The results, reported in tomorrow's issue of Science, may lead to hardier, higher-yielding, more nutritious strains of the grain, which is the leading food crop on the planet.
"Populations are continuing to grow, but arable land is being eaten away. Yields are going to have to go up by some means or another," says Gane Ka-Shu Wong, a research scientist at the University of Washington who headed up one of the sequencing teams.In fact, a preliminary sketch of the rice genome has been available to scientists for more than a year, and researchers are already reaping rewards.
Anna McClung, a geneticist at the U.S. Department of Agriculture's Rice Research Unit, says this information "makes our breeding process much more effective and efficient."
Humans have been cultivating rice for at least 7,000 years, but with conventional crossbreeding it takes a decade to bring a new strain to market, McClung says. Thanks to genetics, "we've already been able to trim two to three years off of that process."
McClung's lab is using the new information to build "pyramids," or clusters of rice genes with traits crop strains should have, such as disease-resistant proteins. She and her colleagues are also trying to exploit rice's vast natural diversity to find genes that code for better tolerance to dry conditions and salty water, better milling and cooking quality, and increased nutritional value.
"I think we're going to be more effective in delivering grasses that have new combinations of traits that would have been extremely difficult or impossible to produce before," she says.
Rice, or Oryza sativa, is a group of grains whose two main food varieties are indica and japonica. Indica, which is grown in warmer climates, accounts for about 80 percent of rice production, while japonica, farmed in northern regions, makes up the rest.
Indica has 466 million base pairs -- the "letters" of matched molecules that make up DNA -- and between 45,000 and 56,000 genes. That's 3.7 times more than Arabidopsis, or mustard, the only other plant genome yet sequenced, but about seven times fewer than humans.
Japonica, meanwhile, has 420 million base pairs and 32,000 to 50,000 genes, says Stephen Goff, director of genome technology at Torrey Mesa Research Institute in San Diego and leader of that sequencing team. Torrey Mesa is an arm of Syngenta, a Swiss biotech firm that is seeking to patent the genes it has discovered.
Goff said that of the 32,000 to 50,000 genes in japonica rice, only "a few hundred" are likely to be commercially interesting.
In addition to helping rice producers grow the crop more efficiently and with specific traits, Goff says knowing the genome of the strains will help farmers of other grains, especially corn and wheat.
"They're all related to each other," he says, so rice is a "great model" for the genetic makeup of the other species. Together, these three plants make up 60 percent to 70 percent of the world's food crops, and a 600 million ton-a-year harvest.
Wong, who led the indica sequencing team, says the two strains share about 99.5 percent of the same genes. Humans and chimps are about 99 percent identical. However, when comparing their large stretches of junk DNA, the two rice forms appear almost unrecognizable, Wong says.
Kevin McGilton, director of government relations for the U.S.A. Rice Federation, a Virginia-based industry group, says rice breeders have the most to gain by the sequencing effort: "It could speed up the whole breeding process."
Perhaps the most well known example of the blend of genetics and rice is so-called "golden rice." This grain, still in an experimental phase, has been modified to carry genes that boost its production of vitamin A. Deficiencies in this nutrient cause blindness and death worldwide, so knitting it into a common food is an ideal way to provide the vitamin, advocates argue.
Opponents of the crop have decried it as unnatural tinkering with nature, and fear it may infiltrate the rice supply with grave consequences.
In the United States, genetic engineering and rice don't mix, at least for the moment.
Of the 21.3 billion pounds of rice grown commercially here last year, none was genetically modified, McGilton says. The reason: America exports about 40 percent of its crop, and foreign countries, especially those in Europe, have staunchly opposed so-called "GM" foods.
"We can't take the chance that [GM] rice will get commingled" with natural strains, he says.
However, McClung says knowing the grain's genome could make genetic engineering unnecessary.
"We're using the genomic information in a different way," to capitalize on rice's own abilities, she says.