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Apomixis: it could revolutionize plant breeding.

It looked like any ordinary forage grass, but it turned out to be a genetic treasure for E.C. Bashaw. For the buffelgrass plant discovered in a Texas pecan orchard about 30 years ago gave Bashaw, a U.S. Department of Agriculture plant geneticist, a way to use a trait called apomixis in plant breeding.

The word apomixis (pronounced AP-o-MIX-sis) comes from the Greek apo (from) and mixis (a mingling). It is sexual reproduction through seed. In apomictic plants, the embryos grow from vegetative cells without being fertilized by pollen, which contains the male sperm in plants.

Today, apomixis is being pursued by scientists with USDA's Agricultural Research Service as a tool for creating hybrids that produce generation after generation of seed that retains its vigor and produces plants identical to the female parent.

Researchers have identified apomixis in more than 35 families of plants--including more than 300 species. Apomixis is known to exist in many subtropical and tropical forage grasses, in citrus, and in wild relatives of beets, strawberries, mangos, corn, and wheat.

Since the early 1950's, Bashaw had been working with limited success on apomixis. The problem was that he hadn't been able to find a sexual plant of buffelgrass, a warm-season, drought-tolerant forage grass native to Africa. All buffelgrasses were thought to be apomictic--which ruled out cross-breeding, since the plants reproduced asexually. Without a sexual buffelgrass, there was no way to transfer genes from one apomictic plant to another--leading one prominent geneticist to call apomixis a "dead end."

And it might have been, it not for the curiosity of Pat Higgins, a seed grower who spotted what turned out to be a very rare buffelgrass plant growing in his pecan orchard in Southerland Springs, Texas, around 1960.

Finding the One in a Million

The plant Higgins found in the pecan orchard was next to a field of buffelgrass introduced from Africa and planted for seed production. Since researchers believed that all buffelgrasses were apomictic, Higgins expected all the plants to be identical. They were--except for that one stray plant.

"Pat noticed that it was different," Bashaw recalls. "If he hadn't noticed that, we might have missed a great discovery."

Higgins went a step further, sowing the seeds from the stray plant. The result was dozens of plants of different sizes and shapes. He called Bashaw, who later confirmed that Higgins had found the first buffelgrass plant ever discovered to reproduce sexually. Among the wide variety of offspring were both sexual and apomictic plants.

It was a breakthrough for Bashaw, who understood the potential of apomixis but hadn't been able to take full advantage of it. Today, Bashaw and other scientists are closer to turning apomixis from a little-known reproductive mechanism into a new tool for developing improved plant varieties that retain acquired traits indefinitely. That could mean a boost in seed quality--and a more bountiful food supply.

Plant breeders want to use apomixis to lock in traits such as high yields, disease and insect resistance, and other key improvements into plants such as corn, wheat, rice, and forage crops.

Bashaw and other breeders realized that apomixis could help overcome the shortcomings of hybrids, which only retain their vigor and uniform genetic traits for one generation--meaning a farmer must buy and plant new hybrid seed each year.

If an apomictic gene could be placed in a hybrid, it wouldn't be necessary to produce new hybrid seed each year. That's because a key trait of apomictic plants is that they produce generation after generation of seed that, in turn, yield identical offspring--since they have the genetic characteristics of only their single parent. Each offspring is just as vigorous as its original female parent.

"Here was a system that could hold beneficial genes in place forever in a hybrid," Bashaw says. "And there is also great potential for transferring apomictic genes from wild species into cultivated corn, rice, wheat, and other plants."

It was considered a long shot until Higgins found the sexual buffelgrass, which could be used as the female parent in buffelgrass breeding, to unlock traits that had been fixed in apomictic buffelgrasses.

Bashaw and graduate students at Texas A&M University in College Station were eventually able to manipulate the apomictic gene in buffelgrass using the sexual plant discovered by Higgins. It was the first time plant breeders had ever accomplished this.

In fact, the grass that Higgins found became the parent of 'Higgins' buffelgrass, released in 1967. The first apomictic variety to be developed, 'Higgins' produced higher yields and more extensive roots, and it was more persistent than earlier varieties. In 1981, ARS and Texas A&M University released two more apomictic hybrid buffelgrasses, Nueces and Llano. They had even greater yields and cold tolerance than 'Higgins' and were higher in digestibility for cattle--meaning the animals would gain weight faster.

The next apomictic crop to be developed by breeders could be pearl millet, a drought-tolerant grain, says Wayne Hanna, an ARS plant geneticist at Tifton, Georgia.

Hanna got his first taste of apomixis as a graduate student at Texas A&M, where he--like Higgins--saw something that aroused his curiosity. Hanna noticed an unusual looking sorghum plant and brought it to the attention of his major professor, Keith Schertz, and to Bashaw. The sorghum plant turned out to be the first apomictic plant found in a grain crop.

Based at the Georgia Coastal Plain Experiment Station, Hanna has been working since 1978 on apomixis in pearl millet, a dryland grain crop similar to sorghum that is planted in the southern United States and on millions of acres in Africa and Asia.

