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Abstract

Flavonols are widely distributed plant metabolites that inhibit microbial growth. Yet many pathogens cause disease in flavonolcontaining
plant tissues. We investigated how Sclerotinia sclerotiorum, a necrotrophic fungal pathogen that causes disease in a
range of economically important crop species, is able to successfully infect flavonol-rich tissues of Arabidopsis (Arabidopsis
thaliana). Infection of rosette stage Arabidopsis with a virulent S. sclerotiorum strain led to the selective hydrolysis of flavonol
glycosidic linkages and the inducible degradation of flavonol aglycones to phloroglucinol carboxylic and phenolic acids. By
chemical analysis of fungal biotransformation products and a search of the S. sclerotiorum genome sequence, we identified a
quercetin dioxygenase gene (QDO) and characterized the encoded protein, which catalyzed cleavage of the flavonol carbon
skeleton. QDO deletion lines degraded flavonols with much lower efficiency and were less pathogenic on Arabidopsis leaves
than wild-type S. sclerotiorum, indicating the importance of flavonol degradation in fungal virulence. In the absence of QDO,
flavonols exhibited toxicity toward S. sclerotiorum, demonstrating the potential roles of these phenolic compounds in protecting plants against pathogens.