Role of the stress-activated MAP kinase module in regulation of photomorphogenesis in Botrytis cinerea

Botrytis cinerea is a plant pathogen that exhibits prominent light responses including the formation of the reproduction structures, tropisms and entrainment of a circadian clock. A complex regulatory network of photoreceptors, transcription factors (TFs), chromatin modifiers and conserved signal transduction cascades is supposed to initiate, transmit, and fine-tune the responses to different light qualities 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 blue light-sensing White Collar complex) and BcLTF1 are important regulators as their deletions result in light-independent conidiation (always conidia) due to the deregulation of BcLTF2. The stress-activated MAP kinase BcSAK1 required for coping with osmotic, oxidative and temperature stress, full virulence and the formation of conidia has recently been shown to be an integral component of the light response system. BcSAK1 becomes transiently phosphorylated after light exposure, and mutation of its dual phosphorylation motif resulted in the Δbcsak1 phenotype (never conidia). In the absence of BcLTF1, BcSAK1 is hyper-phosphorylated which at least partially accounts for the Δbcltf1 phenotype i.e. the hypersensitivity to light, oxidative and osmotic stress as well as deregulated conidiation (always conidia). The activation of the BcSAK1 module by applying osmotic or heat stress – but not by oxidative and cold stress – bypasses the requirement of light for induction of conidiation. However, full conidiation is only achieved by light exposure (near-UV alone, or a mixture of blue and red light) and depends on BcSAK1. The role of the three red/far-red light-sensing histidine kinases/phytochromes (BcPHY1-3) in activation of the MAPK module will be discussed.