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

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 (HO) 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 HO in guard cells and induced stomatal opening in darkness. Additionally, cytokinins not only reduced exogenous HO levels in guard cells caused by exposure to light, but also abolished HO that had been generated during a dark period, and promoted stomatal opening, as did ascorbic acid (ASA, an important reducing substrate for HO removal). However, unlike cytokinins, auxins did not reduce exogenous HO, did not abolish HO 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 HO-generating enzyme NADPH oxidase). Taken together our results indicate that cytokinins probably reduce the levels of HO in guard cells by scavenging, whereas auxins limit HO levels through restraining HO generation, inducing stomatal opening in darkness.