PYR/RCAR Receptors Contribute to Ozone-, Reduced Air Humidity-, Darkness-, and CO2-Induced Stomatal Regulation

Rapid stomatalclosure induced by changes in the environment, such as elevation of CO 2 , reduction of air humidity, darkness, and pulses of the air pollutant ozone (O 3 ), involves the SLOW ANION CHANNEL1 (SLAC1). SLAC1 is activated by OPEN STOMATA1 (OST1) and Ca 2+ -dependent protein kinases. OST1 activation is controlled through abscisic acid (ABA)-induced inhibition of type 2 protein phosphatases (PP2C) by PYRABACTINRESISTANCE/REGULATORY COMPONENTS OF ABA RECEPTOR (PYR/RCAR) receptor proteins. To address the role of signaling through PYR/RCARs for whole-plant steady-state stomatal conductanceand stomatalclosure induced by environmental factors, we used a set of Arabidopsis ( Arabidopsis thaliana ) mutants defective in ABA metabolism/signaling. The stomatal conductancevalues varied severalfold among the studied mutants, indicating that basal ABA signaling through PYR/RCAR receptors plays a fundamental role in controlling whole-plant water loss through stomata. PYR/RCAR-dependent inhibition of PP2Cs was clearly required for rapid stomatalregulation in response to darkness, reduced air humidity, and O 3 . Furthermore, PYR/RCAR proteins seem to function in a dose-dependent manner, and there is a functional diversity among them. Although a rapid stomatalresponse to elevated CO 2 was evident in all but slac1 and ost1 mutants, the bicarbonate-induced activation of S-type anion channels was reduced in the dominant active PP2C mutants abi1-1 and abi2-1 . Further experiments with a wider range of CO 2 concentrations and analyses of stomatalresponse kinetics suggested that the ABA signalosome partially affects the CO 2 -induced stomatalresponse. Thus, we show that PYR/RCAR receptors play an important role for the whole-plant stomataladjustments and responses to low humidity, darkness, and O 3 and are involved in responses to elevated CO 2 .