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Calmodulin (CaM), the predominant Ca(2+) receptors, is one of the best-characterized Ca(2+) sensors in all eukaryotes. In this study the role of CaM and the possible interrelationship between CaM and hydrogen peroxide (H(2)O(2)) in abscisic acid (ABA) induced antioxidant defense were investigated in the seedling of Panax ginseng. Treatment of ABA (100 μM) and H(2)O(2) (10 mM) increased the expression of Panax ginseng calmodulin gene (PgCaM) and significantly enhanced the expression of the antioxidant marker genes such as superoxide dismutase, ascorbate peroxidase, glutathione reductase and the activities of chloroplastic and cytosolic antioxidant enzymes. Pretreatments with two CaM antagonists, trifluoperazine (TFP), N-(6-aminohexyl)-5-chloro-1-naphthalene sulfonamide hydrochloride (W7) and inhibitor or scavenger, diphenyleneiodonium chloride, and dimethylthiourea of reactive oxygen species almost completely suppressed the up-regulation of antioxidant and PgCaM gene. Moreover, H(2)O(2) production and CaM content was almost completely inhibited by pretreatments with two CaM antagonists. In addition, the expressions of PgCaM gene under different biotic stress were analyzed at different time intervals. Thus it may suggests that CaM are involved in ABA-induced increased expression of PgCaM which triggers H(2)O(2) production through activating trans-plasma membrane NADPH oxidase, resulting in up-regulation of defense related antioxidant gene and also plays a pivotal role in defense response against pathogens.