Sugar gene helps rice tolerate drought, salt, cold

November 25, 2002

ITHACA, N.Y. -- A new strategy to genetically engineer rice and other crops to make them more tolerant of drought, salt and temperature stresses, while improving their yields, is being reported by molecular biologists at Cornell University.

In releasing their research, the biologists emphasize that the technique, which involves adding genes to synthesize a naturally occurring sugar called trehalose, should satisfy critics of genetically modified foods because the chemical composition of edible parts of plants, such as rice grains, remains unchanged.

The biologists describe the new strategy to help plants overcome three of the main causes of crop failure in Proceedings of the National Academy of Sciences (PNAS ), published the week of Nov. 25, 2002.

"We have demonstrated the feasibility of engineering rice for increased tolerance of major environmental stresses and for enhanced productivity," says Ray J. Wu, Cornell professor of molecular biology and genetics. He is director of a laboratory in the university's College of Agriculture and Life Sciences where stress-tolerant rice has been under development since 1996 with support from the Rockefeller Foundation.

The Cornell biologists showed stress tolerance by introducing the genes for trehalose synthesis into Indica rice varieties, which represent 80 percent of rice grown worldwide and include the widely eaten basmati rice. But the same strategy, they note, should also work in Japonica rice varieties, as well as in a range of other crops, including corn, wheat, millet, soybeans and sugar cane.

The researchers plan to report on their claims of increased food productivity from the resulting transgenic rice plants in a subsequent article. They say the trehalose gene technology will be placed in the public domain -- instead of being sold exclusively to commercial seed companies -- so that improved crop varieties can be cultivated in resource-poor parts of the world where the need is greatest.

Co-authors of the PNAS report, "Trehalose accumulation in rice plants confers high tolerance levels to different abiotic stresses," include Cornell biologists Ajay K. Garg, research associate and lead author of the article; Thomas G. Owen, associate professor of plant biology; Anil P. Ranwala, a horticulture research associate; and Leon V. Kochian, research leader at the U.S. Department of Agriculture-Agricultural Research Service Plant, Soil and Nutrition Laboratory, located on the Cornell campus. Other authors are South Koreans Ju-Kon Kim, a biologist at Myongji University, and Yang D. Choi of Seoul National University's School of Agricultural Technology.

Garg, a plant molecular biologist, explains why trehalose (generally pronounced TREE-hal-lows) was chosen in the first place: "Trehalose is a simple sugar that is produced naturally in a wide variety of organisms -- from bacteria and yeasts to fungi, including mushrooms, and in many invertebrates, particularly insects. But there is normally not much trehalose in plants, with the exception of the so-called resurrection plants that can survive prolonged droughts in the desert. Drought-stressed resurrection plants look like they are dead and gone forever; then they pop back to life when moisture is available," Garg says. "That's the power of trehalose in combating stress, and it gave us an idea to help important crop plants survive stress."

In their experiment, the Cornell biologists used two different E. coli genes that are fused together and are responsible for trehalose synthesis in bacteria. (Previous attempts in other laboratories had used only one type of trehalose gene and had been less successful because the resulting transgenic plants showed so-called pleiotrophic effects, including stunted growth, and had little tolerance for stresses.)

The Cornell biologists also learned how to add custom-designed "promoter" sequences to the fused genes, to allow precise "when-and-where" control over gene expression. Depending on the need, the trehalose genes can be turned on in the transgenic plants when stresses occur -- the onset of colder temperatures, for example. Or the gene sequence can be regulated to make trehalose in particular parts of the plant -- such as the leaf but not the edible grains.

So far the transgenic rice plants with the trehalose-enhancement gene sequences have been tested through five generations -- from seed-producing plants to seedlings and more seed-producing plants, again and again -- and the desirable, stress-tolerance characteristics have held true. Compared with non-engineered rice plants that lack the trehalose-enhancement gene sequences, the transgenic rice plants are much more robust under a variety and combination of environmental stresses.

Even when the transgenic plants are not under stress, their processes of photosynthesis (converting light to energy) are more efficient, the Cornell scientists report, accounting, in part, for the increased productivity. Better utilization of soil micronutrients, such as zinc and iron, also has been noted in the transgenic plants.

All the benefits -- and any potential liabilities -- of trehalose have yet to be fully explored, Garg notes. At the cellular level in plants, trehalose helps maintain individual cell structure and function during severe environmental stresses that would kill most plants. Then the sugar appears to help plant cells regain function and efficiency when stress is gone. But, Garg adds, "We still have a lot to learn about trehalose in important crop plants."

Likewise, several years of research-and-development work, safety testing and certification are ahead before large-scale production and distribution of transgenic rice seeds to farmers can begin. The Cornell scientists are seeking patent protection of the trehalose-enhancement technologies, not to control the market and profit from the work, but to ensure that the technologies can be offered in the public domain, Wu says.

"World population continues to increase at an explosive rate, our arable land is deteriorating, fresh water is becoming scarce and increasing environmental stresses pose ever more serious threats to global agricultural production and food security," notes Wu. "Anything we can do to help crop plants cope with environmental stresses will also raise the quality and quantity of food for those who need it most."
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Cornell University

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