The tomato FT ortholog triggers systemic signals that regulate growth and flowering and substitute for diverse environmental stimuli

Abstract

The systemic model for floral induction, dubbed florigen, was conceived in photoperiod-sensitive plants but implies, in its ultimate form, a graft-transmissible signal that, although activated by different stimuli in different flowering systems, is common to all plants. We show that SFT (SINGLE-FLOWER TRUSS), the tomato ortholog of FLOWERING LOCUS T (FT), induces flowering in day-neutral tomato and tobacco plants and is encoded by SFT. sft tomato mutant plants are late-flowering, with altered architecture and flower morphology. SFT-dependent graft-transmissible signals complement all developmental defects in sft plants and substitute for long-day stimuli in Arabidopsis, short-day stimuli in Maryland Mammoth tobacco, and light-dose requirements in tomato uniflora mutant plants. The absence of donor SFT RNA from flowering receptor shoots and the localization of the protein in leaf nuclei implicate florigen-like messages in tomato as a downstream pathway triggered by cell-autonomous SFT RNA transcripts. Flowering in tomato is synonymous with termination of the shoot apical meristems, and systemic SFT messages attenuate the growth of apical meristems before and independent of floral production. Floral enhancement by systemic SFT signals is therefore one pleiotropic effect of FT orthologs.

Fig. 1.

The tomato FT gene is mutated in the late-flowering sft mutant and induces premature flowering in day-neutral tomato and tobacco. (A) The primary shoot of tomato is terminated by a determinate inflorescence (I-1) after 8–12 leaves. The apparent main axis then consists of a reiterated array of SUs, each arising from the most proximal axillary bud of the preceding one and consisting of three vegetative nodes (numbered) and a terminal inflorescence (marked I-2 and I-3). (B) Enlarged view of a SU. Note that the SU unites with the basal part of leaf that subtends it (no. 3), thus placing it above the inflorescence and, in addition, displacing the inflorescence (Inf) sideways. (C) In the sft mutant shoot, the first terminal inflorescence is formed after 15–20 leaves and, in addition, delays the release of the prospective SU, thus maintaining its own pole position. The sft VI is initiated with a single flower and is subsequently composed of a mix of single flowers (∗) and leaves (L). (Inset) sft flower with its enlarged leafy sepal. (D) WT plant of a day-neutral tobacco (Samsun) flowers after 26–28 leaves. (E) Samsun plants expressing the 35S:SFT flower after only four to six leaves. (F) 35S:SFT#3 tomato plants flower after three leaves (numbered), display early release of lateral shoots (LS), and form thinner stems and smaller simpler leaves. (G) Primary precocious termination with a single terminal flower (TF) in 35S:SFT#2 is accompanied by the formation of a simple leaf (SL) and a delayed release of the first sympodial bud (arrow). The circle marks a delayed axillary bud. (H). Increasing SFT RNA levels are correlated with earlier flowering in progenies of 35S:SFT#3/+ plants. Number of leaves to flowering is indicated above each lane of the Northern blot. (Scale bars: A, D, E, and F, 10 cm; B, C, and G, 2 cm.)

Fig. 2.

Continuous SFT-stimulated systemic signals rescue flowering and morphogenetic defects of sft. (A) A demonstration graft. A donor 35S:SFT is grafted (boxed) onto a receptor sft rootstock with dormant axillary buds (AXL). Flowering response is followed in the released lateral shoots of the rootstock. (B) Complementation of sft by graft-transmissible 35S:SFT-stimulated systemic signals. A receptor sft shoot (right) with three successive normal SUs, each terminated by a regular inflorescence (arrows), is shown. Leaves of the rescued SUs are numbered. (C) Reversion of a rescued sft shoot after removal of the 35S:SFT donor. The last normal inflorescence (Inf) and the subsequent sft VI (Inset) are shown. (Scale bars: 3 cm.)

Fig. 3.

