As molecular plant pathology research advances, more and more genes are being identified with a plant's ability to defend itself against disease (see Powell et al. 2006). To enhance blight resistance in the American chestnut tree, we are studying several gene products, regulatory regions of genes to control expression (promoters), and the possibility of combining two or more (gene stacking or pyramiding) for better and more sustainable resistance.
Sampling tissues at the margin of a blight canker on an American chestnut tree.
The first gene being tested in transgenic American chestnut encodes an enzyme called oxalate oxidase. This particular gene comes from wheat, but is found in many grass species. We have shown that this gene can enhance resistance to a fungal pathogen in transgenic poplar (see Liang et al. 2001). The oxalate oxidase gene is also being tested by other researchers to enhance pathogen resistance in many other plant systems such as peanut, soybean, sunflower, and oilseed rape and also to possibly increase salt tolerance in tomato. This gene was chosen to be used in American chestnut because the blight causing fungus, Cryphonectria parasitica, produces large amounts of oxalate at the margin of a canker. The oxalate oxidase enzyme can detoxify the oxalate by breaking it down to carbon dioxide and hydrogen peroxide. The hydrogen peroxide might have a second function which helps strengthen the lignin in the barrier produced by the chestnut in an effort to wall-off the fungal infection. We have some evidence that the oxalate oxidase enzyme can enhance lignin formation (see Welch et al. 2006). The first transgenic American chestnut trees planted in field tests contain the wheat oxalate oxidase gene controlled by a a vascular promoter isolated from soybean so that the enzyme is predominantly expressed in the stem tissues where infections occur.
Other genes that will be tested include a gene encoding an antimicrobial peptide which we have shown to enhance fungal resistance in poplar (see Liang et al 2002), a stacked (or pyramid) construct encoding the enzymes chitinase and oxalate oxidase (see Liang et al 2005), among other genes and gene promoters. We have also embarked on a multi-university collaborative effort to identify the resistant genes in Chinese chestnut (see Fagaceae genome project), which could be used to transform American chestnut in the future.
This is an exciting time for the transgenic American chestnut project as we begin our field testing of the trees. Over the next few years we will discover which genes will provide the highest levels of blight resistance while maintaining the characteristic and environmentally beneficial phenotype of the American chestnut.