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 H ₂O₂ 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 H ₂O₂ 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.