| Summary: | <i>Botrytis cinerea</i> possesses a complex light-sensing system composed of eleven photoreceptors. In <i>B. cinerea</i>, <i>bcwcl1</i> encodes for the BcWCL1 protein, the orthologue of the blue-light photoreceptor WC-1 from <i>Neurospora crassa</i>. The functional partner of BcWCL1 is the BcWCL2 protein, both interacting in the nucleus and forming the <i>B. cinerea</i> white collar complex (BcWCC). This complex is required for photomorphogenesis and circadian regulation. However, no molecular evidence shows a light-dependent interaction between the BcWCC components or light-sensing capabilities in BcWCL1. In this work, by employing a yeast two-hybrid system that allows for the <i>in vivo</i> analysis of protein–protein interactions, we confirm that BcWCL1 and BcWCL2 interact in the absence of light as well as upon blue-light stimulation, primarily through their PAS (Per-Arnt-Sim) domains. Deletion of the PAS domains present in BcWCL1 (BcWCL1<sup>PAS∆</sup>) or BcWCL2 (BcWCL2<sup>PAS∆</sup>) severely impairs the interaction between these proteins. Interestingly, the BcWCL1<sup>PAS∆</sup> protein shows a blue-light response and interacts with BcWCL2 or BcWCL2<sup>PAS∆</sup> upon light stimulation. Finally, we demonstrate that BcWCL1 and BcWCL1<sup>PAS∆</sup> respond to blue light by introducing a point mutation in the photoactive cysteine, confirming that both proteins are capable of light sensing. Altogether, the results revealed the complexity of protein–protein interactions occurring between the core elements of the <i>B. cinerea</i> circadian clock.
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