notably Fe, Zn, Mn, and Cu) and consequently to a nutritional imbalance, as documented in wheat and rice cultivars [28,29]. The atmospheric CO₂ concentration has also been reported to impact macro and micronutrient (e.g., Fe, Zn, and Cu) contents in several crop species such as Lactuca sativa, Spinacia oleracea, Oryza sativa and Triticum aestivum [30,31,32]. The analysis of several minerals in C₃ crops reveals that free-air enrichment and open-top chamber studies aimed at enhancing atmospheric CO₂ concentrations result in a decrease (up to −17%) in the concentration of some micronutrients, notably Zn and Fe [33] together with an increased sensitivity of major crops to diseases [34]. Similarly, elevated ozone (O₃) negatively affects the quality of fruits and vegetable crops [35]. High temperature induces alteration in the composition of carbohydrates, mainly glucose and sucrose, inside the seed [36] and reduces vitamin contents in several crop species [37].

In summary, extreme global climatic changes weaken plant performance and increase the frequency/severity of pests and pathogen attacks, leading to a significant loss of crop yield and quality [38]. The decrease in the concentration of essential micronutrients in cereals is of major concern for human health and populations in developing countries, which rely on cereals in their daily diet.

3. Significance of Micronutrients in Plant Physiology and Biochemistry

3.1. What Is the Difference between Micronutrients and Heavy Metal Polluants?

Micronutrients are required by plants, animals, and humans in small quantities [5,39]. In plants, these essential elements are involved in all vital metabolic and cellular functions and, thus, cannot be substituted by other elements in their specific functions. A deficiency in one or more microelements results in an alteration of many cellular metabolic pathways.

Micronutrients are distinct from heavy metals in many ways. The term heavy metals refers to elements that are widespread in nature and generally present in soils or derived from magmatic, sedimentary, or metamorphic rocks [40]. Some heavy metals are crucial for living organisms, while others like cadmium (Cd), mercury (Hg), silver (Ag), lead (Pb), and chromium (Cr) are biologically non-essential and are toxic even at low concentrations. Even if essential, trace elements such as Co, Cu, Fe, Mn, Mo, Ni, and Zn can be toxic when exceeding a certain threshold and, hence, are termed heavy metals [41]. Given the similarity between non-essential and essential metals, competition between both elements can occur, leading to an alteration of many biological functions. Another difference between micronutrients and heavy metals is that the former improves the productivity and nutritional value of crops and, therefore, human health, while the latter has many side effects on living organisms, notably toxicity [40,41].

3.2. Specific Roles of Micronutrients in Plant Metabolism

Although present in very small quantities, micronutrients play big roles in plant growth and development. They are also key elements in a wide range of cellular functions, including control of gene expression, cell elongation, signal perception and transduction, energy metabolism, and primary and secondary metabolism [42]. In this way, some micronutrients act as catalytically active cofactors of many enzymes or as activators of several enzymes, while others are involved in protein stabilization [43]. Some micronutrients (i.e., Mn, Fe, and Cu) operate in mitochondria and chloroplasts as integral components of the electron transport chain [44]. Cl, Cu, Fe, and Mn are essential for several steps of photosynthesis, whereas Zn, Cu, Fe, and Mn are especially associated with diverse enzymatic activities. Still, micronutrients play a big role in oxidative stress responses and redox reactions in plant cells [45]. For example, Se takes part in glutathione peroxidases and thioredoxin reductase, while Fe, Zn, and Cu are cofactors of different superoxide dismutase isoforms. Zn has both structural and catalytic functions as it is a cofactor of more than 80 proteins, as well as its involvement in protein synthesis. This was confirmed since protein contents are severely restricted in plants enduring Zn shortage. Zn deficiency results in a decrease in starch, proteins, and mineral contents and an inhibition of some crucial biochemical processes during the germination phase [46]. Moreover, in the field, plant sensitivity to biotic and abiotic stresses is partly explained by low Zn seed contents, resulting in low biomass production and productivity [47]. Mn is a structural element of several photosynthetic proteins and enzymes [48], while B is involved in various processes (i.e., the synthesis of proteins, cell respiration, transport of assimilates, carbohydrate and hormones metabolism, cell wall structure and synthesis) and is associated with carbohydrate metabolism and reproductive phase of the plants along with photosynthesis or enzymatic activities [49]. The specific function of I is the regulation of gene expression involved in the plant defense against biotic or abiotic stress [50]. I fulfills a functional role, notably as a part of proteins present in chloroplasts, ensuring an adequate function of photosynthesis [48]. Fe and Cu are associated with the photosynthetic electron transport chain since 80% and 50% of Fe and Cu were found in chloroplasts [51,52]. Fe is a cofactor of several enzymes, including those involved in N metabolism (i.e., nitrogenase and nitrate reductase), antioxidative defense system (i.e., catalases, peroxidases, and SODs), and TCA cycle. Fe is also a key element of respiration and is involved in both chlorophyll and DNA synthesis [44]. Mn is required by the photosystem II water oxidizing system under the photosynthetic process functioning [53]. Mn, as a cofactor of antioxidative enzymes, is crucial for mitigating oxidative damage induced by abiotic stressors. As shown recently by Bhat et al. [42], micronutrients influence secondary metabolism by affecting their functions, as in the case of Se, which boosts the accumulation of secondary metabolites in higher plants [54]. Recent studies suggest the involvement of several micronutrients in conferring stress tolerance because they activate several metabolic pathways, regulate the whole plant perception, and induce the adaptive response to stress conditions [55]. For example, during abiotic stress exposure, Se modulates gene expression and secondary metabolite biosynthesis and accumulation [56]. Overall, given their abovementioned versatile roles, an adequate supply of micronutrients is crucial for maintaining optimal rates of photosynthesis and respiration, both ensuring basic plant growth processes and optimal plant yield [51].

