A network of light-responsive transcription factors regulates asexual differentiation n Botrytis cinerea

B. cinerea and other filamentous fungi colonize their substrates in the form of vegetative mycelia and produce asexual spores (macroconidia and microconidia) and sexual spores in high quantities for their distribution and survival. As both processes, vegetative growth and reproduction, are competing for the same limiting resources, they should not occur simultaneously and need therefore be tightly coordinated by genetic programming and environmental conditions. For instance, excessive conidiation (e.g. microcycle conidiation) is disadvantageous as it may prevent vegetative growth and entering the sexual cycle. In this context, light serves as an important cue for the gray mold fungus: It induces the formation of conidia and at the same time represses the formation of sclerotia that represent asexual survival structures and are required for sexual recombination. As conidiation is induced by light, we considered the 24 identified light-responsive transcription factors (LTFs) as possible regulators of the conidiation program in B. cinerea. Nine LTFs have been functionally characterized by deletion and overexpression analyses so far: While BcLTF1 has various functions (growth in light, ROS homoeostasis, virulence, differentiation), other LTFs specifically regulate certain developmental stages such as the initiation of conidiophore development (BcATF1, BcLTF2, BcLTF7) or conidiogenesis (BcLTF3, BcREG1), or do not have obvious functions (BcLTF4, BcLTF5, BcLTF6). The C2H2-TF BcLTF2 has an outstanding function as its expression is necessary and sufficient to induce conidiation and simultaneously to suppress sclerotial development. In contrast, expression of BcLTF7 promotes conidiation but is not required for the process. By expression profiling of the remaining LTFs in mutants exhibiting deregulated conidiation, we identified five further candidates (bcltf12, bcltf15, bcltf16, bcltf20, bcltf21) that may act downstream of the major regulator BcLTF2 in the network of LTFs governing the asexual differentiation programs in the gray mold fungus