Central functions of bicarbonate in S-type anion channel activation and OST1 protein kinase in CO2 signal transduction in guard cell

Plants respond to elevated CO2 via carbonic anhydrases that mediate stomatal closing, but little is known about the early signalling mechanisms following the initial CO2 response. It remains unclear whether CO2, HCO3− or a combination activates downstream signalling. Here, we demonstrate that bicarbonate functions as a small-molecule activator of SLAC1 anion channels in guard cells. Elevated intracellular [HCO3−]i with low [CO2] and [H+] activated S-type anion currents, whereas low [HCO3−]i at high [CO2] and [H+] did not. Bicarbonate enhanced the intracellular Ca2+ sensitivity of S-type anion channel activation in wild-type and ht1-2 kinase mutant guard cells. ht1-2 mutant guard cells exhibited enhanced bicarbonate sensitivity of S-type anion channel activation. The OST1 protein kinase has been reported not to affect CO2 signalling. Unexpectedly, OST1 loss-of-function alleles showed strongly impaired CO2-induced stomatal closing and HCO3− activation of anion channels. Moreover, PYR/RCAR abscisic acid (ABA) receptor mutants slowed but did not abolish CO2/HCO3− signalling, redefining the convergence point of CO2 and ABA signalling. A new working model of the sequence of CO2 signalling events in gas exchange regulation is presented.