Mechanism of Non-Host Flavonoid Nobiletin in Inhibiting Phytopathogen <i>Sclerotinia sclerotiorum</i>

Sclerotinia sclerotiorum is a devastating phytopathogenic fungus with a broad host range, which is partly attributed to its ability to detoxify a wide array of host-derived secondary metabolites. Among these, flavonoids are one of the most widespread classes of plant secondary metabolites and are ubiquitously present in both host and non-host plants of S. sclerotiorum. Understanding the mechanisms by which non-host flavonoids inhibit this pathogen is therefore crucial, not only for elucidating the biochemical basis of non-host resistance but also for discovering novel plant-derived fungicides. In this study, we compared the differential inhibitory effects of various flavonoid types—including isoflavones, flavonols, flavanones, and citrus-derived polymethoxyflavones (PMFs) —on S. sclerotiorum. Our results revealed differential sensitivities of the fungus to these compounds, corresponding with its varying metabolic capabilities. Furthermore, transcriptomic analysis demonstrated that the non-host flavonoid nobiletin and the host-derived, degradable flavonol quercetin elicit fundamentally different gene expression responses in the fungus. Further physiological and biochemical analyses showed that nobiletin induced ROS accumulation in S. sclerotiorum at later stages of treatment, but had no significant effects on plasma membrane integrity or carbohydrate metabolism. The partial rescue of growth inhibition by ascorbic acid suggests that ROS accumulation contributes to the antifungal activity of nobiletin. These findings provide new insights into the antifungal mechanisms of PMFs and support their potential application in fungicide development.