Cryptochromes (CRY) are photolyase-like blue-light receptors that mediate light responses in plants and animals. How plant cryptochromes act in response to blue light is not well understood. We report here the identification and characterization of the Arabidopsis CIB1 (cryptochrome-interacting basic-helix-loop-helix) protein. CIB1 interacts with CRY2 (cryptochrome 2) in a blue light–specific manner in yeast and Arabidopsis cells, and it acts together with additional CIB1-related proteins to promote CRY2-dependent floral initiation. CIB1 binds to G box (CACGTG) in vitro with a higher affinity than its interaction with other E-box elements (CANNTG). However, CIB1 stimulates FT messenger RNA expression, and it interacts with chromatin DNA of the FT gene that possesses various E-box elements except G box. We propose that the blue light–dependent interaction of cryptochrome(s) with CIB1 and CIB1-related proteins represents an early photoreceptor signaling mechanism in plants.

Cryptochrome is a group of flavin-type blue light receptors that regulate plant growth and development. The function of Arabidopsis cryptochrome 2 in the early photomorphogenesis of seedlings was studied by using transgenic plants overexpressing CRY2 protein, and cry2 mutant plants accumulating no CRY2 protein. It is found that cryptochrome 2 mediates blue light-dependent inhibition of hypocotyl elongation and stimulation of cotyledon opening under low intensities of blue light. In contrast to CRY1, the expression of CRY2 is rapidly down-regulated by blue light in a light-intensity dependent manner, which provides a molecular mechanism to explain at least in part that cryptochrome 2 functions primarily under low light during the early development of seedlings.

Cryptochromes are photosensory receptors mediating light regulation of growth and development in plants. Since the isolation of the Arabidopsis CRY1 gene in 1993, cryptochromes have been found in every multicellular eukaryote examined. Most plant cryptochromes have a chromophore-binding domain that shares similar structure with DNA photolyase, and a carboxyl terminal extension that contains a DQXVP-acidic-STAES (DAS) domain conserved from moss, to fern, to angiosperm. In Arabidopsis, cryptochromes are nuclear proteins that mediate light control of stem elongation, leaf expansion, photoperiodic flowering, and the circadian clock. Cryptochromes may act by interacting with proteins such as phytochromes, COP1, and clock proteins, or/and chromatin and DNA. Recent studies suggest that cryptochromes undergo a blue light–dependent phosphorylation that affects the conformation, intermolecular interactions, physiological activities, and protein abundance of the photoreceptors.

Plant photoreceptors mediate light suppression of the E3 ubiquitin ligase COP1 (CONSTITUTIVE PHOTOMORPHOGENIC 1) to affect gene expression and photomorphogenesis. However, how photoreceptors mediate light regulation of COP1 activity remains unknown. We report here that Arabidopsis blue-light receptor cryptochrome 1 (CRY1) undergoes blue-light-dependent interaction with the COP1-interacting protein SPA1 (SUPPRESSOR OF PHYTOCHROME A). We further show that the CRY1–SPA1 interaction suppresses the SPA1–COP1 interaction and COP1-dependent degradation of the transcription factor HY5. These results are consistent with a hypothesis that photoexcited CRY1 interacts with SPA1 to modulate COP1 activity and plant development.

Cryptochromes are photoreceptors that regulate entrainment of the circadian clock by light in plants and animals. They are related to DNA photolyases and have similar three-dimensional structures, characterized by a α/β domain and a helical domain and including a chromophore, flavin adenine dinucleotide.