Indolic metabolites are required for antifungal defense of the Arabidopsis mlo2 mutant

Arabidopsis thaliana genes MLO2 (Mildew resistance locus-O 2), MLO6 and MLO12 exhibit unequal genetic redundancy with respect to the modulation of defense responses against powdery mildew fungi and the control of developmental phenotypes such as premature leaf decay. We show that early chlorosis and necrosis of rosette leaves in mlo2 mlo6 mlo12 mutants reflects an authentic but untimely leaf senescence program. Comparative transcriptional profiling revealed that transcripts of several genes encoding tryptophan/indole biosynthetic enzymes hyper-accumulate during vegetative development in the mlo2 mlo6 mlo12 mutant. Elevated expression levels of these genes correlate with altered steady-state levels of several indolic metabolites, including the phytoalexin camalexin and indolic glucosinolates, during development in the mlo2 single and the mlo2 mlo6 mlo12 triple mutant. Results of genetic epistasis analysis suggest a decisive role for indolic metabolites in mlo2-conditioned antifungal defense against both biotrophic powdery mildews and a camalexin-sensitive strain of the necrotrophic fungus, Botrytis cinerea. The wound- and pathogen-responsive callose synthase Powdery mildew resistance 4/Glucan-synthase-like 5 (PMR4/GSL5) was found to be responsible for the spontaneous callose deposits in mlo2 mutant plants but dispensable for mlo2-conditioned penetration resistance. Our data strengthen the notion that powdery mildew resistance of mlo2 genotypes is based on the same defense execution machinery as innate antifungal immune responses that restrict invasion of non-adapted fungal pathogens.

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Consonni C, Bednarek P, Humphry M, Francocci F, Ferrari S, Harzen A, Ver Loren van Themaat E, Panstruga R