Seed Priming as an Effective Technique for Enhancing Salinity Tolerance in Plants: Mechanistic Insights and Prospects for Saline Agriculture with a Special Emphasis on Halophytes

Abstract

Seed priming is a simple, inexpensive, and effective pre-sowing technique that enables plants to better tolerate abiotic stresses, including high soil salinity, which is a major limiting factor in the establishment of halophytes for saline agriculture, as germinating seeds and early seedlings of many halophytes are sensitive compared to the mature vegetative stage. This article attempts to provide an overview of the research on the seed priming effects on halophyte seeds and subsequent seedlings/plants. Different physio-chemical and molecular processes, including the induction of priming/stress memory, which enhance salinity tolerance following seed priming, have also been discussed. This review also covers the aspects of reactive oxygen species (ROS), and nitric oxide (NO) signaling(s) that are activated as a result of seed priming. Finally, the limitations and prospects of seed priming to enhance the agronomic potential of halophytes for saline agriculture have been discussed.

1. Introduction

Salinity is a serious agricultural problem which is being increasingly intensified due to the combined effects of global climate changes and fast expanding human population [1,2,3]. Rising temperatures and weather pattern shifts are leading to anomalous increase in frequency and intensity of drought and flooding, which are escalating the salinization of arable land and water resources [1,4]. Salinization on a global scale is transforming about 3 hectares of arable land into unproductive areas every minute, hence potentially reducing 10–20 million hectares of arable land to zero productivity annually [5]. It is anticipated that if current trends continue, more than half of arable land will become saline by 2050 [6]. Most plants including our major conventional crops are sensitive to even as low as 4 dS m⁻¹ salinity, equivalent to 40 mM NaCl [7]. As a result, salinity adversely affects crop survival and productivity, thereby compromising food security, especially in semiarid and arid areas of the world, which constitute about one-third of the Earth’s surface [8,9]. This is contradictory to achieving food security in the future, as the world’s population may exceed 10 billion by 2050, requiring twice as much food as it does today [10,11]. It is therefore crucial to understand the complex interactions among climate change, salinization of lands/water, and agricultural sustainability to design effective strategies for sustainable food production and resilient agricultural systems in the world.

Since conventional crops are sensitive to salinity, it is imperative to investigate alternate approaches to ensure food security in salt-affected regions [12,13]. Plants known as halophytes are naturally salt-tolerant and can produce enormous amounts of biomass in hostile saline environments such as salt marshes, salt deserts, and coastal regions [14,15,16]. These remarkable plants not only endure saline environments, but can also facilitate in reclamation of saline lands, improve biodiversity, and mitigate land degradation [17,18]. Hence, cultivation of halophytes on salt-hit barren lands with brackish/saline water irrigation for food, fodder, fuel, fiber, and other economic uses appears both an ecologically and economically feasible solution to supplement demands of a burgeoning population [18,19,20]. By harnessing the economic potentials of halophytes, farmers of the arid–saline regions can diversify their cropping systems, enhance resilience to environmental stresses, and ultimately contribute to food security in areas plagued by salinity/aridity [18,21]. Therefore, understanding and promoting the cultivation of halophytes is essential for the success of saline agriculture and long-term sustainability of agricultural practices in arid–saline regions.

Although halophytes have a high tolerance to salinity at the growth stage, most of them have low salinity tolerance during seed germination [22]. For example, quinoa, a fast-emerging halophyte crop, can tolerate high salinity during growth but is susceptible to salinity during germination [23]. Similarly, increasing salinity decreases seed germination of a halophyte fodder candidate Panicum turgidum [24]. Furthermore, high salinity may also result in seed mortality in some annual dicots and grasses [22,25]. Hence, there is a need to develop techniques for improving seed germination of halophytes under high salinity.