4. Factors Affecting Crop Nutritional Characteristics with Special Consideration on Cereals with Importance in Human Diet

Widely cultivated worldwide (covering 40% of all arable land), cereals represent a major source of human food and animal feed [57,58]. The world’s most important cereals are Zea mays, T. aestivum, O. sativa, Hordeum vulgare, Sorghum bicolor, Avena sativa, and Secale cereale, with a global production estimated at ca. 2626 million metric tons in 2020, representing over 60% of the world food consumption [59]. Agricultural areas dedicated to these staple crops amount to ca. 728 million hectares of the 1.5 billion ha of agricultural area [60].

The significance of cereals and their derived products is generally assessed in terms of consumption and nutritional value [58]. Staple cereals and their products have remained the most essential dietary component of human food over the past several decades [61]. Interestingly, cereal grains have been the principal component and primary source of the human diet for thousands of years since they provide humans with energy (more than 50% of their daily caloric intake) [57,62]. Furthermore, cereals and their derived products are precious sources of vitamins, proteins, carbohydrates, phospholipids, fibers, antioxidants, and minerals [57,63]. Recently, Laskowski et al. [64] documented that cereals represent an excellent source of macroelements, notably phosphorus (P) and potassium (K), and certain micronutrients like Mn (64%), Fe (34%), and Cu (31%). Cereals also have a high proportion of thiamin, polyunsaturated fatty acids, and vitamins, particularly vitamin B6 (Vit B6).

Several cereal grain processing methods, such as fermentation and the use of yeast in derived cereal products, were shown to improve cereal nutritional value [65,66]. This is particularly true for the increased vitamin and mineral intake, notably Zn and Fe [67]. Cereal-based products are generally rich in several minerals, notably Fe, Zn, and Mn [57,58]. Since they are rich in proteins, vitamins, minerals, carbohydrates, and other components, several healthy benefits for humans were ascribed to cereals. Nevertheless, cereals show limitations in the amounts of some essential amino acids such as lysine, threonine, and tryptophan. Thus, an over-reliance on cereals is directly linked to a number of disease-related risk factors, justifying the importance of improving the amino acid composition of such staple crops.

According to Brouns et al. [66], cereal grain consumption reduces the risk of many emerging diseases, such as cancers, diabetes, and cardiovascular disease. A diet based on carbohydrate-rich foods with high fiber content, particularly whole-grain products, may contribute to preventing and lowering the metabolic syndrome “type 2” of diabetes and several cardiovascular diseases [66,68,69,70]. Recent studies [71,72] point out that dietary fiber plays a pivotal role in modulating tissue immune responses and inflammation, notably in the intestine. Grains of some cereal varieties are anthocyanin-rich products, and their health benefits include anti-oxidation, anti-cancer, glycemic and bodyweight regulation, neuroprotection, retinal protection, hypolipidaemia, hepatoprotection, and anti-aging effects [73]. Hence, they greatly contribute to the human diet and are considered as “dietary staple”. Cereals such as wheat, oats, and barley are also precious sources of antioxidants. Their whole grains are rich in many bioactive phytochemical compounds with high antioxidant activity, notably flavonoids, carotenoids, phenols, and tocopherols [74,75,76]. The health benefits of such antioxidant compounds were recently reviewed by Polonkiy et al. [72]. Cereal lipids are rich in essential fatty acids that cannot be synthesized by the human body itself [77]. This was evidenced by Slama et al. [78], who found that cereal oils are characterized by their high essential fatty acid contents playing definite functions in the human body, such as cellular activities, structural integrity and fluidity of the cell membrane, and regulation of blood pressure and the nervous system, among other purposes [79].

