Constans (CO) is is a Zinc finger containing a domain known as B-boxes. Cysteine and histidine coordinate zinc. Mutations identified within the residues resulted in late flowering which is disadvantageous to reproduction patterns, and suggests they are highly conserved. B-boxes are involved in protein-protein interactions. The C-terminal contains 40 main amino acids (C-terminal contains 70-80 in entirety). In arabidopsis, 32 B-box transcription proteins have been identified thus far. CO, which is considered a BBX1, activates the expression of FT by binding to the CORE1 and CORE2 of the FT promoter. Each B-box transcription protein serves to regulate flowering by stimulating or repressing FT hormone. BBX28 is known to decrease FT transcription, specifically in the late afternoons and dark periods. BBX28 is able to interact with CO through the N-terminus. This works by decreasing recruitment of CO to the FT locus. Constans-like genes (COLs) are very genetically related homologs to Constans (CO). Specifically, CO1 has a greater than 80% amino acid genetic similarity to CO. While identity is similar, function of CO1 varies among identified organisms.
Function
As flowers serve as the reproductive structure of all Angiosperms, the regulation of flowering plants is crucial in a plant’s survival and viability. CONSTANS (CO) serves as the key regulator protein of the photoperiodic flowering times of Arabidopsis (Arabidopsis thaliana). CONSTANS (CO) is able to modulate flowering times through the regulation of the florigen hormone FT (FLOWERING LOCUS T). CO regulation is incredibly specific to the growth stage, season, and time of day. In the presence of light stimulus, CO increases the production of FT (FLOWERING LOCUS T) which then begins the process of flower differentiation. Signals from the environment such as temperature and light availability can inversely act as inhibitors for FT. In complete darkness, CO is completely degraded and flowering is halted. Different lighting periods can result in varying accumulations of CO, which can reduce or increase the speed of flowering depending on light cues. Regulation of CO mRNA is controlled by the circadian rhythm of arabidopsis, particularly in enhancement of long days (LD). The nuclear protein GIGANTEA (GI) interacts with FLAVIN BINDING, KELCH REPEAT, F-BOX PROTEIN 1 (FKF1) to begin the degradation process with the use of DOF transcription factors known as Cycling Dof Factors (CDFs). CDFs are then able to bind to the CO promoter region and inhibit its expression during the morning, thus inhibiting flowering times. Since flowering times are incredibly specific, post translational CO activity is critical as peak mRNA expression is not necessarily concurrent with the exact time of CO activity. CO stability varies depending on light conditions. In the presence of blue light, CO stability is high as blue light impacts the photoreceptors PHYTOCHROME B (phyB) and CRYPTOCHROME 2 (CRY2). In red light, CO stability was poor as PHYB activity restricts flowering from occurring. Another key regulator of CO stability is proteasome. When there is darkness present and during the morning hours, proteasome works to degrade CO and thus prevent flowering from occurring. In light periods CO is stable and able to then activate FT to induce flowering.
Evolutionary Conservation
The CO/FT regulatory module is a highly conserved pathway that is seen in all flowering plants. The flowering period for any given plant is incredibly important for reproductive success and survival, thus flowering period regulating genes are highly conserved. Green algae contain the most preserved characteristics in the plant kingdom and contain Constans-like genes (COLs). Mutated genes in Chlamydomonas revealed an encoded protein that shared likeness to COLS. This protein had a conserved zinc finger region and two N-terminal B-boxes. When comparing the presence of these proteins to the evolutionary age of compared plants, the presence of COL genes in green algae is consistent with the hypothesis that these genes appeared prior to or shortly after the photosynthetic endosymbiotic event. While not proven, it is suggested that COLs play some role in photoperiod regulation. For example, in Chlamydomonas, the protein CrCO was identified. MRNA accumulation did not correlate with the photoperiod of Chlamydomonas, however, there was a correlation observed between the peak of the short day period and CrCO mRNA, suggesting that there is some retained photoperiod influence. Overall, phylogenetic evidence suggests the COLs evolved from a singular b-box and CCT to two b-boxes with an extensive CCT region. In other plants, the CO-FT pathway serves in a variety of development regulation for plant propagation and extremity development. In potatoes, the CO-FT pathway is involved in tuberization. In other plants such as in Chlamydomonas, CrCO also helps to regulate growth and cellular stability. Flowering regulation through CO and its homolog COLS suggests a strong conservation of photoperiod pathways.
Relevance
Much is still unknown about the pathway of the FT hormone when it is activated by CO. As the CO-FT model is a highly conserved pathway, understanding how modifications to the CO protein could further enhance agricultural yield and commercial value. As climate change continues to threaten weather conditions and seasonal length, photoperiods may be altered. These alterations can significantly disrupt flowering periods which is why it is critical to understand the regulation pathway of Constans (CO). In the future, it may be necessary to genetically alter flowering periods to correlate to accommodate changing climates and seasonsonal length change to preserve ecological reproduction patterns. Rupturing the intricate ecological patterns can cause significant ramifications to the food chain and overall health of an ecosystem.
CONSTANS 3D structures
3D structures of constants
References
1. Federico Valverde, CONSTANS and the evolutionary origin of photoperiodic timing of flowering, Journal of Experimental Botany, Volume 62, Issue 8, May 2011, Pages 2453–2463, https://doi.org/10.1093/jxb/erq449
2. Khanna, Rajnish et al. “The Arabidopsis B-box zinc finger family.” The Plant cell vol. 21,11 (2009): 3416-20. doi:10.1105/tpc.109.069088
3. Kim, S. Y., Yu, X., & Michaels, S. D. (2008). Regulation of CONSTANS and FLOWERING LOCUS T expression in response to changing light quality. Plant physiology, 148(1), 269–279. https://doi.org/10.1104/pp.108.122606
4. Liu, Y., Lin, G., Yin, C. et al. B-box transcription factor 28 regulates flowering by interacting with constans. Sci Rep 10, 17789 (2020). https://doi.org/10.1038/s41598-020-74445-7
