We have used potato and potato virus X (PVX) as a model system in which to investigate disease resistance in plants. We identified a gene in potato - Rx - that confers resistance against many strains of PVX. We also established that the resistance is induced when the Rx protein interacts with the viral coat protein. Our model for how the resistance operates involves conformational changes in Rx that are elicited by the coat protein. The conformational changes would either allow Rx to recruit cofactors that are required for resistance. Alternatively they would allow Rx to release cofactors so that they can mediate processes required for resistance. This model divides the resistance mechanism into separate recognition and response phases.
Indirect evidence indicates that the recognition process involves the LRR domain. To test this idea we randomly mutagenised the LRR of Rx and, consistent with the LRR recognition hypothesis, we identified mutants with novel recognition specificity. The mutations would have modified residues on the predicted solvent exposed face of the LRR domain in Rx as indicated by **** in this model of a repeat in the LRR.
This approach to modification of disease resistance may be generally applicable because there is a large family of NBS-LRR proteins in plants conferring disease resistance against all types of pests and pathogens including fungi, oomycetes, bacteria, aphids and nematodes. We intend to explore this possibility.
We also intend to explore other approaches to virus resistance in plants including those in which we exploit RNA silencing mechanisms described elsewhere on this website. A project in the current planning stage involves sweep potato virus disease in Africa. If our application for funding is successful this project will involve collaborators in Africa and at the International Potato Research Centre in Peru.
Recent and key(*) publications on disease resistance.
Tameling, W. I. L. and D. Baulcombe (2007). "Physical association of the NB-LRR resistance protein Rx with a Ran GTPase activating protein is required for extreme resistance to potato virus X." Plant Cell 19: 10.1105/tpc.107.050880
*Farnham, G. and D. Baulcombe (2006). "Artificial evolution extends the spectrum of viruses that are targeted by a disease-resistance gene from potato." Proceedings Of The National Academy Of Sciences Of The United States Of America 103: 18828-18833.
Mestre, P. and D. Baulcombe (2006). "Elicitor-mediated oligomerization of the Tobacco N Disease Resistance Protein." The Plant Cell 18: 491-501.
Peart, J., P. Mestre, et al. (2005). "NRG1, a CC-NB-LLR protein mediates resistance against tobacco mosaic virus together with N, a TIR-NB-LRR protein." Current Biology 15: 968-973.
*Moffett, P., G. Farnham, et al. (2002). "Interaction between domains of a plant NBS-LRR protein in disease resistance-related cell death." EMBO Journal 21(17): 4511-4519.
Peart, J. R., G. Cook, et al. (2002). "An EDS1 orthologue is required for N-mediated resistance against tobacco mosaic virus." Plant Journal 29(4): 569-579.
Peart, J. R., R. Lu, et al. (2002). "Ubiquitin ligase-associated protein SGT1 is required for host and nonhost disease resistance in plants." Proceedings Of The National Academy Of Sciences Of The United States Of America 99(16): 10865-10869.
Bendahmane, A., M. Querci, et al. (2000). "Agrobacterium transient expression system as a tool for the isolation of disease resistance genes: application to the Rx2 locus in potato." Plant Journal 21(1): 73-81.
*Bendahmane, A., K. Kanyuka, et al. (1999). "The Rx gene from potato controls separate virus resistance and cell death responses." Plant Cell 11: 781-791.
van der Voort, J. R., K. Kanyuka, et al. (1999). "Tight physical linkage of the nematode resistance gene Gpa2 and the virus resistance gene gene Rx on a single segment introgressed from the wild species Solanum tuberosum subsp. andigena CPC 1673 into cultivated potato." Molecular Plant-Microbe Interactions 12(3): 197-206.
Gilbert, J., C. Spillane, et al. (1998). "Elicitation of Rx-mediated resistance to PVX in potato does not require new RNA synthesis and may involve a latent hypersensitive response." Molecular Plant-Microbe Interactions 8: 833-835.
Bendahmane, A., B. A. Köhm, et al. (1995). "The coat protein of potato virus X is a strain-specific elicitor of Rx1-mediated virus resistance in potato." Plant J. 8(6): 933-941.
Goulden, M. G., B. A. Köhm, et al. (1993). "A feature of the coat protein of potato virus X affects both induced virus resistance in potato and viral fitness." Virology 197: 293-302.
*Köhm, B. A., M. G. Goulden, et al. (1993). "A Potato Virus-X Resistance Gene Mediates an Induced, Nonspecific Resistance In Protoplasts." Plant Cell 5(8): 913-920.