The composition and nutritional characteristics of cereals can be suboptimal, especially in arid and semi-arid areas, if plants are cultivated in soils with low micronutrient content or with low nutrient availability (notably Fe, Se, Cu, and Zn). For instance, cereals grown in arid regions characterized by high pH (especially calcareous soils) show symptoms of Fe or Zn deficiency [80], and their grains are generally poor in micronutrients, notably Zn [81]. According to Cakmak and Kutman [82], the major cause of Zn deficiency in humans is the intensive use of cereal-based foods, knowing that cereals have the lowest Zn concentration and thus do not meet human biological requirements.

The intensive application of macronutrient fertilizers such as nitrogen (N), P, and K and the related enhancement of growth may even aggravate the problem of grains with low micronutrients (especially Fe and Zn) at least in H. vulgare plants [83,84]. According to Prom-u-thai et al. [85], one-third of the world’s population is suffering from micronutrient deficiency, especially in developing countries, partly because of the undiversified diet principally based on cereals. Considering these concerning data, it is essential to improve cereal nutritional content and quality using new sustainable and cost-effective methods.

5. Main Biofortification Strategies Employed to Counteract Nutrient Deficiencies in Plants Including Cereals

In developing countries, cereals are the main source of micronutrients in human food. It is a major challenge to develop solutions to produce more crops with better micronutrient content in grains without affecting their nutritional quality, especially for plants grown on micronutrient-poor soils [26]. The fortification of staple foods can be achieved through genetic fortification (conventional and molecular breeding and genetic engineering methods), preharvest (i.e., fertilization and foliar spray), and postharvest methods (i.e., soaking, coating, dusting, sonication, and extrusion). Biofortification refers to the process by which the nutritional value of staple foods is improved by increasing their contents of one or more essential micronutrients [86,87]. Food fortification and biofortification, targeted agronomic enrichment of micronutrients, is a promising and sustainable approach ensuring both higher yield and optimal grain quality (especially Zn and Fe) [88]. Biofortification is currently the main agricultural technique to address hunger and malnutrition in developing countries [89]. This technique, used to fortify cereals in particular [86], is based on the application of exogenous organic/inorganic fertilizers or biofertilizers, either directly in the soil or via foliar pulverization (foliar spray) or pretreatment (soaking) of seeds with nutrients, a practice better known as nutri-priming [90]. The different methods used in agronomic biofortification are presented in Figure 1.

As shown in Figure 1A, micronutrients can be directly applied to soils to correct micronutrient deficiency in both soils and plants. Yet, it presents some risks since, generally, the elements are immediately fixed by different components present in the soil into insoluble forms, hence not being translocated to the consumable parts of the plant.

Unlike fortification via fertilization, nutri-priming is an easy and cost-effective method without side effects on both humans and the environment (Figure 1B). This approach is of major environmental and socio-economic significance because it ensures the production of crops with high micronutrient content but also with high tolerance to several environmental stresses [15]. Nutri-priming might contribute to efficiently attenuating climate change impact, too. Micronutri-priming of seeds led to faster and more synchronized germination and higher resistance to abiotic stresses at advanced developmental stages [91]. Given that plants suffering micronutrient deficiencies are more sensitive to the severity of abiotic stressors [92], maintaining nutrient homeostasis via nutri-priming is crucial for plant survival under the ongoing climate change [93].

Foliar fortification with micronutrients is another technique used to supply trace elements to plants (Figure 1C). It improves the micronutrient content of the different plant organs, notably seeds and grains. When sprayed, micronutrients enter leaves through stomata and are then transported to different plant organs via both apoplastic and symplastic pathways. The efficiency of micronutrient pulverization is affected by the leaf’s aptitude to absorb the supplied micronutrient, which in turn depends on several factors, such as light, leaf age, and transpiration. These factors generally result in a loss of the element itself since the mobility of the foliar-applied nutrient interrupts its translocation to the different plant organs. Furthermore, an excess of foliar application leads to abnormalities in crop growth.

Other techniques are used to enhance the micronutrient content of edible crops and are called food postharvest fortification, such as coating and extrusion [94]. Extrusion refers to the use of cereal grain coproducts like flour or bran to make derived products with high micronutrient contents. Since this technique can inactivate several antinutritional factors, it was used to enhance the bioavailability of some nutrients, notably Fe and Zn, to customers. However, it represents some inconveniences, such as low-moisture and high-shear conditions that can destroy substantial amounts of heat-labile nutrients [95] in addition to being costly. Seed coating consists of covering seeds with adhesive micronutrients to enhance seed performance and plant establishment under both non-stressful and stressful conditions [96]. Since seeds are directly intended for human consumption, a coating process using high amounts of micronutrients was added to seeds, followed by other processes, including drying seeds at ambient temperatures to restore their original moisture content. The main disadvantage of seed coating is the cost.

5.1. Seed Nutri-Priming Methodology

Nutri-priming implies soaking seeds in an adequate nutrient solution for an optimized duration before rinsing seeds several times with distilled water and drying them until their weight gets closer to the initial one [97]. Different priming agents and methods (Figure 2 and

Table 1. Diversity of the nutri-priming agents used for biofortification. For the same agent, concentration and exposure time depend on the species and the experimental context.

