The transition to flowering is controlled by genetic pathways which integrate environmental cues and the developmental state of the plant. In Arabidopsis thaliana, the photoperiod, vernalization, and autonomous pathways converge at the level of transcriptional regulation of the floral integrator gene FLOWERING LOCUS T (FT). Only under inductive long-day (LD) conditions CONSTANS (CO) protein accumulates in the leaf vasculature and activates FT expression. As part of the systemic flowering signal, FT protein moves through the phloem to the shoot apex where it initiates meristem identity changes. To understand the molecular mechanism of flowering time regulation mediated by FT, cis-regulatory sequences of FT were identified in the present study. A FT promoter region between 4.0 and 5.7 kb upstream of the start codon was found to be essential for FT expression. This region contains a sequence stretch of 430 bp (block A) that is highly conserved within Brassicacea. The FT locus is associated with the transcriptional repressor TERMINAL FLOWER 2 (TFL2) but the conserved block A in the promoter coincides with a locally TFL2-depleted region. Expression analysis of FT promoter deletion constructs in tfl2 background revealed that TFL2 mediates FT repression via sequences 1.0 to 4.0 kb upstream of FT. The proximal promoter of FT contains a 360 bp region that is highly conserved within Brassicacea (block D). Mutational analysis of short conserved �shadows� within this region suggested a role in the CO-mediated activation of FT based on transient expression studies. Analysis of the mutated elements in the context of the full-length FT promoter in stably transformed plants confirmed that a 6 bp motif (named S1) is essential for FT expression. Endogenous signals and vernalization promote flowering through repression of FLOWERING LOCUS C (FLC). FLC has been proposed to repress FT by binding to a region of intron 1 of FT. Analysis of transgenes either containing or lacking the first intron of FT in high FLC expressing plants, revealed that FLC can repress FT through the promoter sequences also. Interestingly, a genomic FT construct containing the full-length FT promoter and the genomic region with all introns but lacking the 3�-untranslated region is not expressed and cannot complement the ft mutant phenotype. These data demonstrate a negative regulatory role conferred by the structural FT gene and indicate that positive regulatory regions are present downstream of FT
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