For achieving food security particularly in harsh arid–saline regions, seed priming techniques can be used as an effective and low-cost way to enhance seed germination and salinity tolerance of halophyte crop candidates [26,27,28,29]. Seed priming is an easy cost-effective pre-sowing treatment that involves hydrating seeds to a controlled extent followed by drying with the aim to activate some of the physiological processes of germination to initiate without radicle emergence [29,30,31]. It provides a head-start for quick germination and vigorous seedling formation and also induces tolerance to a number of abiotic stresses, including salinity [28,32,33,34]. This technique is especially critical in arid–saline environments where water scarcity and soil salinity substantially hinder agricultural productivity [29,34,35]. Seed priming techniques include various methods such as hydropriming (i.e., priming with water), osmopriming (i.e., priming with osmotic agents), biopriming (i.e., priming with biological agents like rhizobacteria or fungi), and redox priming (i.e., priming involving antioxidants). Each method aims to initiate metabolic processes within the seed without causing germination, effectively preparing them for future stress conditions. The selection of these solutions depends on factors such as the type of agent used, crop species, seed quality, and the specific stress being addressed [28,36,37]. In order to meet the increasing global food demand, adopting seed priming techniques can help achieve food security in vulnerable areas, encourage sustainable agricultural production, and mitigate the effects of global climate change and soil degradation. This review article aims to present a detailed overview of various seed priming methods for halophytes, exploring their physiological and biochemical effects, advantages, and limitations, as well as the underlying mechanisms that contribute to improved tolerance in saline conditions.

2. Methodology for Literature Search

For the literature search, we employed the approach outlined in [38] with slight deviations. A number of electronic databases, namely Google Scholar, Web of Science, Crossref, ScienceDirect, and Scopus, were used for the literature search. The search was performed by using the keyword filters like “seed priming”, “seed priming, salinity tolerance”, “seed priming, halophytes”, “halophytes, salinity tolerance, carbon sequestration”, and “physiochemical changes, seed priming, halophyte seeds” and “molecular changes, seed priming, halophytes”, “seed priming, priming memory”, and “signaling, seed priming, halophyte”, etc. Only reviews, research articles, and book chapters as document type with a timespan option of “all years” were selected. Article titles and abstracts were manually screened to exclude irrelevant and duplicate articles. After screening the top 100 publications for each of the aforementioned keywords, we selected articles most relevant to this review. Generally, articles published during the last fifteen years were preferred. However, some older articles were also chosen, if found essential.

3. General Procedure and Types of Seed Priming

Seed priming is a pre-sowing treatment that involves immersing seeds in a suitable solution or water for a certain period of time (depending on the seeds/species) and drying them to original moisture content (on fresh weight basis) followed by brief dry storage until use (Figure 1). Concentration of priming agent solution and soakage duration may vary among species and hence needs to be optimized prior to actual experiment. Based on the type of soaking agent used, seed priming techniques can be categorized into various methods. Most common seed priming types are hydropriming, osmopriming, hormonal priming, redox priming, biopriming, nanopriming, etc. Each method has distinct mechanisms and applications, making them suitable for different crops and environmental conditions. Hydropriming involves soaking seeds in water to initiate metabolic activity while preventing germination [39]. This easy and cheap priming process allows seeds to imbibe water, softens the seed coat, and activates metabolism responsible for germination, hence making seeds ready for prompt germination and vigorous seedling formation [40,41]. Hydropriming improved seed germination and/or early seedling growth of rapeseed [42], maize [43], wheat [19], sunflower [44], and basil [45] under salinity stress. Osmopriming that involves soaking seeds in low water potential (ranging from about −1.0 to −2.0 MPa) [46] solutions of various osmotic agents (e.g., polyethylene glycol- PEG and mannitol, etc.) was effective in enhancing performance of soybean [47], spinach [48], and tomato [49]. Halopriming, which is a variant of osmopriming involving seed imbibition in salt solutions (e.g., NaCl, KCl, CaCl₂, MgCl₂, etc.), improves germination and/or stress tolerance of wheat [50], okra [51], rice [40], and marigold [52]. Hormonal priming with different phytohormones (e.g., gibberellins, auxins, cytokinins, etc.) was productive for wheat [53], pigeon pea [54], maize [55], and hot pepper [56]. Redox priming that involves soaking seeds in solutions of redox-active agent (e.g., hydrogen peroxide, nitric oxide/sodium nitroprusside, ascorbic acid, tocopherol, etc.) improves the performance of sunflower [57], rice [58], pea [59], and wheat [60,61]. Biopriming integrates biological agents (such as beneficial bacteria and fungi) into the priming process and was reportedly effective in barley [62], finger millet [63], wheat [64,65], and maize [66,67]. Nanopriming involves the use of nanoparticles which can facilitate the delivery of nutrients or growth regulators to seeds and stimulate metabolic processes [11,68,69]. Nanopriming improved salinity tolerance of maize [70], wheat [71], rice [72], and rapeseed [73]. Most studies on priming involve seeds of cultivated plants, and this information on halophyte seeds is limited to just a few studies [28,74,75,76].

