Another Scientific American article (William Powell, March 2014, pp54-57) inspired me to write this post, which details a story of genetic engineering a plant, but not a crop, but a tree! First, a quick background of why….
Apparently, in 1876 an unfortunate situation developed following importation of chestnut seeds from Japan, it turned out that these seeds were contaminated with spores from a fungus (Cryphonectria parasitica) to which American Chestnuts were highly sensitive, but the Japanese Chestnuts were immune. This fungus effectively strangles the tree through growth of its mycelial fans, which produce oxalic acid that destroys the bark of the tree while allowing growth of the fungus. It is this dead wood, produced by the action of the oxalic acid, that leads to strangulation of the tree as it tightens its grip on the trunk of the tree. only 50 years after the initial import of this deadly fungus more than 3 billion trees were dead!
A programme of research was initiated to produce hybrid trees by crossing Chinese variants, which are also resistant to the fungus, with American trees to produce a hardy hybrid, but this work will take many years. Therefore, in parallel a project was initiated to make use of, what at the time was a novel approach, genetic engineering of the plant. As is often the case in science this idea was built around a fortunate coincidence in which a group had isolated a wheat gene for oxalate oxidase, and introduced this gene into other plants using a well described engineering system called Agrobacterium. This enzyme was, of course, ideal for the proposed project as it breaks down oxalic acid the primary cause of the blight. In addition, they had available genes that produce antimicrobial peptides (AMPs) that disrupt C. parasitica infection and, as time passed, genome sequencing projects have pointed to the genes in Chinese Chestnut trees that are responsible for resistance to the fungus. The future looks promising for genetically engineering the tree instead of depending upon hybrids.
The use of the soil bacterium Agrobacterium tumefaciens is an interesting story in itself and a subject I enjoyed teaching about as a perfect example of using a natural system for advanced technology. The odd thing about this bacterium is that is has the ability to infect plants with its own DNA that makes the plant produce unusual amino acids, which it cannot synthesise itself. the result of this infection of foreign DNA is that the plant develops small tumours, but the bacterium benefits from the availability of these biomolecules. Genetic engineers were able to manipulate this system so that they could insert “foreign” DNA into the bacterial plasmid, in place of the tumour-forming components, and enable the bacterium to transfer this foreign DNA into a wide variety of plants in a stable a predictable manner. Eventually, the research group were able to develop the mechanisms for tissue culture of the genetically altered plant cells and a model system based on poplar trees was available to initiate the experimental approach to overcoming the blight infection.
There are now more than 1,000 transgenic Chestnut trees growing in field sites, public acceptance of this approach to restoring a small piece of biodiversity is good and the future holds a promising approach for further such experimentation. My own view is that this is a piece of genetic engineering that all sounds very good and very promising for the future. My only caution, also expressed by the researchers is the spread of the genetically modified seeds, which may help remaining trees recover from infection, may also lead to cross pollination with closely related plants. However, there are few trees closely related to the American Chestnut, so this seems unlikely. A good story that supports genetic engineering in plants!