MAPK kinase and CDP kinase modulate hydrogen peroxide levels during dark-induced stomatal closure in guard cells of Vicia faba

We used 2'-amino-3'-methoxyflavone (PD98059) (an inhibitor of mitogen-activated protein kinase kinase, MEK) and Trifluoperazine (TFP) (a specific inhibitor of calcium-dependent protein kinase, CDPK) to investigate the role of MEK/CDPK and its effects on H2O2 levels of guard cells in the dark-induced stomatal closure in Vicia faba. We provide evidence that both PD98059 and TFP reduced H2O2 levels in guard cells and promoted stomatal opening significantly in the dark, implying that MEK/CDPK mediated dark-induced stomatal closure by influencing H2O2 levels of guard cells. In addition, like ascorbic acid (ASA), an important reducing substrate for H2O2 removal, but unlike diphenylene iodonium (DPI), an inhibitor of the H2O2-generating enzyme NADPH oxidase, PD98059 and TFP not only reduced exogenous H2O2 levels in guard cells in light, but also eliminated the H2O2 that had been generated during a dark period and promoted stomatal opening. The results suggest MEK and CDPK are probably involved in restraining the H2O2 scavenging enzyme and elevating H2O2 levels in guard cells during dark-induced stomatal closure. Of course, the probability of MEK and CDPK acting as the target downstream of H2O2 in the signaling transduction chain is not excluded.