Abscisic acid is a key inducer of hydrogen peroxide production in leaves of maize plants exposed to water stress

The histochemical and cytochemical localization of water stress-induced H2O2 production in the leaves of ABA-deficient vp5 mutant and wild-type maize (Zea mays L.) plants were examined, using 3,3-diaminobenzidine and CeCl3 staining, respectively, and the roles of endogenous ABA in the production of H2O2 induced by water stress were assessed. Water stress induced by polyethylene glycol resulted in the accumulation of H2O2 in mesophyll cells, bundle-sheath cells and vascular bundles of wild-type maize leaves, and the accumulation was substantially blocked in the mutant maize leaves exposed to water stress. Pre-treatments with several apoplastic H2O2 manipulators abolished the majority of H2O2 accumulation induced by water stress in the wild-type leaves. The subcellular localization of H2O2 production was demonstrated in the cell walls, xylem vessels, chloroplasts, mitochondria and peroxisomes in the leaves of wild-type maize plants exposed to water stress, and the accumulation of H2O2 induced by water stress in the cell walls and xylem vessels, but not in the chloroplasts, mitochondria and peroxisomes, was arrested in the leaves of the ABA mutant or the ABA biosynthesis inhibitor (tungstate)-pre-treated maize plants. Pre-treatments with the apoplastic H2O2 manipulators also blocked the apoplastic but not the intracellular H2O2 accumulation induced by water stress in the leaves of wild-type plants. These data indicate that under water stress, the apoplast is the major source of H2O2 production and ABA is a key inducer of apoplastic H2O2 production. These data also suggest that H2O2 generated in the apoplast could not diffuse freely into subcellular compartments.