Abstract
Flowering time of summer annual Arabidopsis thaliana accessions is largely determined by the timing of FLOWERING LOCUS T (FT) expression in the leaf vasculature. To understand the complex interplay between activating and repressive inputs controlling flowering through FT, cis-regulatory sequences of FT were identified in this study. A proximal and an approximately 5-kb upstream promoter region containing highly conserved sequence blocks were found to be essential for FT activation by CONSTANS (CO). Chromatin-associated protein complexes add another layer to FT regulation. In plants constitutively overexpressing CO, changes in chromatin status, such as a decrease in binding of LIKE HETEROCHROMATIN PROTEIN1 (LHP1) and increased acetylation of H3K9 and K14, were observed throughout the FT locus, although these changes appear to be a consequence of FT upregulation and not a prerequisite for activation. Binding of LHP1 was required to repress enhancer elements located between the CO-controlled regions. By contrast, the distal and proximal promoter sequences required for FT activation coincide with locally LHP1 and H3K27me3 depleted chromatin, indicating that chromatin status facilitates the accessibility of transcription factors to FT. Therefore, distant regulatory regions are required for FT transcription, reflecting the complexity of its control and differences in chromatin status delimit functionally important cis-regulatory regions.