Higher endogenous bioactive gibberellins and α-amylase activity confer greater tolerance of rice seed germination to saline-alkaline stress

Seed germination is the most critical phase in plant life cycle and sensitive to saline-alkaline stress. However, few studies have investigated the mechanisms by which rice seeds respond and tolerate to saline-alkaline stress. We explored physiological and molecular mechanisms underlying tolerance to saline-alkaline stress during seed germination using two rice genotypes (Dongdao-4 and Jigeng-88) differing in their tolerance to saline-alkaline stress. Seed germination in Dongdao-4 rice was less inhibited by saline-alkaline stress than Jigeng-88 rice, as evidenced by higher ratio of seed germination rate between saline-alkaline stress and control in Dongdao-4 than that in Jigeng-88. Higher endogenous concentrations of gibberellins (GA(1) and GA(4)) and greater expression levels of GA biosynthetic genes and lower expression levels of genes involved in inactivation of GAs in Dongdao-4 than in Jigeng-88 seeds were observed when challenged by saline-alkaline stress, suggesting that a higher concentration of bioactive GAs may underpin the greater tolerance of Dongdao-4 to saline-alkaline stress than that of Jigeng-88. We further demonstrated that exogenous application of GA and GA biosynthetic inhibitors enhanced and suppressed seed germination under conditions of saline-alkaline stress, respectively. Moreover, we demonstrated a higher activity of alpha-amylase in Dongdao-4 than in Jigeng-88 seeds under saline-alkaline stress. These findings highlight that a greater amount of endogenous bioactive GAs and a higher activity of alpha-amylase in seeds may account for the greater tolerance of Dongdao-4 to saline-alkaline stress during seed germination.