Regulation of CONSTANS and FLOWERING LOCUS T Expression in Response to Changing Light Quality

In addition to pathways that regulate flowering in response to environmental signals such as photoperiod or cold temperatures (vernalization), flowering time is also regulated by light quality. In many species, far-red (FR) light is known to accelerate flowering. This is environmentally significant because leaves absorb more red light than FR light; thus, plants growing under a canopy experience light that is enriched in FR light. In this article, we have explored the promotion of flowering by FR-enriched light (FREL) in Arabidopsis (Arabidopsis thaliana). Previous work has shown that the floral promoter CONSTANS (CO) plays a critical role in day-length perception and exhibits complex regulation; CO mRNA is regulated by the circadian clock and CO protein is stabilized by light and degraded in darkness. We find that plants grown under FREL contain higher levels of CO mRNA in the early part of the day than plants under white light. Furthermore, transgenic plants expressing CO under the control of a constitutive promoter accumulate higher levels of CO protein under FREL, indicating that FREL can increase CO protein levels independently of transcription. Consistent with the model that FREL promotes flowering through CO, mutants for co or gigantea, which are required for CO transcript accumulation, are relatively insensitive to FREL. Because the red:FR ratios used in these experiments are in the range of what plants would experience under a canopy, these results indicate that the regulation of CO by light quality likely plays a key role in the regulation of flowering time in natural environments.