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The near-extinct American chestnut looks set to make a comeback. Genetically modified trees, which are resistant to a deadly fungus that has decimated the species, have produced the first resistant chestnuts. From these seeds, countless resistant trees could be grown in the wild.

An estimated 4 billion American chestnut trees () once covered the US, accounting for . But in around 1900, a lethal fungus called was accidentally imported in chestnut trees from Asia, and by the 1950s it had almost completely wiped out the American chestnut.

Over the past 20 years, the has been trying to turn the situation around. Led by and of the State University of New York in Syracuse, the team has used genetic engineering to create a strain of fungus-resistant chestnuts called Darling4.

The modified trees contain a gene from wheat called OxO, which makes an enzyme called oxalate oxidase that destroys the toxic oxalic acid made by the fungus, preventing cankers from forming on the tree. By-products from the enzyme’s action help the tree’s own natural defences to fight off the fungus.

Keep off my tree

Powell and Maynard’s group have now shown that the Darling4 trees and their first-generation offspring are more resistant than unaltered American chestnuts, but less so than the naturally resistant .

The team tested leaves and stems that were deliberately infected with the fungus. Powell says it would be better to test entire trees, but that will have to wait until the trees grow larger, which may take another two years.

The best news is that the resistance seems to be heritable through the chestnut seeds. This will make restoration simpler and faster, because growing trees from seedlings is faster than the current practice – growing trees from tissue-cultured plantlets derived from embryos found in the few surviving trees.

Newer strains are proving more resistant, outstripping the Chinese chestnuts. In a 2013 study, the outperformed Chinese chestnuts in the leaf and stem tests. In one experiment, leaves from conventional American chestnut trees sustained damage on average over 119 square millimetres, and Chinese chestnut leaves over 38, but the best new strains limited the damage to just 5 square millimetres ().

The chestnuts on this tree, enclosed in bags, are genetically modified to defy a deadly fungus (Image: William A. Powell/American Chestnut Research and Restoration Project)

The ultimate goal is to release the modified chestnuts into the wild. The team planted the first Darling4 chestnut in 2006, and there are now over 1000 modified trees at various sites in New York state, says Maynard.

“We hope to obtain regulatory approval for trees to be grown outside permitted plots within three to five years, at which point our transgenic trees could potentially be planted anywhere in the US,” says Maynard. “Once approved, they’ll be distributed to the public in a not-for-profit programme to restore the American chestnut tree.”

Breed them strong

There is also progress in . The in Asheville, North Carolina, is developing resistant American chestnuts by crossing them with Chinese chestnuts. This is a painstaking job: the crosses with most resistance must be picked out from each generation, and the Chinese component gradually bred out.

The latest generation is 94 per cent American chestnut, and some strains have resistance approaching that of the Chinese trees. “We can’t exceed it like Bill’s doing, we can only match it,” says of Pennsylvania State University in University Park. But she says the conventionally bred chestnuts may be more popular in areas where genetically modified crops are not normally grown.

Genetic modification has one key advantage, says Maynard. The wheat OxO gene alone confers strong resistance, whereas resistance in Chinese chestnuts relies on at least three genes, so it is harder to transfer. For extra security, his team is now creating strains with both the OxO gene and the Chinese chestnut genes.

Journal reference:

Leader: “Open your mind to the gene-modified American chestnut“