Down‐regulation of cladofulvin biosynthesis is required for biotrophic growth of Cladosporium fulvum on tomato

Fungal biotrophy is associated with a reduced capacity to produce potentially toxic secondary metabolites (SMs). Yet, the genome of the biotrophic plant pathogen Cladosporium fulvum contains many SM biosynthetic gene clusters, with several related to toxin production. These gene clusters are, however, poorly expressed during colonisation of tomato. The sole detectable SM produced by C. fulvum during in vitro growth is the anthraquinone cladofulvin. Although this pigment is not detected in infected leaves, cladofulvin biosynthetic genes are expressed throughout the pre-penetration phase and during conidiation at the end of the infection cycle, but they are repressed during the biotrophic phase of tomato colonization. It was suggested that tight regulation of SM gene clusters is required for C. fulvum to behave as a biotrophic pathogen, while retaining potential fitness determinants for growth and survival outside its host. To address this hypothesis, we analysed the disease symptoms caused by mutant C. fulvum strains that do not produce or over-produce cladofulvin during the biotrophic growth phase. Non-producers infected tomato similar to wild type, suggesting that cladofulvin is not a virulence factor. In contrast, the cladofulvin over-producers caused strong necrosis and desiccation of tomato leaves, which in turn, arrested conidiation. Consistent with the role of pigments in survival against abiotic stresses, cladofulvin protects conidia against UV light and low temperature stress. Overall this study demonstrates that repression of cladofulvin production is required for C. fulvum to sustain its biotrophic lifestyle in tomato, while its production is important for survival outside its host.