TWIN SISTER OF FT (TSF) Acts as a Floral Pathway Integrator Redundantly with FT

In Arabidopsis, several genetic pathways controlling the floral transition (flowering) are integrated at the transcriptional regulation of FT, LFY and SOC1. TSF is the closest homolog of FT in Arabidopsis. TSF expression was induced rapidly upon activation of CONSTANS (CO). The mRNA levels of TSF and FT showed similar patterns of diurnal oscillation and response to photoperiods: an evening peak, higher levels in long day (LD) than in short day (SD) conditions, and immediate up-regulation upon day-length extension. These observations suggest that TSF is a direct regulatory target of CO. tsf mutation delayed flowering in SD conditions and enhanced the phenotype of ft in both LD and SD conditions. TSF and FT also shared similar modes of regulation by FLC, an integrator of autonomous and vernalization pathways, and other factors such as EBS and PHYB. Consistently, TSF overexpression caused a precocious flowering phenotype independent of photoperiods or CO, or FLC. These observations suggest that TSF is a new member of the floral pathway integrators and promotes flowering largely redundantly with FT but makes a distinct contribution in SD conditions. TSF and FT seem to act independently of each other and of LFY, and partially upstream of SOC1. Interestingly, the expression patterns of TSF and FT in seedlings did not overlap, although both were expressed in the phloem tissues. Our work revealed additional complexity and spatial aspects of the regulatory network at the pathway integration level. We propose that the phloem is the site where multiple regulatory pathways are integrated at the transcriptional regulation of FT and TSF.