The final stage in testing new varieties of rice that can withstand harsh flooding conditions has been successfully completed in India and Bangladesh.
Researchers at UC Davis, UC Riverside and the International Rice Research Institute in the Philippines proved that flood tolerance can be bred into any rice plant by introducing a single gene. The breeding strategy has broad implications for health and livelihood in flood prone regions of the world where rice is a dietary staple.
Four million tons of rice crop, enough to feed 30 million people, are destroyed per year by seasonal flooding in India and Bangladesh. Although farmers normally grow rice in flooded paddies, most cultivated varieties die if submerged completely underwater for more than three days. The new-engineered varieties can tolerate deep flooding for up to two weeks.
“Breeders have tried to develop some varieties to growers that are flood tolerant,” said Pamela Ronald, a professor in the department of plant pathology at UC Davis. “But they didn’t have other qualities that the farmers really wanted and that’s because other unwanted genes [from the parental rice varieties] came in. This is typical of conventional breeding in which large sets of uncharacterized genes are mixed in search of a particular trait.“
By identifying a single gene called Sub1A that confers flood tolerance under controlled lab and field conditions, Ronald and colleagues were able to provide breeders with the necessary information to bring only this favored trait into rice varieties using a genetic engineering technique called “precision breeding.“
The Sub1A gene, which is rapidly switched on when rice plants are submerged in water, does not typically exist in modern cultivated varieties. The product of this gene slows down metabolism so that the plant can hold on to vital nutrients needed for growth until floodwaters recede.
Without Sub1A, the activity of a nearby gene called Sub1C promotes rice plants to grow taller in an attempt to escape submergence, which can exhaust nutrient reserves and lead to death if the water is too deep for too long.
“This type of response [with Sub1C] is successful when the plants experience very shallow flooding of a few centimeters,” said Julia Bailey-Serres, a professor of genetics at UC Riverside who is studying the control of metabolism by Sub1 genes in rice plants. “The tolerant lines [with Sub1A] …seemingly wait out the flood. This strategy is especially effective if there is no chance of growing out of a deep flood.… The Sub1A gene is the solution for rice.“
After researchers confirmed the flood tolerant trait in field stations across the Philippines, Bangladesh and India, the new Sub1A rice varieties were distributed to volunteer farmers who tested the plants in their own fields for three years. The results look excellent, Ronald said, after she and colleagues toured the test plots in early November.
“I think the farmers are just concerned about whether there are any differences between Sub1A and parental rice varieties,” said David Mackill, a senior rice breeder at the International Rice Research Institute who collaborated with Ronald and Bailey-Serres to develop the new varieties.
Researchers did not observe any differences other than flood tolerance in the Sub1A varieties, which will need to be certified by the Indian and Bangladeshi governments before being released for general distribution to farmers.
In addition to those cultivated in South Asian countries, the Sub1A gene will probably be introduced into varieties in Southeast Asia and other places where rice farmers have flooding problems, Ronald said.
In the US, new Sub1A rice varieties could benefit the environment and the organic farming industry, which prohibits the use of herbicides to control weeds that limit crop yield.
These varieties could enable farmers to use controlled flooding as a strategy to selectively kill weeds, Ronald said.
“We haven’t tested this [idea] yet, but we would like to,” she said.
The full story behind the effort to develop flood tolerant rice was published earlier this year in a book coauthored by Ronald, Tomorrow’s Table: Organic Farming, Genetics and the Future of Food.
ELAINE HSIA can be reached at email@example.com.