4. Seed Priming and Salinity Tolerance in Halophytes

As mentioned above, studies on efficacy of various seed priming methods on seed germination, seedling growth, and salinity tolerance of halophytes are scant ([77]; Supplementary Tables S1 and S2). Some studies have reported beneficial effects of hydro-priming on germination, growth, and salinity tolerance of halophytes. For instance, hydro-priming improved seed germination of Atriplex canescens, with longer (4-day) durations of hydro-priming, that resulted in higher germination percentages and rates [78]. Similarly, the 8-day hydro-priming treatment resulted in the highest germination percentage and rate of Nitraria schoberi seeds under 100 mM salinity and enhanced root length and biomass of seedlings [79]. Hydropriming for 12 h resulted in enhanced germination percentages in Giza02 and Q102 quinoa seeds and also improved growth parameters with higher chlorophyll content compared to unprimed control plants in salt stress conditions [80]. Meanwhile, halopriming with KNO₃ and MgSO₄ effectively mitigated the negative impacts of salinity in quinoa [80]. Similarly, seed halopriming priming with CaCl₂, KCl, and KNO₃ enhanced germination and growth of Hordeum maritimum under salinity, primarily by reducing sodium ion accumulation and oxidative stress [81]. Halopriming also improved salinity tolerance of Distichlis spicata at low to moderate salinity levels [82]. Osmopriming seeds with low water potential solutions improved germination in salinity for Amaranthus caudatus, whereas osmopriming seeds with relatively higher water potential solutions was effective for C. quinoa [83]. Osmopriming seeds with proline enhanced germination and seedling establishment of Cakile maritima even at high salinity, and increases in proline and carbohydrate levels were found in the plant tissues under saline conditions compared to unprimed plants [84].

A number of studies have shown that redox priming can also improve salinity tolerance of halophytes during both seed germination and growth stages. For instance, seed priming with H₂O₂ enhanced germination (both rate and total percentage) and reduced mean germination time of halophyte crop Chenopodium quinoa with concomitant stimulation in amylase activity, increased starch breakdown, enhanced amino acid and protein content following exposure to salinity [85]. Seed priming with H₂O₂ and sodium nitroprusside (SNP, a nitric oxide donor) effectively overcame primary dormancy that enhanced seed germination of an annual halophyte Zygophyllum simplex in moderate- and high-salinity conditions compared to unprimed control, hydropriming, and other redox priming treatments [20]. Seed priming with H₂O₂ ameliorated growth and antioxidant activities of Cakile maritima and Eutrema salsugineum under drought and salinity [74]. Likewise, H₂O₂ priming improved performance of Crithmum maritimum under salinity [86]. In addition, Khan et al. [87] reported that seed priming with antioxidant ascorbic acid (AsA) enhanced germination of Suaeda fruticosa and Atriplex stocksii but not of Arthrocnemum macrostachyum, Haloxylon stocksii, Desmostachya bipinnata, and Aeluropus lagopoides under saline conditions. Hence, redox priming efficacy could be species- and chemical-specific, warranting more studies in this area.

