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UCR Finding Could Lead to More Chilling-Tolerant Crops

UCR Finding Could Lead to More Chilling-Tolerant Crops

(November 8, 1999)

Scientists at the University of California, Riverside report they have identified a gene in blackeye beans that can enable the crop's seed to germinate and the plants to emerge in cool soil. The discovery could lead to the development of crops with less chance of failing when spring temperatures turn cold after sowing.

The finding, reported in the Nov. 9 issue of Proceedings of the National Academy of Sciences, also holds promise for improving other, major warm-season crops such as soybean, cotton, corn, rice and sorghum through classical plant breeding, and possibly through genetic engineering.

Only a moderate amount of blackeye dry beans -- a type of cowpea -- are produced in California with a gross value to farmers of $30 million per year. Cowpea also is grown in several southern U.S. states and it is widely grown in West Africa, Latin America and Asia. In Africa, it is particularly important due to its strong resistance to the droughts that have caused widespread failures of other crops and hunger.

Ironically, it was research to breed heat-tolerant varieties of blackeye beans that led to a serendipitous discovery by UCR plant physiologists and breeders Abdelbagi M. Ismail and Anthony E. Hall, and geneticist Timothy J. Close that the gene pool of cowpea plants also contains genetic variation for seedling emergence during cool weather.

"The importance of this finding is not for West Africa, but for places like California with subtropical conditions and cool spring soil," said Ismail, a postgraduate researcher from the Sudan. "If we can breed a chilling-tolerant variety of blackeye bea ns, its ability to emerge under cool soil conditions would offer a considerable advantage to farmers. Early sowing can result in plants with higher yields because plants sown earlier begin flowering before mid season hot weather occurs which can reduce p od set."

The problem with many warm-season plants that are sown early in the growing season is that if the soil is too cool, most seeds may fail to emerge and very young seedlings become damaged, requiring that the field be replanted, said Hall, professor of plant physiology.

The observation of a link between a type of stress protein, called a dehydrin, that Associate Professor Close has been studying and genetic variation in chilling tolerance in cowpea occurred in Riverside during an unusually cool spring in 1995. Ismail observed that one cowpea breeding line had emerged more successfully than a closely related line and several other lines and varieties.

Ismail had found just a few months prior that this line was also unique in that it contained a large quantity of a specific dehydrin, while the protein could not be detected in seeds of the other lines. Once the chilling tolerance trait had been observed in the field, the trio then hypothesized that there may be a cause and effect relationship between the dehydrin protein and the chilling tolerance trait, and they set out to test the hypothesis through conventional genetics and physiological studies.

They reported their initial observations and hypothesis in 1997 in the journal Crop Science. Earlier this year, they followed up with another publication in the journal Plant Physiology, where they reported results from studies involving purification of the dehydrin protein.

But, a missing link in the story was that the UCR scientists did not know what gene actually controlled the production of the protein. "Was it the gene that directly encodes the dehydrin protein, or was it a gene located elsewhere in the cowpea genome th at might encode something that flips the on/off switch for the expression of the dehydrin gene?" Close said.

Genes are the hereditary units contained in the DNA of every living cell. Each gene contains a specific sequence of chemicals, called bases, that directs a specific function in the organism, such as the manufacture of a protein or the triggering of an on/off switch. Not all genes are present or are expressed in each plant, however, which accounts for plant varieties that differ from one another in their ability to withstand such environmental stresses as drought, high temperatures, or cool soil conditions.

The publication in Proceedings of the National Academy of Sciences provides strong evidence that the gene which directly encodes the dehydrin protein determines the expression of the protein in the seed.

"This is a good example of the elegance and efficiency of nature in finding ways to help plants grow in diverse environments," said Close. "Solutions to problems in plant improvement are out there waiting to be discovered, and they can continue to be us ed in plant breeding, a genetic improvement method that has served humanity for centuries."

The study also confirmed another hypothesis made by the UCR scientists, that the chilling tolerance conferred by the presence of the dehydrin protein in the seed seems not to act through effects on plasma membrane leakage. The plasma membrane is the semi-permeable barrier at the outer edge of cells that helps them retain minerals and other ingredients. When the plasma membrane is damaged, these cellular ingredients leak out. This form of leakage is a common symptom of chilling damage that has been extensively studied by other scientists as a means to enhance chilling tolerance.

"This knowledge provides a new approach to working on improving chilling tolerance. With a specific gene now implicated in the ability of cowpea seeds to germinate in relatively cool soil temperature, scientists have a new tool for improving cowpeas and perhaps other crops", Hall said.

The UCR researchers said it is possible that the gene pools of other major warm-season crops contain similar forms of a dehydrin gene that would endow their seeds with tolerance to cool soil temperatures. "It certainly is worth looking through the germplasm of these other crop plants for this trait." Close said. "We are now able to target the search to a particular protein and part of the genome," Ismail added.

If other crops do contain a similar form of a dehydrin gene, those plants also could be bred to develop varieties with seeds that germinate and produce vigorous plants under cool soil conditions in the early spring. Through a classical plant breeding technique known as backcrossing, a new variety could be developed in as little as three years, Hall said. "We have he ability to remove a small piece from a single seed, determine if the relevant dehydrin protein is present in this piece, plant that seed, and it will still grow. This saves us one generation in each cycle of backcrossing," he said.

Should the gene pool of other crops not contain a similar gene for the chilling-tolerance protein, it may be possible to transfer the cowpea gene to other crops through genetic engineering so that they also produce seeds that contain the dehydrin gene responsible for chilling-tolerance during emergence, Close said.

"There are complexities with such 'transgenic' approaches, however, that may make it take longer and be more expensive to achieve a production-ready variety than would conventional plant breeding." Close said.

The University of California, Riverside ( is a doctoral research university, a living laboratory for groundbreaking exploration of issues critical to Inland Southern California, the state and communities around the world. Reflecting California's diverse culture, UCR's enrollment has exceeded 21,000 students. The campus opened a medical school in 2013 and has reached the heart of the Coachella Valley by way of the UCR Palm Desert Center. The campus has an annual statewide economic impact of more than $1 billion.

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