Hypoxia induces H2O2 production and activates antioxidant defence system in grapevine buds through mediation of H2O2 and ethylene

Abstract

Paradoxically, in eukaryotic cells, hydrogen peroxide (H2O2) accumulates in response to oxygen deprivation (hypoxia). The source of H2O2 under hypoxia varies according to the species, organs, and tissue. In nonphotosynthetic tissues, H2O2 is mainly produced by activation of NAD(P)H-oxidases or by disruption of the mitochondrial electron transport chain (m-ETC). This study showed that hypoxia, and inhibitors of respiration like potassium cyanide (KCN) and sodium nitroprusside (SNP), trigger the production of H2O2 in grapevine buds. However, diphenyleneiodonium, an inhibitor of NAD(P)H-oxidase, did not reduce the H2O2 levels induced by KCN, suggesting that, under respiratory stress, H2O2 is mainly produced by disruption of the m-ETC. On the other hand, g-aminobutyric acid (GABA), a metabolite that in plants alleviates oxidative stress by activating antioxidant enzymes, reduced significantly the levels of H2O2 induced by KCN and, surprisingly, repressed the expression of genes encoding antioxidant enzymes such as ASCORBATE PEROXIDASE (VvAPX), GLUTATHIONE PEROXIDASE (VvGLPX), SUPEROXIDE DISMUTASE (VvSOD), and one of the CATALASE isoforms (VvCAT1), while VvCAT2 was upregulated. In contrast to GABA, hypoxia, H2O2, and ethylene increased dramatically the expression of genes encoding antioxidant enzymes and enzymes of the alternative respiratory pathway such as ALTERNATIVE NADH-DEHYDROGENASES (VvaNDs) and ALTERNATIVE OXIDASES (VvAOXs). Hence, it is concluded that H2O2 production is stimulated by respiratory stress in grapevine buds, that H2O2 and ethylene act as signalling molecules and activate genes related to the antioxidant defence system, and finally that GABA reduces H2O2 levels by up-regulating the expression of VvCAT2.