A COP1‐PIF‐HEC regulatory module fine‐tunes photomorphogenesis in Arabidopsis. (11th July 2020) - Kathare, Praveen K.
Summary: Light responses mediated by the photoreceptors play crucial roles in regulating different aspects of plant growth and development. An E3 ubiquitin ligase complex CONSTITUTIVELY PHOTOMORPHOGENIC1 (COP1)1/SUPPRESSOR OF PHYA (SPA), one of the central repressors of photomorphogenesis, is critical for maintaining skotomorphogenesis. It targets several positive regulators of photomorphogenesis for degradation in darkness. Recently, we revealed that basic helix‐loop‐helix factors, HECATEs (HECs), function as positive regulators of photomorphogenesis by directly interacting and antagonizing the activity of another group of repressors called PHYTOCHROME‐INTERACTING FACTORs (PIFs). It was also shown that HECs are partially degraded in the dark through the ubiquitin/26S proteasome pathway. However, the underlying mechanism of HEC degradation in the dark is still unclear. Here, we show that HECs also interact with both COP1 and SPA proteins in darkness, and that HEC2 is directly targeted by COP1 for degradation via the ubiquitin/26S proteasome pathway. Moreover, COP1‐mediated polyubiquitylation and degradation of HEC2 are enhanced by PIF1. Therefore, the ubiquitylation and subsequent degradation of HECs are significantly reduced in both cop1 and pif mutants. Consistent with this, the hec mutants partially suppress photomorphogenic phenotypes of both cop1 and pifQ mutants. Collectively, our work reveals that the COP1/SPA‐mediated ubiquitylation and degradation of HECs contributes to the coordination of skoto/photomorphogenic development in plants. Significance Statement: Proper transition from skotomorphogenesis to photomorphogenesis is crucial for the survival of plants. Several key regulators, including CONSTITUTIVELY PHOTOMORPHOGENIC1 (COP1), SUPPRESSOR OF PHYA (SPA) and PHYTOCHROME‐INTERACTING FACTORs (PIFs), control this transition. Here, we identify a new target of COP1 called HECATEs (HECs), which promote photomorphogenesis, whereas COP1, SPA and PIFs inhibit photomorphogenesis in part by promoting the degradation of HEC2 in darkness. The present study contributes to the growing list of COP1 substrates in plants that are key regulators of plant growth and development.