Cryptochromes are blue light receptors that mediate light regulation of gene expression in all major evolution lineages, but the molecular mechanism underlying cryptochrome signal transduction remains not fully understood. It has been reported that cryptochromes suppress activity of the multifunctional E3 ubiquitin ligase CONSTITUTIVE PHOTOMORPHOGENIC 1 (COP1) to regulate gene expression in response to blue light. But how plant cryptochromes mediate light suppression of COP1 activity remains unclear. We report here that Arabidopsis CRY2 (cryptochrome 2) undergoes blue light-dependent interaction with the COP1-interacting protein SUPPRESSOR OF PHYTOCHROME A 1 (SPA1). We demonstrate that SPA1 acts genetically downstream from CRY2 to mediate blue light suppression of the COP1-dependent proteolysis of the flowering-time regulator CONSTANS (CO). We further show that blue light-dependent CRY2-SPA1 interaction stimulates CRY2-COP1 interaction. These results reveal for the first time a wavelength-specific mechanism by which a cryptochrome photoreceptor mediates light regulation of protein degradation to modulate developmental timing in Arabidopsis.
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