Mechanisms of Seed-Priming Induced Drought Tolerance

Seed priming involves three diﬀerent phases viz.,

(I) imbibition phase, where the rapid uptake of water will occur on dry tissues;

(II)activation phase or lag phase, where the uptake of water is reduced and slight increase in fresh weight, rejuvenation of metabolic events, and repairing events at cellular level occur.

The seeds are physiologically and metabolically very active in this phase, which helps in the development of mitochondria for ATP synthesis and synthesis of proteins from new mRNA and thereby, mobilization of the food material for radical growth.

(III) Growth and cell elongation phase or germination phase, the germination gets started with resumption of the radical in addition with rapid water uptake .

Seeds are usually exposed to a variety of abiotic stresses in the initial stage of the germination process and, thus, oxidative damage of lipids, proteins, and nucleic acids is not an unusual event . However, seed water uptake and the subsequent growth of the embryo are controlled by water transportation .

Seed priming induces rapid imbibition of seeds with a limitedamountofwatertostartthepre-requisitemetaboliceventsforpre-germinationwithoutradical protrusion. The priming process can extend the lag phase and also prevent the start of the log phase through restriction of the movement of water for completing the radical protrusion. The hydration treatment allows the water movement up to 50% to reinitiate the metabolic events. Due to the rapid uptake of water, the induction of germination and the emergence and establishment of primed seeds occurs earlier compared to that of control seeds .

After seed priming treatment, the synchronized and earlier germination of seeds might be due to the decline of lag phase, increased enzyme activity, increased germination-promoting metabolites, and better osmotic regulation of primed seeds.

Ataphysiologicalstate,seedpriminginducesvariousmetabolicchangesintheseedwiththeinitiation of the imbibition process . The rehydration of primed seeds brings major cellular changes in seeds such as de novo synthesis of nucleic acids and proteins, ATP synthesis, activation of sterols and phospholipids, and repairing of DNA. DNA repairing mechanism is an essential component of the pre germinative metabolism; proper repairing of damaged DNA permits the embryo cells to restart the cell cycle and DNA replication.

Seed priming induces antioxidant activity and storage protein solubilization and minimizes lipid peroxidation . A proteomic analysis of primed Arabidopsis seeds revealed the increase of various storage proteins and enzymes such as isocitrate lyase and amylases during germination. Priming signiﬁcantly increases the quantity of mitochondria and related proteins (α- and β-tubulin) that are responsible for cell division.

According to [193], hydro priming of wheat seeds improved the WUE, which ultimately improved the yield. Itwasfoundthat,hydroprimingtreatmenthydrolyzingtheendospermtissuesof Solanum lycocarpum and also enhancing the seed germination, seedling growth, and development by endo-β-mannanase activity, favored breaking the mechanical resistance for cell elongation in embryo and embryonic cover. The up regulation of xyloglucan endo trans glycosylase during priming treatment was responsible for cell wall loosening and restructuring.

Yan et al. reported that rice seed primed with melatonin was observed under a transmission electron microscope and it was found that seed priming restored the integrity of the cellular structure .

Cytoskeleton restructuring is essential for cell elongation to promote radical emergence . The primed seeds began cell division in advance to induce the accumulation of β-tubulin and to start the replication of DNA for early radical protrusion.

Seedprimingreinforcesthecellulardefenseresponseandimprovesthetoleranceagainstbioticand abiotic stress through latent defense protein accumulation . Aquaporins, a major intrinsic protein, regulate the water movement across the cell membrane via the intercellular region in both monocot and dicot families . The deep understanding of the positive role of water on germination and the proﬁle of aquaporin expression genes were studied during imbibition and primary embryo growth stages in many species such as Arabidopsis , Brassica napus , and Oryza sativa .

