Impact of Light on Differentiation and Virulence of Botrytis cinerea

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The ascomycetous fungus Botrytis cinerea has a worldwide distribution and is the causal agent of gray mold diseases in more than 235 plants species including high-value crops such as grape vine and strawberry. The fungus may reproduce asexually by forming macroconidia for dispersal (summer cycle) and sclerotia for survival (winter cycle); the latter also participate in sexual reproduction by bearing the apothecia after fertilization by microconidia has taken place. Light induces the differentiation of conidia and apothecia, while sclerotia are exclusively formed in the absence of light. Field populations are highly diverse with regard to different phenotypic traits and can be divided into two groups: strains that undergo photomorphogenesis (LIGHT-RESPONSIVE) and BLIND strains that produce either sterile mycelia (FLUFFY) or the same reproductive structures under different light conditions (ALWAYS CONIDIA, ALWAYS SCLEROTIA). However, the inability to form certain reproductive structures such as conidia as the major source of inoculum or sclerotia as prerequisite for sexual recombination is expected to decrease the overall fitness of the pathogen, and thus, it is questionable why BLIND strains are abundantly found in the field. In fact, the BLIND phenotype may accompanied by reduced virulence, as shown for the ALWAYS CONIDIA phenotype of strains T4 and 1750 that is caused by single nucleotide polymorphisms (SNPs) in the gene bcvel1. The identification of bcltf1 (Light-responsive Transcription Factor 1) as new virulence-associated gene by random mutagenesis, further supports our hypothesis of an interrelationship between light signaling and virulence. Accordingly, we assume that the same mechanisms (e.g. expression of detoxifying enzymes, formation of pigments) are activated and required for light tolerance and virulence by protecting the fungus from oxidative stress arising during illumination (photooxidative stress) and host infection (oxidative burst). In view of the impact of light on the life cycle of B. cinerea and the limited knowledge about light signaling in plant pathogenic fungi in general, we aim to investigate the network of photoreceptors and light-responsive transcription factors that regulates the differentiation programs in response to the light, pigment formation and virulence. In addition, a FORWARD GENETICS approach is intended to identify further components of the light signaling machinery in B. cinerea.