Plants respond to light with numerous changes in developmental and physiological processes. However, the exact process by which the light signal is transduced into developmental alterations has remained obscure. Using a variety of assays in Arabidopsis, Wang et al. demonstrate a physical interaction between the blue-light photoreceptors, cryptochromes, and COP1, a ubiquitin ligase that can promote degradation of proteins by the 26S proteasome. Among the proteins that can be targeted for destruction by the proteasome is HY5, which is a transcription factor that can bind to and regulate promoters of light-inducible genes. The demonstration of a physical connection between these components helps explain the signaling pathway by which blue light affects plant growth and development.