A new B-BOX containing protein regulates red light photomorphogenesis ($) (PNAS)

As the primary red light photoreceptor in Arabidopsis, phyB affects various developmental processes, such as seed germination, shade avoidance, etc. PIF3 is a key repressor of phyB-mediated photomorphogenesis that is degraded in red light. Recently Heng et al. identified a positive regulator of red light signaling designated BBX4. In darkness, BBX4 is targeted by COP1 for degradation, but under red light, phyB interferes with COP1’s function thus enhances BBX4’s abundance.  BBX4 accumulation in turn inhibits PIF3’s transcription activation, therefore promoting photomorphogenesis. As the authors state, “this evidence supports the conclusion that phyB promotes photomorphogenesis in R light not only by promoting degradation of negative regulators like PIF3, but also by facilitating the accumulation of positive regulators like BBX4.” (Summary by Nanxun Qin) PNAS 10.1073/pnas.1915149116