A large number of studies have been conducted in the recent past on isolation and characterization of salt-tolerant plant-growth-promoting rhizobacteria (ST-PGPRs) and fungi from the roots/root zones of halophytes and their utilization as bio-priming agents for improving the salinity tolerance of crops [88,89,90]. However, studies on bio-priming effects on seed germination, seedling growth, and salinity tolerance of halophytes are actually scant. Fahad and Sohaib [91] reported that the seeds of Chenopodium quinoa treated with Pseudomonas spp. displayed 100% germination in sand surface and no salinity, whereas seeds treated with Azotobacter spp. alone and in combination with Azospirrilum spp. and/or Pseudomonas spp. resulted in a better germination response at high salinity. More studies are therefore required to unveil efficacy and underlying mechanisms of bio-priming in halophytes.

Nanopriming that involves soaking seeds in solutions of nanoparticles has been used in a number of studies for improving salinity tolerance of crops [62,69,71,92]. Many studies have also used halophyte plants for the green synthesis of nanoparticles with potential as a seed priming agent and improving salinity tolerance of crop plants [93,94,95]. However, little is known about nanopriming effects on seed germination and salinity tolerance of halophytes. Recently, Ashraf Ganjouii et al. [96] reported that nanopriming with selenium nanoparticles can improve growth and morphological parameters of C. quinoa plants under salinity stress at low concentrations. Lack of information thus warrants detailed studies on halophytes.

5. Comparative Analysis of Seed Priming Techniques

The efficacy of various seed priming strategies may vary across species, priming agents, and doses/duration. For instance, halopriming with KNO₃, hormonal priming with GA₃, and halopriming with MgSO₄ effectively mitigated the negative impacts of salinity in C. quinoa compared to hydropriming and CaCl₂ halopriming treatments [80]. Seed priming with halo-priming CaCl₂, KCl, and KNO₃ improved H. maritimum germination and growth under salt stress by reducing sodium ion accumulation and oxidative stress. Seed halopriming priming with CaCl₂ was the most effective, followed by KNO₃, and priming with KCl was the least effective in improving germination and growth of H. maritimum under salt stress [81]. Priming with SNP and H₂O₂ resulted in more significant improvement in seed germination under saline conditions than the AsA priming in annual halophyte Z. simplex [20]. In addition, hydropriming resulted in only marginal improvement in some germination and seedling growth attributes of Z. simplex in comparison to redox priming treatments [20]. The dose of priming also influences the beneficial effects of seed priming. For instance, osmopriming seeds with low water potential solutions led to improved germination of A. caudatus seeds under salinity; however, for the seed of C. quinoa, hydropriming and osmopriming seeds with high water potential solutions was effective [83]. Hussain et al. [28] reported that the low (5 and 100 µM) concentrations of melatonin (MT) were most effective as priming agents in improving seed dormancy, germination, and early seedling growth of five warm subtropical halophytes under saline conditions. Under non-saline conditions, all MT doses improved germination of S. fruticosa following longer (34 h) priming duration, whereas only low (5 and 100 µM) MT doses improved germination of Portulaca oleracea seeds only after short (12 h) priming [28]. In the context described above, identifying the optimal balance of these factors specific to each halophytic species appears important for enhancing seed germination and seedling establishment in saline environments.