Priming Agent Form	Concentration	Duration	Reference
ZnSO4	100–200 ppm 0.3 and 0.5% 0.1, 0.3, 0.5, 0.7% 0.01, 0.05, 0.1, 0.5, 1.0 M 4 mM	1 h 10 h 6 h 12 h 24 h	[98] [99] [100] [101] [102]
ZnCl2	0.01, 0.05, 0.1, 0.5, 1.0 M	12 h	[101]
Zn(Gln), Zn(Arg), Zn(His)	40 mg Zn kg−1 soil	12 h	[103]
ZnO NPs	0, 25, 50, 75, 100 mg/L	24 h	[104]
CuSO4	100–200 ppm 0.03 and 0.06%	1 h 10 h	[98] [99]
CUO-NPs	3.33, 44.4, 5.55 mg/L	8 h	[105]
Na2SeO4	10 μM	30 mn, 24 h	[106]
MnSO4	0.1, 0.2, 0.3, 0.4%	6 h	[107]
H3BO3	0.01%	8 h	[108]
FeEDTA	50 µmol/L	12 h	[109]
Fe NPs	0, 5, 10, 15, 20 mg/L	24 h	[104]

) were used to increase cereal micronutrient content depending on the plant species. Since these micronutrients are needed in very small quantities, the choice of the concentration of the priming agent should be thoroughly tested to avoid possible nutrient toxicity that can hamper germination (Table 1). According to the literature, organic or inorganic forms, chelated micronutrient sources, and nanoparticles are the most sources commonly used as priming agents (Figure 2).

5.1.1. Inorganic Agents

Inorganic agents such as zinc sulfate, manganese sulfate or ferrous iron, zinc or iron sulfate heptahydrate, and other forms are widely used for the nutri-priming process (Figure 2). This method is advantageous as it stimulates both plant growth and nutrient uptake [110]. Yet, understanding absorption mechanisms, translocation, and utilization of the micronutrient in plants prior to the selection of its suitable sources are determinant factors for nutri-priming [111]. As shown in Table 1, several micronutrients were used in their inorganic forms for optimal priming duration and at adequate concentrations, and they were efficient in improving several plant physiological processes and micronutrient uptake. Based on the literature, we found that Zn might be applied via its inorganic form as zinc sulfate (ZnSO₄), zinc sulfate heptahydrate (ZnSO₄, 7H₂O), zinc chloride (ZnCl₂) zinc nitrate (ZnNO₃), zinc carbonate (ZnCO₃) and zinc oxide (ZnO) (Figure 2). However, the two major forms of Zn commonly used in nutri-priming are ZnSO₄ and ZnSO₄ 7H₂O because of their high solubility and their low soil reactivity. For example, soaking sugar beet seeds in ZnSO₄ and ZnCl₂ solutions enhances Zn concentration in plant tissues and was correlated with an increase in photosynthetic rate, transpiration, and stomatal conductance [112].

Figure 2 and Table 1 depict that other inorganic forms can be used as nutri-priming agents and include sodium molybdate (Na₂MoO₄), cobalt sulfate (CuSO₄, 7H₂O), and cobalt nitrite (CoN₂O₄), ferrous iron (FeSO₄, 7H₂O), and manganese sulfate (MnSO₄). In common wheat, nutri-priming with ZnSO₄, 7H₂O improves grain zinc concentration [100]. Similarly, seed pretreatment with FeSO₄, 7H₂O increases seed Fe concentration in rice [113]. Seed priming with (NH₄)6Mo₇O₂₄, 4H₂O for 10 h enhances the net CO₂ assimilation rate, chlorophyll content, biological N fixation, and grain yield [114].

5.1.2. Organic Micronutrient Chelators

Micronutrients can be provided to seeds as organic compounds, mainly in their chelated forms (Figure 2). Such organic complexing products (i.e., amino acids and polysaccharides) boost the content of micronutrients in the plant’s edible parts [115]. For instance, synthetic Zn–amino acid chelates are widely used as natural and important sources of Zn in seed priming [103]. In this case, Zn is associated with diverse chelators, such as amino acids, forming complexes such as Zn(His)2, Zn(Met)2, Zn(Gln)2, Zn(Gly)2, and Zn(Arg)2 (Table 1), which are widely used in the priming process [116]. The potential of some Zn–amino acid chelates in improving plant growth performance was investigated, and the main results show that Zn-Ala and Zn-Gly increase the plant dry weight and shoot length and promote Zn uptake by common wheat grown in calcareous soil [115]. More recently, Hussaan et al. [117] demonstrated that Zn-Lys seed priming mitigates the negative impact of Cd stress by improving several attributes, including nutrient uptake, chlorophyll synthesis, biomass accumulation, and antioxidant defense system and reducing Cd uptake by roots. Likewise, Zn(His)2 and Zn(Arg)2 seed priming increases grain Zn content in wheat [103]. Synthetic Fe–amino acid chelates are also used in the priming process as Fe (Gly)2 and Fe (Met)2 amino chelates with beneficial effects on the nutritional quality of crops grown in Fe-deficient soils [118].

