Bonham Smith: Using Non-host Species To Identify Novel Genes For Durable Clubroot Resistance in Canola
Date: December 2017
Term: 5 years
Status: Completed
Researcher(s): Peta Bonham-Smith, Tagnon Missihoun, Chris Todd, Yangdou Wei, University of Saskatchewan; Gary Peng, Bruce Gossen Agriculture and Agri-Food Canada, Saskatoon SK; and Simeon Kotchoni, Rutgers University, Camden NJ
SaskCanola Investment: n/a
Total Project Cost: n/a
Funding Partners: ADF
Project Summary
Clubroot, a serious threat to canola (Brassica napus) yields across Canada, is caused by the intracellular parasite Plasmodiophora brassicae. Researchers conducted a four-year study based on previous studies to identify the differences in host and non-host (Brachypodium distachyon) physical and genetic responses to infection by P. brassicae, and to identify durable resistance that may be transferred from non-host plants to canola. Overall, the results of the project established B. distachyon as a non-host species for clubroot disease, however, in general B. distachyon does not respond at the genomic level to an infection by P. brassicae. Therefore, researchers recommend that understanding the compatible host-pathogen system is a higher priority for the identification of plant resistance than understanding non-host systems.
Clubroot, a serious threat to canola (Brassica napus) yields across Canada, is caused by the intracellular parasite Plasmodiophora brassicae. Most existing sources of plant resistance to clubroot have been found to be pathotype-specific with no overarching resistance gene(s) identified. To date, 20 pathotypes of P. brassicae have been identified, with a rapid rate of pathotype evolution being driven by the monogene resistance selection pressure of current commercial canola germplasm. Moreover, the number of resistance genes available to canola breeders is limited.
Researchers at the University of Saskatchewan, in collaboration with Agriculture and Agri-Food Canada and Rutgers University conducted a four-year study (2014-2017) to identify the differences in host and non-host physical and genetic responses to infection by P. brassicae. The two main objectives of the project were to verify Brachypodium distachyon (temperate wild grass species recognised as the new model plant for temperate grasses and herbaceous energy crops) as a non-host species for P. brassicae infection and subsequent clubroot development; and to profile the transcriptome of the pathogen and non-host plant during infection to identify non-host plant genes likely involved in the mechanism of disease prevention.
Researchers based their approach on previous studies in which expression of non-host species ‘resistance’ genes in susceptible hosts resulted in improved disease tolerance in these crop plants (e.g. wheat, soybean and barley). This is believed to be the first time that such an approach to identify resistance to clubroot in canola, caused by P. brassicae, has been attempted. Researchers utilised an established tool box of histochemical staining techniques for the in planta identification of the various life cycle stages of P. brassicae developed in a previous project (CGDP.SCDC.ADF2010-98).
Researchers first established the primary infection by P. brassicae in root hairs of B. distachyon. However, unlike control B. napus host plants, infected B. distachyon plants did not produce root galls. PCR analysis of root tissue from control and infected B. distachyon and B. napus confirmed the absence of a secondary infection in B. distachyon, thereby establishing B. distachyon as a non-host species for P. brassicae infection. This suggests that B. distachyon is able to recognize and either eliminate the pathogen from the plant or prevent the completion of the pathogen life cycle, preventing the onset of clubroot disease and the formation of root galls.
Researchers next conducted transcriptome sequencing of P. brassicae-infected non-host B. distachyon plants to highlight the genomic difference between a host plant susceptible to clubroot and a non-host plant resistant to clubroot disease. RNA-seq was carried out on root tissue of B. distachyon inoculated with fresh P. brassicae pathotype 3 resting spores. Infected and control non-infected or mock-treated plants were sampled at 1, 2, 5 and 7 days post inoculation (dpi). Comparative heat map analyses of the transcriptomes showed that both B. distachyon and P. brassicae transcript levels demonstrated very little difference between infected and mock-infected plants. These results suggest that while B. distachyon does ‘host’ a primary infection, it prevents a secondary infection by P. brassicae producing no root galls, thereby preventing the establishment and progression of clubroot disease. Researchers will be comparing the 40 ‘highest reads’ from this study with the ‘highest reads’ from the primary infection stage in the host plant Arabidopsis in another study (ADF #20160138).
Overall, the results of the project established B. distachyon as a non-host species for clubroot disease, however, in general B. distachyon does not respond at the genomic level to an infection by P. brassicae. Without fully understanding the compatible interaction between the Brassicacea and P. brassicae it is very difficult to understand this non-host interaction, particularly in light of very little genome activity from both the non-host and the pathogen. Therefore, researchers recommend that understanding the compatible host-pathogen system is a higher priority for the identification of plant resistance than understanding non-host systems.
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