Role and interrelationship of MEK1-MPK6 cascade, hydrogen peroxide and nitric oxide in darkness-induced stomatal closure.

Pharmacological data have suggested the involvement of mitogen-activated protein kinase (MPK) cascades in dark-induced stomatal closure, but which specific MPK cascade participates in the darkness guard cell signaling and its relationship with hydrogen peroxide (H2O2) and nitric oxide (NO) remain unclear. In this paper, we observed that darkness induced activation of MPK6 in leaves of wild-type Arabidopsis (Arabidopsis thaliana) and mutants for nitrate reductase 1 (NIA1), but this effect was inhibited in mutants for MPK Kinase 1 (MEK1) and ATRBOHD/F. Mutants for MEK1, MPK6 and NIA1 showed defect of dark-induced NO production in guard cells and stomatal closure, but were normal in the dark-induced H2O2 generation, while stomata of mutant AtrbohD/F showed defect of dark-induced H2O2 and NO production and subsequent closure. Moreover, exogenous NO rescued the defect of dark-induced stomatal closure in mutants of AtrbohD/F, mek1 and mpk6, while exogenous H2O2 could not rescue the defect of dark-induced stomatal closure in mutants of mek1, mpk6 and nia1. These genetic and biochemical evidences not only show that MEK1-MPK6 cascade, AtRBOHD/F-dependent H2O2 and NIA1-dependent NO are all involved in dark-induced stomatal closure in Arabidopsis, also indicate that MEK1-MPK6 cascade functions via working downstream of H2O2 and upstream of NO.

Mitogen-Activated Protein Kinase Phosphatases Affect UV-B-Induced Stomatal Closure via Controlling NO in Guard Cells.

Ultraviolet B (UV-B) radiation induces the activation of MITOGEN-ACTIVATED PROTEIN KINASE PHOSPHATASE1 (MKP1) and its targets MPK3 and MPK6, but whether they participate in UV-B guard cell signaling is not clear. Here, evidence shows that UV-B-induced stomatal closure in Arabidopsis (Arabidopsis thaliana) is antagonistically regulated by MKP1 and MPK6 via modulating hydrogen peroxide (H2O2)-induced nitric oxide (NO) production in guard cells. The mkp1 mutant was hypersensitive to UV-B-induced stomatal closure and NO production in guard cells but not to UV-B-induced H2O2 production, suggesting that MKP1 negatively regulates UV-B-induced stomatal closure via inhibiting NO generation in guard cells. Moreover, MPK3 and MPK6 were activated by UV-B in leaves of the wild type and hyperactivated in the mkp1 mutant, but the UV-B-induced activation of MPK3 and MPK6 was largely inhibited in mutants for ATRBOHD and ATRBOHF but not in mutants for NIA1 and NIA2 mpk6 mutants showed defects of UV-B-induced NO production and stomatal closure but were normal in UV-B-induced H2O2 production, while stomata of mpk3 mutants responded to UV-B just like those of the wild type. The defect of UV-B-induced stomatal closure in mpk6 mutants was rescued by exogenous NO but not by exogenous H2O2 Furthermore, double mutant mkp1/mpk6 and the single mutant mpk6 showed the same responses to UV-B in terms of either stomatal movement or H2O2 and NO production. These data indicate that MPK6, but not MPK3, positively regulates UV-B-induced stomatal closure via acting downstream of H2O2 and upstream of NO, while MKP1 functions negatively in UV-B guard cell signaling via down-regulation of MPK6.