The effects of different ambient pH on the pathogenicity of Fusarium sulphureum and reactive oxygen species metabolism in F. sulphureum inoculation muskmelon fruits

Fusarium sulphureum is an important plant pathogenic fungus, which can lead to fusarium rot of muskmelon. During fungal infection host plant, environmental pH plays crucial role by secreting acid or base to regulates the arsenals of enzymes which contribute to fungal pathogenicity. In this study, in order to define the acid-alkali property of F. sulphureum and determinate the effect of ambient pH values on reactive oxygen species (ROS) metabolism and pathogenicity during F. sulphureum infection muskmelon fruits. F. sulphureum spore suspension was modified with different pH values buffer solution and then was inoculated muskmelon fruits. The change of pH value in lesion tissue, the pathogenicity, as well as ROS metabolism in inoculated muskmelon fruits were investigated and analyzed. The results indicated that the pH value of lesion tissue increased from 5.92 on 0 day to 9.24 on 7 day after inoculation, the lesion diameter in fruit inoculated with spore suspension at pH 6 was significantly higher than those at pH 3, 5, 7 and 9. Meanwhile, the inoculated fruits with pH 6 spore suspension showed higher NOX activity, O2−. production and H2O2 content, cell membrane permeability, as well as Malondialdehyde （MDA）content. However, the activities of antioxidant enzyme (such as superoxide dismutase (SOD), peroxidase (POD), catalase (CAT)) and the activities of key enzymes and the contents of non-enzymatic antioxidants involved in ascorbate-glutathione cycle (AsA-GSH) were remarkedly increased in fruits inoculated with spore suspension at pH 3, comparing with pH 5, 6, 7, 9. Based on the present results, F. sulphureum inoculation led to pH increasing of the tissue around the inoculation site, and weak alkaline environment increased the pathogenicity of F. sulphureum infection muskmelon by stimulating ROS metabolism.