5.1.3. Synthetic Microelements Chelators

As shown in Figure 2, in such a kind of nutri-priming, the metal ion is coordinated with the chelating agent [119]. Among the chelates widely used to produce synthetic microelements chelators, EDTA and DTPA are shown to increase plant metal uptake by entrapping ions such as Zn or Fe and releasing them slowly to be available for plants in a timely manner [120]. Several criteria should be taken into consideration when producing a synthetic microelement chelator, such as the uptake, translocation, and utilization of nutrients in plants [110,121]. This method has many risks because of the slow biodegradability of synthetic chelators, resulting in their persistence in the environment [122]. For example, Zn-EDTA, which is the most synthetic chelated form frequently used in nutri-priming, presents some environmental risks given its low biodegradability and the release of toxic compounds through a process called photodegradability. Other chelated micronutrient sources are also used in nutri-priming, including FeEDTA and CuEDTA. Recently, Hadia et al. [109] used Fe-EDTA (50 µmol/L) for priming seeds of common wheat and found that Fe improved several physiological attributes, including the germination rate, root weight, length and number, and coleoptile length under salt stress (Table 1). In maize, seed priming with Zn-EDTA results in a maximum concentration of Zn in grains as compared to ZnSO₄ [123].

5.1.4. Nanoparticles (NPs)

Nano-priming or seed nutri-priming via nanoparticles is another method used in nutri-priming (Figure 2) and has emerged as a promising approach for increasing crop productivity and ensuring global food security [124]. Increasing attention is given to the use of nanoparticles in nutri-priming because this technique is more advantageous, notably with respect to the high stability and adsorption of NPs and the improvement in seed performance and crop productivity either under optimal or adverse environmental cues [125]. The benefits of seed nutri-priming via NPs have been reviewed recently by Gupta et al. [126]. Nano-priming facilitates the uptake of water and nutrients during the germination process through the formation of nanopores and the induction of aquaporin gene expression. Moreover, NPs, as potential carriers for plant growth regulators, can act as stimulators during germination and early seedling growth phases by mediating ROS production and enhancing the activity of several hydrolyzing enzymes, such as proteases and amylases.

Zn nanoparticles (ZnNP) are, for example, widely used to improve seed quality and seedling establishment. Similarly, particles of nano-Scale Zero Valent Iron (nZVI) are utilized for priming applications (synthesized by sodium borohydride method using ferrous sulfate, FeSO₄, 7H₂O) and EDTA (C₁₀H₁₄N₂Na₂O₈, 2H₂O). Cu nanoparticles (CuNP) are also applied as nutri-priming agents in maize [127]. Synthesizing chitosan nanoparticles containing Cu ions is performed by the ionotropic gelation process. Fe-NP seed priming triggers Fe uptake and increases starch metabolism [128]. In watermelon, the same technique increases non-enzymatic antioxidant system defense and induces jasmonate-dependent defense responses at the early stages of seedling development [129]. As demonstrated by Rizwan et al. [104], the application of ZnO NPs and Fe NPs increases Zn and Fe concentrations in grains (Table 1).

6. Key Mechanisms of Seed Nutri-Priming for Multiple Environmental Stress Mitigation

The mechanisms underlying nutri-priming could be addressed at different developmental stages of the life cycle of any plant species: (i) germination, (ii) vegetative stage, and (iii) fructification stage (Figure 3). During germination (Figure 3A), seed nutri-priming reduces the time period of the germination process and improves both germination rate and seedlings uniformity [130]. In this way, seed priming with B triggers various metabolic activities, leading to the activation of enzymes involved in starch metabolism, notably α-amylase and starch phosphorylase [131]. The activity of both enzymes generates sufficient sugar amounts necessary for protein synthesis and results in faster germination and uniform seedling emergence [108]. According to Alejandro et al. [132], Mn seed priming hastens seed germination via the activation of DNA polymerase (a DNA repair enzyme functioning in plant meristematic and meiotic tissues) and results in an increase in germination speed and rate [133].

As germination is a complex process that includes imbibition, activation of metabolic processes, and intensive embryo growth and radicle growth, the effect of the priming agent is phase-dependent. During imbibition, priming induces DNA repair, respiration, cell cycle initiation, gene transcription, signaling pathways (ROS and phytohormones), and eventually stress-responsive genes such as the Late Embryogenesis Abundant (LEA) proteins [134]. In the second phase, priming triggers significant changes in nucleic acid methylation as well as chromatins and leads to the accumulation of several transcription factors regulating a wide range of metabolic pathways [135]. Simultaneously, other physiological and biochemical modifications occur, such as the enhanced synthesis and accumulation of osmolytes (i.e., proline) and carbohydrates. In addition, enzymes involved either in N metabolism (i.e., nitrate reductase) or the removal of produced reactive oxygen species (i.e., superoxide dismutase and peroxidases) are activated [136]. The third post-germination phase is characterized by the mobilization of stored reserve, including micronutrients, and hence represents a major step in the nutri-priming process, which is followed by several events, notably radical elongation. As documented by Reis et al. [137], Fe and Zn seed priming significantly affects the mitotic index and the regular and irregular division phases of wheat root cells and increases seed Zn and Fe contents, which are later translocated to growing tissues for better seedling development and establishment via the activation of several nutrient transporters (Figure 3A).

