Structure and activity of SLAC1 channels for stomatal signaling in leaves

Abstract

Stomata in leaves regulate gas exchange between the plant and its atmosphere. Various environmental stimuli elicit abscisic acid (ABA); ABA leads to phosphoactivation of slow anion channel 1 (SLAC1); SLAC1 activity reduces turgor pressure in aperture-defining guard cells; and stomatal closure ensues. We used electrophysiology for functional characterizations of <i>Arabidopsis thaliana</i> SLAC1 (<i>At</i>SLAC1) and cryoelectron microscopy (cryo-EM) for structural analysis of <i>Brachypodium distachyon</i> SLAC1 (<i>Bd</i>SLAC1), at 2.97-Å resolution. We identified 14 phosphorylation sites in <i>At</i>SLAC1 and showed nearly 330-fold channel-activity enhancement with 4 to 6 of these phosphorylated. Seven SLAC1-conserved arginines are poised in <i>Bd</i>SLAC1 for regulatory interaction with the N-terminal extension. This <i>Bd</i>SLAC1 structure has its pores closed, in a basal state, spring loaded by phenylalanyl residues in high-energy conformations. SLAC1 phosphorylation fine-tunes an equilibrium between basal and activated SLAC1 trimers, thereby controlling the degree of stomatal opening.