The phytochrome-mediated regulation of photomorphogenesis under red and far-red light conditions involves both positive and negatively acting factors. The positively acting factors (e.g., HY5/HFR1/LAF1 and others) are degraded in the dark to prevent photomorphogenesis. By contrast, the negatively acting factors (e.g., PIFs) are degraded in response to light to promote photomorphogenesis. Here we show that the negatively acting factor, PIF1 is also degraded in the dark by direct heterodimerization with the positively acting factor, HFR1. Conversely, PIF1 also promotes the degradation of HFR1 in darkness. PIF1 enhances the poly-ubiquitination of HFR1 by COP1 in vivo and in vitro. In addition, the reciprocal co-degradation of PIF1 and HFR1 is dependent on the 26S-proteasome pathway in vivo. Genetic evidence shows that the hfr1 mutant partially suppresses the constitutive photomorphogenic phenotypes of the cop1-6 pif1 and pifq both in the dark and far-red light conditions. Taken together, these data uncover a co-degradation mechanism between PIFs and HFR1 underlying photomorphogenic development in Arabidopsis thaliana.
- Received November 12, 2016.
- Accepted March 27, 2017.