6. Mechanistic Insights: Physiochemical and Molecular Footprints

6.1. Physio-Chemical Changes

Priming induces a number of physiological, biochemical, and molecular changes in the seeds and subsequent seedlings, which enable them to efficiently deal with stressful conditions (Figure 2). However, most of the mechanistic information about seed priming is based on glycophytic crops, whereas this information on halophytes is virtually absent. Priming treatments improve hydration ability of seeds by reducing imbibition and lag phases of water absorption upon sowing [56]. The swelling of the embryo in primed seeds accelerates germination by facilitating water absorption and priming also reduces endosperm resistance [97,98]. Nagarajan et al. [99] suggested that improved seed hydration following priming was due to reorganization of seed moisture, changes in seed water-binding properties, and increasing macromolecular hydration water for germination-related metabolic activities. Dry primed seeds showed tissue detachment in X-ray photographs, which improved water flow and tissue hydration by creating free space between the cotyledons and radicle [100]. Similarly, Liu et al. [101] also noticed occurrence of free space around embryo of the dry-primed tomato seeds. Osmopriming increased hydrolysis of the endosperm tissue of Cucumis melo seeds [102] and also decreased endosperm mechanical resistance on the elongating tomato embryo by increasing endo-β-mannanase activity in tomato embryos [98]. Reorganization of the cytoskeleton is also essential for higher cell elongation rates and radicle protrusion. Rapeseed showed increased β-tubulin protein accumulation during PEG treatment and subsequent germination, along with increased expression of γ- and β-tubulins genes [103].

Priming treatments also improve mitochondrial and energy-related activities of the seeds [104,105,106]. Priming enhances ATP synthesis and ATP/ADP ratio during imbibition, with the stimulatory effect increasing with longer duration of osmotic treatment. Priming had a positive effect on tomato seeds by stimulating energy metabolism. The highest improvement occurred when the ATP/ADP ratio and energy charge (EC) were greater than about 0.75 and 1.7, respectively [107]. Seed priming also increases the number of mitochondria, as found in embryonic cells of osmoprimed leek seeds [108]. Osmopriming up-regulated expression of mitochondrial biogenesis genes in Brassica napus seeds, including mitochondrial ribosomal protein, EF2 elongation factor, and TIM10 and TIM23-1 translocases [103].

Priming treatments also improved phyto-hormone balance of the seeds. For instance, CaCl₂ osmopriming improved the hormonal balance of two spring wheat (Triticum aestivum) cultivars by way of reducing abscisic acid (ABA) concentrations and increasing salicylic acid concentrations in the MH-97 cultivar and increasing free indoleacetic and indolebutyric acid content in the Inqlab-91 cultivar [109]. Additionally, there was a strong correlation between the effectiveness of priming and ACC (1-aminocyclopropane-1-carboxylic acid)-dependent ethylene production in sunflower seeds [104]. Several phytohormones, including zeatin, indole-3-acetic acid, and 1-aminocyclopropane carboxylic acid, showed a strong correlation with PsMAPK2 expression in pea seeds pre-treated with H₂O₂ [59]. Nakaune et al. [110] suggested that the beneficial effect of NaCl priming on tomato seed germination was linked to an increase in the GA₄ content via GA biosynthetic gene activation and a subsequent increase in the expression of genes related to endosperm cap weakening. Seed priming significantly improved GA₃ contents in rice seeds by up-regulating the expression of OsGA3ox1 and OsGA20ox1 and decreased the ABA content and the expression of OsNCED1 [111]. Similarly, NaCl osmopriming of tomato seeds up-regulated the expression of GA-biosynthesis genes GA 20-oxidase (GA20ox), GA 3-oxidase (GA3ox), and GA₄ (GA 3beta-hydroxylase) under salinity stress [112]. GA synthesis induced the mobilization of stored seed reserves, particularly starch which is essential for energy generation in germinating seeds and subsequent nascent seedlings. For instance, Nie et al. [111] reported enhanced starch degradation ability in primed rice seeds due to GA-mediated α-amylase expression. Same authors also reported an increase in total soluble sugar content in primed seeds. Another study found that primed true seeds of sugar beet had 0.5–4% more soluble sugar and 1.9–11.5 times higher amylase activity compared to control seeds [113].

Often, primed seeds appear to have higher levels of the osmo-protectant substances. For instance, pre-sowing treatment of Cajanus cajan seeds with KNO₃ or CaCl₂ enhanced protein, free amino acid, and soluble sugar contents during germination under salt stress [114]. The accumulation of osmotically active solutes like amino acids, ammonium compounds, and sugars was also reported in a number of species during priming, which supports seed germination under water stress [60,115,116,117]. Likewise, choline osmopriming enhanced salinity tolerance of wheat by increasing glycinebetaine accumulation [118].