At subsequent stages of plant ontogeny, a beneficial effect of priming with several micronutrients, including Fe and Zn, on early seedling growth was observed in different plant species (Figure 3B). Such a positive effect could be due to the effectiveness of both elements in increasing cell division, expansion, and meristematic activities of plant tissues [138] via the stimulation of phytohormone biosynthesis, notably auxins, which are involved in cell elongation and cell differentiation of meristematic tissues [139,140]. B-nutri-priming promotes several physiological attributes related to plant growth [39] (i.e., root and shoot lengths and seedling fresh and dry weights), given the implication of B in cell elongation, mitosis, and meristem growth. Moreover, the use of B in nutri-priming improves water uptake by increasing the number of root tips and mycorrhizae [141]. Photosynthesis, the major process involved in plant biomass accumulation, is also enhanced by nutri-priming, as revealed by El-Shintinawy et al. [142], who found an improvement of leaf chlorophyll synthesis and stabilization in sunflower following seed treatment with B. Likewise, the use of Mn as a biostimulant induces changes in photosynthetic processes through its positive effect on ATP generation and Ribulose-1,5-Bisphosphate (RuBP) enzyme regeneration [143]. Seed priming with Co enhances ethylene production and triggers several physiological processes involved in seedling establishment and flower initiation [144]. Nutri-priming also affects plant secondary metabolism as some micronutrients such as Mn induce changes in phenolic compounds as this element is a cofactor of phenylalanine ammonia-lyase, a key enzyme of the phenylpropanoid metabolism [143]. Likewise, Zn seed priming improves plant mineral status by promoting nutrient uptake and water use efficiency [144]. This is due to an enhancement of the expression levels and activities of several transporters involved in micronutrient uptake, translocation, and storage. In wheat, seed priming with iron oxide nanoparticles enhances Fe absorption and deposition as well as its translocation within the different plant organs including grains [145]. The use of nanoparticles (NPs) such as Fe₂O₃ NPs16 and AgNPs13 to improve cereal seed development is a successful method since both metal-based nanoparticles trigger Fe acquisition and increase starch metabolism [128].

At the reproductive stage, seeds yielded by plants primed with micronutrients are generally rich in many trace elements and are thus called biofortified seeds (Figure 3C). Recent reports pointed out that nutri-priming improves the nutrient contents of seeds, notably cereals [146]. Seed priming with Zn has received attention due to its role in antiviral immunity. Thus, its use as a supplement for COVID-19 patient treatment [147] made this element a suitable candidate for the nutri-priming process. Several investigations on cereal species documented the importance of seed micronutrient pre-soaking in enhancing seed nutrient reserves [148,149,150]. Interestingly, it was found that seed priming with 4 mM ZnSO₄ results in a 7-fold increase in Zn content in maize grains [149,150]. This could be due to the stimulation of several transporters responsible for Zn uptake and storage. According to Suandria et al. [145], priming wheat grains with Fe significantly improves their Fe content with a noticeable over-accumulation of Fe in the different sections of the seed, which was concomitant to the stimulation of Fe transport. The main transporters involved in Fe uptake, translocation, and storage in relation to Fe biofortification have recently been reviewed by Rehman et al. [151]. Similarly, nutri-priming via B improves its availability and translocation to seeds due to an upregulation of several mechanisms underlying B transport, especially the putative B transporters for xylem loading, which further enhances B supply to wheat grains [152].

Under harsh environmental conditions, seed nutri-priming improves plants’ resilience when challenged with abiotic stresses (Table 2). Pretreatment of seeds with micronutrients boosts several metabolic pathways, leading to a modulation of plant primary (such as amino acid) and secondary (such as phenols and flavonoids) metabolism in response to different stressors [153]. Several molecular mechanisms have been shown to be regulated under priming processes, including an increase in the expression of various stress-related genes and proteins, and generate a stress imprint enabling plants to withstand several environmental cues. According to Bhatia and Gupta [154], nutri-priming is an efficient strategy for boosting the defensive capacity of plants by inducing active or inactive signaling protein accumulation. This assumption was confirmed given that several protein precursors switch from inactive to active forms when plants encounter adverse environmental conditions. Under salinity stress, the expression of several sodium transporters, notably SOS and NHX, was upregulated by seed priming, leading to a restriction of Na⁺ uptake and accumulation in shoots [135]

