A role for calcium‐dependent protein kinases in differential CO2‐ and ABA‐controlled stomatal closing and low CO2‐induced stomatal opening in Arabidopsis

Summary Low concentrations of CO 2 cause stomatal opening, whereas [CO 2 ] elevation leads to stomatal closure. Classical studies have suggested a role for Ca 2+ and protein phosphorylation in CO 2 ‐induced stomatal closing. Calcium‐dependent protein kinases (CPKs) and calcineurin‐B‐like proteins (CBLs) can sense and translate cytosolic elevation of the second messenger Ca 2+ into specific phosphorylation events. However, Ca 2+ ‐binding proteins that function in the stomatal CO 2 response remain unknown. Time‐resolved stomatal conductance measurements using intact plants, and guard cell patch‐clamp experiments were performed. We isolated cpk quintuple mutants and analyzed stomatal movements in response to CO 2 , light and abscisic acid (ABA). Interestingly, we found that cpk3/5/6/11/23 quintuple mutant plants, but not other analyzed cpk quadruple/quintuple mutants, were defective in high CO 2 ‐induced stomatal closure and, unexpectedly, also in low CO 2 ‐induced stomatal opening. Furthermore, K + ‐uptake‐channel activities were reduced in cpk3/5/6/11/23 quintuple mutants, in correlation with the stomatal opening phenotype. However, light‐mediated stomatal opening remained unaffected, and ABA responses showed slowing in some experiments. By contrast, CO 2 ‐regulated stomatal movement kinetics were not clearly affected in plasma membrane‐targeted cbl1/4/5/8/9 quintuple mutant plants. Our findings describe combinatorial cpk mutants that function in CO 2 control of stomatal movements and support the results of classical studies showing a role for Ca 2+ in this response.