Regulation of Em Gene Expression in Rice : Interaction Between Osmotic Stress and Abscisic Acid

Expression of the Em gene was characterized in rice (Oryza sativa L.) suspension cultures following exposure of the cultures to various combinations of abscisic acid (ABA) and salt. Response-saturating concentrations of either ABA (50 micromolar) or NaCl (0.4 molar) rapidly induced (by 60 minutes) the accumulation of Em mRNA, with a maximum accumulation occurring 12 to 24 hours after treatment. NaCl-induced Em expression was accompanied by a doubling of endogenous ABA levels as determined by immunoassay. Inhibition of ABA biosynthesis by fluridone during NaCl treatment reduced the levels of endogenous ABA by fourfold and Em expression by 50%. Desiccation of the cultures to 12 to 15% of their initial fresh weight increased endogenous ABA more than twofold and was accompanied by an increase in Em mRNA levels. Exposure of the cultures to heat shock temperatures, chilling, or ultraviolet light neither increased endogenous ABA levels nor induced Em expression. When a subthreshold or saturating level of NaCl was added in combination with increasing levels of ABA, Em transcripts were detected at ABA concentrations that alone did not induce expression of Em. Treatment with saturating levels of both NaCl and ABA resulted in a doubling of Em transcript levels over the maximum signal for each treatment alone. Hence, our data suggested that salt interacted synergistically with ABA, in part because of the increased sensitivity of rice cells to ABA. The effect of salt stress on Em gene expression in rice suspension cells appeared to operate through two pathways: one is mediated through increases in the level of ABA; the other is via a unique salt response pathway that includes an intermediate that is common to both the salt and ABA response chains.