Abstract
The phytohormone abscisic acid (ABA) regulates stomatal aperture to adapt plants to drought stresses. Recent studies have demonstrated that ABA signaling is initiated by the newly discovered ABA receptors via protein phosphorylation. However, little is known about the downstream events that result in the regulation of transporters in guard cells. We found that ABA rapidly induced phosphorylation of novel bHLH transcription factors in Arabidopsis guard cells. We named them as ABA-responsive kinase substrates (AKS1, 2, 3). In aks1aks2 mutant, the rate of stomatal opening induced by light or fusicoccin was decreased but that of stomatal closure by ABA was not affected. Transcript level of KAT1, a major K+in channel, and the activity of K+in channels were reduced but the H+-ATPase activity was not affected in the mutant, with decrease in the accumulation of K+ in guard cells. AKS1 showed the transcriptional activity that is sensitive to ABA. We revealed that AKS1 was bound to KAT1 promoter and was released by ABA-induced phosphorylation. Our results demonstrate that ABA represses gene expression by phosphorylation of the bHLH transcription factors and inhibits stomatal opening.
