COP1 re‐accumulates in the nucleus under shade

Shade‐avoider plants typically respond to shade‐light signals by increasing the rate of stem growth. CONSTITTUTIVE PHOTOMORPHOGENESIS 1 (COP1) is an E3 ligase involved in the ubiquitin labelling of proteins targeted for degradation. In dark‐grown seedlings, COP1 accumulates in the nucleus and light exposure causes COP1 migration to the cytosol. Here, we show that in Arabidopsis thaliana, COP1 accumulates in the nucleus under natural or simulated shade, despite the presence of far‐red light. In plants grown under white light, the transfer to shade‐light conditions triggers an unexpectedly rapid re‐accumulation of COP1 in the nucleus. The partial simulation of shade by lowering either blue or red light levels (maintaining far‐red light) caused COP1 nuclear re‐accumulation. Hypocotyl growth of wild‐type seedlings is more sensitive to afternoon shade than to morning shade. A residual response to shade was observed in the cop1 mutant background, but these seedlings showed inverted sensitivity as they responded to morning shade and not to afternoon shade. COP1 overexpression exaggerated the wild‐type pattern by enhancing afternoon sensitivity and making morning shade inhibitory of growth. COP1 nuclear re‐accumulation also responded more strongly to afternoon shade than to morning shade. These results are consistent with a signalling role of COP1 in shade avoidance. We propose a function of COP1 in setting the daily patterns of sensitivity to shade in the fluctuating light environments of plant canopies.