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Recessively inherited mutant alleles of Mlo genes (mlo) confer broad-spectrum penetration resistance to powdery mildew pathogens in angiosperm plants. Although a few components are known to be required for mlo resistance, the detailed molecular mechanism underlying this type of immunity remains elusive. In this study, we identified alloxan (5,5-dihydroxyl pyrimidine-2,4,6-trione) and some of its structural analogs as chemical suppressors of mlo-mediated resistance in monocotyledonous barley (Hordeum vulgare) and dicotyledonous Arabidopsis thaliana. Apart from mlo resistance, alloxan impairs nonhost resistance in Arabidopsis. Histological analysis revealed that the chemical reduces callose deposition and hydrogen peroxide accumulation at attempted fungal penetration sites. Fluorescence microscopy revealed that alloxan interferes with the motility of cellular organelles (peroxisomes, endosomes …