These studies conﬁrmed the potential role of aquaporins on seed germination against abiotic stresses. Furthermore, the reduced rate of seed germination by silencing OsPIP1;3 (plasma membrane intrinsic proteins) and improved the germination by overexpression of OsPIP;3 under water deﬁcit conditions. The experimental results concluded that OsPIP;3 is a prerequisite for obtaining the actual germination in rice. Ge et al. demonstrated that, the primed seeds of Brassica napus were studied during the event of germination, it was found that primed seed induced the expression of BnPIP1 and had no eﬀect on Bnγ-TIP2[201]. The transcription levels of Bn-TIP1 and Bnγ-TIP2 were found to be increase din primed seeds, but no such impact was found on unprimed seeds. The gene BNPIP1 is required for water movement to turn on the storage nutrients and enzyme metabolism during germination in brassica seeds, whereas, Bnγ-TIP2 expression depends on cell growth during radical protrusion. Seed priming induces the AP2/ERF transcription factors, which induce the production of secondary metabolites and, thus, improve the drought tolerance.

The seed priming methods induce the physiological mechanisms to increase yield of crops under drought stress. Priming with chemical agents was found to be a better ameliorant for drought conditions. Recently several studies are focusing on exploiting the priming-induced drought tolerance mechanisms in diﬀerent crops. Moreover, seed priming could also improve tolerance mechanisms to subsequent stresses, via the mechanism of “stress memory”, where the plant exposed to a certain stress can induce a stress memory for better and faster response in later stress events. The plants form early exposed to one type of stress(stress priming/hardening)may develop tolerance/protection to another kind of stress through the production of secondary metabolites.

Though, the time gap between the priming and subsequent stress is short, the molecular mechanism of seed-priming-induced drought stress tolerance to a subsequent stress persists for a longer time due to stress memory. Post-translation modiﬁcation (PMT) is a potential stress memory for priming the defense system, by activating the genes for stronger and faster transcription in drought stress response. The induced expression of H3K4me3 (histone H3 lysine 4- trimethylation) and stalled RNAPII(RNA polymerase) are correlated with activation of RD29 Band RAB18,signifying that histone modiﬁcations are involved in the drought priming memory in Arabidopsis. Results of various studies reveal that drought priming could induce the tolerance mechanism at the jointing stage; for example, tolerance to water deﬁcit and heat stress condition sat the grain ﬁlling stage in wheat.

The progeny from drought-priming treatment own the stress tolerance mechanism through enhanced photosynthesis and increased antioxidant capacity, and this might have the potential to protect the plants from heat stress in wheat crop at the time of post-anthesis. Similarly, osmoprimed and hydroprimed seeds showed improved transgenerational drought tolerance by modulating the water relations, osmolytes accumulation, malondialdehyde contents, and lipid peroxidation at the terminal growth stage.

Trans generational memory of drought stress also eﬀects the root and to early exposed of the progeny in spring barley, for instance, the progeny from primed plants have a reduced shoot-to-root fraction and its leads to a reduction in root thickness compared to the progeny from control under drought conditions. Priming before anthesis improved the tolerance to a succeeding drought stress during grain ﬁlling, resulting in enhanced photosynthesis rate, ascorbate peroxidase activity, and higher yield compared to non-primed plants of wheat. Plant hormones also respond to drought by long distance signaling in root and shoot and organize the water transport. Among the plant hormones, ABA has decisive mechanism to induce tolerance against the drought.

Drought stress results in the production of a number of hormones, which could be involved in organizing the various physiological events as gesture molecules in signaling networks. During water deﬁcit conditions, the plants sense the stress by the sensors implicated in response signaling, and it can be transduced through various metabolic cycles through signaling pathways and transcriptional factors. Seeds of Arabidopsis primed with the non-protein amino acid b-aminobutyric acid have shown tolerance against drought stress through ABA accumulation, the expression of stress-responsive genes, and the closing of stomata. High ABA accumulation boosts cytosolic Ca2+ and activates the anion channel in the plasma membrane.

The parental line of rice treated with osmopriming agents helps in the accumulation of a high proline content through proline expression genes along with the removal of a methyl group from nucleotides in DNA in the preceding progeny under drought stress.