CONSTANS, a HUB for all seasons: How photoperiod pervades plant physiology regulatory circuits

Abstract

How does a plant detect the changing seasons and make important developmental decisions accordingly? How do they incorporate daylength information into their routine physiological processes? Photoperiodism, or the capacity to measure the daylength, is a crucial aspect of plant development that helps plants determine the best time of the year to make vital decisions, such as flowering. The protein CONSTANS (CO) constitutes the central regulator of this sensing mechanism, not only activating florigen production in the leaves but also participating in many physiological aspects in which seasonality is important. Recent discoveries place CO in the center of a gene network that can determine the length of the day and confer seasonal input to aspects of plant development and physiology as important as senescence, seed size, or circadian rhythms. In this review, we discuss the importance of CO protein structure, function, and evolutionary mechanisms that embryophytes have developed to incorporate annual information into their physiology.

Figure 1.

Regulation of flowering in LD and SD plants. Arabidopsis A) and rice B) were chosen as models. Arrows indicate positive effects, and blunt ends indicate negative effects. Red rods indicate functional interactions. Rectangles denote gene expression and ovals protein content. Green indicates positive effectors, and red indicates negative effectors. Green lines indicate protein effects, and blue lines indicate transcriptional effects. Blue, red, and brown‒orange sparks represent blue, red, and far-red light, respectively.

Figure 2.

Model of photoperiod reception and response in plants. Seasonal variations in daylength are first sensed by photoreceptors (e.g. cryptochromes, phytochromes) and the circadian clock. This information is transmitted to photoperiod integrators containing B-box and/or CCT elements, such as CO and PRR proteins. These integrators then activate specific targets, which can act as activators or repressors in several photoperiodic processes.

Figure 3.

Structure of the CO protein. A) Structural model for the dimer of Arabidopsis thaliana CO predicted by ColabFold overlapped with the electrostatic potential. B) Location of the different domains and motifs identified in CO in the previous dimeric structure (subunit 1 in blue, subunit 2 in green). C) CO amino acid sequence domains and motifs shown in the 3D models. Graphical visualizations were obtained with PyMOL.