Cytokinin- and auxin-induced stomatal opening involves a decrease in levels of hydrogen peroxide in guard cells of...

Previous studies have shown that cytokinins and auxins can induce the opening of stomata. However, the mechanism of stomatal opening caused by cytokinins and auxins remains unclear. The purpose of this paper is to investigate the relationship between hydrogen peroxide (H₂O₂) levels in guard cells and stomatal opening induced by cytokinins and auxins in Vicia faba. By means of stomatal bioassay and laser-scanning confocal microscopy, we provide evidence that cytokinins and auxins reduced the levels of H₂O₂ in guard cells and induced stomatal opening in darkness. Additionally, cytokinins not only reduced exogenous H₂O₂ levels in guard cells caused by exposure to light, but also abolished H₂O₂ that had been generated during a dark period, and promoted stomatal opening, as did ascorbic acid (ASA, an important reducing substrate for H₂O₂ removal). However, unlike cytokinins, auxins did not reduce exogenous H₂O₂, did not abolish H₂O₂ that had been generated in the dark, and therefore did not promote reopening of stoma induced to close in the dark. The above-mentioned effects of auxins were similar to that of diphenylene iodonium (DPI, an inhibitor of the H₂O₂-generating enzyme NADPH oxidase). Taken together our results indicate that cytokinins probably reduce the levels of H₂O₂ in guard cells by scavenging, whereas auxins limit H₂O₂ levels through restraining H₂O₂ generation, inducing stomatal opening in darkness.