Mar. 12, 2004 A team of researchers at the University of California, Davis, has pieced together a clearer picture of how wheat has been able to adapt to such a wide range of climates and become one of the world's staple food grains.
They accomplished this by isolating and cloning the VRN2 gene in wheat, which controls vernalization -- the cold-weather requirement for triggering flowering. The findings of the study, which have practical implications for improving wheat varieties through manipulation of flowering times, will be reported in the March 12 issue of the journal Science.
The researchers, who last year cloned the first wheat vernalization gene, VRN1, discovered that VRN1 and VRN2 work together to confer the winter growth habit. They showed that loss-of-function mutations in either of these two genes result in spring wheat varieties that don't require cold weather to initiate flowering. These varieties can be planted in spring to grow throughout the warmer months of the year. On the contrary, winter wheat varieties germinate and go through early growth stages in the fall but wait until the very cold winter weather passes before flowering in spring.
"During the 10,000 years of domestication of wheat, different mutations occurred in these two genes," said Professor Jorge Dubcovsky, a wheat breeder and leader of the UC Davis research group. "It is now possible to characterize these different mutations and study their effects on the adaptability of wheat to the different environments.
"These studies will provide breeders with a tool to select the best vernalization gene combinations for particular regions," he added. "An additional application of this discovery will be the experimental manipulation of cereals' flowering time. And a delay in flowering time could also be of particular value for forage grasses."
Working in collaboration with a team of researchers from the U.S. Department of Agriculture's Western Regional Research Center in Albany, Calif., Dubcovsky's group has already produced a transgenic winter wheat that flowers 42 days earlier than the non-transgenic line.
Wheat has developed into one of the world's most important crops. It is estimated by the Food and Agriculture Organization of the United Nations that wheat now provides 23 percent of the food available for daily human consumption around the world.
Wheat is grown not only by such leading producers as China, the European Union, the United States, India and Canada, but also by more than 70 developing nations and on six continents, according to CIMMYT -- The International Wheat and Maize Improvement Center. Although it originated in the mild climates of the Middle East, wheat is now cultivated throughout a wide range of temperatures and environments.
This climatic adaptability is, in large part, responsible for wheat's success as a source of food for both humans and livestock. And key to this adaptive ability is the biological process of vernalization. Winter wheat, for example, requires several weeks of low temperatures, usually in the range of 40-50 degrees, in order to flower and eventually produce grain. This cold-weather requirement prevents flowers from developing during winter when they might be damaged by the cold.
In addition, this vernalization system is very flexible. During the domestication of wheat, barley and other temperate cereals, different loss-of-function mutations occurred in the vernalization genes and were selected by humans, resulting in spring varieties better adapted to certain regions.
Last year, Dubcovsky and colleagues reported detailed genetic and physical maps for the VRN1 region in wheat, rice and sorghum. By comparing the sequences from these species, they determined that the wheat VRN1 vernalization gene was involved in the regulation of the transition from vegetative to reproductive growth. This gene is similar to a gene found in Arabidopsis, a model plant commonly used in research. The VRN1 findings were published in the May 2003 issue of the Proceedings of the National Academy of Sciences
In the more recent study to be published in Science, the UC Davis researchers also used detailed genetic and physical maps to discover the VRN2 gene. They determined that VRN2 is a new type of gene involved in the regulation of other flowering genes. In winter wheat varieties, the VRN2 gene prevents the plant from developing flowers. But when the plant is exposed to cold weather during vernalization, the gene is "down-regulated" -- its activity diminished -- thus allowing the plant to proceed with flower formation. The researchers found that experimental down-regulation of the VRN2 gene accelerated flowering time in genetically modified wheat plants by more than a month.
They also found that, unlike the VRN1 gene, the VRN2 is distinctly different than a gene in Arabidopsis that has a similar function.
"This suggests that as they evolved, Arabidopsis and the temperate grasses developed different vernalization pathways, including both similar and very different genes," said Dubcovsky. "For those of us involved in plant genetics research, this serves as a reminder that while model plant systems like Arabidopsis are extremely valuable, we must not neglect the study of the crop species that feed our world."
Funding for this research was provided by the U.S. Department of Agriculture's National Research Initiative and the National Science Foundation.
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