Scientists discover that ancient genes dictate flowering, fall bud set

It often requires many years of growth before a tree is ready to flower -- a delay that makes tree breeders impatient. Now, scientists at universities in Sweden and the United States have discovered genes that are responsible for initiation of flowering.

In the annual plant Arabidopsis, the first plant to have its genome sequenced, the genes Constans (CO) and Flowering Locus T (FT) induce flowering in response to day length. It turns out that Populus trees (aspens, cottonwoods, poplars), the first tree to have been sequenced, also have CO and FT genes and can be forced to flower in months, rather than years.

The discovery is being reported in Science magazine on-line (Science Express www.sciencexpress.org) on May 4, 2006, in the article, "The Conserved CO/FT Regulatory Module Controls Timing of Flowering and Seasonal Growth Cessation in Trees," by Henrik Bohlenius, Tao Huang, Laurence Charbonnel-Campaa, and Ove Nilsson of the Umea Plant Science Centre at the Swedish University of Agricultural Sciences; Amy M. Brunner of the forestry department at Virginia Tech; Stefan Jansson of the Umea Plant Science Centre at Umea University; and Steven H. Strauss of the forest science department at Oregon State University.

"In this study, we looked at a poplar homolog of the gene FT, which has been shown to regulate flowering in annual plants. Surprisingly, we found that this gene not only controls the multi-year delay in flowering in trees, but also controls seasonal growth cessation and bud set," said Nilsson, the corresponding author of the study.

The Swedish scientists over expressed FT in poplar, and observed normal flowers on six month old trees -- in a tree species that ordinarily takes 8 to 20 years to flower.

Why does it take trees so many years to flower? Brunner and Strauss -- who have been doing research to make genetically engineered trees sterile so that they cannot cross with wild trees -- used consecutive years of poplar clones to study the multi-year delay in flowering. The Science paper reports their discovery that the expression of the FT gene increases with age. "This gradual increase might be part of the mechanism by which trees become adults," Brunner said.

The Umea Plant Centre scientists also looked at the genetics that signal fall growth cessation and bud set in trees. They discovered that CO accumulates in response to long days and initiates the formation of FT, and in the short days of fall, the pattern of CO accumulation changes so that FT is not activated.

They also observed that the same species of tree at different latitudes will respond to local conditions in order to become dormant before the risk of frost damage. Because the short daylengths occurring in fall induce bud set, the scientists wondered if FT and CO also controlled this process. When they grew trees originating from different latitudes in Europe in a growth chamber, the Umea Plant Centre team observed that "This response is under strong genetic control and is maintained when trees are moved," the article reports.

However, they did observe that levels of CO and FT genes could be made to respond to artificially imposed day length. Most importantly, they observed that under the same day length, CO and FT levels accumulated differently in trees from northern latitudes compared to those from southern latitudes. Because winter arrives earlier at northern latitudes, trees need to set bud and enter dormancy earlier than trees at more southern latitudes.

The article concludes that the CO and FT genetic pathway is key to understanding adaptation to climate change, as well as to speeding tree breeding.

Brunner, associate professor of molecular genetics in the College of Natural Resources at Virginia Tech, whose expertise includes genomics of tree development and the genetics of tree maturation and flowering, has collaborated with Nilsson, of the Umea Plant Science Centre, for a number of years on research related to flowering in trees. She is also a co-principal investigator (PI) on a National Science Foundation-funded plant genome project using transgenics and microarray analysis to understand the genetic networks of the multi-year transition to maturity and seasonal flowering in trees. Others on the project are lead investigator Dawn Luthe of Pennsylvania State University, co-PI Cetin Yuceer of Mississippi State University, John Carlson and Claude dePamphilis of Penn State, and Grier Page of the University of Alabama.