The American Chestnut, Castanea dentata, was once a proud denizen of eastern U.S. deciduous forests, and prized for its wood. Then, in the early 1900s, the fungus “chestnut blight,” Cryphonectria parasitica, was introduced to the U.S. from Japanese nursery stock. Within a few decades, it wiped out around 4 billion chestnut trees. Since the fungus is airborne, a few adult trees have survived in the East if they’re several kilometers from the nearest tree, and some trees survive outside the natural range; but the species isn’t coming back. When a tree dies or is cut down within the natural ranges, saplings will sprout from the roots, but before the tree can reproduce it’s invariably killed by the fungus.
Restoring the tree has been a tough problem, as the fungus persists. The American Chestnut Foundation (ACF), a pretty big organization, has done its best by spreading seeds from fungus resistant trees and so on, but now there’s additional hope—thanks to genetic engineering. (Reader Hempenstein is responsible for sending me this brand-new information.) The chestnut is in fact now a Genetically Modified Organism (GMO), with a gene injected into the DNA that makes the tree resistant to blight.
First, check out the photo below, which has just been made public. Left: American chestnuts showing effect of blight. Center: GMO (transgenically engineered) chestnuts infected with blight. Right: Chinese chestnut (Castanea mollissima), showing its susceptibility to blight.
This is a small stem blight resistance assay of Ellis 1 wild type American chestnut (left), Darling 54 transgenic American chestnut (center), and Qing Chinese chestnut (right). The Ellis 1 and Darling 54 lines are clonal except that the Darling 54 has the oxalate detoxifying enzyme gene protecting it. All were infected with a highly virulent strain of the blight fungus, EP155. After one month, all the Ellis1 were wilted, all the Darling 54 survived (and are still surviving today), and five of the six Qing eventually wilted. This is demonstrates the high level of blight resistance in the Darling 54 line.
Interestingly, we can still isolate the blight fungus from the Darling 54, showing that the OxO doesn’t hurt the fungus. It just neutralizes its weapon, oxalate. This is important because by not killing the fungus it greatly reduces the selective pressure to select fungal mutations that may overcome the resistance. Therefore it should be a very sustainable resistance.
This resistance is heritable as a dominant trait and therefore when outcrossing with surviving wild type trees, half the offspring will be fully resistant. We also have a easy leaf disk assay that can identify which offspring carry the resistance gene. This will allow rescuing the genetic diversity of American chestnut that still survives in the forests.
You can see much more information (and a video of the blight-resistance assay) here. If the FDA, EPA, and USDA approves this (and I’m hopeful), the resistant seeds will be distributed for planting, and perhaps these giants will grace our forests again. I wonder if there will be a public outcry against the use of GMO chestnuts.
The fact that the resistance is dominant is a good thing, for any tree with the gene will survive, and those lacking it will not. That means that there’s no barrier to the spread of the resistant trees, even if the added gene gives them reduced fitness compared to the susceptible trees in the absence of the blight.
Here’s Powell talking about the significance of this tree,and describing the restoration project in a nice 15-minute TEDx lecture: