Understanding the Molecular Basis of Disease Resistance in Plants
Posted 13th October 2017 by Jane Williams
There has been tremendous progress in understanding the molecular basis of disease resistance in plants in the last twenty years. However, translation of this knowledge into practical use has been slow.
In many cases, the resistance of a plant to a particular pathogen strain occurs by recognition of pathogen proteins called effectors, which are virulence factors delivered into the host cell by the pathogen in the infection process. Effector recognition is mediated by nucleotide-binding, leucine-rich repeat (NLR) proteins encoded in the plant genome, and this recognition initiates defence responses by the plant. This recognition-based resistance can be very effective, but it can also rapidly be overcome if the pathogen evolves to lose or modify the recognised effector.
Rather than deploying genes individually in a race with pathogen evolution, a more effective strategy is to deploy multiple resistance genes at once in a gene stack, such that multiple independent mutations are necessary for a pathogen to overcome the resistance. Transgenic resistance gene stacks are effective against late blight in the field in potato, whereas individually deployed resistance genes can be overcome within a single season (Haverkort et al., 2016).
Renewed threat of wheat stem rust
Wheat is one of the most important crops on earth and, since its domestication, it has been beset by disease epidemics. A particularly important pathogen of wheat is stem rust which, under the right conditions, can wipe out an entire crop.
Stem rust resistant wheat lines were introduced during the Green Revolution of the 1950s. However, since 1999, new strains of stem rust have arisen in Africa that overcame this resistance. Since then, renewed efforts have focused on identifying new sources of resistance in related species. These resistance genes can be introduced into wheat through wide crosses, but it is a long and difficult process to get rid of unwanted characteristics that have been introduced from the wild species to create a new commercial wheat strain.
Developing durable rust resistance
2Blades and our collaborators are taking advantage of new, cutting-edge molecular tools and high-quality genome sequence information to isolate genes conferring stem rust resistance from wheat and its relatives and to create gene stacks for durable resistance. Methods such as sequence capture of NLR gene populations (Jupe et al., 2013, Steuernagel et al., 2016) and chromosome sorting (Sanchez-Martin et al., 2016) from resistant and susceptible wheat lines have increased the speed at which disease resistance genes can be isolated. We have introduced gene stacks containing up to six different stem rust resistance genes in a single construct into wheat, and these new lines are currently being characterised.
In the future, gene editing may allow the creation of gene stacks at a specific chromosomal site, or “landing pad,” facilitating the addition of new resistance genes to keep pace with pathogen evolution. Improved pathogen genomic information will allow monitoring of pathogen populations for effector loss, and knowledge of resistance gene-effector specificity will enable prediction of the best resistance genes to deploy. Appropriate deployment of resistance genes enabled by technology can allow us to stay one step ahead in the race against wheat stem rust and many other diseases.
Dr Lynne Reuber holds a Ph.D. in Biology from Massachusetts Institute of Technology and a B.A. in Biology and Chemistry from Drury University. She joined the 2Blades Foundation as Program Director in 2014.
To learn more about the future of gene editing technologies, listen to Dr. Lynne Reuber speak at the 5th Plant Genomics & Gene Editing Congress: USA. View the agenda here.
You can find a full list of references here.