A light-governed cascade of ubiquitin modifications regulates cotton fiber development by coordinating PIN3a proteolysis.

PIN-mediated auxin transport is crucial for light-regulated plant organogenesis; however, how light modulates PIN localization remains elusive. Cotton (Gossypium hirsutum), a key textile crop, requires ample sunlight for optimal growth and fiber development. Yet, the mechanism underlying light-regulated fiber development is obscure. Our research shows that light promotes fiber initiation and elongation through inhibiting ubiquitylation degradation of GhPIN3a and subsequently enhancing GhPIN3a plasma-membrane localization. In fiber cells, where GhPIN3a undergoes preferential ubiquitylation, GhCOP1 was identified to control ubiquitylation degradation of GhPIN3a in response to light. Dark-stabilized GhCOP1 targets GhUCH3, which interacts with GhPIN3a to balance its stability through deubiquitylation. This regulatory cascade converts light signals into developmental cues in cotton fibers. Intriguingly, while GhCOP1 promotes GhUCH3 degradation via the ubiquitin-proteasome system (UPS), GhUCH3 modulates GhPIN3a proteolysis through both the UPS and the vacuolar degradation pathway. Our findings reveal a light-regulated GhPIN3a stability mechanism through the GhCOP1-GhUCH3 module, consequently influencing cotton fiber development.