At the molecular level, seed nutri-priming induces changes in the plant genome, including activation or repression of either transcription factors or RNA polymerase, allowing the plant species to acquire a stress memory [135]. In his regard, several micronutrients were shown to regulate the activity of several Na⁺ and K⁺ transporters, resulting in better tolerance to salt stress. In rice treated with Na₂SeO₄ and subjected to salt stress, an increase in OsNHX1 transcript levels is noted, resulting in a higher K⁺/Na⁺ ratio and better performance under hyper-osmotic salinity stress [155]. ZnSO₄ triggers the accumulation of soluble sugar and proline, leading to an improvement in the mineral status of crops exposed to salinity [156]. The adverse impact of nutrient deficiency can be mitigated via Zn seed nutri-priming, as Zn promotes the translocation of nutrients from seeds to the different plant organs, resulting in an improvement in Zn and K uptake [18]. In maize, Zn seed priming improves the whole plant growth and the grain yield as well as Zn grain contents when plants grow in Zn-deficient soils [17] or at low temperatures [150].

To decipher the role of seed nutri-priming under limiting N conditions, it is worth revisiting N metabolism in plants, which is known to involve three major steps: N uptake by plant roots, its translocation and assimilation, and finally, its remobilization within the different plant organs [157]. In the rhizosphere, nitrate is taken up by roots and reduced to nitrite before its reduction to ammonium or storage in shoot or root vacuoles. These two steps are catalyzed by nitrate reductase and nitrite reductase. The abovementioned mechanisms were shown to be stimulated by seed nutri-priming under low N availability and resulted in a better response to N shortage. Recently, seed priming combining N and Zn was effective in mitigating the impact of N deficiency in rice [158]. This effect is due to the key role of Zn in (i) promoting N translocation and distribution from roots to shoots, (ii) the induction of the expression of N transporter genes, and (iii) the increase in N assimilation in leaves [159]. Furthermore, the combined effect of Zn-N seed priming results in the activation of protein synthesis and root activity for efficient nutrient uptake and translocation during the germination process, favorizing root and coleoptile development and hence seedling growth under limiting Zn and N conditions [158]. In rice grown under N deficiency, seed priming with Se improves the uptake of primary macro-nutrients, reduces lipid peroxidation and free radical (hydrogen peroxide, superoxide anion, and hydroxyl free radicals) accumulation and enhances both enzymatic (superoxide dismutase, peroxidase, and glutathione peroxidase and glutathione reductase) and non-enzymatic antioxidants (reduced glutathione and vitamin C) [160]. Still in rice, short and long-term arsenic-induced N deficiency stress effects have been mitigated via silicon seed priming with beneficial impacts on the expression of genes encoding several enzymes and transporters involved in the absorption and utilization of N [161]. Indeed, the expression of N absorption and assimilation-related genes, notably those encoding enzymes involved in N metabolism (nitrate reductase, nitrite reductase, glutamine synthetase, glutamate synthase) and transport (NRT2 and AMT1, respectively, a high-affinity nitrate transporter and a high-affinity ammonium transporter protein) are upregulated in shoots and roots following plant exposure for 7 and 15 days to arsenic-induced N shortage stress. Seed priming combining selenium and other agents, including salicylic acid, also significantly increases root fresh and dry weights of rice grown under N shortage [162]. Interestingly, the same pretreatment of seeds lowers ROS accumulation and lipid peroxidation, stimulates both enzymatic (superoxide dismutase, catalase, and peroxidase) and non-enzymatic (glutathione) components of the antioxidative defense machinery in shoots and enhances nutrient uptake under the interactive effect of N deprivation and Pb stress [162]. Recently, Pereira et al. [163] found that nutri-priming with 2 mM NH₄NO₃ promotes rice root length, root ammonium content, and total N content in shoots, which were concomitant to an increase in glutamine synthetase activity in the same organs. Moreover, root architecture, which generally displays an array of modifications to enhance N uptake and absorption under limiting N conditions, has been shown to be positively affected by low and high N priming (2 and 6 mM NH₄NO₃).

Abiotic stressors negatively affect photosynthesis. Seed nutri-priming, especially with Zn, Mg, and Mn promotes chlorophyll synthesis and mitigates such negative impacts [143,164]. Water shortage generally reduces the uptake of nutrients, including Mg, which is the core element of chlorophyll molecules and hinders the activity of enzymes involved in its biosynthetic paths. Zn or Mg seed soaking stimulates the synthesis of chlorophyll by affecting the abovementioned attributes [164,165] and enhances the production of osmolytes, involved in osmotic adjustment and, thereby, the maintenance of adequate photosynthetic functioning and plant growth rate [166].

