Seminars

The CONSTANS (CO) gene of Arabidopsis acts at the core of the photoperiodic pathway by promoting the flowering response to long days. The circadian clock and light perception combine to confer day-length regulation on CO activity. CO mRNA oscillates under LD and SD, but under LDs is increased in abundance at the end of the day. At this time post-translational regulation also increases the abundance of CO protein. Photoreceptors PHYTOCHROME A (PHYA), CRYPTOCHROME 1 (CRY1) and CRYPTOCHROME 2 (CRY2) stabilize CO protein by antagonizing the activity of the ubiquitin ligase CONSTITUTIVE PHOTOMORPHOGENIC 1 (COP1) and its interacting protein SUPPRESSOR OF PHYTOCHROME A 1 (SPA1). It was previously shown that CO protein level is reduced in the dark through the activity of COP1, which interacts with and ubiquitinates CO protein. We have now demonstrated that CO protein is also phosphorylated, which raises the question whether phosphorylation and ubiquitination are related. We have shown that the phosphorylated form of CO protein preferentially accumulates in cop1 mutants suggesting that this form is the substrate for COP1. To try and define COP1 binding domains we tested the abundance of the two forms of CO protein in various transgenic lines expressing mutated or truncated forms of CO protein. Our results indicate that a C-terminal region containing the CCT domain of CO is sufficient for its degradation and light regulation. Also, CO contains a putative recognition site for Casein Kinase 2 and several candidate phsphorylation sites that we are currently evaluating the relevance of. The significance of CONSTANS phosphorylation in the model of photoperiodic flowering will be discussed.