**3. Mechanisms of seed enhancements**

#### **3.1. Physiological and biochemical aspects**

Improved crop performance through pre-sowing treatments depends on the nature of compounds used for priming and their accumulation under abiotic stresses. These compounds include inorganic salts, osmolytes, phytohormones, tertiary amino compounds such as glycinebetaine, amino acids and sugar alcohols including bioactive compounds from microorganisms. For instance, the application of compatible solutes as seed priming improves salinity resistance by cytosolic osmotic adjustment indirectly by enhancing regulatory functions of osmoprotectants [110, 111]. Chilling-induced cross-adaptation salt tolerance in wheat is associated with enhanced accumulation of beneficial mineral elements (K+ and Ca2+) in the roots and reduced uptake of toxic Na+ in the shoots through ionic homeostasis and hormonal balance with greater concentrations of indoleacetic acid, abscisic acid, salicylic acid and spermine in chilled wheat seeds [112]. In flooded soils, improved stand establishment in rice through seed priming is related to enhanced capacity of superoxide dismutase (SOD) and catalase (CAT) activities to detoxify the reactive oxygen species in seeds and greater carbohy‐ drate mobilization. These effects are more pronounced in tolerant genotypes that emphasize to combine crop genetic tolerance with appropriate seed treatments to improve seedling establishment of rice sown in flooded soils [113].

Such enhanced remobilization efficiency in seed embryos of cereals coated with hydroabsrobers is related to change in activities of enzymes for sucrose breakdown upon moisture absorption. Coated seeds absorb more moisture that creates anoxic conditions in developing embryos but genetic difference are found for sucrose breakdown in rye, barley and wheat with change in invertase activities due to difference in timing of imbibitions [114].

Beneficial effects of magnetic seed stimulation are associated with various biochemical, cellular and molecular events [115]. Pre-sowing magnetic seed treatment also increases ascorbic acid contents [33] by stimulating the activity of the enzymes and proteins [116]. Physiological and biochemical properties also increase due to enhanced metabolic pathway by the free move‐ ment of ions [117]. However, its biochemical and physiological mechanisms are still poorly understood [118].

### **3.2. Molecular aspects**

Seed coating demands uniform application of inert material over the seed surface. This also helps to protect the seed from soil and seed-borne pathogens [17]. Pharmaceutical industry uses seed polymer coating for a constant application of numerous materials to seeds. The commercially available plasticizers, polymers and colourants (commercially they are readily available to be used as liquid) are applied as film formulations [108]. However, the exact composition of coating material is a carefully guarded secret by the companies who develop them. Usually, coating material contains binders, fillers (e.g., polyvinyl alcohol, gypsum and clay) and an intermediate layer (e.g., clay, polyvinyl acetate and vermiculite). Seed agglom‐ eration is an alternate coating technology with the purpose to sow multiple seeds of the same

Improved crop performance through pre-sowing treatments depends on the nature of compounds used for priming and their accumulation under abiotic stresses. These compounds include inorganic salts, osmolytes, phytohormones, tertiary amino compounds such as glycinebetaine, amino acids and sugar alcohols including bioactive compounds from microorganisms. For instance, the application of compatible solutes as seed priming improves salinity resistance by cytosolic osmotic adjustment indirectly by enhancing regulatory functions of osmoprotectants [110, 111]. Chilling-induced cross-adaptation salt tolerance in

hormonal balance with greater concentrations of indoleacetic acid, abscisic acid, salicylic acid and spermine in chilled wheat seeds [112]. In flooded soils, improved stand establishment in rice through seed priming is related to enhanced capacity of superoxide dismutase (SOD) and catalase (CAT) activities to detoxify the reactive oxygen species in seeds and greater carbohy‐ drate mobilization. These effects are more pronounced in tolerant genotypes that emphasize to combine crop genetic tolerance with appropriate seed treatments to improve seedling

Such enhanced remobilization efficiency in seed embryos of cereals coated with hydroabsrobers is related to change in activities of enzymes for sucrose breakdown upon moisture absorption. Coated seeds absorb more moisture that creates anoxic conditions in developing embryos but genetic difference are found for sucrose breakdown in rye, barley and wheat with

