2018 ASHS Annual Conference

Genetic resistance is the most preferred management strategy for powdery mildew (Podosphaera xanthii) and downy mildew (Pseudoperonospora cubensis) in cucurbits. However, traditional breeding for resistance to the two diseases is resource intensive, often requiring decades’ long phenotyping and selection processes. As an alternative, durable and broad-spectrum resistance to powdery and downy mildew can be obtained through loss-of-function of susceptibility genes in elite breeding material. Susceptibility genes for powdery mildew [Mildew-Locus-O (MLO) and Powdery Mildew Resistance (PMR)] and downy mildew [Downy Mildew Resistance (DMR)] have been functionally proven in model plant species. Previous studies have reported candidate MLO genes for Citrullus lanatus and Cucurbita. pepo, but none for C. maxima and C. moschata. However, no PMR or DMR candidate genes have been identified for C. lanatus or any of the Cucurbita species. The current study used bioinformatics approaches based on sequence similarity, phylogenetic relationships and presence of conserved domains to predict candidate MLO genes in C. maxima and C. moschata and PMR and DMR genes in C. lanatus, C. pepo, C. maxima and C. moschata. Four MLO homologs in C. maxima and five in C. moschata clustered within Clade V, a clade containing all MLO susceptibility genes in dicots, and had highly conserved transmembrane domains and C-terminal PM interaction motif. Sixty-three candidate PMR genes were identified among the four species, 16 of which had close similarity to functionally proven PMR homologs in model species. Similarly, 37 candidate DMR genes were identified 12 among which clustered with functionally proven DMR homologs in model species. Functional analysis of the genes identified in the current study will reveal their role in pathogenesis and assess their potential for manipulation through gene editing methods to generate novel resistant plant genotypes.