Seed priming also reduced salinity-induced oxidative damages in primed seeds and subsequent seedlings by stimulating their antioxidant defense system. For example, salicylic acid primed seeds of Vicia faba showed enhanced catalase, peroxidase, ascorbate peroxidase, and glutathione reductase activities under salinity stress [119]. Similarly, various priming treatments increased the activities of antioxidant enzymes [100,110,120,121] and contents of antioxidant compounds [120,121,122]. Hence, primed seeds and/or subsequent seedlings showed lower malondialdehyde (MDA, a common indicator of oxidative membrane damage) accumulation under salinity [34,70,123].

An enhanced/efficient nutrient uptake and utilization was also evident under salinity stress following seed priming. For instance, enhanced salinity tolerance of wheat after choline priming coincided with maintained beneficial elements (K⁺ and Ca²⁺) and lower toxic elements (Na⁺ and Cl⁻), alongside higher glycinebetaine accumulation [118]. Similarly, priming of V. faba seeds with melatonin augmented beneficial elements (K⁺ and Ca²⁺) levels and their ratios compared with Na⁺ (i.e., K⁺/Na⁺ and Ca²⁺/Na⁺) in the plant leaves under salinity [124]. Biopriming of wheat seeds with beneficial fungus Trichoderma harzianum enhanced agronomic nitrogen use efficiency by 3.36% under 1/4th nitrogen along with regular phosphorus and potassium levels [125].

Seed priming is also known to stimulate photosynthetic performance in the primed seedlings. For example, seedlings of V. faba developed from melatonin-primed seeds had more photosynthetic pigments compared to unprimed control [124]. Recently, Hussain et al. [28] reported a greater level of photosynthetic pigments in seedlings of some warm subtropical halophytes developed from melatonin-primed seeds. Seed preconditioning increased photosynthetic capacity in adult hexaploid wheat plants subjected to salt [126]. Melatonin priming improved leaf photosynthetic efficiency by stimulating Pn, Gs, Ci, Tr, and chlorophyll contents under salinity in cotton seedlings [127]. Redox priming with H₂O₂ increased Fv/Fm, transpiration, and carboxylation efficiency under salinity in Zea mays plants [128].

6.2. Molecular Changes

Seed priming induces a series of molecular level changes that prepare the seeds for subsequent growth. For instance, a number of germination-related and stress-responsive genes are up-regulated alongside down-regulation of dormancy-inducing and stress-sensitive genes [111,112,129]. A prominent increase in the expression of cell-cycle and DNA repair-related genes can be seen in primed seeds. Flow cytometry-based studies have revealed that DNA replication is initiated in the radicle tip cells during the osmopriming of tomato seeds [130,131]. Beginning with a 3-day incubation period in PEG, the 4C DNA signal in root tip cells of tomato enhanced and remained unaltered when the seeds were dried back to their original moisture content [132]. Interestingly, priming treatment requires at least 5% oxygen to stimulate germination, which is equivalent to the oxygen concentration needed for DNA replication [131] and hence signifies the importance of DNA replication for priming. Osmopriming of B. napus seeds increased the expression of various proteins, including cell division control protein 48 homolog C, cyclin P4;1, cyclin-related protein, topoisomerase II, and proliferating-cell nuclear antigen 2 [103]. The study also reported increased expression of microtubule-associated proteins, tubulin subunits, and kinesin family proteins in B. napus seeds during and after osmopriming. Sivritepe and Dourado [133] reported that priming facilitates the repair of chromosomal damage aged pea seeds. Priming stimulated the expression of 952 genes and 75 proteins in B napus, which are required for seed germination and the start of transcriptional activity for DNA replication, repair, and cell cycle regulation [103]. In addition, Varier et al. [134] suggested that DNA damage repair occurs in primed seeds before replication primarily through DNA synthesis. This DNA damage repair is essential in enhancing seed physiological quality [135].

