bHLH Transcription Factors That Facilitate K ⁺ Uptake During Stomatal Opening Are Repressed by Abscisic Acid Through Phosphorylation

Abstract

Stomata open in response to light and close after exposure to abscisic acid (ABA). They regulate gas exchange between plants and the atmosphere, enabling plants to adapt to changing environmental conditions. ABA binding to receptors initiates a signaling cascade that involves protein phosphorylation. We show that ABA induced the phosphorylation of three basic helix-loop-helix (bHLH) transcription factors, called AKSs (ABA-responsive kinase substrates; AKS1, AKS2, and AKS3), in Arabidopsis guard cells. In their unphosphorylated state, AKSs facilitated stomatal opening through the transcription of genes encoding inwardly rectifying K⁺ channels. aks1aks2-1 double mutant plants showed decreases in light-induced stomatal opening, K⁺ accumulation in response to light, activity of inwardly rectifying K⁺ channels, and transcription of genes encoding major inwardly rectifying K⁺ channels without affecting ABA-mediated stomatal closure. Overexpression of potassium channel in Arabidopsis thaliana 1 (KAT1), which encodes a major inwardly rectifying K⁺ channel in guard cells, rescued the phenotype of aks1aks2-1 plants. AKS1 bound directly to the promoter of KAT1, an interaction that was attenuated after ABA-induced phosphorylation. The ABA agonist pyrabactin induced phosphorylation of AKSs. Our results demonstrate that the AKS family of bHLH transcription factors facilitates stomatal opening through the transcription of genes encoding inwardly rectifying K⁺ channels and that ABA suppresses the activity of these channels by triggering the phosphorylation of AKS family transcription factors.

GOLDEN 2-LIKE transcription factors for chloroplast development affect ozone tolerance through the regulation of stomatal movement

“…The kat1 single mutant does not affect or only slightly inhibits Fc-dependent stomatal opening (21,35). By contrast, the dominant negative form of KAT1 or KAT2 impaired the activity of K + in channels in guard cells and clearly inhibited light-dependent stomata opening (36,37), indicating that the activity of K + in channels in guard cells is essential and might be much higher than the threshold necessary for stomata opening (35)(36)(37)(38).…”

“…3), suggesting that GLKs act as positive regulators of K + in channel genes and stomatal movement. Similar to the GLKs, the AKSs transcription factors positively regulate stomatal opening and activate KAT1 expression (21). The aks1 aks2 double mutants have reduced K + in channel activity and lower rates of light-or fusicoccin (Fc)-dependent stomata opening; AKSs also function in an ABA-dependent manner and are regulated by phosphorylation (21).…”

“…They facilitate K + uptake through positive regulation of KAT1, but abscisic acid (ABA) represses their transcriptional activation activity through phosphorylation (21). The aks1 aks2 mutants exhibit a weak closed-stomata phenotype and weak down-regulation of KAT1 (21). These phenotypes suggest that other, functionally redundant transcription factor(s) may regulate the expression of the genes for K + in channels and stomatal opening.…”

“…The bHLH transcription factors ABA-RESPONSIVE KINASE SUBSTRATES1 (AKS1), AKS2, and AKS3 function as positive regulators of stomatal opening (21). They facilitate K + uptake through positive regulation of KAT1, but abscisic acid (ABA) represses their transcriptional activation activity through phosphorylation (21). The aks1 aks2 mutants exhibit a weak closed-stomata phenotype and weak down-regulation of KAT1 (21).…”

“…MYB44, MYB15, ERF7, and NFYA5 also participate in stomatal movement in A. thaliana (17)(18)(19)(20). The bHLH transcription factors ABA-RESPONSIVE KINASE SUBSTRATES1 (AKS1), AKS2, and AKS3 function as positive regulators of stomatal opening (21). They facilitate K + uptake through positive regulation of KAT1, but abscisic acid (ABA) represses their transcriptional activation activity through phosphorylation (21).…”