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Mothproofing walnut trees of the future.

Mothproofing Walnut Trees of the Future

Tomorrow's walnut trees may be armed with a new weapon that better equips them to battle insect enemies.

Genetic engineers have given experimental walnut trees a mothproofing gene borrowed from the soil-dwelling bacterium Bacillus thuringiensis, or Bt. The gene may enable trees to manufacture a powerful Bt protein that kills many destructive insects when they are in their caterpillar stage.

The protein, however, is harmless to humans and other mammals, as well as to birds, fish, many insects, and other forms of life.

The gene might protect walnuts against attack by pests that chomp on leaves or ruin nut kernels. These insects include codling moth (also a major pest of apples), navel orangeworm, and Indianmeal moth, says Patrick V. Vail, an ARS research entomologist at Fresno, California.

At the Horticultural Crops Research Laboratory, and in the small experimental orchard about 200 yards from the back door of his laboratory, Vail plans to test trees that contain the new gene.

Vail says his experiments, planned for 1992, will reveal whether the present version of the Bt gene is powerful enough to knock out walnut's persistent pests. If it isn't, biotechnologists can rework the gene so that it packs more of a punch. The genetic engineers can modify the gene, for example, by attaching different promoters - segments of genes that turn its activity on or off, much like a light switch.

Vail's tests with leaves from the young orchard and, in about 3 years, the first nuts of the transgenic trees, will tell biotechnologists how well the gene is working and what parts of the trees contain the special protein.

That's important, Vail says, because right now scientists don't know where the protein ends up in a tree and how much of it the tree produces.

For the Bt-based strategy to work, trees must produce large enough amounts of the protein in nut kernels or perhaps in the hull - the thick outer layer that protects the familiar nutshell - to stop the voracious insects.

To start an experimental orchard of transgenic walnut trees at Fresno, Vail will rely on budwood cut from genetically engineered trees now growing in an orchard at the University of California at Davis. The first of its kind, the Davis orchard - now 2 years old - is the work of two UC Davis researchers - Abhaya M. Dandekar and former ARS scientist Gale H. McGranahan.

According to Vail, Bt has been widely used for about 30 years in spray formulas to kill caterpillars that attack home gardens, farm crops, and forest trees. The Bt protein targets caterpillars stomachs and plays such havoc with their digestion that insects simply stop eating and eventually starve to death.

By putting the Bt gene into walnut's genetic makeup, UC Davis genetic engineers may have sidestepped the need for some chemical sprays currently used in walnut orchards. Further, the team may have cut back the need for chemicals that today protect stored walnuts from insect attack in the warehouse, supermarket storeroom, or home pantry.

The idea of giving plants a Bt gene isn't new: Scientists elsewhere have already moved it into cotton and tomatoes, for example.

To transfer the gene to walnut, the Davis scientists bathed the tiny walnut embryos is a solution that contained the gene. In the laboratory, some, but not all, of the laboratory embryos took up the gene. The scientists nurtured offspring of those embryos into seedlings. Later, they grafted shoots from the seedlings onto rootstocks in the campus orchard.

The trio of walnut pests that the Bt protein might fend off causes at least $9 million in losses each year to the U.S. walnut industry. That's in spite of using the most effective chemicals available. California growers produce 99 percent of this country's walnut crop, worth about $191 million each year.

The codling moth's cream-colored caterpillar, about five-eights of an inch long, sneaks into walnuts through a soft, narrow passageway in the pointed tip of the developing walnut. This natural route allows the hungry caterpillar to reach the center of the nut.

Navel orangeworm's similarly sized, yellowish-white caterpillar may attack next. The caterpillar enters walnuts once the hull splits open. That happens naturally as the nut matures.

Unlike the codling moth and navel orangeworm, which typically attack walnuts still in the orchard, the Indianmeal moth becomes a nuisance once nuts are harvested and moved indoors. The moth's caterpillar, whitish and about a quarter inch long, wriggles into damaged walnuts. The worm can also crawl into poorly sealed bags of nutmeats.

If the 1992 Fresno tests show that Bt protects walnuts from these insect culprits, the scientists will have trimmed years off a conventional walnut breeding program. "Genetic engineering allows you to insert the gene of your choice directly into walnut," say Gary L. Obenauf of California's Walnut Marketing Board, cosponsor of the research. "And in the case of Bt, you're inserting a gene that you probably couldn't even get into walnut through traditional breeding." - By Marcia Wood, ARS.

PHOTO : English walnut trees.
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Author:Wood, Marcia
Publication:Agricultural Research
Date:Jun 1, 1991
Words:838
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