Export of FT protein from phloem companion cells is sufficient for floral induction in Arabidopsis

Several endogenous and environmental factors need to be integrated to time the onset of flowering [1-3]. Genetic and molecular analyses, primarily in Arabidopsis thaliana and rice, have shown that CONSTANS (CO) and FLOWERING LOCUS T(FT) play central roles in photoperiod-dependent flowering [4-13]. The overall picture is that CO acts in the phloem companion cells of leaves and that its main effect is to induce FT mRNA in these cells [11, 12, 14-19]. Surprisingly, FT, a small globular protein of 20 kDa, interacts at the shoot apex with the bZIP transcription factor FLOWERING LOCUS D (FD) to induce downstream targets [17, 18]. Given that green fluorescent protein (GFP), which as a monomer is 27 kDa, can be easily exported to sink tissue including flowers when expressed in phloem companion cells, the latter finding strongly implied that FT protein is the mobile floral-inductive signal [17-19]. In agreement with this hypothesis, an FT-GFP fusion, just like GFP, can be exported from the phloem of both rice and Arabidopsis [20, 21]. It has been unknown, however, whether mobile FT protein is sufficient for transmitting the flowering signal. Here we show that FT mRNA is required in phloem companion cells where it acts partially redundant with its paralog TWIN SISTER OF FT (TSF) to induce flowering. Furthermore, we have devised a method that uncouples FT mRNA and protein effects in vivo. We demonstrate that export of FT protein from phloem companion cells is sufficient to induce flowering.