Rescue of the Maryland Mammoth tobacco by the graft-transmissible tomato SFT
protein
The hypothesis that the FT mRNA may be the florigen was first challenged by
Eliezer Lifschitz and colleagues at the Technion Institute of Technology (Haifa, Israel) in
a paper published in the April 18th 2006 issue of the Proceedings of the National
Academy of Sciences (Lifschitz et al., 2006). In this paper, Eliezer and his co-workers
reported a study of the functional homolog of FT in tomato called SFT (SINGLEFLOWER
TRUSS). Tomato sft is a late-flowering mutant isolated long ago and mapped
to the same chromosome position as that of a homologous gene of Arabidopsis FT.
Sequence analyses of multiple sft alleles revealed missense or deletion mutations of
this FT-like gene in those sft alleles, indicating that SFT is most likely the FT ortholog in
tomato.
Transgenic tomato and tobacco plants constitutively expressing either the
Arabidopsis FT gene or the tomato SFT gene under the 35S promoter (35S::FT or
35S::SFT) flower earlier, although the wild type tomato is day-neutral and insensitive to
2
photoperiod. The researchers then tested whether the SFT signal is graft transmissible.
Although the wild type donor failed to promote flowering in the receptor sft mutant, a
transgenic donor expressing 35S::SFT rescued the mutant sft receptor. The
interpretation was that the wild type donor may not have enough SFT to donate to the
receptor sft mutant, that the transgenic 35S::SFT grafting stock expressed SFT at a
higher concentration, and that the transgenic SFT mRNA or protein was systemically
transmitted across the grafting union to promote flowering in the sft mutant receptor.
Interestingly, the authors could detect the SFT protein, by immunoblot, in the receptor
samples. In contrast, no transgenic SFT mRNA was detected, by nested RT-PCR, in
the RNA samples isolated from the grafting receptor samples, although SFT mRNA was
detected in 1/2500 as much RNA isolated from the donor samples. These results
demonstrated that at least in tomato, SFT protein is more likely than the SFT mRNA to
act as the graft transmissible signal that promotes flowering.
The researchers went further and used the heterograft technique to test
background information (Yu et al., 2006).