ABSCISIC-ACID SIGNAL-TRANSDUCTION IN THE BARLEY ALEURONE IS MEDIATED BY PHOSPHOLIPASE-D ACTIVITY

The plant hormones abscisic acid (ABA) and gibberellic acid (GA) are i mportant regulators of the dormancy and germination of seeds. In cerea ls, GA enhances the synthesis and secretion of enzymes (principally al pha-amylases) in the aleurone cells of the endosperm, which then mobil ize the storage reserves that fuel germination, ABA inhibits this enha nced secretory activity and delays germination, Despite the central ro le of ABA in regulating germination, the signal transduction events le ading to altered gene expression and cellular activity are essentially unknown, We report that the application of ABA to aleurone protoplast s increased the activity of the enzyme phospholipase D (PLD) 10 min af ter treatment, The product of PLD activity, phosphatidic acid (PPA), a lso increased transiently at this time, The application of PPA to aleu rone protoplasts led to an ABA-like inhibition of alpha-amylase produc tion, and induction of the ABA upregulated proteins ASI (amylase subti lisin inhibitor) and RAB (responsive to ABA). Inhibition of PLD activi ty by 0.1% 1-butanol during the initial 20 min of ABA treatment result ed in inhibition of ABA-regulated processes, This inhibition coincided with the timing of PLD activation by ABA and was overcome by simultan eous addition of PPA, These results suggest that ABA activates the enz yme PLD to produce PPA that is involved in triggering the subsequent A BA responses of the aleurone cell.