CHARACTERIZATION OF ION-CHANNEL MODULATOR EFFECTS ON ABA-INDUCED AND MALATE-INDUCED STOMATAL MOVEMENTS - STRONG REGULATION BY KINASE AND PHOSPHATASE INHIBITORS, AND RELATIVE INSENSITIVITY TO MASTOPARANS

In the present study abscisic acid-induced stomatal closing, and malat e effects on stomatal apertures were analysed in the presence of guard cell ion channel regulators. A recent study has suggested that abscis ic acid (ABA) activation of protein kinases and/or inhibition of prote in phosphatases may be central to activation of guard cell slow anion channels and mediation of stomatal closing in Vicia faba (Schmidt ef a l., 1995). These findings were confirmed and extended in the present s tudy showing that both in Vicia faba and in Commelina communis ABA-ind uced stomatal closings were abolished by kinase inhibitors and enhance d by the protein phosphatase inhibitor okadaic acid. Further detailed studies demonstrate that very high 40 mM extracellular malate concentr ations are required to close stomata only partially and that okadaic a cid also enhances malate-induced stomatal closing. In addition, when s tomata are widely opened, even at 40 mM malate concentrations, no mala te effect on stomatal apertures was observed. This finding may be expl ained by a complete inactivation of guard cell anion channels when sto matal apertures are opened very widely and suggests that extracellular malate cannot function as a primary CO2 signal in stomatal regulation . The G-protein regulators mastoparan and mas7 as well as neomycin sho wed no significant effects on light-induced stomatal opening and ABA-i nduced stomatal closing. Findings reported here correlate closely to r ecent findings on slow anion channel regulation in guard cells and sup port the hypothesis that activation of these anion channels by phospho rylation events and complete inactivation by dephosphorylation events is a rate-limiting component in guard cell signal transduction. Furthe rmore, the presented data support a model in which ABA-activation of p rotein kinases and/or inhibition of okadaic acid-sensitive protein pho sphatases is central to ABA regulation of stomatal movements in Vicia faba and Commelina communis.