CRY1 inhibits COP1‐mediated degradation of BIT1, a MYB transcription factor, to activate blue light‐dependent gene expression in Arabidopsis

Summary Cryptochromes (CRY) are one of the two major classes of photoreceptors that perceive light stimuli in the UV‐A to blue light region and they are involved in multiple aspects of plant growth and development. However, knowledge regarding their signaling transduction components and mechanisms remains limited. Here, we report that a MYB transcription factor Blue Insensitive Trait 1 (BIT1), plays an important role in controlling blue light responses. Hypocotyl growth responses indicate that BIT1 functions as a positive element in blue light signaling, since BIT1 antisense and knock‐out lines show a reduced light response in blue light. BIT1 controls blue light‐dependent expression of various genes such as PsbS, a member of the light‐harvesting complex gene family. A transactivation assay showed that BIT1 regulates promoter activity of PsbS in a blue light‐dependent manner and that it requires CRY1 for activation of the PsbS promoter. BIT1 undergoes degradation in darkness and CRY1 functions to stabilize BIT1 in a blue light‐dependent manner. In contrast, COP1 binds to BIT1 and mediates its degradation. We propose that the PsbS promoter is activated in blue light via the blue light‐dependent stabilization of BIT1 by CRY1, while in darkness BIT1 is degraded by COP1‐mediated proteolysis.