A Theory Blossoms

Researchers unfold a key step in the process that tells plants to flower, findings that could one day benefit agriculture.

Fluorescent FT protein in the phloem of an Arabidopsis plant.
Courtesy of Laurent Corbesier and George Coupland

Few acts of nature seem simpler than flowers blooming on the outstretched tips of a plant’s shoots. But the induction of that seemingly simple process baffled plant biologists for almost 60 years.

In the 1930s, Cornell University plant scientist James Knott coined the term “florigen” for a mysterious signal that instructs flowers to begin growing at the tips of stems, called apical meristems.1 Researchers knew and had demonstrated that changes in day length and temperature caused plants to flower, a process essential to plant reproduction. Knott tracked the unidentified florigen traveling through the vascular system of a spinach plant, and other scientists worked out parts of the molecular pathway...

Then in 2007, researchers at the Max Planck Institute for Plant Breeding Research in Cologne, Germany, cracked the case. Plant geneticist George Coupland and colleagues showed that florigen, at least in the flowering Arabidopsis plants they were studying, was a protein encoded by the gene FLOWERING LOCUS T (FT), which behaves like certain types of kinase inhibitors in plant cells. “People were not expecting [florigen] to be a protein or a nucleic acid,” Coupland recalls. “They were expecting it to be a hormone or small molecule,” which typically act as chemical signals in plants.

“It was very clear that FT was the signal,” says Jorge Dubcovsky, a University of California, Davis, plant geneticist who was not involved with the study. Nailing down the identity of florigen meant that science finally identified all the key molecular puzzle pieces involved in flowering—a process of great interest to agriculturalists, for whom an understanding of the molecular machinery of flowering could lead to greater control of their crops by speeding up or slowing down blossom production. “We’ve got the major mysteries solved,” says Amasino. Since Coupland’s long-awaited discovery, his lab and others have continued to answer other lingering questions in flowering pathways, while other plant biologists have corroborated his results and pinpointed the identity of florigen in other, more agriculturally relevant, plants.

False positive

As Coupland and his group sought to provide support for their hypothesis that florigen was a protein, Swedish researchers in 2005 claimed that they had discovered that florigen was mRNA transcribed from the FT gene.2 But the Swedish paper showed an extremely low level of FT mRNA in the meristem, which Coupland says that his group saw as a potential artifact of the real-time PCR method used to detect it. Other researchers shared Coupland’s misgivings. Meanwhile, Science, which published the 2005 paper, was heralding it as a “breakthrough of the year,” and plant biology textbooks began reporting that florigen was FT mRNA.

Coupland and his colleagues continued their experiments, but the 2005 Science paper did cause Coupland to be extra careful about assembling his evidence that the FT protein was florigen. “We couldn’t do exactly the same experiments” as the authors of that paper, such as using PCR to show which molecule was moving from leaves to stem tips to induce flowering, Coupland says.

The Coupland group tagged the FT protein with a fluorescent protein, and using microscopy, showed FT moving from leaves through the phloem—a central vascular tissue that transports water in plants—to the apical meristem, where it induced the growth of Arabidopsis flowers. The researchers also grafted one plant to another and showed that the FT protein was moving from the leaf of one plant to the meristem of the other—further proof that the protein was the long-distance carrier of the flowering signal. They also tracked FT mRNA in these experiments, but failed to find those molecules crossing the junction between the grafted plants.

They submitted their paper to Science; later it came to light that the FT mRNA paper contained serious flaws and was retracted from the journal. The first author on the paper was accused of fudging some crucial data. “There was some fearsome competition there,” says Dubcovsky, “and some people put a priority on speed over certainty.”

Crop confirmation

In the same issue of Science where Coupland published his results, a Japanese team identified an ortholog of the FT protein as the florigen in rice (another Hot Paper).3 William Lucas, a UC Davis plant biologist, confirmed that the FT protein was florigen in pumpkins,4 while Dubcovsky identified a florigen protein homologous to FT protein in wheat.5 The FT protein has also been shown to induce flowering in poplar trees.6

But more mysteries about flowering physiology remain. “[Identifying florigen is] a very important piece of the puzzle, but there’s still a lot to be done,” says Dubcovsky. For example, there is no direct evidence to show how the protein moves through the phloem, according to Coupland. His group is working to characterize some of the mechanistic links between the FT protein and other players in the flowering pathway, such as CONSTANS, a transcriptional regulator that triggers the transcription of FT in leaves and the bZIP transcription factor FD, which interacts with the FT protein to deliver the flower induction signal to its target in the meristem. “There may well be other things to find out in the details of this model,” he says.

Data derived from the Science Watch/Hot Papers database and the Web of Science (Thomson ISI) show that Hot Papers are cited 50 to 100 times more often than the average paper of the same type and age.
L. Corbesier et al., “FT protein movement contributes to long-distance signaling in floral induction of Arabidopsis,” Science, 316:1030–33, 2007. (Cited in 144 papers)

1. J. Knott, “Effect of a localized photoperiod on spinach,” Proceedings of the Society for Horticultural Science, 31:152–54, 1934.
2. T. Huang et al., “The mRNA of the Arabidopsis gene FT moves from leaf to shoot apex and induces flowering,” Science, 309:1694–96, 2005.
3. S. Tamaki et al., “Hd3a protein is a mobile flowering signal in rice,” Science, 316:1033–36, 2007.
4. M. Lin et al., “FLOWERING LOCUS T protein may act as the long-distance florigenic signal in the cucurbits,” Plant Cell, 19:1488–506, 2007.
5. C. Li et al., “Wheat FT protein regulates VRN1 transcription through interactions with FDL2,” Plant Journal, 55:543–54, 2008.
6. T. Igasaki et al., “The FLOWERING LOCUS T/TERMINAL FLOWER 1 family in Lombardy poplar,” Plant Cell Physiology, 49:291–300, 2008.

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