Seed germination is the initial step of plant development. Seed priming with salt promotes seed germination in tomato (Solanum lycopersicum L.); however, the molecular and physiological mechanisms underlying the enhancement of seed germination by priming remain to be elucidated. In this study, we examined the following in seeds both during and after priming treatment: the endogenous abscisic acid (ABA) and gibberellin (GA) concentrations; the expression of genes encoding ABA catabolic and GA biosynthesis enzymes, including 8'-hydroxylase (CYP707A), copalyl diphosphate synthase (CPS), GA 20-oxidase (GA20ox) and GA 3-oxidase (GA3ox); and endosperm cap weakening enzymes, including expansin (EXP), class I β-1,3-glucanase (GulB), endo-β-mannanase (MAN) and xyloglucan endotransglucosylase (XTH). Tomato seeds were soaked for 24 h at 25 °C in the dark in 300 mM NaCl (NaCl-priming) or distilled water (hydro-priming). For both priming treatments, the ABA content in the seeds increased during treatment but rapidly decreased after sowing. Both during and after the priming treatments, the ABA levels in the hydro-primed seeds and NaCl-primed seeds were not significantly different. The expression levels of SlGA20ox1, SlGA3ox1 and SlGA3ox2 were significantly enhanced in the NaCl-primed seeds compared to the hydro-primed seeds. The GA(4) content was quantifiable after both types of priming, indicating that GA(4) is the major bioactive GA molecule involved in tomato seed germination. The GA(4) content was significantly higher in the NaCl-primed seeds than in the hydro-primed seeds 12 h after sowing and thereafter. Additionally, the peak expression levels of SlEXP4, SlGulB, SlMAN2 and SlXTH4 occurred earlier and were significantly higher in the NaCl-primed seeds than in the hydro-primed seeds. These results suggest that the observed effect of NaCl-priming on tomato seed germination is caused by an increase of the GA(4) content via GA biosynthetic gene activation and a subsequent increase in the expression of genes related to endosperm cap weakening.
Seed germination is the initial step of seedling development in plants. Seed priming with salts has been used to synchronise seed germination. In general, a long-term treatment with a relatively high salt concentration, such as 1 M NaCl, is employed. To improve the e ciency of this treatment, we examined the e ect of seed priming with a lower NaCl concentration than conventional method in tomato (Solanum lycopersicum). Tomato seeds were soaked for 24 h at 25°C in the dark in 100-1000 mM of NaCl solution (NaCl-priming) or distilled water (hydro-priming). To estimate the e ect of NaCl-priming on seed germination and subsequent seedling growth, the germination rate, seedling emergence, plant height, and hypocotyl and root length were investigated under NaCl-, hydro-and non-priming treatments. At 4 d a er sowing, the seedling emergence was markedly promoted by 300 mM of NaCl-priming. e NaCl-priming also signi cantly enhanced the seed germination rate at 48 h a er sowing. Seedling growth, as indicated by plant height, stem diameter and hypocotyl and root length, was promoted by NaCl-priming. ese results suggest that priming with low saline has similar e ects as conventional priming methods. A comprehensive gene expression analysis showed that the genes related to seedling growth and stress responses were up-regulated by NaCl-priming at 144 h a er the start of the treatment, followed by advanced and uniform seed germination. e seedlings exhibited an increased tolerance to Ralstonia solanacearum, the causative agent of bacterial wilt of tomato, compared with the hydro-primed and non-primed seedling.
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