Strelkov: Studies on the Genetic and Molecular Basis for Clubroot Resistance in Canola
Date: September 2015
Term: 5 years
Status: Completed
Researcher(s): Stephen Strelkov, Habibur Rahman, University of Alberta; Sheau-Fang Hwan, Jie Feng, Alberta Agriculture and Forestry; Gopalan Selvaraj, National Research Council Plant Biotechnology Institute
SaskCanola Investment: n/a
Total Project Cost: n/a
Funding Partners: ACPC
Project Summary
Clubroot, caused by Plasmodiophora brassicae, is a destructive soilborne disease that is now prevalent on canola (Brassica napus) in central Alberta and appears to be spreading to other regions of the Prairies. The results of this five-year project have generated information and tools that will benefit clubroot resistance breeding activities, help guide the development of recommendations for proper resistance stewardship and, in the long term, enable rational approaches to clubroot management through an improved understanding of clubroot pathogenesis and the host response.
Clubroot, caused by Plasmodiophora brassicae, is a destructive soilborne disease that is now prevalent on canola (Brassica napus) in central Alberta and appears to be spreading to other regions of the Prairies. Several clubroot resistant (CR) canola hybrids have been developed, however the resistance in these hybrids is most likely based on single CR- genes and single gene-based resistance may eventually become eroded or breakdown altogether. Therefore, as part of a proactive strategy for durable clubroot resistance, it will be important to: (1) pyramid or stack resistance genes in canola cultivars and (2) rotate resistance genes in clubroot infested fields.
The goal of this five-year project (2010 to 2015) was to improve knowledge of the genetic and molecular basis for clubroot resistance (CR) in canola, in order to facilitate the development of new resistant durable cultivars and other innovative clubroot management strategies. The project included four main objectives:
Identification of major resistance genes and the development of molecular markers.
Clarification of the behavior of these genes in different genetic backgrounds.
Evaluation of the feasibility of resistance gene pyramiding and rotation.
Identification and investigation of the biological function of host and pathogen genes differentially expressed during the infection process
Researchers first conducted a phenotypic evaluation of about 200 doubled haploid lines from two crosses between a clubroot susceptible and two clubroot resistant lines. The results showed that a major gene is involved in the control of clubroot resistance in these two populations, and will be an important germplasm for breeding programs in Canada. Researchers identified 12 molecular markers that demonstrated robustness in marker assisted selection, including in CR gene pyramiding for durable clubroot resistance, as well as in map-based cloning of the genes. Additional studies with a population stemming from another cross between a susceptible and a resistant parent identified a resistance locus that is linked to the clubroot resistance gene CRa; the resistance in this B. napus population is controlled either by the combined effect of this new CR gene and CRa, or by the new CR gene alone.
Researchers then evaluated the change in virulence of the population and single-spore isolate following five successive infection cycles on the same host cultivars; and assessed the potential durability of resistance in the current clubroot resistant canola cultivars. A total of seven Brassica genotypes were included in the study, including susceptible and resistant commercial hybrids currently on the market (designated CV-S and CV-R, respectively). The results clearly demonstrated the capacity of local P. brassicae populations to adapt to the selection pressure imposed by the planting of clubroot resistant canola genotypes. Under greenhouse conditions, genetic resistance was significantly eroded in as few as two cycles of exposure to the same single-spore isolate or population of the pathogen.
Figure 1: Clubroot disease index (severity) in Brassica host genotypes showing susceptibility [Chinese cabbage (ECD 05); commercial canola hybrid (CV-S)], moderate resistance [kale (ECD 15); commercial canola breeding line (8N823)], and resistance [Polish rape (ECD 02 and ECD 04); commercial canola hybrid (CV-R)] to Plasmodiophora brassicae. The host genotypes were inoculated, assessed for clubroot disease severity, and re-inoculated over a total of five cycles.
Source: Modified from LeBoldus et al. 2012
The significance of these tests has increased considerably since the 2013 and 2014 growing seasons, as several fields that had been sown to clubroot resistant cultivars developed higher levels of clubroot than expected in those years. The quick erosion or defeat of clubroot resistance that was observed under greenhouse conditions through the pathogen cycling experiments seems to have accurately predicted what would happen under field conditions. The resistance, at least on the present set of clubroot resistant canola cultivars available, does not seem to be durable and will have to be managed carefully. This highlights the need for careful resistance stewardship, in particular longer rotations out of canola in fields where clubroot is an issue.
