UC San Diego

Increases in CO₂ concentration in plant leaves due to respiration in the dark and the continuing atmospheric [CO₂] rise cause closing of stomatal pores, thus affecting plant-water relations globally. However, the underlying CO₂/bicarbonate (CO₂/HCO₃^-) sensing mechanisms remain unknown. [CO₂] elevation in leaves triggers stomatal closure by anion efflux mediated via the SLAC1 anion channel localized in the plasma membrane of guard cells. Previous reconstitution analysis has suggested that intracellular bicarbonate ions might directly up-regulate SLAC1 channel activity. However, whether such a CO₂/HCO₃^- regulation of SLAC1 is relevant for CO₂ control of stomatal movements in planta remains unknown. Here, we computationally probe for candidate bicarbonate-interacting sites within the SLAC1 anion channel via long-timescale Gaussian accelerated molecular dynamics (GaMD) simulations. Mutations of two putative bicarbonate-interacting residues, R256 and R321, impaired the enhancement of the SLAC1 anion channel activity by CO₂/HCO₃^- in Xenopus oocytes. Mutations of the neighboring charged amino acid K255 and residue R432 and the predicted gate residue F450 did not affect HCO₃^- regulation of SLAC1. Notably, gas-exchange experiments with slac1-transformed plants expressing mutated SLAC1 proteins revealed that the SLAC1 residue R256 is required for CO₂ regulation of stomatal movements in planta, but not for abscisic acid (ABA)-induced stomatal closing. Patch clamp analyses of guard cells show that activation of S-type anion channels by CO₂/HCO₃^-, but not by ABA, was impaired, indicating the relevance of R256 for CO₂ signal transduction. Together, these analyses suggest that the SLAC1 anion channel is one of the physiologically relevant CO₂/HCO₃^- sensors in guard cells.