Pathogenic Roles of Polyketide Synthase CLPKS18 and (R)-(-)-Mellein from Curvularia lunata in Maize Leaf Spot

<i>Curvularia lunata</i> (Wakkre) Boedijn is an important pathogenic fungus that causes maize leaf spot, a prevalent disease that caused significant yield losses in maize-growing areas in China in the 1990s. <i>Clpks18</i>, a polyketide synthase (CLPKS18) gene, has been identified as a crucial virulence-related gene in <i>C. lunata</i>. However, the impact of <i>Clpks18</i> and its biosynthesized virulence factor (<i>R</i>)-(-)-mellein on the expression of maize genes related to the defense signal pathway has never been determined. In this study, it was found that <i>Clpks18</i> and (<i>R</i>)-(-)-mellein significantly interfere with the signaling pathways of JA and IAA in maize leaves but in different ways and in a time-dependent manner. While CLPKS18 inhibited the maize's JA and IAA signaling pathways through its related secondary metabolite, (<i>R</i>)-(-)-mellein inhibited the JA signaling pathway but stimulated IAA accumulation in maize leaves. In summary, understanding this novel virulence effector's mechanism of interference with maize resistance enriches the pathology of <i>Curvularia</i> leaf spot in maize on the one hand and provides a foundation for screening the resistance germplasm and chemical fungicides against the disease on the other.