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 that allowed plants to sense environmental changes and respond by producing flowers. But the chemical identity of florigen eluded discovery. “It was a technical challenge to put that last nail in the coffin,” says Richard Amasino, a plant scientist at the University of Wisconsin in Madison.

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.

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.”

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)

Drs. Romanov and Aksenova,

Thank you very much for your comment. You are correct in stating that Mikhail Chailakhyan coined the term "florigen," and that he conducted some of the seminal research into the signal's movement through plant tissues. We are correcting the story as per your suggestions. I sincerely apologize for the error.

Bob Grant

We are looking into your claims on the origin of "florigen," and will respond shortly.

Alison McCook
Deputy Editor

This article of Mr. B. Grant disseminates false data on the history of florigen investigation. Firstly, ?Cornell University plant scientist James Knott? never proposed the term ?florigen?. The term ?florigen? has been firstly introduced upon his extensive studies by Michail Chailakhyan (1936) who was working in the Moscow Institute of Plant Physiology. Since that time the term ?florigen? is linked for ever with the name of Chailakhyan, and the florigen theory of M. Chailakhyan is worldwide recognized for tens of years, cited in thousand of books, reviews, experimental articles and encyclopedia. Even in papers quoted by of Mr. Grant in his article early works of M. Chailakhyan and not of J. Knott are extensively cited in relation to florigen.
In fact, Knott (quoted paper of 1934) stimulated spinach leaves with a photoperiod favorable for flowering and produced flower formation on non-stimulated stem apex. Knott concluded that ??the part played by the foliage of spinach in hastening the response to a photoperiod favorable to reproductive growth may be in the production of some substance, or stimulus, that is transported to the growing point?. It was nothing else from Knott on this subject, so the statement of Mr Grant that ?? Knott tracked the unidentified florigen traveling through the vascular system?? is totally wrong. It was just Chailakhyan who produced that time hundreds of experiments using different plants, making grafts between the same and different species, girdling stems and branches, etc. It was just Chailakhyan who definitely proved that florigen exists as a real substance and moves over long distances through the vascular system (floem) in both directions, up and down. Chailakhyan was the first to show that florigen is not species- or photoperiod specific, he provided first estimate for a speed of the florigen translocation. Chailakhyan proposed the hormonal nature of this blossom forming hormone, in his famous paper ?New facts in support of the hormonal theory of plant development? (Comptes Rendus (Doklady) de l?Academie des Sciences de l?URSS. 1936, vol. IV (XIII), No.2 (106), pp. 79-83) he wrote ?? we may term this blossom forming or blossom hormone, more concisely, florigen, meaning ?blossom-former?, which expresses the basic function of this substance in the vegetable organism?. This phrase from Michail Chailakhyan was the first mentioning of the term ?florigen? in the scientific history.

To conclude, we claim to eliminate as soon as possible this gross error in the article ?A theory blossom? which might lead to the science history falsification, or delete this article from the website of ?The Scientist?. We very regret that so known and estimated edition as ?The Scientist? has published such incorrect data. We also claim the true history of florigen discovery to be published in ?The Scientist? (see, as example, the concise review of Aksenova et al. ?Florigen goes molecular: seventy years of the hormonal theory of flowering regulation?, Russian J. of Plant Physiology, 2006, vol. 53, No.3, pp. 401-406).

Georgy Romanov, Prof., Institute of Plant Physiology, Moscow
Nina Aksenova, Leading Scientist, Institute of Plant Physiology, Moscow

The idea of a florigen is more an association as than a complete answer. The central question for an agriculturist is what hastens and what delays the flowering. A seasonal plant, e.g., a monocot, that takes longer to flower grows larger and yields better. The answer to this is in the physiology of the meristem, whose metabolism is primarily dependent on respiration as these cells are not photosynthetic. Meristematic events are few and far between in a seasonal plant like rice or even Arabidopsis. At the time of germination and the time of budding and branching, the meristematic activity manifests itself. Flowering is but one aspect of budding.
The problem was how to measure the respiratory activity in the meristem of a growing plant that is relatively undisturbed. Respiration was the answer. In seeds, faster the seeds respire, faster is their germination. Faster the germination, faster was the onset of the flowering leading to a smaller plant with lower yield. It so happens that the mitochondrial oxidative phosphorylation is the most osmotically sensitive activity thus far seen in biology with a signature value of 10-13 L/mole. Since pressure sensitivity must imply a relevant thermodynamic volume, it gives us a critical pressure which can be measured as the limiting pressure at which this biological activity ceases. The product of the volume and pressure yields an energy related term. Budding and branching were seen to be most sensitive with an activation volume matching the mitochondrial signature when measured for osmotic sensitivity in vivo. They also have the highest energy value, as the product term. Our published results have been attempted to be scooped in Annals of Botany except that their thermodynamics was non-existent and the physical chemistry was all wrong. See LINK
Should we demand minimal academic standards to be maintained in terms of correct science even when scooping?

The most interesting aspect of this plant biology is that respiration decides plant size and yield and not the other way round. In so far as we believe that there must a genomically manipulable answer, we will find a florigen idea attractive. As an idea, there is nothing wrong with it and is even interesting. But imagine the situation where you design a plant for a faster flowering?it will lead to a smaller plant with lesser yield. If the climate does not match the life cycle of the plant and its variable environment, you have a disaster on hand. What does the plant itself do? We would see that even a single tiller would have variation in seed size, smaller ones breathing less and larger ones respiring more (per seed), with consequences on germination and flowering. Thus retaining variation in respiration sidewise is an excellent strategy to cope with climate variations and therefore climate change. In stead, our worship of all products of genetic engineering makes us forget that the evolutionary strategies proven and tested over millennia (even in this year of Darwin) are ignored. Sad indeed!