Figure 6.

Working model for UVR8‐mediated regulation of hypocotyl elongation through degradation of PIF4 and PIF5.

In the absence of UV‐B, nuclear COP1 exerts it role as an E3 ubiquitin ligase by targeting HY5 for ubiquitination and proteasomal degradation, as well as facilitating accumulation of PIF4 and PIF5. PIF4 and PIF5 mediate the expression of genes associated with hypocotyl elongation (e.g. XTR7, YUC8, IAA19 and IAA29), thereby promoting elongation. In the presence of UV‐B, UVR8 monomers interact with and inhibit COP1, resulting in stabilization of HY5 and HY5‐dependent induction of genes associated with UV‐B acclimation, as well as degradation of PIF4 and PIF5 and hence reduced hypocotyl elongation. Note that UVR8‐mediated repression of brassinosteroid‐promoted plant growth is omitted for simplicity (see Liang et al., 2018).