FLOWERING LOCUS T Protein May Act as the Long-Distance Florigenic Signal in the Cucurbits

Abstract

Cucurbita moschata, a cucurbit species responsive to inductive short-day (SD) photoperiods, and Zucchini yellow mosaic virus (ZYMV) were used to test whether long-distance movement of FLOWERING LOCUS T (FT) mRNA or FT is required for floral induction. Ectopic expression of FT by ZYMV was highly effective in mediating floral induction of long-day (LD)-treated plants. Moreover, the infection zone of ZYMV was far removed from floral meristems, suggesting that FT transcripts do not function as the florigenic signal in this system. Heterografting demonstrated efficient transmission of a florigenic signal from flowering Cucurbita maxima stocks to LD-grown C. moschata scions. Real-time RT-PCR performed on phloem sap collected from C. maxima stocks detected no FT transcripts, whereas mass spectrometry of phloem sap proteins revealed the presence of Cm-FTL1 and Cm-FTL2. Importantly, studies on LD-and SD-treated C. moschata plants established that Cmo-FTL1 and Cmo-FTL2 are regulated by photoperiod at the level of movement into the phloem and not by transcription. Finally, mass spectrometry of florally induced heterografted C. moschata scions revealed that C. maxima FT, but not FT mRNA, crossed the graft union in the phloem translocation stream. Collectively, these studies are consistent with FT functioning as a component of the florigenic signaling system in the cucurbits.

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.…”

Agrobacterium-mediated transient MaFT expression in mulberry (Morus alba L.) leaves

“…18) FT genes of many plants have been isolated, and overexpression of FT homologous genes from Arabidopsis, rice, poplar, tomato, peas, cucurbits, sunflower, apple, and alfalfa have resulted in precocious flowering. [19][20][21][22][23][24] However, functional study of Morus alba flowering locus T (MaFT) gene in mulberry is very limited. Thus, the Agrobacterium-mediated transient expression system in mulberry was firstly optimized.…”

Graft-transmissible action of Arabidopsis FLOWERING LOCUS T protein to promote flowering

“…[12][13][14][15][16]19,21 However, temporal aspects of the transport and spatial distribution of the FT in the shoot apex remain to be investigated. A local, transient induction system by single-leaf blade heat treatment to express FT-T7 or FT:EGFP proteins 21 will provide useful tools.…”

“…10,11 Recent studies from several laboratories have provided strong evidence for the FT protein in Arabidopsis and corresponding proteins in other species being an important part of the florigen. [12][13][14][15][16] These were mainly based on (1) detection in the shoot apex or its vicinity of GFP-fusion or Myc-tagged proteins expressed by promoters with preferential activity in the phloem tissues in Arabidopsis and rice, [12][13][14] (2) detection of FT-like proteins in the phloem sap from cucurbits, 16 and (3) transmission of FT:GFP protein through a graft junction from donor transgenic plants expressing the fusion protein in the vasculature to recipient ft plants with concomitant promotion of flowering. 12 Because key observations in Arabidopsis 12,14 were based on the effect of cumulative expression by SUCROSE TRANSPORTER 2 (SUC2) promoter which has strong activity in the vasculature within a short distance from the shoot, 12,14,17 it is difficult to analyze the temporal aspects of the transport and to exclude the possibility of short-distance, cellto-cell transport without entering into the phloem.…”