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New flood-tolerant rice offers relief for world's poorest farmers
Davis, California and Los Baños, The Philippines
August, 2006

A gene that enables rice to survive complete submergence has been identified by a team of researchers at the International Rice Research Institute in the Philippines and at the University of California's Davis and Riverside campuses. The discovery allows for development of new rice varieties that can withstand flooding, thus overcoming one of agriculture's oldest challenges and offering relief to millions of poor rice farmers around the world.

While rice thrives in standing water, like all crops it will die if completely submerged for more than a few days. The development and cultivation of the new varieties is expected to increase food security for 70 million of the world's poorest people, and may reduce yield losses from weeds in areas like the United States where rice is seeded in flooded fields. Results of this study will appear in the Aug. 10 issue of the journal Nature.

"Globally, rice is the most important food for humans, and each year millions of small farmers in the poorest areas of the world lose their entire crops to flooding," said Pamela Ronald, a rice geneticist and chair of UC Davis' Plant Genomics Program. "Our research team anticipates that these newly developed rice varieties will help ensure a more dependable food supply for poor farmers and their families. And, in the long run, our findings may allow rice producers in the United States to reduce the amount of herbicides used to fight weeds."

Background

Rice is the primary food for more than 3 billion people around the world. Approximately one-fourth of the global rice crop is grown in rain-fed, lowland plots that are prone to seasonal flooding. These seasonal flash floods are extremely unpredictable and may occur at any growth stage of the rice crop.

While rice is the only cereal crop that can withstand submergence at all, most rice varieties will die if fully submerged for too long. When the plant is covered with water, its oxygen and carbon dioxide supplies are reduced, which interferes with photosynthesis and respiration. Because the submerged plants lack the air and sunlight they need to function, growth is inhibited, and the plants will die if they remain under water for more than four days.

During any given year, yield losses resulting from flooding in these lowland areas may range from 10 percent to total destruction, depending on the water depth, age of the plant, how long the plants are submerged, water temperature, rate of nitrogen fertilizer use and other environmental factors. Annual crop loss has been estimated at more than $1 billion.

"For half a century, researchers have been trying to introduce submergence tolerance into the commonly grown rice varieties through conventional breeding," said rice geneticist and study co-author David Mackill, who heads the Division of Plant Breeding, Genetics, and Biotechnology at the International Rice Research Institute. "Several traditional rice varieties have exhibited a greater tolerance to submergence, but attempts to breed that tolerance into commercially viable rice failed to generate successful varieties.

"We're especially pleased that we have been able to use the latest advances in molecular biology to help improve the lives of the world's poor," Mackill added. "We're confident that even more important discoveries like this are in the pipeline."

Results of this study

Using genetic mapping techniques, the research team identified a cluster of three genes that appeared closely linked to the biological processes that either make rice plants vulnerable to flooding or enable them to withstand the total submergence that occurs during flooding.

The researchers then focused their attention on one of those three genes, known as the Sub1A gene. They found that when this gene is over-expressed, or hyper-activated, a rice variety that is normally intolerant of submergence becomes tolerant.

Further studies indicated that the Sub1A gene is likely successful in conferring submergence tolerance to rice because it affects the way the plants respond to hormones, such as ethylene and giberellic acid, that are key to the plant's ability to survive even when inundated with water.

Going one step further, the researchers introduced the Sub1A gene into a rice variety that is especially suited for growing conditions in India. The resulting rice plants were not only tolerant of being submerged in water but also produced high yields and retained other beneficial crop qualities. Development of submergence-tolerant varieties for commercial production in Laos, Bangladesh and India is now well under way.

In addition to providing a more stable supply of rice in developing countries, the researchers are hoping that the new gene will be useful in suppressing weeds and reducing herbicide applications for conventional and organic rice farmers in developed countries like the United States. If water can be left on the rice for an additional week, it is expected that weed populations will be reduced.

The research team is now trying to identify all the genes that are regulated by Sub1A and to use this information to further improve tolerance to flooding and other stresses.

