Abstract: 【Objective】To study the effects of salicylic acid (SA) priming on the vigor of seed germination and physiological response of rice seeds at low temperature, and to reveal the induction mode of SA priming on genes related to abscisic acid (ABA) and gibberellin acid (GA) metabolic pathways and cell wall relaxation, which can provide a theoretical basis for the study of low temperature germination of rice seeds.【Method】Using indica hybrid rice Taifengyou 208 seeds as materials, the effects of SA on seed germination vigor and physiology were analyzed by seed priming, and the expression patterns of ABA, GA and expansion protein genes in response to SA were analyzed by qRT-PCR.【Result】Low temperature (15 ℃) significantly delayed the germination process of rice seeds. The concentration of endogenous SA in seeds germinated at low temperature for 1 day was 1.7 times that at room temperature (28 ℃). However, the SA concentration at low temperature was only 0.6% of that at normal temperature for 5 d seedlings. SA could effectively improve germination vigor of seeds at low temperature, especially 2 000 μmol·L^-1 SA, which significantly increased germination index, vigor index, bud length, root length, fresh weight and dry weight of seeds at low temperature, and vigor index of seeds at low temperature was 3 and 2 times of that of non-induced seeds (CK1) and water-induced seeds (CK2), respectively. In terms of physiological indexes, SA priming increased the contents of soluble sugar, proline and active oxygen, enhanced the activities of total amylase, β-amylase, superoxide dismutase (SOD) and catalase (CAT), and decreased the content of malondialdehyde (MDA). Compared with CK1, 2 000 μmol·L^-1 SA decreased the ABA content by 79%, and increased the IAA and GA1 contents by 32.2% and 74.7%. In terms of gene expression, the expressions of ABA synthesizing genes OsNCED2 and OsNCED3 were decreased by 94.26% and 90.24% compared with CK1 in seeds primed by 2 000 μmol·L^-1 SA, respectively. The expression levels of ABA decomposing genes OsABA8 'ox2 and OsABA8' ox3 were 5.9 and 3.9 times that of CK1, respectively. Compared with CK1, SA significantly up-regulated the expression levels of GA synthesizing genes OsCPS1, OsKAO and OsGA20ox1, and significantly down-regulated the expression levels of GA decomposing genes OsGA2ox2 and OsGA2ox6. In several candidate genes encoding cell wall relaxation protein, e.t. expansin, except OsEXPB11, the other homologous genes were triggered and upregulated to some extent. Compared with CK1, 2 000 μmol·L^-1 SA increased the expression levels of OsEXPA2, OsEXPB4 and OsEXPB6 to 12.2, 5.9 and 6.1 times, respectively.【Conclusion】SA induction could significantly alleviate the effects of low temperature on seed germination and seedling growth of rice. It was probably because SA increased the activity of antioxidant enzymes such as SOD and CAT, decreased the production of MDA, increased the contents of soluble sugar and proline, and thus enhanced the tolerance of seeds and seedlings to low temperature. On the other hand, SA prime decreased endogenous ABA content, increased GA₁ content, enhanced the activities of total amylase and β-amylase, and promoted the expression of genes related to cell wall relaxation, thus facilitating seed germination and seedling growth at low temperature.

Key words: rice, low temperature stress, seed priming, salicylic acid (SA), physiological index, genes expression