ABA signal transduction in the barley aleurone is mediated by phospholipase D activity

Sian Ritchie and Simon Gilroy
Department of Biology, 208 Mueller Lab, The Pennsylvania State University, University Park, PA 16802

Abstract.
The plant hormones abscisic acid (ABA) and gibberellic acid (GA) are important regulators of the dormancy and germination of seeds. In cereals, GA enhances the synthesis and secretion of enzymes (principally a-amylases) in the aleurone cells of the endosperm which then mobilize the storage reserves that fuel the germination process. ABA inhibits this enhanced secretory activity and delays germination. However, despite their central role in regulating the germination process, the molecular events transducing the ABA signal to altered gene expression and cellular activity are essentially unknown. We report that the activity of the enzyme phospholipase D (PLD) increased 10 min after the application of ABA, but not GA, to barley aleurone protoplasts. The product of PLD activity, phosphatidic acid, also increased transiently at this time. The application of phosphatidic acid to aleurone protoplasts led to an ABA-like inhibition amylase production, and induction of the ABA up-regulated proteins ASI and RAB. Inhibition of PLD activity by 1-butanol during the initial 20 min of ABA treatment resulted in inhibition of ABA regulated processes. This inhibition coincided with the timing of PLD activation by ABA and was overcome by simultaneous addition of phosphatidic acid. These results suggest that ABA activates the enzyme PLD to produce phosphatidic acid which is involved in triggering the subsequent ABA responses of the aleurone cell.

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