Funding for this study was provided by the U.S. Department of Agriculture's Cooperative State Research, Education and Extension Service - National Research Initiative; the U.S. Agency for International Development; and the German Federal Ministry for Economic Cooperation and Development.

In addition to Ronald and Mackill, this international research team included Kenong Xu, Xia Xu and Patrick Canlas, all of UC Davis; Takeshi Fukao and Julia Bailey-Serres, both of UC Riverside; and Reycel Maghirang-Rodriguez, Sigrid Heuer and Abdelbagi Ismail, all of the International Rice Research Institute in the Philippines.


Nature
Rice made to breathe underwater
http://www.nature.com/news/2006/060807/full/060807-8.html


Source: SciDev.Net

Gene discovery could lead to flood-resistant rice

Hepeng Jia, SciDev.Net

Scientists have identified a gene that enables rice to survive for up to two weeks underwater, raising the possibility of breeding varieties that can withstand what would otherwise be damaging floods.

A quarter of the world's rice grows in areas prone to flooding, which costs rice farmers in South and South-East Asia more than US$1 billion a year. 

Although rice thrives in standing water, most varieties die within a week of being completely submerged. But others can tolerate being totally submerged for up to two weeks.

The researchers, who published their findings today (10 August) in Nature, studied the DNA of one such variety. They found it has a gene that intolerant varieties lack.

When they introduced the gene into a high-yielding rice variety grown widely in Asia, they found it kept its high yield but could also tolerate being totally submerged.

David Mackill of the International Rice Research Institute in the Philippines, and Pamela Ronald of the University of California in Davis, United States, led the international team.

Mackill told SciDev.Net that scientists would be able to crossbreed submergence-tolerant rice with varieties that are already popular with farmers.

Link to full paper in Nature

Link to related commentary article in Nature

RELATED RELEASE: UC Riverside researchers develop flood-tolerant California rice


Source: University of California Davis
http://indica.ucdavis.edu/research/index.php?research=tolerance

RESEARCH OVERVIEW

Submergence tolerance in rice

More than 22 million hectares of rice paddies are prone to transient flooding due to seasonal rains. Cultivars that can survive temporal flooding of fields are highly desired to enhance yield. Furthermore, sowing of submergent tolerant seed into fields flooded to 15 to 20 cm would effectively limit weed growth and eliminate the need for herbicides.

An international effort, focused on the characterization of the genetic determinants of submergence tolerance, identified an indica landrace, FR13A, as the world's most tolerant of flooding. In collaboration with D. Mackill (International Rice Research Institute) we have now cloned the submergence tolerance (Sub1) locsu from FR13A using a map-based cloning approach. The Sub1 locus encodes three putative transcription regulators of the AP2 class and RNA blot analysis indicates that the Sub1A mRNA levels increase dramatically in response to O2 deprivation in sub1 seedlings; whereas Sub1C levels decrease. The sub1A-2 allele possesses a single nucleotide change as compared to the Sub1A-1 allele. All five sequenced sub1A-2 (intolerant) alleles encode a proline at this position whereas the Sub1A-1 alleles from three tolerant lines examined encode a serine. Transgenic lines overexpressing the Sub1A-1 gene into a submergence intolerant line display enhanced submergence tolerance.

Once we confirm these results, we will examine the expression profiles of the Sub1 vs sub1 lines, determine the significance of the serine to proline mutation and investigate whether the submergence tolerance phenotype of rice involves differential regulation of other signal transduction pathways (in collaboration with Julia Bailey-Serres, UCR). These studies will help us determine if genetic manipulation of this pathway can be used to improve seedling survival and seed development under submergence stress.

Xu K, Xu X, Ronald PC, Mackill DJ. 2000. A high-resolution linkage map in the vicinity of the rice submergence tolerance locus Sub1. MGG. 263. 681-689 (pdf)
Kenong Xu, Xia Xu, Takeshi Fukao, Patrick Canlas, Sigrid Heuer, Julia Bailey-Serres, Abdel Ismail, Ronald, P.C, David J. Mackill. 2006. Sub1A encodes an ethylene responsive-like factor that confers submergence tolerance to rice. Provisional Acceptance Nature.

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