Flowering in many plant species is controlled by photoperiod, which represents the most reliable seasonal change in nature. The FLOWERING LOCUS T (FT) gene is crucial for the accelerated flowering in response to long days. FLOWERING LOCUS T (FT) regulates the floral transition in many plant species by integrating environmental seasonal signals and internal cues. Previous study showed that the 5.7-kb region upstream of the transcription start site (TSS) of FT was necessary to initiate FT expression in long day (LD) conditions and contained three sequence blocks, A, B and C conserved among Brassicaceae species. However, it is not clear why such a long promoter evolved to drive photoperiod-dependent expression in leaves.
In a study conducted by Dr. LIU Liangyu of Xishuangbanna Tropical Botanical Garden (XTBG) and his teachers from Max Planck Institute for Plant Breeding Research, Germany, it showed that Block C and Block A (the two core regulatory regions) were necessary and sufficient to confer responsiveness to long days (LD), while the intermittent sequences were required to suppress background expression in short days (SDs).
They have shown that the minimal photoperiod-responsive unit of FT promoter was composed of elements from two regulatory regions separated by a non-responsive intermittent region of variant length in Arabidopsis accessions (4.7−3.8 kb). The distal regulatory region, Block C, acted as a general enhancer in transient assays and was able to increase the base-line expression of a reporter gene driven by the pNOS minimal promoter. In contrast, in stably transformed plants, Block C required the FT proximal promoter (Block A) as an obligatory partner to induce transcription in response to LD .
The intermittent promoter region, in particular, Block ID (a third region) seemed to play a role in mediating communication between Blocks A and C. Natural variation caused by large insertion-deletions (indels) overlapping with Block ID was widespread and correlated with geographical clines. Three major FT promoter variants caused by indels showed differences in allele usage in F1 hybrids. Furthermore, the necessity to fine-tune the response to photoperiod in different growth habitats had led to the accumulation of three FT promoter structural variants among natural Arabidopsis populations.
The researchers proposed that future field studies are required to clarify whether and how variation at the FT promoter contributes to microadaptation. It will be interesting to see in future studies whether a combination of a limited number of cis-elements is sufficient to reconstruct the LD-induced FT regulon in an artificially designed promoter.
The study entitled “Induced and natural variation of promoter length modulates the photoperiodic response of FLOWERING LOCUS T” has been published in Nature Communications.
