Ithaca, New York
August 16, 2007
By Krishna Ramanujan
Some 10,000 years ago white rice evolved from wild red rice and
began spreading around the globe. But how did this happen?
Researchers at Cornell
University and elsewhere have determined that 97.9 percent
of all white rice is derived from a mutation (a deletion of DNA)
in a single gene originating in the Japonica subspecies of rice.
Their report, published online in the journal PloS (Public
Library of Science) Genetics, suggests that early farmers
favored, bred and spread white rice around the world.
The researchers report that this predominant mutation is also
found in the Indica subspecies of white rice. They have found a
second independent mutation (a single DNA substitution) in the
same gene in several Aus varieties of rice in Bangladesh,
accounting for the remaining 2.1 percent of white rice
varieties. Neither of these two mutations is found in any wild
red rice species.
Both mutations produce shortened versions of the same protein in
which the missing part is responsible for activating the
molecular pathway leading to grain color in rice.
"We think that other domains of this protein are critical for
other functions in the plant, because we never see the protein
entirely deleted, just the part of the molecule that affects the
pathway for grain color," said Susan McCouch, Cornell professor
of plant breeding and genetics and the paper's senior author.
Megan Sweeney, Cornell Ph.D. '06 and postdoctoral associate, was
the paper's lead author.
The researchers speculate that ancient farmers actively bred and
spread white rice varieties first throughout the Himalayan
region and then the rest of the world because the varieties
cooked faster (requiring less fuel), their hulls were easier to
remove compared with red rice, and disease and insects were
easier to see amid the white grains. The farmers also may have
favored one mutation over the other because it may have produced
favorable grains more consistently, the researchers say.
In 2006 the researchers first identified the gene that makes the
rice seed's bran layer, or pericarp, white. This gave rice
breeders and engineers a genetic marker to help develop new
breeds. The Cornell researchers regularly introduce favorable
genes from wild red rices into elite white cultivars to improve
yields and provide better responses to stress, but they
generally select against the gene for red pericarp because it is
associated with such unfavorable "weedy" linked traits as seed
dormancy and "shattering" (where seeds fall easily from the
stalk).
"Breeders can now begin to screen for the red pericarp gene
while selecting against closely linked traits like shattering
and dormancy," said McCouch. The new tools may lead to more
diverse domestic rice varieties.
Also, breeders are interested in using the marker to predict
whether new generations will contain white or red grains, using
DNA from young seedlings, long before the plants set seed.
McCouch noted that due to the genetics of pericarp color in rice
(white grain is recessive and maternally inherited), when white
grains appear in the panicle (the grain clusters on the stems),
it is an indication that all seeds in the clusters will be white
-- and offspring from these seeds will continue to produce
white-grain plants. The researchers theorize that women who
shucked rice for cooking thousands of years ago would have
recognized the value of the white seeds and may have set aside
selected panicles for breeding and planting.
Scott Williamson and Carlos Bustamante, both researchers in
Cornell's Department of Biological Statistics and Computational
Biology, were also co-authors on this paper. The researcher was
supported by the Plant Genome Program of the National Science
Foundation, the Generation Challenge Program, Chengbuk National
University and Korea's National Institute of Agricultural
Biotechnology. |
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