The cell biology of the COP/DET/FUS proteins. Regulating proteolysis in photomorphogenesis and beyond?

Plants as sessile organisms have evolved a great deal of developmental plasticity to optimally respond to their immediate environment. Because light is one of the most important cues for plant growth, mechanisms to respond to light conditions are highly elaborated. In particular, the transition from darkgrown (skotomorphogenic) to light-grown (photomorphogenic) development (Fig. 1) in Arabidopsis is steered by a complex molecular network. This network senses the intensity and quality of light and transduces the light signal to downstream effectors that govern the physiological changes that will eventually result in photomorphogenesis. Numerous loci involved in this process have been identified over the last several years by genetic screens (Fig. 2). They include upstream signaling components, like the photoreceptors (for review, see Batschauer, 1998), and intermediate factors transducing the signal to downstream regulators such as EID1, FHY1, FHY3, FIN2, SPA1, FAR1, PAT1, FIN219, RSF1, or HFR1 (Whitelam et al., 1993; Soh et al., 1998; Hoecker et al., 1999; Hudson et al., 1999; Bolle et al., 2000; Büche et al., 2000; Fairchild et al., 2000; Fankhauser and Chory, 2000; Hsieh et al., 2000). The downstream components integrate the light signals from the various photoreceptors and bring about the changes in metabolism and gene expression that eventually lead to photomorphogenesis. More downstream effectors that directly interact with photoreceptors have recently been identified by protein-to-protein interaction approaches (Ni et al., 1998; Choi et al., 1999; Fankhauser et al., 1999). Also, several downstream components identified by their mutant phenotype are negative regulators of photomorphogenesis. They constitute a genetic bottleneck that represses the onset of photomorphogenesis in darkness. Excellent reviews on the signal transduction from photoreceptors to downstream regulators have been published recently (Deng and Quail, 1999; Casal, 2000; Nagy and Schäfer, 2000; Neff et al., 2000). In this update we will thus focus on current progress in the dissection of the molecular function of negative regulators of photomorphogenesis, in particular those of the pleiotropic constitutive photomorphogenic class.