Superinduction of the Em gene in rice suspension cells in the presence of ABA and cycloheximide

The Em gene in rice (Oryzae sativa L.) cell suspensions is strongly induced in the presence of abscisic acid (ABA) or in the presence of an osmoticum such as NaCl. Experiments were conducted to determine the impact of protein synthesis inhibitors on Em mRNA accumulation as part of a study to further characterize regulation of this gene in rice. When rice cell suspensions were incubated in the presence of ABA or NaCl and either 10 mu M cycloheximide (resulting in a 42% inhibition of net protein synthesis) or 100 CIM cycloheximide plus 10 mu M chloramphenicol (93% inhibition of net protein synthesis), Em mRNA accumulated to a level more than twofold greater than in cell suspensions treated with either ABA or NaCl alone. Neither cycloheximide nor chloramphenicol alone or in combination induced Em gene expression. When rice cells were exposed to 50 mu M ABA for 1 h and then transferred to medium without ABA, Em transcript abundance increased until 24 h and then declined. The rate of this decline was slower in the presence of the protein synthesis inhibitors. The results indicate that de novo protein synthesis is not required for ABA or NaCl induction of Em gene expression in rice suspension cells and that superinduction can occur when protein synthesis is only partially blocked in the cell population.