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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, MPK12, have been shown to partially impair CO 2 -induced stomatal closure. Here, we show that mpk12 plants, in which MPK4 is stably silenced specifically in guard cells (mpk12 mpk4GC 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 mpk4GC double-mutants. These findings suggest that MPK12 and MPK4 act very early in CO 2 signaling, upstream of, or parallel to the convergence of CO 2 and ABA signal transduction. The activities of MPK4 and MPK12 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 MPK4 and MPK12 have distinguishable roles in Arabidopsis and that the previously suggested role of RHC1 in stomatal CO 2 signaling is minor, whereas MPK4 and MPK12 act as key components of early stomatal CO 2 signal transduction., (© 2018 The Authors The Plant Journal © 2018 John Wiley & Sons Ltd.)

Summary: Respiration in leaves and the continued elevation in the atmospheric CO2 concentration cause CO2‐mediated reduction in stomatal pore apertures. Several mutants have been isolated for which stomatal responses to both abscisic acid (ABA) and CO2 are simultaneously defective. However, there are only few mutations that impair the stomatal response to elevated CO2, but not to ABA. Such mutants are invaluable in unraveling the molecular mechanisms of early CO2 signal transduction in guard cells. Recently, mutations in the mitogen‐activated protein (MAP) kinase, MPK12, have been shown to partially impair CO2‐induced stomatal closure. Here, we show that mpk12 plants, in which MPK4 is stably silenced specifically in guard cells (mpk12 mpk4GC homozygous double‐mutants), completely lack CO2‐induced stomatal responses and have impaired activation of guard cell S‐type anion channels in response to elevated CO2/bicarbonate. However, ABA‐induced stomatal closure, S‐type anion channel activation and ABA‐induced marker gene expression remain intact in the mpk12 mpk4GC double‐mutants. These findings suggest that MPK12 and MPK4 act very early in CO2 signaling, upstream of, or parallel to the convergence of CO2 and ABA signal transduction. The activities of MPK4 and MPK12 protein kinases were not directly modulated by CO2/bicarbonate in vitro, suggesting that they are not direct CO2/bicarbonate sensors. Further data indicate that MPK4 and MPK12 have distinguishable roles in Arabidopsis and that the previously suggested role of RHC1 in stomatal CO2 signaling is minor, whereas MPK4 and MPK12 act as key components of early stomatal CO2 signal transduction. Significance statement: We show that MPK4 and MPK12 are required for CO2‐triggered stomatal responses but not for ABA‐induced stomatal closure. These results suggest that these two MAP kinases function upstream of the convergence of CO2 and ABA signaling. However, activities of MPK4 and MPK12 were not modulated by CO2/bicarbonate and, thus, direct HCO3−/CO2 sensing mechanisms remain to be identified. [ABSTRACT FROM AUTHOR]