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Magnesium (Mg) deficiency in plants is a widespread problem, affecting productivity and quality in agriculture. The mechanism of Mg deficiency inducing antioxidant enzyme activities has not been elucidated in rice. We examined the relationship among abscisic acid (ABA), H 2 O 2 , and antioxidant enzymes in the leaves of rice seedlings grown under conditions of Mg deficiency. The expression of OsRab16A, an ABA responsive gene, was used to determine the content of ABA. Mg deficiency resulted in increased ABA content in leaves of rice seedlings. The production of H 2 O 2 was examined by 3,3-diaminobenzidine staining and a colorimetric method. Mg deficiency also induced H 2 O 2 production in leaves, which was blocked by dipehnyleneiodonium chloride (DPI), an NADPH oxidase inhibitor. Tungstate (Tu), an ABA biosynthesis inhibitor, was effective in reducing Mg deficiencyincreased ABA content, as well as Mg deficiency-induced H 2 O 2 production. Both Tu and DPI were effective in reducing Mg deficiency-induced activities of superoxide dismutase, ascorbate peroxidase, glutathione reductase, and catalase in the leaves. Mg deficiency-induced ABA accumulation may trigger increased production of H 2 O 2 , which may involve plasma-membrane NADPH oxidase, and, in turn, up-regulates the activities of antioxidant enzymes in leaves of rice seedlings.

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