The Trichoderma atroviride putative transcription factor Blu7 controls light responsiveness and tolerance

Background: Most living organisms use sunlight as a source of energy and/or information about their environment. Consequently, they have developed mechanisms to sense light quality and quantity. In the fungus Trichoderma atroviride blue-light is perceived through the Blue Light Regulator Complex, which in turn up-regulates a set of genes (blu) and down-regulates another set (bld), triggering asexual reproduction. To gain insight into this process, we characterized the blu7 gene, which encodes a protein containing a C2H2 zinc finger domain. Results: Delta blu7 mutants show reduced conidiation at low light fluences, which is still clear even when exposed to saturating light. For the first time we show a genome wide survey of light regulated gene expression in T. atroviride, including RNA-seq analyses of the wild type and the Delta blu7 strains after brief exposure to blue-light. Our data show a reduction in the number of induced genes and an increase in down-regulated genes in the mutant. Light activates stress responses and several metabolic processes in the wild type strain that are no longer activated in the mutant. In agreement with the misregulation of metabolic processes, continuous exposure to white light strongly inhibited growth of the Delta blu7 mutant, in a carbon source dependent fashion. RNA-seq analyses under constant white light using glucose as sole carbon source revealed that localization and transport process present the opposite regulation pattern in the Delta blu7 and wild type strains. Genes related to amino acid, sugar and general transporters were enriched in the induced genes in the mutant and the repressed genes of the wild type. Peptone supplemented in the media restored growth of the Delta blu7 mutant in constant light, suggesting a role of Blu7 in the regulation of nitrogen metabolism in the presence of light. Conclusions: Blu7 appears to regulate light sensitivity in terms of induction of conidiation, and to play a major role in supporting growth under continuous exposure to light. The diminished conidiation observed in Delta blu7 mutants is likely due to misregulation of the cAMP signaling pathway and ROS production, whereas their low tolerance to continuous exposure to light indicates that Blu7 is required for adaptation.