Seed priming also influences gene expression of germination/dormancy-related phyto-hormones. For example, seed priming significantly up-regulated the expression of OsGA3ox1 and OsGA20ox1 and decreased the expression of OsNCED1-related ABA content in rice seeds [111]. Hormonal priming with ethephon (i.e., a source of ethylene) increased ethylene concentration and up-regulated ethylene signaling genes like OsACS1 and OsEIN2 [136].

Membrane proteins called aquaporins, which are specialized water channels, facilitate water movement across membranes. Kubala et al. [103] reported for B. napus seeds that osmopriming increased the expression of two genes encoding tonoplast aquaporins (TIP4.1 and TIP1.2), with TIP1.2 expression increasing 20-fold during post-priming germination. Similarly, the aquaporin gene expression levels were higher in the primed rice seeds than in the control seeds after 24 h of imbibition [137]. Primed seeds of Spinacia oleracea also showed higher expression of aquaporin coding genes, namely SoPIP1;1, SoPIP2;1, and SoδTIP that coincided with their greater drought tolerance compared to unprimed seeds [138].

Late embryogenesis-abundant (LEA) proteins play an important role in protecting cellular structures and macromolecules during dehydration by preventing protein inactivation, aggregation, and membrane loss. In B. oleracea, transcripts of Em6 (encoding LEA group 1 protein) and RAB18 (encoding responsive to ABA 18 protein of LEA group 2) genes were found to decrease during osmopriming, re-accumulate following slow-drying, and degrade again during seed germination [139]. Similarly, after soaking in PEG solution, LEA4-1 and LEA4-5 transcripts and LEA3 protein accumulated in B. napus [103].

Transcriptional profiling of maize seeds nano-primed with silver nanoparticles showed 140 up-regulated and 242 down-regulated differentially expressed genes (DEGs). A number of up-regulated DEGs were related to mitogen-activated protein kinase signaling pathways, hormone signal transduction, receptor-like kinases (RLKs), and many stress transcription factors (especially WRKY, MYB, NAC) [140]. A total of 221 DEGs were observed in Silene sendtneri seeds after seed priming with silicic acid, among which 117 genes were unique to silicic acid-primed seeds and 102 to non-primed seeds [141]. In contrast, over 1300 genes in Medicago truncatula were differently regulated upon PEG osmopriming at −1.7 MPa [142]. In addition, a proteomics study reported accumulation of 18 proteins in seeds of sugarbeet after hydropriming [143].

A number of recent studies have shown that seed priming can also induce epigenetic modifications, which result in heritable changes in gene expression [144,145,146,147]. These modifications do not alter the DNA sequence but instead involve chemical alterations to the DNA (e.g., methylation) or the histone proteins wrapping the DNA strands [147]. Epigenetic variations are inherited via meiosis and mitosis [148]. Hence, epigenetic changes, due to their heritable nature, may act as a “memory” for plants, enabling them to “remember” the stress conditions they experienced during seed development, equipping them with adaptability to cope with similar challenges in their early growth stages [147]. Despite several reports indicating the trans-generational inheritance potential of priming, few studies have actually investigated the role of stability in epigenetic change transmission through generations.

7. ROS and NO as Potential Signaling Cues of Seed Priming

Although little is known, ROS and NO have gained significant attention as signaling molecules following seed priming. The ROS, which have conventionally viewed as harmful byproducts of cellular metabolism, play a paradoxical role in regulating various plant processes plants including regulation of dormancy/germination and stress tolerance by acting as signaling molecules [149,150]. Hydrogen peroxide (H₂O₂, a common ROS) signaling regulated seed germination in ZnO = nanoprimed T. aestivum seeds and improved plant performance under drought stress [151]. Similarly, priming seeds with low levels of H₂O₂ also reportedly stimulated the expression of stress-responsive genes and enhanced seed vigor and germination in crops [122,152] as well as halophytes [20,74]. Nitric oxide (NO) is another important versatile signaling agent in plants that interacts with ROS and various hormonal pathways to promote seedling growth and stress tolerance [153,154]. Magneto-priming stimulated NO synthesis to modulate phyto-hormones, which improves soybean seed germination under salt stress [155]. Another study reported that magneto-priming enhances salt stress tolerance in soybean seedlings through an NR-dependent NO production pathway, with NO donor promoting germination and NO inhibitor inhibiting it [156]. The interaction of these signaling molecules and phytohormones produces a finely tuned response of seeds/plants to environmental cues. However, little is known about key cross-talks, fine details, and downstream molecular targets of these signaling species in primed seeds, particularly of non-crops such as halophytes.

