Thymol Induces Cell Death of Fusarium oxysporum f. sp. niveum via Triggering Superoxide Radical Accumulation and Oxidative Injury In Vitro

Fusarium oxysporum f. sp. niveum (FON) causes watermelon wilt that is one of the major disease-causing yield losses of watermelon. Sustainable development of agriculture requires controlling watermelon wilt disease with good environmental performance. One important approach is to identify environmental-friendly compounds with inhibitory activity against FON. Thymol is a plant-derived compound that is safe for ecology. Little is known about the application of thymol in agriculture. In this study, we studied the inhibitory activity of thymol against FON by using morphological, physiological, and histochemical approaches. Thymol significantly inhibited colony diameter of FON in a dose-dependent manner, with EC50 at 21 mu g/mL. Thymol at 10, 21, and 35 mu g/mL decreased the fresh weight of FON mycelia by 29.0%, 50.6%, and 69.5%, respectively. Microscopic observation revealed irregular damage and loss of shape of mycelia upon thymol exposure. Thymol induced the accumulation of superoxide radical in mycelial cells and accompanied increased activity of antioxidative enzymes (SOD, superoxide dismutase; CAT, catalase). Thymol induced membrane permeability was indicated by lipid peroxidation and electrolyte leakage (increased by 29-58%) in mycelial cells. These results suggested that thymol induced oxidative damage in mycelia, which may be one of the possible reasons for thymol-induced mycelial cell death observed with fluorescent detection. Thymol decreased the production of conidia and inhibited the germination of conidia. Thymol induced superoxide radical accumulation, lipid peroxidation, and cell death in conidia as well. All of these results revealed the inhibitory activity of thymol against FON, which may have resulted from the superoxide radical-induced oxidative injury in both conidia and mycelia of FON.