Science 20 April 2007:
Vol. 316. no. 5823, pp. 350 - 351
DOI: 10.1126/science.316.5823.350

News of the Week
PLANT SCIENCE:
Long-Sought Plant Flowering Signal Unmasked, Again
Elizabeth Pennisi

As elusive as the top quark, the signal that tells plants to flower has befuddled plant biologists for more than a century with many false leads to its identity. Two years ago, researchers created quite a stir with data indicating that this signal was messenger RNA (mRNA) that traveled from the so-called flowering locus T (FT) gene in plant leaves to the growth tip where flowering takes place. But those authors are now retracting that finding (p. 367). Instead, two new reports, published online by Science this week (www.sciencemag.org/cgi/content/abstract/1141752 and www.sciencemag.org/cgi/content/abstract/1141753), have fingered the FT protein itself.

"This is something we have been waiting for a long time," says J. A. D. Zeevaart, an emeritus plant physiologist at Michigan State University in East Lansing. "These two papers will be classics in the field for years to come," adds Philip Wigge, a plant biologist at the John Innes Centre in Norwich, U.K. Others, however, think the evidence is not yet conclusive. "They haven't taken the story any further," says William Lucas, a plant cell biologist at the University of California, Davis.

This story has its roots in a 1930s study by Russian plant physiologist Mikhail Chailakhyan. Based on grafting experiments, Chailakhyan proposed that when leaves sense the appropriate day length, they send a mobile signal called florigen to the plant's growing tip to initiate flowering. But promising leads led to dead ends, and "florigen [became] the pariah of botany, [akin to] Big Foot or intelligent extraterrestrial life," says Brian Ayre, a plant biologist at the University of North Texas in Denton.

In the past decade, researchers armed with molecular tools for manipulating genes and visualizing proteins in live tissue have revived the quest. They pinned down the FT gene, the leaf protein that turns FT on, and a flowering gene that the FT protein controls. Then, in 2005, Tao Huang, a postdoc at the Swedish University of Agricultural Sciences in Umeå, and his colleagues proposed that mRNA was the mobile signal in Arabidopsis, as they saw mRNA from FT build up in both the leaf and the growing tip. They concluded that FT mRNA was produced in the leaf and traveled to the growing tip, where it was translated into the FT protein, which then kicked off flowering (Science, 9 September 2005, p. 1694). This report seemed "an enormously exciting breakthrough," recalls Colin Turnbull, a plant biologist at Imperial College in Wye, U.K.

Figure 1 Peripatetic protein. In Arabidopsis (top), a leaf protein moves from a flowering graft into a nonflowering mutant, causing a stem and blossoms to form. In rice (bottom), the equivalent protein (green) shows up in the shoot apical meristem.

CREDITS (TOP TO BOTTOM): COLIN TURNBULL; S. TAMAKI AND K. SHIMAMOTO

But it has not held up. In the 18 April 2006 Proceedings of the National Academy of Sciences, Eliezer Lifschitz of Technion Israel Institute of Technology in Haifa reported no sign of mRNA from the FT-equivalent gene in the flowering shoots of tomatoes. And in their retraction notice, Huang's collaborators report that their initial analysis excluded some data and gave extra weight to other data. When they redid the experiments, "we could not detect movement of the transgenic FT mRNA," says Ove Nillson, in whose lab Huang did this work. Huang, now at Xiamen University in China, has not agreed to the retraction.

Turnbull and George Coupland of the Max Planck Institute for Plant Breeding Research in Cologne, Germany, working with Arabidopsis, and another team studying rice, have now proposed that the mobile signal is the FT protein itself rather than mRNA.

In rice, the equivalent of the FT gene is called Hd3a. Ko Shimamoto of the Nara Institute of Science and Technology in Japan, his student Shojiro Tamaki, and their colleagues first measured Hd3a mRNA in various tissues. They found that in rice grown with short days (rice requires short days to develop flowers), the mRNA increased in leaves but was present only in very low amounts in the shoot apical meristem, the growing tip. Next, they made a transgenic rice strain by joining the gene for green fluorescent protein (GFP) with that for Hd3a, which made any Hd3a protein visible under a confocal laser scanning microscope. They saw the protein in the vascular tissue of the leaf and the upper stem as well as in the core of the growing tip.

They then attached promoters to the combination GFP/Hd3a gene that caused the genes to turn on in the leaf but not in the growing tip. Flowering still occurred, they report. "The only way [ft] could get there was if it moved," explains Zeevaart.

Like Shimamoto, Coupland and Turnbull focused on the FT protein and used GFP to track its fate, this time in Arabidopsis. Laurent Corbesier, a postdoc in Coupland's lab, added the fused FT/GFP gene to a mutant Arabidopsis strain that lacked the FT gene. They observed the protein first in the vascular tissue of the stem, and 4 days later, at the base of the growing tip.

In another experiment, the team grafted plants carrying the fused gene to mutant plants that could not make FT at all. The FT/GFP protein, but no mRNA, moved across the graft junction and through the mutant plant, they report.

Finally, when they attached two GFP genes to the FT gene, the resulting protein was too big to travel beyond the leaf--and in those plants, no flowers formed. Thus, the researchers could rule out both RNA and the existence of a signal activated by FT.

"The evidence is convincing, especially the grafting experiments," says Ayre. And, strengthening the case, several other researchers are preparing to publish similar results.

But not everyone agrees. Lifschitz calls the evidence in both reports "circumstantial." He, Nilsson, and Miguel Blázquez of the Polytechnic University of Valencia, Spain, point out that neither group tested whether GFP moves through the plant on its own accord. And Lucas doesn't think the authors adequately demonstrated that FT gets into the growing tip from the leaf. For example, in Arabidopsis, one leaf promoter used turns on genes elsewhere in the plant, so it could have turned on FT outside the leaf, Lucas points out. Even Ayre is still cautious. "Florigen has a long history of disappointing people," he says. "We're getting there, but the race is intense, and we need to keep cool heads."