He and researchers with ARS and the University of Georgia are close to developing an apomictic pearl millet. They've found that a few tightly linked genes in a single chromosome are all that may be required to transmit apomixis from the wild variety to the cultivated one. They're using biotechnology techniques such as polymerase chain reaction to mark the apomictic genes in Pennisetum squamulatum, a wild relative of pearl millet, and to see if they are transferred into cultivated pearl millet, P. glaucum, during breeding. If they're successful, it would be the first time an apomictic gene had ever been transferred from a wild species into a cultivated grain crop.

In the summer of 1992, Hanna began screening more than 27,000 pearl millet backcrosses growing at Tifton. Initially, he explains, they developed a hybrid cross between the wild and cultivated pearl millet that contains the apomictic gene. Then, they began backcrossing--which means they crossed that original hybrid with cultivated pearl millet, trying to bring the resulting backcrosses closer to cultivated pearl millet while screening out unwanted traits from the wild relative.

Hanna says, "We have some good-looking plants that we know have the gene for apomixis, but the trouble is that many of those plants also have undesirable genes from the wild species. It takes a long time to narrow it down to the best plants."

Hanna says another benefit of apomixis is that, once an apomictic plant is developed, its traits can be bred further by putting its pollen onto a sexual plant. The breeding process can continue as long as you have a sexual plant to make the cross with.

Hanna and other apomixis researchers are setting out to clone, or duplicate, the genes for apomixis and transfer them to other plants--such as corn, rice, or wheat. "There's no reason to think it can't be done with the help of biotechnology, but it will take time," Hanna says.

Breeding apomixis into wheat is under way at Utah State University and the ARS Forage and Range Research Unit in Logan, Utah, where university and ARS scientists are working with an apomictic relative of wheat called Australian wheatgrass, Elymus rectisetus, a wild grass native to that country. University geneticist John Carman, ARS plant physiologist Jerry Chatterton, and geneticist Richard Wang have crossed the Australian wheatgrass with cultivated wheat and are now making backcrosses. The scientists are confident that, eventually, they'll develop an apomictic wheat. But, like Hanna, they say it will take time.

"I don't think there's any question that we'll be able to do it," Chatterton says. "But it'll take a while for us to get it into a germplasm that can be used to make bread."

The same is true for getting apomictic corn into production. ARS scientists at Temple, Texas, have begun work to transfer apomixis from a wild corn relative, called eastern gamagrass, Tripsacum dactyloides, into cultivated corn. As has been done with pearl millet at Tifton and with wheat at Logan, Temple researchers have made the initial crosses between corn and eastern gamagrass. Crosses were made in 1991 and 1992, and the resulting hybrids--4 the first year and about 50 in the second--are growing in greenhouses, according to Byron Burson, a plant geneticist at Temple.

"The hybrids recently began to flower, and we are in the process of determining if apomixis is expressed in any of them," he says. "We've also begun to make backcrosses with corn to move genetically closer to the commercial variety while hopefully retaining the gene for apomixis."

Burson and plant geneticist Paul Voigt, both 25-year veterans of apomixis studies, have also been investigating the phenomenon in two important forage grasses--lovegrass and dallisgrass.

But while transferring apomixis into corn, wheat, and pearl millet is under way, scientists have not thus far succeeded with rice. Before becoming associate director of the ARS Midsouth Area, J. Neil Rutger worked for 3 years in the agency's Crops Pathology and Genetics Research Unit in Davis, California, trying to find apomixis in rice. Although unsuccessful, Rutger believes someone will eventually succeed and that apomictic hybrids will be developed.

"That would be particularly important in rice, because rice is highly self-pollinating and we haven't been able to produce hybrids in this country," he says. "The Chinese produce rice hybrids, but it's very costly and labor-intensive and their grains aren't of high enough quality for U.S. markets. With apomixis, we could produce higher quality rice hybrids."

Inducing Sexual Reproduction

If sexual varieties can't be found for breeding with apomictic ones, there may be another alternative: inducing sexual reproduction in apomictic plants. At the agency's U.S. Regional Pasture Research Laboratory in University Park, Pennsylvania, scientists have shown that certain salts can actually induce apomictic plants to develop sexual embryos.

Plant pathologist Robert T. Sherwood, plant physiologist David L. Gustine, and microbiologist Yannis Gounaris, in cooperation with Penn State University scientists, were the first to show that an apomictic plant could be made to reproduce sexually by stressing it with salts.

The scientists are not certain why this occurs, but it may be because salt gives the upper hand to sexual embryo sacs inside the plant. These sacs are equivalent to the uterus in the human female. Sherwood says apomictic plants have both sexual and apomictic sacs, but that usually the apomictic embryos are dominant and the sexual sacs wither away and never develop. The salt, however, appears to reverse this, allowing the sexual sacs to outcompete the apomictic ones.

The researchers at University Park have also found that apomixis in buffelgrass-their model system--appears to be controlled by a single gene. "This finding is important because it tells us it should be possible to isolate that gene using biotechnology and then insert it into related sexual plants so that they will have apomictic reproduction," he says.

Those plants may not be producing food until the 21st century--long after Pat Higgins found that stray buffelgrass plant--but at least apomixis is no longer a dead end for breeders.
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Author:Adams, Sean
Publication:Agricultural Research
Date:Apr 1, 1993
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