1. Laboratory of Plant Molecular Biology, Rockefeller University, 1230 York Avenue, New York, New York 10021, USA

Correspondence to: Nam-Hai Chua Correspondence and requests for materials should be addressed to N.-H.C. (Email: chua@mail.rockefeller.edu).

Abstract

Far-red light regulates many aspects of seedling development, such as inhibition of hypocotyl elongation and the promotion of greening¹, acting in part through phytochrome A (phyA). The RING motif protein COP1 is also important because cop1 mutants exhibit constitutive photomorphogenesis in darkness^{2, 3}. COP1 is present in the nucleus in darkness but is gradually relocated to the cytoplasm upon illumination⁴. Here we show that COP1 functions as an E3 ligase ubiquitinating both itself and the myb transcription activator LAF1, which is required for complete phyA responses⁵. In transgenic plants, inducible COP1 overexpression leads to a decrease in LAF1 concentrations, but is blocked by the proteasome inhibitor MG132. The coiled-coil domain of SPA1, a negative regulator of phyA signalling⁶, has no effect on COP1 auto-ubiquitination but facilitates LAF1 ubiquitination at low COP1 concentrations. These results indicate that, in darkness, COP1 functions as a repressor of photomorphogenesis by promoting the ubiquitin-mediated proteolysis of a subset of positive regulators, including LAF1. After the activation of phyA, SPA1 stimulates the E3 activity of residual nuclear COP1 to ubiquitinate LAF1, thereby desensitizing phyA signals.