Plants adapt to light by modulating gene expression in response to photoreceptor stimulation. COP1 is a nuclear protein that acts downstream of multiple photoreceptors and negatively regulates light-dependent developmental processes, such as hypcotyl elongation and chlorophyll accumulation, by suppressing positive regulators in the nucleus. Yamamoto et al. have identified a third nuclear COP1-interacting protein called CIP4 that also appears to be a positive regulator of light-responsive processes. Because CIP4 does not contain DNA binding motifs, it may likely be a transcriptional coactivator. CIP4 is expressed at a high level in the light and antisense inhibition of CIP4 expression in transgenic plants suppressed photomorphogenic responses. The authors suggest that COP1 interacts directly with, and negatively regulates, multiple transcription regulators and hence, relays signals perceived by multiple light-sensing photoreceptors to these multiple binding partners. Y.Y. Yamamoto, X.-W. Deng, M. Matsui, CIP4, a new COP1 target, is a nucleus-localized positive regulator of Arabidopsis photomorphogenesis. Plant Cell 13 , 399-411 (2001). [Abstract] [Full Text]