Dual role of cytosolic GSH in the ABA signaling pathway and plasma membrane ion channel regulation in guard cells of Vicia faba

Abscisic acid (ABA) induces stomatal closure in higher plants under drought stress. Glutathione (GSH) negatively regulates ABA-induced stomatal closure and reactive carbonyl species (RCS) play a role as signal mediators downstream of reactive oxygen species production in ABA signaling pathway in Arabidopsis thaliana. Activation of slow (S-type) anion channels and inhibition of inward-rectifying potassium ion (K_in⁺) channels in the plasma membrane are essential for ABA-induced stomatal closure. However, there is limited evidence regarding role of GSH in the activation of S-type anion channels and the inhibition of K_in⁺ channels. We used Vicia faba to clarify the regulation of these ion channels by GSH and RCS. Pretreatment of guard-cell protoplasts with the GSH-supplementing agent, glutathione monoethyl ester (GSHmee), suppressed the activation of S-type anion channels and the inactivation of K_in⁺ channels induced by ABA. The pretreatment with the RCS scavenger carnosine suppressed the activation of S-type anion channels and the inactivation of K_in⁺ channels by ABA. On patch clamping guard-cell protoplasts, the addition of GSH to the pipette (cytosolic) buffer decreased the S-type anion currents and increased the K_in⁺ currents. These results suggest that cytosolic GSH is involved in ABA-induced stomatal closure via negative regulation of ABA signaling and via direct regulation of ion channel activities in V. faba.