S-Type Anion Channels SLAC1 and SLAH3 Function as Essential Negative Regulators of Inward K+ Channels and Stomatal Opening in Arabidopsis

Drought stress induces stomatal closure and inhibits stomatal opening simultaneously. However, the underlying molecular mechanism is still largely unknown. Here, we show that the slow-type (S-type) anion channels SLAC1 and SLAH3 mainly inhibit the inward-rectifying K+ channel KAT1 by protein-protein interaction and consequently prevent stomatal opening in Arabidopsis thaliana. Voltage-clamp results demonstrated that SLAC1 inhibited KAT1 dramatically but did not inhibit KAT2. SLAH3 inhibited KAT1 to a lesser extent than did SLAC1. Both the N and C termini of SLAC1 inhibited KAT1, but the inhibition by the N terminus was stronger. The C terminus was essential for SLAC1-mediated inhibition of KAT1. Furthermore, drought stress strongly upregulated the expression of SLAC1 and SLAH3 in Arabidopsis guard cells, and the overexpression of wildtype and truncated SLAC1 dramatically impaired inward K+ (K-in(+)) currents of guard cells and light-induced stomatal opening. Additionally, the inhibition of KAT1 by SLAC1 and KC1 only partially overlapped, suggesting that SLAC1 and KC1 inhibited K+ in channels via different molecular mechanisms. Taken together, these findings reveal a novel regulatory mechanism for stomatal movement, in which signaling pathways for stomatal closure and opening are directly coupled by protein-protein interaction between SLAC1/SLAH3 and KAT1 in Arabidopsis.