Developing Blight-Tolerant American Chestnut Trees

Abstract

An invasive fungal pathogen has reduced the American chestnut (Castanea dentata), once a keystone tree species within its natural range in the eastern United States and Canada, to functional extinction. To help restore this important canopy tree, blight-tolerant American chestnut trees have been developed using an oxalate oxidase-encoding gene from wheat. This enzyme breaks down oxalate, which is produced by the pathogen and forms killing cankers. Expressing oxalate oxidase results in blight tolerance, where the tree and the fungus can coexist, which is a more evolutionarily stable relationship than direct pathogen resistance. Genetic engineering (GE) typically makes a very small change in the tree's genome, potentially avoiding incompatible gene interactions that have been detected in some chestnut hybrids. The GE American chestnut also retains all the wild American chestnut's alleles for habitat adaptation, which are important for a forest ecosystem restoration program.

Intentional introgression of a blight tolerance transgene to rescue the remnant population of American chestnut

“…In 1990, an effort was initiated to genetically engineer blight resistance into the American chestnut using only a few genes, with the goal of introgressing this blight resistance trait into the surviving remnant population (Powell, Newhouse, & Coffey, 2019). This effort follows the same intended consequence as earlier efforts: genetically altering the chestnut tree to be resistant to the blight.…”

“…The same mechanism of fungal toxin degradation is expressed in many crop plants (Lane, 2002), and a similar mechanism is present in Chinese chestnuts: in both cases the pathogen resistance has been stable and durable, without increases in pathogen virulence. We expect blight tolerance in GE chestnuts to be similarly robust, but we are nevertheless testing combinations of other traits (through either GE or backcross breeding) or treatments (such as biocontrols) that should provide additional durability and resilience across temporal and ecological scales (Powell et al, 2019; Westbrook, 2018).…”

Medical biotechnology as a paradigm for forest restoration and introduction of the transgenic American chestnut

“…A transgenic chestnut tree has been developed that has an inserted gene from the wheat plant that codes for the oxalate oxidase enzyme (OxO) (Powell et al. 2019). In this tree, the biochemistry of the OxO gene product provides a strong rationale to allay concerns.…”

“…The OxO gene is widely present across plant species already. The transgenic chestnut would simply increase by one the number of wild and cultivated plants known to contain the OxO gene; there are more than 30 such plants (Powell, et al 2019).…”

“…As noted above, a transgenic American chestnut tree has been developed that has an inserted gene from the wheat plant that codes for OxO (Powell et al. 2019). Oxalic acid is produced by the blight fungus, and it destroys chestnut tree tissue, forming killing cankers.…”