Wheat, New and Improved

By Betsy HansonDec 1, 1992 6:00 AM


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Tomatoes have been done, and potatoes, too. Tobacco is old hat. In the decade since the first foreign gene was successfully introduced into the first plant, even rice and maize have yielded to the manipulations of genetic engineers. But wheat, their sister cereal--the most widely planted grain in the world (more than half a billion acres each year)--has resisted all efforts to improve its genome. So to make the first genetically engineered wheat, succeeding where they themselves and others had failed, Indra Vasil and Vimla Vasil of the University of Florida in Gainesville had to try something a little different.

Splicing genes into a tomato plant has become almost routine. To isolate tomato cells that have the ability to grow into a whole new plant-- you don’t want them to just languish in a petri dish after you’ve inserted your new gene in them--all you have to do is wound a tomato leaf. One way is to punch a piece out of it with a standard office hole punch. The cells around the wound lose their identity as leaf cells; they regress to an undifferentiated state in which they are capable of becoming stem or root as well as leaf.

Even better, they become vulnerable to infection by a bug called Agrobacterium, which slips one of its own genes into their DNA. If you first add a gene to the right part of the bacterial DNA, Agrobacterium will ferry it into the wounded cells on the leaf piece. Bathe the leaf piece in the proper nutrients and hormones, and it will sprout shoots, each one carrying the foreign gene.

Neither element of this strategy, however, works in the case of wheat. For one thing, Agrobacterium doesn’t infect grasses like wheat. For another, it’s hard to grow a wheat plant from a leaf snippet; instead of de-differentiating, the cells around the wound usually just die.

The Vasils and their colleagues solved the latter problem with an approach that had already worked with rice and corn. They took very young cells, cells that had not yet differentiated, from seed embryos on wheat flowers. The embryos have to be removed at a very specific stage of development, about 10 to 12 days after the flowers have been pollinated, says Indra Vasil. In test-tube cultures, the embryo cells continued to grow, dividing into clumps of undifferentiated cells.

To add a gene to the growing cells, the Vasils first tried using an enzyme to disintegrate the cells’ outer barrier--the cell wall--and an electric shock to punch holes in the inner barrier, the cell membrane. That allowed loops of bacterial DNA containing the desired gene--one that would make the wheat cells resistant to a common herbicide called Basta--to enter the cells. It’s a very good system, Vasil says, and it has worked for rice, corn, and many other plants.

Yet it didn’t work for wheat. The researchers could grow whole plants from cells without the new gene. And they could engineer the gene into wheat cells. But for reasons they don’t understand, they couldn’t coax cells carrying the new gene to grow into plants. Somehow, during the Vasils’ long process of selecting the cells and culturing them and getting the gene inside them, the cells had lost their ability to become full-grown wheat.

Faced with this dead end, the researchers decided on a violent shortcut. They went back to the clumps of undifferentiated cells, walls and all. Then they reached for their gene gun--a laboratory device in which a gunpowder discharge drives tiny gold pellets (so tiny they can penetrate a cell without damaging it) down a plastic .22-caliber barrel. The Vasils coated the pellets with DNA containing the Basta-resistant gene and fired on a petri dish full of wheat cells from a point-blank range of five inches.

This time they scored. When the researchers added Basta to the cells’ nutrients, cells without the gene died, while the ones that had taken it up grew into Basta-resistant plants. The plants flowered, and when they were crossed with normal wheat plants, their progeny also carried the gene.

This first type of transgenic wheat may prove eminently useful: farmers in the future may be able to spray Basta on their wheat fields, knowing that only the weeds will be killed. And now that the Vasils know how to do it, they would like to add other new genes to wheat. They envision wheat impervious to fungi and bacteria. They foresee high-protein wheat that yields more nutritious bread. The good that such new and improved wheat could do, especially in areas of the world where men must live by bread alone, would make the long quest to create it well worth the effort.

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