OKADAIC ACID, A PROTEIN PHOSPHATASE INHIBITOR, BLOCKS CALCIUM CHANGES, GENE-EXPRESSION, AND CELL-DEATH INDUCED BY GIBBERELLIN IN WHEAT ALEURONE CELLS

The cereal aleurone functions during germination by secreting hydrolas es, mainly a-amylase, into the starchy endosperm, Multiple signal tran sduction pathways exist in cereal aleurone cells that enable them to m odulate hydrolase production in response to both hormonal and environm ental stimuli. Gibberellic acid (GA) promotes hydrolase production, wh ereas abscisic acid (ABA), hypoxia, and osmotic stress reduce amylase production, In an effort to identify the components of transduction pa thways in aleurone cells, we have investigated the effect of okadaic a cid (OA), a protein phosphatase inhibitor, on stimulus-response coupli ng for GA, ABA, and hypoxia, We found that OA (100 nM) completely inhi bited all the GA responses that we measured, from rapid changes in cyt osolic Ca2+ through changes in gene expression and accelerated cell de ath. OA (100 nM) partially inhibited ABA responses, as measured by cha nges in the level of PHAV1, a cDNA for an ABA-induced mRNA in barley, In contrast, OA had no effect on the response to hypoxia, as measured by changes in cytosolic Ca2+ and by changes in enzyme activity and RNA levels of alcohol dehydrogenase. Our data indicate that OA-sensitive protein phosphatases act early in the transduction pathway of GA but a re not involved in the response to hypoxia, These data provide a basis for a model of multiple transduction pathways in which the level of c ytosolic Ca2+ is a key point of convergence controlling changes in sti mulus-response coupling.