Karthikeyan Ponnienselvan

I'm a simple man. If I see a field trial of genetically-engineered trees, I read it. A recent preprint reports the largest field trial to-date of American Chestnut trees engineered to resist a blight. And the results look promising. For context: The U.S. was once filled with American Chestnut trees. There were billions of them, each with a natural lifespan of 500+ years. In the late 1800s, however, an import of Asian chestnut trees unintentionally introduced a hitchhiking fungus into U.S. nurseries. That fungus, Cryphonectria parasitica, was first identified on an American chestnut tree growing in New York in 1904 by a forester at the Bronx Zoo, named Hermann Merkel. The fungus spread through the U.S. in the early 20th century (its spores can float on the wind for several miles). By the 1950s, this fungus had infected basically every American Chestnuts growing in the U.S., killing at least 4 billion trees. (There are still some mature, intact trees in the U.S., but they are really rare. Maine has most of them, it seems, including one American Chestnut tree in Lovell that is 115 feet tall.) When the fungus latches onto a tree (in a small wound), the fungus grows and colonizes the inner bark. It chokes the tree, forms cankers, and prevents it from growing outwards, thus cutting off phloem transport to tissues above the wound site. The fungus eats the tree using a chemical called oxalic acid. In the mid-2010s, researchers at SUNY made a transgenic American Chestnut tree that can fend off the fungus. These researchers took a gene from wheat, called oxalate oxidase (or OxO), and spliced it into an American chestnut sapling. This gene expresses an enzyme that breaks oxalate into hydrogen peroxide and carbon dioxide, thus neutralizing it. And, importantly, the gene is driven by a promoter that expresses this protein in basically every tissue of the tree. The OxO gene does not make the trees immune; it merely helps them tolerate the fungus. Trees can still get cankers and spread the fungus. But anyway, onto this paper. This is a two-year field trial of transgenic saplings vs. wildtype siblings. It is the largest such trial to date; prior papers used like 3 trees. Whereas in this study, 261 trees were inoculated with the fungus in three replicates. Transgenic trees (carrying the OxO gene) “consistently outperformed their [wildtype] siblings” and also Chinese chestnut trees. Every year, these researchers inoculated the trees and then, 90 days later, measured the lengths of cankers (or fungal growth) around the trees. The trees were planted in an orchard in Cape Elizabeth, Maine. The key result is that transgenic trees fared better than their wildtype siblings on basically every metric. They had smaller cankers, and also released fewer fungal spores from those cankers. Growth was normal, despite one year of fairly significant drought conditions. This is a really promising sign for the American Chestnut, perhaps saved by genetic engineering.