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To Bloom Or Not To Bloom? Researchers Clone A Key Gene That Separates Annual Plants From Biennials

Many crops, flowers and weeds don”t bloom their first summer. Known as biennials, these plants only flower and set seed after a prolonged period of near-freezing temperatures.

In the Oct. 13 issue of the journal Science, researchers from the John Innes Centre in Norwich, England and the University of Wisconsin-Madison report that they have cloned FRIGIDA, one of two genes that makes Arabidopsis a biennial. A small, weedy member of the mustard family, Arabidopsis is the model organism for studying plant genetics.

“This is an exciting breakthrough as flowering is a fascinating and crucial part of plant growth and development” says Caroline Dean, who led the team at the John Innes Centre. “Having this gene will enable us to both understand more about the biology and evolution of flowering in plants, and potentially lead to improvements in farmers” crops.”

The research team included Urban Johanson, Joanne West, Clare Lister, and Nigel Hartley at the John Innes Centre, and Richard Amasino and Scott Michaels at the UW-Madison.

“The biennial nature of crops such as wheat, cabbage, and sugar beet is critical to their value,” says Amasino, a plant molecular biologist in the College of Agricultural and Life Sciences. “The biennial nature of winter wheat enables planting in fall and rapid growth in spring. The same trait in cabbage or sugar beet ensures that only roots and leaves are produced during the first growing season. Flowering then would drain energy from the vegetative parts of these crops and destroy their value.”

Amasino says the discovery may help plant breeders block flowering so plants produce more vegetative parts. Inhibiting flowering in alfalfa, for example, might lead to varieties with greater yields.

The biennial habit is a common adaptation that allows plants to flower and produce seed in early spring, according to Amasino. That seed often remains dormant until late summer. There are many biennial weeds that overwinter as small plants and put on a burst of growth in spring.

One of those biennials is Arabidopsis. However, a few types of Arabidopsis are annuals and bloom their first summer. Comparing the two types was the key to finding the genes responsible for biennialism.

Amasino and his former graduate student Ilha Lee found that the biennial and annual types differed by only two genes.

“Biennial types of Arabidopsis have forms of the genes FRIGIDA (FRI) and FLOWERING LOCUS C (FLC) that actively express proteins until the plants experience several weeks of cold,” Amasino says. “The expression of those genes blocks flowering until the plants are cold-treated. We showed that these genes fully account for the difference between annual and biennial varieties of Arabidopsis.”

In 1999 Amasino and Michaels, who are in the Department of Biochemistry, cloned and sequenced FLC. Dean and her colleagues at the John Innes Centre led the effort to clone and sequence the FRI gene, which is described in this week”s Science article.

According to Amasino, studies of when the two genes are actively expressed in annual and biennial Arabidopsis types suggest that FLC inhibits flowering and FRI acts to increase the expression of FLC. A period of cold inactivates FLC expression.

“It appears that FLC expression blocks flowering and this flower-inhibiting level of expression requires the FRI gene,” he says. As long as FLC is active, biennial Arabidopsis types do not flower.

In the Science article, Dean and her colleagues show that the annual types of Arabidopsis they studied have mutations in the FRI gene that render it nonfunctional.

If biennials are kept without exposure to cold they continue to grow. To show what this looks like, Amasino has a photo of a shrub-sized Portuguese cabbage that has been growing for seven years in a campus greenhouse.

Amasino is not sure how universal the FLC-FRI mechanism is in biennials. He and Thomas Osborn, a geneticist and plant breeder in the UW-Madison Department of Agronomy, have found that the major gene that distinguishes some biennial forms in the cabbage family from their rapid-flowering annual relatives is very similar to the Arabidopsis FLC gene. Amasino is looking for similar genes in other biennials, such as winter wheat and sugar beet.

There is still much more work to be done, according to Amasino. “We have the end of the pathway that determines biennialism,” he says. “Now we plan to work back to determine how other genes regulate FLC.” Amasino is particularly interested in the genes that sense prolonged cold and override the flower-inhibiting effects of FRI and FLC.

In 1999 Amasino received the Alexander von Humboldt Award – one of the most prestigious awards for agricultural research – for his studies of processes that regulate plant flowering and senescence.

Amasino”s research was supported by state funding to the College of Agricultural and Life Sciences, and a grant from the National Science Foundation and the USDA National Research Initiative.