While rice thrives in standing water, like all crops it will die ifcompletely submerged for more than a few days. The development andcultivation of the new varieties is expected to increase foodsecurity for 70 million of the world's poorest people, and may reduceyield losses from weeds in areas like the United States where rice isseeded in flooded fields. Results of this study will appear in theAug. 10 issue of the journal Nature.
"Globally, rice is the most important food for humans, and each yearmillions of small farmers in the poorest areas of the world losetheir entire crops to flooding," said Pamela Ronald, a ricegeneticist and chair of UC Davis' Plant Genomics Program. "Ourresearch team anticipates that these newly developed rice varietieswill help ensure a more dependable food supply for poor farmers andtheir families. And, in the long run, our findings may allow riceproducers in the United States to reduce the amount of herbicidesused to fight weeds."
Rice is the primary food for more than 3 billion people around theworld. Approximately one-fourth of the global rice crop is grown inrain-fed, lowland plots that are prone to seasonal flooding. Theseseasonal flash floods are extremely unpredictable and may occur atany growth stage of the rice crop.
While rice is the only cereal crop that can withstand submergence atall, most rice varieties will die if fully submerged for too long.When the plant is covered with water, its oxygen and carbon dioxidesupplies are reduced, which interferes with photosynthesis andrespiration. Because the submerged plants lack the air and sunlightthey need to function, growth is inhibited, and the plants will dieif they remain under water for more than four days.
During any given year, yield losses resulting from flooding in theselowland areas may range from 10 percent to total destruction,depending on the water depth, age of the plant, how long the plantsare submerged, water temperature, rate of nitrogen fertilizer use andother environmental factors. Annual crop loss has been estimated atmore than $1 billion.
"For half a century, researchers have been trying to introducesubmergence tolerance into the commonly grown rice varieties throughconventional breeding," said rice geneticist and study co-authorDavid Mackill, who heads the Division of Plant Breeding, Genetics,and Biotechnology at the International Rice Research Institute."Several traditional rice varieties have exhibited a greatertolerance to submergence, but attempts to breed that tolerance intocommercially viable rice failed to generate successful varieties.
"We're especially pleased that we have been able to use the latestadvances in molecular biology to help improve the lives of theworld's poor," Mackill added. "We're confident that even moreimportant discoveries like this are in the pipeline."
Results of this study
Using genetic mapping techniques, the research team identified acluster of three genes that appeared closely linked to the biologicalprocesses that either make rice plants vulnerable to flooding orenable them to withstand the total submergence that occurs duringflooding.
The researchers then focused their attention on one of those threegenes, known as the Sub1A gene. They found that when this gene isover-expressed, or hyper-activated, a rice variety that is normallyintolerant of submergence becomes tolerant.
Further studies indicated that the Sub1A gene is likel y successful inconferring submergence tolerance to rice because it affects the waythe plants respond to hormones, such as ethylene and giberellic acid,that are key to the plant's ability to survive even when inundatedwith water.
Going one step further, the researchers introduced the Sub1A geneinto a rice variety that is especially suited for growing conditionsin India. The resulting rice plants were not only tolerant of beingsubmerged in water but also produced high yields and retained otherbeneficial crop qualities. Development of submergence-tolerantvarieties for commercial production in Laos, Bangladesh and India isnow well under way.
In addition to providing a more stable supply of rice in developingcountries, the researchers are hoping that the new gene will beuseful in suppressing weeds and reducing herbicide applications forconventional and organic rice farmers in developed countries like theUnited States. If water can be left on the rice for an additionalweek, it is expected that weed populations will be reduced.
The research team is now trying to identify all the genes that areregulated by Sub1A and to use this information to further improvetolerance to flooding and other stresses.