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

Stomata of plant leaves open to enable CO2 entry for photosynthesis and close when CO2 in the leaf is elevated. CO2 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 CO2-H2CO3 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 CO2-dependent, but binding is mediated by amino acids distal from the CO2-binding site of CA4 and is separable from carbonic-anhydrase activity. CA4 mutants impaired in channel binding eliminate the CO2-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 CO2-response mechanics regulating SLAC1 at near-ambient CO2 in guard cells and to stomatal kinetics in the plant.