Arabidopsis BIC1 inactivates CRY2 by suppressing the blue light-dependent cryptochrome dimerization

Cryptochromes are blue-light receptors in plants and animals. Homodimers are the physiologically active form of plant cryptochromes that mediates blue light regulation of gene expression and photomorphogenesis, but how light regulates the photoreceptor activation and inactivation remains unclear. We identified an Arabidopsis protein BIC1 (Blue-light Inhibitor of Cryptochromes 1) that inhibits all photoreactions of plant cryptochrome 2 (CRY2) examined, allowing direct tests of the photoactivation and inactivation mechanisms of CRY2. We found that blue light stimulates homodimerization of CRY2, whereas BIC1 binds to CRY2 to suppress CRY2 dimerization and the interaction of CRY2 with its signaling partners. We further show that cryptochromes and phytochromes mediate light induction of BIC1 transcription, evoking the negative feedback circuitry to control homeostasis of the active cryptochromes under the broad spectra of solar radiation in nature. RNA-seq of BIC1 overexpression (BIC1ox), cry1cry2 mutant, and wild type seedlings exposed to 2h blue light or dark.