Mitogen‐activated protein kinases MPK4 and MPK12 are key components mediating CO₂‐induced stomatal movements

Summary Respiration in leaves and the continued elevation in the atmospheric CO 2 concentration cause CO 2 ‐mediated reduction in stomatal pore apertures. Several mutants have been isolated for which stomatal responses to both abscisic acid ( ABA ) and CO 2 are simultaneously defective. However, there are only few mutations that impair the stomatal response to elevated CO 2 , but not to ABA . Such mutants are invaluable in unraveling the molecular mechanisms of early CO 2 signal transduction in guard cells. Recently, mutations in the mitogen‐activated protein ( MAP ) kinase, MPK 12 , have been shown to partially impair CO 2 ‐induced stomatal closure. Here, we show that mpk12 plants, in which MPK 4 is stably silenced specifically in guard cells ( mpk12 mpk4 GC homozygous double‐mutants), completely lack CO 2 ‐induced stomatal responses and have impaired activation of guard cell S‐type anion channels in response to elevated CO 2 /bicarbonate. However, ABA ‐induced stomatal closure, S‐type anion channel activation and ABA ‐induced marker gene expression remain intact in the mpk12 mpk4 GC double‐mutants. These findings suggest that MPK 12 and MPK 4 act very early in CO 2 signaling, upstream of, or parallel to the convergence of CO 2 and ABA signal transduction. The activities of MPK 4 and MPK 12 protein kinases were not directly modulated by CO 2 /bicarbonate in vitro , suggesting that they are not direct CO 2 /bicarbonate sensors. Further data indicate that MPK 4 and MPK 12 have distinguishable roles in Arabidopsis and that the previously suggested role of RHC 1 in stomatal CO 2 signaling is minor, whereas MPK 4 and MPK 12 act as key components of early stomatal CO 2 signal transduction.