8. Limitations and Applications of Seed Priming in Saline Agriculture

Seed priming is an effective seed enhancement approach that increases stress tolerance of seeds and subsequent seedlings under saline conditions, making it an important strategy for ensuring future food security. However, currently, there are a few limitations in its large-scale applications. For instance, some priming procedures may increase the contamination chance of the medium by microorganisms, which could significantly impede consequent seed germination [157]. Some researchers have reported that primed seeds have shorter longevity than unprimed seeds [158,159,160]. In a worst-case scenario, priming-induced benefits may disappear after only 14 days of storage, leading to inferior seedling performance compared to unprimed seeds [161]. However, many studies based on classical long-term storage strategies found that priming may not always reduce longevity [158]. Ma et al. [162] found that treating seeds of perennial grass Leymus chinensis with GA₃ significantly improved seed germination and plant growth, with a growth-promoting effect lasting for at least two years. Initial financial investment in adopting seed priming such as for acquiring priming agents, equipment, and storage facility cannot be overlooked, especially for smallholder farmers with tight budgets [163]. To overcome these challenges, further research, optimization of priming methods, and practical training and resources are needed. Most studies on seed priming approaches focus on seeds and young seedlings under controlled conditions, but information about the seedling and adult plant performance under natural environments is unclear, necessitating urgent focus on mature plants developed from the primed seeds, particularly at the reproductive phase, to evaluate long-term effects on various life cycle stages. Our understanding about epigenetic factors influencing “priming memory” duration following seed priming is far from comprehensive. Hence, research in this domain is needed to establish “long-term-primed” plants, enhance stress tolerance, and improve crop productivity for future food security. Another important challenge is the variability in response among different plant species and even among varieties within the same species, making it difficult to implement a one-size-fits-all approach. Last but not least, our understanding of seed priming techniques and underlying mechanisms is mostly based on crops, with little information available regarding halophytes, particularly those with crop potential. Yet, the advantages of seed priming may outweigh the disadvantages. However, further research, optimization of priming methods, and practical training for farmers are crucial to overcome the shortcomings of current priming protocols for securing food supplies and enhancing saline agriculture in the face of global challenges. Global warming and climate changes are causing a decrease in precipitation and water quality, forcing farmers to utilize brackish/saline water for irrigation. Since most crops are sensitive to salinity, utilization of halophytes that can withstand high salinity levels appears a sustainable solution for future agriculture. However, despite high tolerance at the growth stage, the sensitivity of germinating seeds and nascent seedlings could be a bottleneck in the success of halophyte-based saline agriculture in the future. Seed priming, in this context, could be a game-changer for enhanced establishment and productivity of halophytes in stressful barren areas. Nonetheless, more focused research is required to fill the aforementioned gaps and technical hurdles.

9. Conclusions

Seed priming is an easy, low-cost, and effective approach to improve salinity tolerance of early sensitive life-cycle stages of plants including the halophytes, which are the plants naturally found in saline environments. This treatment triggers a number of physiological, biochemical, and molecular changes in seeds, leading to faster germination, improved seedling establishment, and increased resilience under salinity. However, challenges include optimizing priming parameters for different species, reducing seed viability post-priming, and finding cost-effective methods for large-scale agricultural applications. Despite these obstacles, seed priming has significant potential in halophyte cultivation, thereby contributing to sustainable farming practices in degraded saline/dry lands.