Ubiquitination in the control of photoperiodic flowering.

Triggering flowering at the appropriate time is a key factor for the successful reproduction of plants. Daylength perception allows plants to synchronize flowering with seasonal changes, a process systematically analyzed in the model species Arabidopsis thaliana. Characterization of molecular components that participate in the photoperiodic control of floral induction has revealed that photoreceptors and the circadian oscillator interact in a complex manner to modulate the floral transition in response to daylength and in fact, photoperiodic flowering can be regarded as an output pathway of the circadian oscillator. Recent observations indicate that besides transcriptional regulation, the promotion of flowering in response to photoperiod appears to be also regulated by modulation of protein stability and degradation. Therefore, the ubiquitin/26S proteasome system for targeted protein degradation has emerged as a key element in photoperiodic flowering regulation. Different E3 ubiquitin ligases are involved in the proteolysis of a variety of photoperiod-regulated pathway components including photoreceptors, clock elements and flowering time proteins, all of which participate in the control of this developmental process. Given the large variety of plant ubiquitin ligase complexes, it is likely that new factors involved in mechanisms of protein-targeted degradation will soon be ascribed to various aspects of flowering time control.