A molecular pathway for CO2 response in Arabidopsis guard cells

Increasing carbon dioxide (CO2) levels in the atmosphere have caused global metabolic changes in diverse plant species. CO2 is not only a carbon donor for photosynthesis but also an environmental signal that regulates stomatal movements and thereby controls plant–water relationships and carbon metabolism. However, the mechanism underlying CO2 sensing in stomatal guard cells remains unclear. Here we report characterization of Arabidopsis RESISTANT TO HIGH CO2 (RHC1), a MATE-type transporter that links elevated CO2 concentration to repression of HT1, a protein kinase that negatively regulates CO2-induced stomatal closing. We also show that HT1 phosphorylates and inactivates OST1, a kinase which is essential for the activation of the SLAC1 anion channel and stomatal closing. Combining genetic, biochemical and electrophysiological evidence, we reconstituted the molecular relay from CO2 to SLAC1 activation, thus establishing a core pathway for CO2 signalling in plant guard cells.