Similarly, a study on the impact of resistant cultivars on soil inoculum levels demonstrated how even relatively small percentages of susceptible plants within a resistant crop can help to maintain inoculum levels, reducing the effectiveness of rotations. These results are being used to formulate recommendations for farmers.
To improve the understanding of the mechanisms associated with clubroot disease development and resistance, researchers analyzed RNA from resistant and susceptible canola genotypes. They also investigated the roles of primary and secondary resting spores (zoospores) of P. brassicae in host/non-host resistance and clubroot pathogenesis. Researchers found that secondary zoospores produced on a nonhost (ryegrass) were able to infect a host (canola). Moreover, primary zoospores were observed to cause secondary infection when the host already was under primary infection, suggesting that P. brassicae uses primary infection to overcome the basal resistance of the plant to cortical infection.
Significant insights were obtained on the role of primary and secondary infection on clubroot pathogenesis, host and non-host resistance, and molecular and histological changes associated with resistance and susceptibility to P. brassicae. This information will facilitate efforts to gain a better understanding of the mechanisms of host resistance (and pathogen virulence), which will facilitate the rational development of novel and sustainable clubroot management strategies.
Collectively, the research conducted in this project has generated information and tools that will benefit clubroot resistance breeding activities, help guide the development of recommendations for proper resistance stewardship and, in the long term, enable rational approaches to clubroot management through an improved understanding of clubroot pathogenesis and the host response. This project will also help facilitate the development of new and durable varieties of CR- resistant canola.
Scientific Publications
Rahman, H., Fredua-Agyeman, R., Kulkarni, M., and Selvaraj, G. xxxx. Studies on the genetic and molecular basis for clubroot resistance in spring canola introgressed from European winter canola cv. Mendel. Euphytica (submitted).
Zhang, H., Feng, J., Hwang, S.F., Strelkov, S.E., Falak, I., Huang, X., and Sun, R. 2015. Mapping of clubroot (Plasmodiophora brassicae) resistance in canola (Brassica napus). Plant Pathol., In Press (Available Online), Doi: 10.1111/ppa.12422.
Feng, J., Cao, T., Rennie, D.C., Strelkov, S.E., and Hwang, S.F. 2014. Host-parasite interactions in clubroot of crucifers. Can. J. Plant Pathol. 36(S1): 113-121.
Strelkov, S.E., and Hwang, S.F. 2014. Clubroot in the Canadian canola crop: 10 years into the outbreak. Can. J. Plant Pathol. 36(S1): 27-36.
Rahman, H., Peng, G., Yu, F., Falk, K.C., Kulkarni, M., and Selvaraj, G. 2014. Genetics and breeding for clubroot resistance in Canadian spring canola (Brassica napus L.). Can. J. Plant Pathol. 36(S1): 122-134.
Feng, J., Hwang, S.F., and Strelkov, S.E. 2013. Studies into primary and secondary infection processes by Plasmodiophora brassicae on canola. Plant Pathol. 62:177- 183.
Hwang, S.F., H.U. Ahmed, Q. Zhou, A. Rashid, S.E. Strelkov, B.D. Gossen, G. Peng and
G.D. Turnbull. 2013. Effect of susceptible and resistant canola plants on Plasmodiophora brassicae resting spore populations in the soil. Plant Pathol. 62: 404-412.
Feng, J., S.F. Hwang and S.E. Strelkov. 2012. Analysis of expressed sequence tags derived from a compatible Plasmodiophora brassicae-canola interaction. Can. J. Plant Pathol. 34: 362-574.
LeBoldus, J.M., Manolii, V.P., Turkington, T.K., and Strelkov, S.E. 2012. Adaptation to Brassica host genotypes by a single-spore isolate and population of Plasmodiophora brassicae (clubroot). Plant Dis. 96:833-838.
Feng, J., Xiao, Q., Hwang, S.F., Strelkov, S.E., and Gossen, B.D. 2012. Infection of canola by secondary zoospores of Plasmodiophora brassicae produced on a nonhost. Eur. J. Plant Pathol. 132: 309-315.
Full Report PDF: Studies on the Genetic and Molecular Basis for Clubroot Resistance in Canola