Light-induced gene expression in Botrytis cinerea involves GATA-transcription factors and the stress-activated MAP kinase module.

Botrytis cinerea is a plant pathogen that exhibits prominent light responses including the formation of the reproduction structures (photomorphogenesis), secondary metabolites/ pigments, and antioxidant enzymes. A complex regulatory network of photoreceptors, transcription factors (TFs) and chromatin modifiers is supposed to initiate, transmit, and fine-tune the responses to different wavelengths of light on the transcriptional level that finally leads to the observable phenotypes. As the formation of the reproduction structures is strictly regulated by light in this fungus - conidia are formed in the light, sclerotia in the dark - the output can be easily monitored. The GATA-type TFs BcWCL1 (as part of the White Collar complex (WCC)) and BcLTF1 are important regulators as their deletions result in light-independent conidiation (“always conidia”) due to the deregulation of BcLTF2 (Schumacher et al. 2014; Canessa et al. 2013; Cohrs et al. 2016). Study of light-induced gene (LIG) expression in both deletion mutants highlighted the role of the TFs in activating EARLY and in repressing LATE LIG expression, including bcltf2 encoding the master regulator of conidiation and further LTFs that may function downstream or in parallel with BcLTF2 in regulating the conidiation process. As the group of LIGs also contained genes that are induced by various stresses in a BcSAK1-dependent manner, the phosphorylation status of the stress-activated MAPK BcSAK1 was studied and shown to increase after exposure to light. Deletion of bcsak1 impairs LIG expression suggesting that BcSAK1 functions as a co-activator of the WCC in inducing EARLY genes, and as a component releasing the LATE promoters (e.g. bcltf2) from repression by BcLTF1 and the WCC resulting in the expression of the conidiation genes. This preliminary model is in agreement with the observations that the deletion of BcSAK1 results in a “never conidia” phenotype (Segmüller et al. 2007), and that the exposure of the wild type to heat and osmotic stress bypasses the requirement of light for conidiation