A Reductase from the Phytopathogenic Fungus Botrytis cinerea Degrades Plant Antimicrobial Coumarins to Promote Infection.

Phytopathogenic fungi must contend with host immune responses to successfully colonize plants. These responses include the production of heterogenous antimicrobial compounds (phytoalexins), which plants deploy to intoxicate infecting microbes. Among these host defense compounds are the coumarins, a family of aromatic lactones widely present in dicotyledonous plants (dicots). The fungal necrotroph Botrytis cinerea (gray mold) infects a plethora of dicots, causing large losses in the produce, viticulture, and ornamental flower industries. B. cinerea detoxifies numerous host defense compounds to support its virulence, including the coumarin scopoletin. However, B. cinerea coumarin detoxification factors have remained uncharacterized. Here, we employed transcriptomics in scopoletin-treated B. cinerea followed by heterologous expression of scopoletin-induced genes in budding yeast (Saccharomyces cerevisiae) to identify a reductase, BCIN14g01070 (Bcsaf1), which B. cinerea deploys to degrade scopoletin. Using liquid chromatography-tandem mass spectrometry, we demonstrate that Bcsaf1 reduces a carbon-carbon double bond within the coumarin backbone, converting coumarins to their nontoxic melilotic acid derivatives. We further show that Bcsaf1 orthologs are present in numerous fungi of the family Sclerotiniaceae and that an ortholog from another notorious phytopathogen, Sclerotinia sclerotiorum (white mold), possesses similar coumarin detoxification capacity. We constructed ΔBcsaf1 mutants that displayed compromised scopoletin degradation and increased scopoletin susceptibility. The ΔBcsaf1 mutants further displayed reduced virulence in Nicotiana tabacum (tobacco), which exerts a scopoletin defense response, whereas their virulence was only modestly compromised in Helianthus annuus (sunflower) seedlings, which produce much less scopoletin. Thus, Bcsaf1 is an important virulence factor that counters host coumarins to support B. cinerea infection. [Formula: see text] Copyright © 2026 The Author(s). This is an open access article distributed under the CC BY 4.0 International license.