STRONG REGULATION OF SLOW ANION CHANNELS AND ABSCISIC-ACID SIGNALING IN GUARD-CELLS BY PHOSPHORYLATION AND DEPHOSPHORYLATION EVENTS

Recent evidence suggests that slow anion channels in guard cells need to be activated to trigger stomatal closing and efficiently inactivate d during stomatal opening. The patch-clamp technique was employed here to determine mechanisms that produce strong regulation of slow anion channels in guard cells, MgATP in guard cells, serving as a donor for phosphorylation, leads to strong activation of stow anion channels, Sl ow anion-channel activity was almost completely abolished by removal o f cytosolic ATP or by the kinase inhibitors K-252a and 117, Nonhydroly zable ATP, GTP, and guanosine 5'-[gamma-thio]triphosphate did not repl ace the ATP requirement for anion-channel activation. In addition, dow n-regulation of slow anion channels by ATP removal was inhibited by th e phosphatase inhibitor okadaic acid, Stomatal closures in leaves indu ced by the plant hormone abscisic acid (ABA) and malate were abolished by kinase inhibitors and/or enhanced by okadaic acid, These data sugg est that ABA signal transduction may proceed by activation of protein kinases and inhibition of an okadaic acid-sensitive phosphatase, This modulation of ABA-induced stomatal closing correlated to the large dyn amic range for up- and down-regulation of slow anion channels by oppos ing phosphorylation and dephosphorylation events in guard cells, The p resented opposing regulation by kinase and phosphatase modulators coul d provide important mechanisms for signal transduction by ABA and othe r stimuli during stomatal movements.