A guard cell carbonic anhydrase binds and regulates SLAC1 separate from its catalytic activity

Abstract Stomata of plant leaves open to enable CO 2 entry for photosynthesis and close when CO 2 in the leaf is elevated. CO 2 is thought to promote stomatal closure in part by activating the SLAC1 anion channel at the guard cell plasma membrane. Carbonic anhydrases (CAs) contribute to this activation, but their contribution as distinct from CO 2 -H 2 CO 3 catalysis remains controversial. Here we show that the β-carbonic anhydrase CA4 binds selectively with the guard-cell anion channel SLAC1 to enhance channel current. The interaction is CO 2 -dependent, but binding is mediated by amino acids distal from the CO 2 -binding site of CA4 and is separable from carbonic-anhydrase activity. CA4 mutants impaired in channel binding eliminate the CO 2 -sensitivity of SLAC1 in vivo and slow stomatal kinetics with a commensurate loss in water use efficiency. The findings demonstrate that CA4 contributes directly to the CO 2 -response mechanics regulating SLAC1 at near-ambient CO 2 in guard cells and to stomatal kinetics in the plant.