Abscisic acid signal transduction in guard cells is mediated by phospholipase D activity

In guard cells, the plant hormone abscisic acid (ABA) inhibits stomatal ope ning and induces stomatal closure through the coordinated regulation of ion transport. Despite this central role of ABA in regulating stomatal functio n, the signal transduction events leading to altered ion fluxes remain inco mpletely understood. We report that the activity of the enzyme phospholipas e D (PLD) transiently increased in guard cell protoplasts at 2.5 and 25 min after ABA application. Treatment of guard cell protoplasts with phosphatid ic acid (PtdOH), one of the products of PLD activity, led to an inhibition of the activity of the inward K+ channel. PtdOH also induced stomatal closu re and inhibited stomatal opening when added to epidermal peels. Applicatio n of l-butanol (1-buOH), a selective inhibitor of PtdOH production by PLD, inhibited the increase in PtdOH production elicited by ABA. 1-BuOH treatmen t also partially prevented ABA-induced stomatal closure and ABA-induced inh ibition of stomatal opening. This inhibitory effect of buOH was enhanced by simultaneous application of nicotinamide, an inhibitor of cADP ribose acti on. These results suggest that in the guard cell. ABA activates the enzyme PLD, which leads to the production of PtdOH. This PtdOH is then involved in triggering subsequent ABA responses of the cell via a pathway operating in parallel to cADP ribose-mediated events.