Role of abscisic acid in chilling tolerance of rice (Oryza sativa L.) seedlings.

We have investigated the role of abscisic acid (ABA) in the regulation of polyamine biosynthesis in rice (Oryza sativa L.) seedlings exposed to chilling (5°C). In a chilling-tolerant cultivar (cv. Tainung 67, TNG.67), levels of free putrescine and activity of arginine decarboxylase (ADC, EC 4.1.1.19) in both shoots and roots, and levels of free spermidine/spermine and activity of S-adenosylmethionine decarboxylase (SAMDC, EC 4.1.1.50) in shoots increased after exposure to chilling. In a chilling-sensitive cultivar (cv. Taichung Native 1, TN.1), level of free putrescine and activity of ADC in shoots increased slightly after exposure to chilling while those in roots decreased. Activity of ornithine decarboxylase (ODC, EC 4.1.1.17) in both cultivars remained unchanged after exposure to chilling. α-Difluoromethylarginine (DFMA), an irreversible inhibitor of ADC, but not α-difluoromethylornithine (DFMO), an irreversible inhibitor of ODC, at 0.5 mM inhibited free putrescine accumulation in chilled TNG.67 and resulted in a decrease in chilling tolerance; a decrease in survival and an increase in electrolyte leakage. The effects of DFMA were partially reversed by the addition of 0.5 mM putrescine. In TNG.67, chilling induced an increase of free ABA levels first, then ADC activity and finally free putrescine levels. Fluridone, an inhibitor of ABA synthesis, at 0.5 mM inhibited the increase of free ABA levels, ADC activity and free putrescine levels in chilled TNG.67, and resulted in a less tolerance of TNG.67 to chilling; these effects could be reversed by the pre-chilling treatment of 10 μM ABA. Application of 10 μM ABA for 48 h before chilling not only rendered TN.1 tolerant to chilling but also induced a change of polyamine pattern to be similar to chilled TNG.67. It seems that, for the materials used in this study, one of strategies of rice seedlings to resist chilling stress is to raise ABA levels and, in turn, one of ABA's functions is to enhance the ADC-mediated putrescine synthesis.