Beneficial effects of magnetic seed stimulation are associated with various biochemical, cellular and molecular events [115]. Pre-sowing magnetic seed treatment also increases ascorbic acid contents [33] by stimulating the activity of the enzymes and proteins [116]. Physiological and biochemical properties also increase due to enhanced metabolic pathway by the free move‐ ment of ions [117]. However, its biochemical and physiological mechanisms are still poorly

change in invertase activities due to difference in timing of imbibitions [114].

and Ca2+)

in the shoots through ionic homeostasis and

wheat is associated with enhanced accumulation of beneficial mineral elements (K+

seed lot, or multiple seeds of different seed lots, varieties or species [109].

56 New Challenges in Seed Biology - Basic and Translational Research Driving Seed Technology

**3. Mechanisms of seed enhancements**

**3.1. Physiological and biochemical aspects**

in the roots and reduced uptake of toxic Na+

establishment of rice sown in flooded soils [113].

understood [118].

Favourable effects of priming at cellular level include RNA and protein synthesis [22]. Seed priming induces several biochemical changes within the seed needed for breaking seed dormancy, water imbibition, enzymes activation, hydrolysis of food reserves and mobilization of inhibitors [119]. At cellular level priming initiates cell division transportation of storage protein [120]. Higher germination rate and uniform emergence of primed seed is due to metabolic repair with increased production of metabolite required for the germination [121, 122] during the imbibition process. Priming increased the production and activity of α-amylase within germinating seeds, thus increased the seed vigour [65].

Several proteins and their precursors for regulation involved in different steps of seed germination or priming have been identified using model plant Arabidopsis. The expression of these proteins such as actin isoform or a WD-40 repeat protein occurs in imbibition and cytosolic glyceraldehyde-3-phosphate dehydrogenase in the seed dehydration process [123]. Priming-induced changes in proteins levels have been identified as peroxiredoxin-5, 1-Cys peroxiredoxin, embryonic protein DC-8, cupin, globulin-1 and late embryogenesis abundant protein. The expression of these proteins led to improved seed germination and the expression of these embryo proteins remained unchanged even after priming [124].

A major quantitative trait locus (QTL) Htg6.1 of seed germination responsive to priming under high temperature stress using a recombinant inbred line (RIL) of lettuce (*Lactuca sativa* L.) has been identified. The expression of this QTL at high temperature is coded by a gene *LsNCED4* encoding the key enzyme, i.e., 9-cis-epoxycarotenoid dioxygenase, of the abscisic acid biosynthetic pathway and maps precisely with Htg6.1. However, *LsNCED4* gene expression was higher in non-primed seeds after 24 h of imbibition at high temperature compared to the expression of *LsGA3ox1* and *LsACS1* genes encoding enzymes of gibberellins and ethylene biosynthetic pathways, respectively. *LsNCED4* gene expression was reduced after priming and when imbibition was carried out at the same temperature . The seed response to priming in terms of germination and temperature sensitivity is associated with temperature regulation of hormonal biosynthetic pathways [125].

Osmopriming induced quantitative expression of stress-responsive genes such as CaWRKY30, PROX1, Osmotin for osmotic adjustment, Cu/Zn SOD for antioxidant defence and CAH for phenylpropanoid pathway. The same genes were induced earlier or at higher levels in response to thiourea priming at low temperature. The expression of these genes imparts cold tolerance in capsicum seedlings [126]. Notably, high levels of other plant-growth hormones, such as indolyl-3-acetic acid (IAA) and abscisic acid (ABA), were also observed. The authors suggested that *Bacillus* strains have dual effect on plant-growth promotion and accumulation of cytokinins by increasing other routes of synthesis of hormones such as IAA and ABA, as well as interfering in other hormonal balance synthesis such as gibberellins (GA).

Using advanced molecular tools such as proteomics may help to detect protein markers that can be used to unravel complex development process of seed vigour of commercial seed lots, or analysis of protein changes occur in industrial seed priming treatments to accelerate seed germination and improve seedling uniformity.
