Export of FT Protein from Phloem Companion Cells Is Sufficient for Floral Induction in Arabidopsis

Abstract

Several endogenous and environmental factors need to be integrated to time the onset of flowering. 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. 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. 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. 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. In agreement with this hypothesis, an FT-GFP fusion, just like GFP, can be exported from the phloem of both rice and Arabidopsis. 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.

Long-distance movement of ArabidopsisFLOWERING LOCUS TRNA participates in systemic floral regulation

“…In addition, the movement of FT protein is sufficient to trigger floral initiation, which suggests that the FT protein is a key part of the florigenic signaling pathway. 9,10,[20][21][22][23] Parallel proteomic analyses showed that FT protein is translocated through phloem sap, which is consistent with the notion that the florigen is transported along the phloem translocation stream. 24 Thus, FT protein is clearly part of the florigen.…”

“…9,23,25 Expression of immobilized FT protein with a nuclear localization signal or three copies of yellow fluorescent protein in companion cells failed to rescue the late flowering of ft mutants, which suggests that the movement of FT RNA is not sufficient to promote flowering. 20,21 In addition, the expression of an artificial microRNA (amiRNA) against the FT RNA in the apex did not affect floral initiation, suggesting that FT RNA in the apex may not be necessary for flowering. 21 On the contrary, use of an RNA mobility assay system based on movement-defective viruses and agroinfiltration revealed that FT RNA moves systemically in Arabidopsis and Nicotiana benthamiana.…”

“…20,21 In addition, the expression of an artificial microRNA (amiRNA) against the FT RNA in the apex did not affect floral initiation, suggesting that FT RNA in the apex may not be necessary for flowering. 21 On the contrary, use of an RNA mobility assay system based on movement-defective viruses and agroinfiltration revealed that FT RNA moves systemically in Arabidopsis and Nicotiana benthamiana. 26 Furthermore, the cisacting elements required for FT RNA movement were localized to nucleotides 1-102 of the FT coding sequence.…”

Arabidopsis NPCC6/NaKR1 Is a Phloem Mobile Metal Binding Protein Necessary for Phloem Function and Root Meristem Maintenance

“…The gibberellin (GA) pathway plays a relatively independent role in floral transition under noninductive photoperiods (Araki, 2001). Among these pathways, the photoperiod pathway is largely dependent on phloem activity, since the signals induced by photoperiod, CONSTANS and its downstream target FLOWERING LOCUS T (FT) are expressed in the vasculature and the mobile flowering signal FT needs to be transported through the phloem to the meristem (Corbesier et al, 2007;Mathieu et al, 2007). The floral transition times of nakr1-1 and Col-0 were measured under different floral promotion conditions.…”

Effects of Light Quality on Vegetative Growth and Flower Initiation in Phalaenopsis

“…There are some genes involved in flowering and among those genes, FLOWERING LOCUS T (FT) is a fl oral integrator that plays an important role in controlling fl owering time. Most recently, the nature of FT and its ortholog as fl origen, a mobile fl owering signal, has been proposed in tomato, rice, arabidopsis and cucurbits (Lifschitz, et al, 2006;Tamaki et al, 2007;Corbesier et al, 2007;Jaeger and Wigge, 2007;Mathieu et al, 2007;Lin et al, 2007;Notaguchi et al, 2008;Zeevart, 2008). At present, the isolation of FT gene in orchid (Oncidium Gower Ramsey) has been reported.…”