Inhibition of abscisic acid-induced stomatal closure by ethylene is related to the change of hydrogen peroxide levels in guard cells in broad bean

We analysed the role and relationship between hydrogen peroxide (H2O2) reduction and the inhibition of abscisic acid (ABA)-induced stomatal closure by ethylene. Like ascorbic acid (ASA), the most important reducing substrate for H2O2 removal, catalase, one of the H2O2 scavenging enzymes and diphenylene iodonium, an inhibitor of the H2O2-generating enzyme NADPH oxidase, both ethylene-releasing compound 2-chloroethylene phosphonic acid (ethephon, ETH) and 1-aminocyclopropane-1-carboxylic acid (ACC), the immediate precursor of ethylene, were found to inhibit stomatal closure by ABA and to reduce H2O2 levels by ABA in guard cells, indicating that ethylene-caused inhibition of ABA-induced stomatal closure involves reduction of H2O2 levels in guard cells. Additionally, similar to ASA and catalase, ACC/ETH not only suppressed H2O2-induced stomatal closure and H2O2 levels in guard cells treated with exogenous H2O2 in light, but also reopened the stomata which had been closed by ABA and reduced H2O2 levels that had been generated by ABA. The above mentioned effects of ACC and ETH were dissimilar to that of diphenylene iodonium, an inhibitor of the H2O2-generating enzyme NADPH oxidase, which not only had incapability to reduce H2O2 levels by exogenous H2O2 but also could not abolish H2O2 that had been generated by ABA. So we suggest that ethylene probably induces H2O2 removal and reduces H2O2 levels in Vicia faba guard cells, and finally inhibits stomatal closure induced by ABA.