Downy mildew caused by the oomycete pathogen Pseudoperonospora cubensis is a devastating foliar disease of cucurbits worldwide. We previously demonstrated that the wild melon line PI 124111F (PI) is highly resistant to all pathotypes of P. cubensis . That resistance was controlled genetically by two partially dominant, complementary loci. Here, we show that unlike other plant disease resistance genes, which confer an ability to resist infection by pathogens expressing corresponding avirulence genes, the resistance of PI to P. cubensis is controlled by enhanced expression of the enzymatic resistance ( eR ) genes At1 and At2 . These constitutively expressed genes encode the photorespiratory peroxisomal enzyme proteins glyoxylate aminotransferases. The low expression of At1 and At2 in susceptible melon lines is regulated mainly at the transcriptional level. This regulation is independent of infection with the pathogen. Transgenic melon plants overexpressing either of these eR genes displayed enhanced activity of glyoxylate aminotransferases and remarkable resistance against P. cubensis . The cloned eR genes provide a new resource for developing downy mildew-resistant melon varieties.

Downy mildew caused by the oomycete Pseudoperonospora cubensis, is a devastating foliar disease of cucurbits. The wild melon PI 124111F (PI) is highly resistant to this disease while BU21/3 and Hemed are susceptible. In a previous study we showed that resistance in PI is metabolic, resulting from enhanced activity of glyoxylate aminotransferase encoded by the genes At1 and At2. When either gene from PI was transformed into a susceptible plant it became resistant. Here we show that the nearly silent At1 and At2 in the susceptible Hemed confer resistance to downy mildew when overexpressed (by CaMV S35 promoter) in the susceptible BU21/3. The cisgenic plants, overexpressing either At1 or At2, exhibited enhanced activity of glyoxylate aminotransferase and resistance against P. cubensis. Northern and western gel blot analyses suggested that the low expression of At1 and At2 in the susceptible melons is modulated by transcriptional inhibition.