Long-range SFT signals substitute for three distinct environmental stimuli. (A) A typical late-flowering uf plant grown under low irradiance. (Inset) A PS with a rare flower. (B) A uf 35S:SFT plant grown under low-irradiance conditions. The primary shoot is terminated by a flowering PS after three leaves. (C) Floral induction in shade-grown 35S:SFT//uf grafts (boxed). uf lateral shoots of the receptor stock formed the first flowering PS (arrow) after only seven leaves. (D) An ever-vegetative MM plant grown under long (18-h light/6-h dark) days. (E) Early flowering in long-day grown MM plants expressing the 35S:SFT transgene. (F) Tomato 35S:SFT donor scions grafted onto leaf petioles induces flowering in MM under long-day conditions. (G and H) Transactivation of ER-GFP by means of the BLS promoter as detected by CLSM before (G) and after (H) flowering. Earliest ER-GFP expression is detected in the P5 primordium but not in the SAM, flowers (F), or the inflorescence meristem (IM) of Arabidopsis apices. (I) Arabidopsis plants expressing BLS:SFT and grown under short days flowered after only 6 rather than 19 leaves. (Scale bars: A and C–F, 10 cm; B and I, 2 cm; G and H, 50 μm.)

Fig. 4.

Cellular localization of SFT, organ distribution of SFT-interacting proteins, and SFT transcript assay in receptor graft tissues. (A) Expression profile of a GUS-tagged SFT promoter. Shown are a SAP (Left), the youngest leaf with detected expression (Center), and a series of developing leaves (Right). (Scale bars: 1 mm.) (B) Diurnal RNA expression of SFT-interacting proteins in 3- to 4-cm leaves and SAPs of tomato. (C) Correlation between SFT-MYC antigen (Upper) and SFT-MYC RNA (Lower) in progenies of a 35S:SFT-MYC/+ Samsun plant segregating for flowering time. (D–F) The SFT-MYC antigen is localized primarily in the nucleus. (D) Intracellular localization of SFT-MYC antigen in a sepal of 35S:SFT-MYC tobacco (rhodamine staining). (E and F) An optical confocal section and Nomarski image (alkaline phosphatase), respectively, of tobacco cells expressing the SFT-MYC antigen. The arrow points to the spindle of a decorated dividing cell. (G) Calibration of RT-PCR detection levels of 35S:SFT-born transcripts (Ω and RI primers) in RNA from leaves of a 35S:SFT donor and a rescued sft-k receptor. Lanes 1–6, 35S:SFT donor RNA template in 1/5 dilution series starting with 5 μg of RNA; lanes 7 and 8, 5 μg of template RNA from WT and rescued sft receptor SAPs, respectively. (H) RT-PCR detection of endogenous SFT RNA in rescued receptor sft organs (5 μg of RNA; FII and RII primers). Lanes 1–4, template RNA of sft-k receptor from young leaves, SAPs, stem, and flowers, respectively; lanes 5 and 6, WT and donor 35S:SFT leaves, respectively; lane 7, no template control. (I) Nested RT-PCR detection (primers FII and RII) of 35S:SFT transcripts (odd lanes, first-round primers Ω and RI) in comparison with SFT transcripts (even lanes, first-round primers FI and RI) in the four rescued sft recipient organs as above (lanes 1–8), in WT (lanes 9 and 10), and in 35S:SFT donor (lanes 11 and 12). Dilution ratios of first-round PCR were 1/1,000 for the 35S:SFT and 1/10⁵ for the endogenous SFT transcripts. (J) A scheme of the 35S:SFT transgene showing the primers used in the PCR experiments.

Fig. 5.

Meristem termination, a target for systemic SFT/FT signals. (A) FIL≫GUS is expressed in young tomato leaves but not in the SAM proper (Inset) or underlying stem. (B) Expression of FIL≫ER-GFP is detected in P1 leaf primordia but not in the SAM of tomato plants. (C) Early flowering, simple leaves (SL), short internodes, and inflorescences with single flowers (arrows) in the FIL≫SFT#1. (D) Early flowering with single flower, meristem arrest, and simple leaves in FIL≫FT#5 plant. The arrested primary apex consists of two leaves and a terminal flower (TF). Subsequent laterals (AXLI and AXLII) and their axillary derivatives will also form attenuated shoots with arrested apices and give rise to plants similar to that shown in C. In contrast, WT SAP (Inset) displays continuous growth upon flowering. (E) A complete arrest of the primary SAM in extreme FIL≫FT#7. The primary apex was consumed after forming three leaves. An axillary subtended by cotyledon, which consists of only one leaf, is indicated. (F) Tomato seedlings expressing the BLS:SFT transgene. Note the early flowering (upper left Inset) and the complete attenuation of the preflowering second leaf (lower left Inset). Arrows denote expression driven by transactivation. (Scale bars: A, 2 mm; B, 100 μm; C, 5 cm; D–F, 1 cm.)