Long-Distance, Graft-Transmissible Action of Arabidopsis FLOWERING LOCUS T Protein to Promote Flowering

Abstract

Day length perceived by a leaf is a major environmental factor that controls the timing of flowering. It has been believed that a mobile, long-distance signal called florigen is produced in the leaf under inductive day length conditions, and is transported to the shoot apex where it triggers floral morphogenesis. Grafting experiments have shown that florigen is transmissible from a donor plant that has been subjected to inductive day length to an uninduced recipient plant. However, the nature of florigen has long remained elusive. Arabidopsis FLOWERING LOCUS T (FT) is expressed in cotyledons and leaves in response to inductive long days (LDs). FT protein, with a basic region/leucine zipper (bZIP) transcription factor FD, acts in the shoot apex to induce target meristem identity genes such as APETALA1 (AP1) and initiates floral morphogenesis. Recent studies have provided evidence that the FT protein in Arabidopsis and corresponding proteins in other species are an important part of florigen. Our work shows that the FT activity, either from overexpressing or inducible transgenes or from the endogenous gene, to promote flowering is transmissible through a graft junction, and that an FT protein with a T7 tag is transported from a donor scion to the apical region of recipient stock plants and becomes detectable within a day or two. The sequence and structure of mRNA are not of critical importance for the long-distance action of the FT gene. These observations led to the conclusion that the FT protein, but not mRNA, is the essential component of florigen.

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.…”

“…Previous Arabidopsis micrografting experiments with young seedlings grafted on hypocotyls failed to detect the movement of FT RNA across the graft junction. 9,23 However, because the trafficking of phloem-mobile RNA is highly dynamic and depends on the sink-source strength between the stock and scion, 27 it is possible that the methods of graft may affect the detection of mobile RNA on the scions. To test this possibility, we used the Arabidopsis cleft-grafting system and continually removed mature leaves from the recipient scion to enhance the translocation of phloem derived from the stock.…”

“…8 Experiments with tomato grafting and Arabidopsis micrografting failed to detect FT RNA in the scion. 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.…”

Tissue-Specific Expression of FLOWERING LOCUS T in Arabidopsis Is Maintained Independently of Polycomb Group Protein Repression

“…Auxin is key to the development of the vascular tissue, and important genes involved in the biosynthesis, transportation, perception, and signaling of this hormone are among PcG targets (Notaguchi et al, 2008;Lafos et al, 2011;Rizzardi et al, 2011). Several genes that play an important role in the auxin pathway are affected in swn clf (Figure 3; see Supplemental Data Set 1 online) (Lafos et al, 2011).…”

TheFT-LikeZCN8Gene Functions as a Floral Activator and Is Involved in Photoperiod Sensitivity in Maize

“…Until recently, the biochemical nature of florigen remained elusive until breakthrough experiments in tomato (Solanum lycopersicum; , Arabidopsis thaliana (Corbesier et al, 2007;Jaeger and Wigge, 2007;Notaguchi et al, 2008), and rice (Oryza sativa; Tamaki et al, 2007) revealed that the protein encoded by the FLOWERING LOCUS T (FT) gene had florigenic activity. The FT protein and other related proteins share sequence similarity with mammalian phosphatidylethanolamine binding protein (PEBP) (Kardailsky et al, 1999), which has been annotated as a kinase regulator (Banfield and Brady, 2000).…”