Enhanced responsiveness and sensitivity to blue light by blr-2 overexpression in Trichoderma atroviride

Light is an environmental factor that regulates pivotal processes in living organisms, and appropriate perception is key to adaptation to the environment. Blue light activates asexual reproduction in Trichoderma atroviride through transcription factors BLR-1 and BLR-2 which regulate light-responsive genes. Here, we show that blr-2 expression is a limiting factor for photo-perception and photo-transduction. Overexpression of blr-2 resulted in increased photoconidiation and stronger expression of light-induced genes. In contrast, overexpression of blr-1 resulted in reduced photoconidiaton and weaker expression of light-induced genes. blr-2 overexpression caused a marked reduction of growth when the fungus was grown under defined photoperiods, including a period of strong sensitivity to light, followed by a period of insensitivity. Long periods of incubation under this condition permitted recovery of a rhythmic growth similar to that of the wild-type. In addition, blr-2 expression is apparently regulated at the post-transcriptional level through the BLR proteins and its expression level is BLR-1-dependent even in the dark. Finally, we demonstrated that blr-2 overexpression caused higher sensitivity to blue light and we therefore propose that the preformation of BLR-1/BLR-2 complexes is key to adequate light perception in T. atroviride.