Under dark conditions, the E3 ubiquitin ligase COP1 is enriched in the nucleus, where it contributes to the degradation of various transcriptional regulators involved in the photomorphogenic response, including HY5 (ELONGATED HYPOCOTYL 5). Thus, in Arabidopsis, COP1 is a negative regulator of photomorphogenesis. Oravecz et al. report that COP1 has a positive role in plant responses to low-level ultraviolet B (UV-B) irradiation through a mechanism that includes stimulation of expression of the HY5 gene. Microarray analysis and phenotypic analysis of wild-type and hy5-1 mutant plants indicated that HY5 was necessary for the stimulation of the expression of genes involved in tolerance to UV-B. The nonlethal COP1 mutant cop1-4 also exhibited decreased UV-B-mediated stimulation of several genes, including HY5, and about half of the genes that required COP1 also required HY5. Plants respond to UV-B by stimulation of the phenylpropanoid biosynthetic pathway, which produces "sunscreen" flavonoids. Both hy5-1 and cop1-4 plants were deficient in this response to UV-B; however, plants with another COP1 mutant allele, cop1eid6, did not show this deficiency, which suggests that the structural requirements and functions of COP1 in visible light (photomorphogenesis) and UV-B signaling (flavonoid biosynthesis) are different. Exposure of light-grown plants to supplemental UV-B irradiation promoted the nuclear accumulation of COP1 (in just light-grown plants, COP1 is excluded from the nucleus). HY5 is also abundant in the nucleus of light-grown plants exposed to supplemental UV-B irradiation, which is unexpected given that COP1 promotes its degradation under dark conditions when both proteins are present in the nucleus. How COP1 can mediate degradation of HY5 in the dark, yet be required for HY5 to accumulate in response to UV-B irradiation, remains to be determined.
A. Oravecz, A. Baumann, Z. Máté, A. Brzezinska, J. Molinier, E. J. Oakeley, E. Ádám, E. Schäfer, F. Nagy, R. Ulm, CONSTITUTIVELY PHOTOMORPHOGENIC1 is required for the UV-B response in Arabidopsis. Plant Cell 18, 1975-1990 (2006). [Abstract] [Full Text]