Pathogen-induced expression of a blight tolerance transgene in American chestnut.

American chestnut (Castanea dentata) is a susceptible host of the invasive necrotrophic fungus Cryphonectria parasitica, which causes chestnut blight disease. The fungal pathogen attacks chestnut stems by invading wounded tissue and secreting oxalate. This process leads to the death of infected host cells and the formation of cankers, eventually girdling stems and killing the tree above the infections. To reduce damage caused by fungal oxalate, American chestnut has been genetically engineered to express a wheat oxalate oxidase (OxO). This enzyme degrades the oxalate produced by the pathogen and confers elevated tolerance to Cryphonectria parasitica infection. We report new lines of transgenic American chestnut that have been developed with the win3.12 inducible promoter from poplar (Populus deltoides) driving OxO expression. This promoter is responsive to both wounding and pathogen infection, with a low level of baseline expression. Targeted expression of OxO to wounded and infected tissue is sought as an alternative to constitutive expression for potential metabolic resource conservation and transgene stability over the long lifetime of a tree and over successive generations of breeding. Transgenic Castanea dentata lines harbouring the win3.12-OxO construct were evaluated for transgene expression patterns and tolerance to chestnut blight infection. OxO transcript levels were low in uninfected plants, but robust infection-induced expression levels were observed, with one transgenic line reaching levels comparable to those of previously characterized CaMV35S-OxO lines. In chestnut blight infection bioassays, win3.12-OxO lines showed elevated disease tolerance similar to blight-resistant Chinese chestnut (Castanea mollissima) controls.