Research Magazine :: Summer 1997 : Spreading Chestnuts Across the Land

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Spreading Chestnuts Across the Land
by Judy Bolyard Purdy

The amorphous blobs in Scott Merkle's test tubes don't look like much yet. But given enough time, coaxing and good, old-fashioned luck, the tiny yellow clumps one day may grow into towering American chestnut trees.

Experimental Cloning Strategy for Blight Resistant Chestnuts

STEP 1. Collect immature chestnut burs and dissect developing seeds.

Earlier this century, American chestnuts spread their massive limbs throughout the forests of Eastern North America. But that was before the chestnut blight fungus stowed away on a shipment of young trees from Japan and nearly wiped out chestnuts from Maine to Georgia.

STEP 2. Put immature seeds (embyos) in culture with plant growth regulators. Some embryos will clone themselves.

"Although a few have sprouted back from the original stumps or, more rarely, from mature nuts, most don't get large enough to reproduce," said Merkle, a UGA forestry professor.

STEP 3. Use a gene gun to bombard embryos with gold particles that carry the blight-resistance gene and an antibiotic "selection" gene, which allows blight-resistant clones to grow on an antibiotic medium.

A successful chestnut cloning program would enable scientists to convert a few cells into a forest of chestnut trees valued for both their beauty and their lumber. Merkle already has successfully cloned other native hardwood trees: sweetgums for the pulp and paper industry, native magnolias for plant nurseries and experimental yellow-poplars for environmental cleanup.

STEP 4. Select for "transformed" clones with the resistance gene by growing them on an antibiotic medium (right) and then grow clones into transformed embryos.

He and his graduate students start by cutting immature fruit or flower parts - in this case nuts - into small pieces and treating them with various combinations of plant growth regulators. Among their sources for immature chestnuts is the American Chestnut Foundation, which, along with the UGA Daniel B. Warnell School of Forest Resources, helps fund the research.

STEP 5. Transformed embryos grow into plantlets.

It turns out chestnut cloning is a lot harder than Merkle had anticipated. Merkle said he thinks the problem stems from the large size of chestnut fruit compared with other trees he has cloned. Since 1990, his group has been trying to find the precise combination of laboratory conditions that will unlock the chestnut's genetic codes at just the right times.

STEP 6. Plantlets begin to look like seedlings.

"The big bottleneck is going from an embryo to early plantlet growth," Merkle said. "We've never gotten the embryos to look normal. It's hard to get them to root, much less develop shoots."

Because chestnut blight is still a threat, long-term survival of chestnuts also could depend on finding and then inserting a blight-resistance gene into the clones. Merkle's team has successfully inserted foreign genetic material into chestnut clones in anticipation of the day when such a gene is found. They use a gene "gun" to bombard embryos with microscopic particles of gold coated with bacterial genes, including marker genes that enable scientists to know which embryos contain the inserted genes.

STEP 7. Plantlets grow into blight-resistant saplings.

Another approach to long-term survival could lie in cloning chestnut trees that already are blight-resistant, such as American chestnuts crossbred with blight-resistant Chinese chestnuts. Merkle recently started a new program to grow embryo cells using flower parts from trees produced from just such crosses.

STEP 8. Years later, mature blight-resistant chestnut trees develop. Illustrations by Don Bagwell.

"Any of these techniques would enable us to grow thousands of individual trees with desired genetic traits," he said. "We might be able to re-establish the American chestnut tree in mature Appalachian forests and in people's yards. Companies might even take a chance on it for commercial growth for lumber."

And unlike the cloning of Dolly, the sheep, Merkle's chestnut cloning research hasn't caused a commotion.

"You can do lots of genetic engineering with trees," he said, "and in most cases no one gets upset."

For more information access http://www.uga.edu/~wsfr/, or e-mail Scott Merkle at smerkle@uga.cc.uga.edu.

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