The E3 ubiquitin ligase LOP1 targets PIF4 to control plant morphogenesis

Light signals orchestrate plant morphogenic development, yet the mechanisms linking photoreceptor signaling to growth regulation remain incompletely understood. Here, we report that the RING-finger E3 ubiquitin ligase long petiole 1 (LOP1) acts as a negative regulator of petiole elongation and stem elongation at both vegetative and adult stages, thereby controlling plant architecture. Loss of LOP1 function leads to elongated petioles and stems, whereas overexpression of LOP1 displays dwarfed phenotypes showing dramatically shortened petioles and stems. The red-light photoreceptor phytochrome B (phyB) physically interacts with LOP1, and they mutually stabilize each other at the protein level. Genetic analysis revealed that LOP1 inhibits petiole and stem growth interdependently of phyB but in a PIF4-dependent manner. Mechanistically, LOP1 directly targets PIF4 for ubiquitination and proteasomal degradation. Conversely, PIF4 binds to the G-box cis-elements in the LOP1 promoter to activate its transcription, thereby establishing a reciprocal negative feedback loop that precisely controls petiole and stem elongation. These findings reveal a molecular framework wherein phyB-LOP1-PIF4 forms a self-regulatory module that precisely controls plant architecture through mutual protein stabilization and transcriptional feedback.