Abiotic stress-induced oxidative damage is a common disorder in plants. Nutri-priming reduces the effects of oxidative stress by improving the antioxidative defense system [135] via gene expression reprogramming [167]. In common wheat and common mustard, Zn or Se seed priming increases the activities of several antioxidative enzymes as well as the expression levels of their stress-responsive genes [98,168]. Recent investigations [168] point out that seed priming with Se promotes the expression of stress-responsive genes, notably those encoding SOD, CAT, APX, and POD, resulting in the alleviation of salt stress in common mustard. In sweet peppers, seed priming with 0.5 mg/L Mn²⁺ increases the activity of MnSOD in seedlings grown under salt stress by 5.51-fold [169]. According to Salehi et al. [143], the general mechanism underlying Mn seed priming in sweet annie is the stimulation of SOD activity and the adjustment of proline and phenolic compound production. The stimulation of antioxidative defense by seed priming attenuates oxidative damage by lowering the degree of lipid peroxidation, as shown for common wheat primed with Mg and Zn [164]. In Table 2, we provide a list of references about the use of nutri-priming at diverse abiotic stresses such as salinity, drought, nutrient deficiency, and low temperature.

Table 2. Examples of agents used as part of the nutri-priming approach for improving crop the responses of cereals and other plant species to osmotic (drought, salinity), thermal, and nutrient (Fe, Zn, and Mn) deficiency stresses.

Priming Agent	Plant Species	Stress	Reference
FeSO4	Beta vulgaris Vigna Radiata Glycine max Stevia rebaudiana	Salinity Salinity Iron deficiency Drought	[170] [171] [172] [173,174]
ZnSO4	Zea mays Triticum aestivum Glycine max Green bean Avena sativa	Salinity	[149] [98] [156] [175] [176]
	Zea mays Hordeum vulgare Glycine max Cicer arietinum	Zn deficiency	[17] [148] [172] [177]
	Zea mays Spinacia oleracea	Low temperature	[150] [178]
	Zea mays Triticum durum Stevia rebaudiana Nigella sativa Vicia faba	Drought	[179] [180] [173,174] [181] [182]
MnSO4	Glycine max	Zn and Mn deficiency	[183]
Na2SeO4	Oryza sativa Zea mays, Triticum durum Oryza sativa	Drought Drought Salinity	[184] [179,185,186] [155]
CuSO4	Brassica rapa Avena sativa	Salinity	[187] [176]

Table 2. Examples of agents used as part of the nutri-priming approach for improving crop the responses of cereals and other plant species to osmotic (drought, salinity), thermal, and nutrient (Fe, Zn, and Mn) deficiency stresses.

Priming Agent	Plant Species	Stress	Reference
FeSO4	Beta vulgaris Vigna Radiata Glycine max Stevia rebaudiana	Salinity Salinity Iron deficiency Drought	[170] [171] [172] [173,174]
ZnSO4	Zea mays Triticum aestivum Glycine max Green bean Avena sativa	Salinity	[149] [98] [156] [175] [176]
	Zea mays Hordeum vulgare Glycine max Cicer arietinum	Zn deficiency	[17] [148] [172] [177]
	Zea mays Spinacia oleracea	Low temperature	[150] [178]
	Zea mays Triticum durum Stevia rebaudiana Nigella sativa Vicia faba	Drought	[179] [180] [173,174] [181] [182]
MnSO4	Glycine max	Zn and Mn deficiency	[183]
Na2SeO4	Oryza sativa Zea mays, Triticum durum Oryza sativa	Drought Drought Salinity	[184] [179,185,186] [155]
CuSO4	Brassica rapa Avena sativa	Salinity	[187] [176]

7. Conclusions

The intensity and rapidity of changes currently affecting climate, natural resources availability, and associated risks (including health) provide strong evidence that the current century will be challenging for humanity. Among the lessons learned from recent global crises (geopolitical and COVID-19 issues), ensuring food security by enhancing cereal production and quality must be included in the future management programs of governments worldwide, especially in developing countries where the diet is principally based on cereals. It is therefore urged to reflect on how to develop or promote sustainable and affordable alternatives to mitigate these environmental issues. This notably includes developing innovative and environmentally friendly methods like nutri-priming that could improve crop micronutrient content and make humans ready for fighting future pandemics. Nutri-priming, a new, easy, and cost-effective approach, should be introduced in agricultural practices with promising applications in mitigating the ongoing crisis, including climate change, the effects of conflicts, and pandemic emergence. This approach is not the only alternative to deal with the abovementioned issues, but it proved to be efficient in increasing mineral reserves and their translocation during plant development, and also improving crop resistance to stress combination and crop micronutrient content, which benefits the human immune system (and hence, health), not only in food-insecure regions. As key actors of such an approach, farmers need to be convinced by the importance of nutri-priming. Therefore, increasing awareness about the multidimensional benefits of biofortification in general and nutri-priming in particular outside the scientific community has to be achieved. Finally, it has to be stated that the acceptance of such crops might only happen if the seeds of these crops are available and affordable.