Objective
- Screen the Brassica napus TILLING population for resistance to sclerotinia stem rot (~700 mutant lines) and additional ~400 remaining lines for clubroot resistance (ie they screened some lines already during Peng CARP2023.22). Mutant lines with reduced susceptibility or enhanced resistance, and/or broad-spectrum efficacy against these pathogens will be identified.
- Characterize candidate S genes associated with the selected resistant mutant lines through genetic mapping and sequence analysis to determine their identity and potential function. Genomic regions corresponding to these S genes will be defined, and their sequences and associated molecular functions will be characterized.
- Validate the function of top S-gene candidates and assess the feasibility of genome-editing approaches for their application in disease-resistance breeding. Genetic markers tightly linked to key S genes will be developed. Canola lines homozygous for beneficial S-gene alleles are generated through marker-assisted selection and evaluated against diverse pathotypes or strains of both pathogens.
Project Description
Host resistance and susceptibility represent two contrasting facets of plant responses to pathogen infection. Historically, crop disease management has primarily relied on the deployment of resistance (R) genes, which recognize specific pathogen patterns/effectors and trigger defense responses. However, resistance conferred by R genes is often short-lived, as pathogens rapidly evolve new virulence alleles to overcome recognition. In contrast, host susceptibility (S) genes are intrinsic host factors that pathogens exploit to promote infection or that act as negative regulators of plant immunity. Disrupting such genes can impair pathogen growth and/or colonization, thereby providing durable resistance that can be broad-spectrum. Notable examples include the mlo allele in barley, which has provided powdery mildew resistance for decades, xa13 in rice conferring resistance to Xanthomonas bacteria, and eIF4E variants that block infection by potyviruses. Recent genome-editing studies in wheat, tomato, cucumber, and grapevine confirm that targeted S-gene disruption (e.g., MLO homologs) can yield durable mildew resistance without major yield penalties.
In Arabidopsis and related Brassica species, several S genes have been implicated in susceptibility to pathogens. For instance, PMR4/GSL5, a callose synthase gene, negatively regulates defense; loss-of-function mutants exhibit enhanced resistance to powdery mildew. Another study reported this year identified GSL5 as a key S gene for clubroot in Brassica crops and showed that genome editing to inactivate GSL5 confers broad-spectrum, durable resistance to P. brassicae without yield penalties. In our current collaboration with the University of Saskatchewan, we also found that inactivation of PMR4 by genome editing significantly reduced the susceptibility of B. napus to multiple P. brassicae (clubroot) pathotypes. Similarly, PEN1, PEN2, and PEN3 are required for penetration resistance against multiple fungal pathogens, and their mutation alters the susceptibility of host plants. Members of the SWEET sugar transporter family act as susceptibility hubs by providing carbon to pathogens; disruption of SWEET homologs in rice and Brassica reduces bacterial and fungal colonization. These findings demonstrate that S genes often act at fundamental host–pathogen interfaces such as cell wall reinforcement, nutrient transport or immune suppression. Despite these advances, relatively little is known about S genes in canola. To address this gap, we have been collaborating with the University of Saskatchewan to investigate UBC-13 and PMR4 (ADF- 20210893; ADF-20240861) as potential susceptibility targets for clubroot, with PMR4 emerging as a more promising S gene in terms of canola resistance response. Its resistance spectrum and agronomic trade-offs are being evaluated for potential deployment in breeding programs. In parallel, an ongoing project (CARP 2023.22) led by Dr. Peng has screened ~700 TILLING (Targeting Induced Local Lesions in Genomes) mutants generated from the AAFC’s susceptible B. napus doubled-haploid (DH) line 12075 by researchers at UBC. Several mutants showed markedly reduced susceptibility to blackleg under controlled conditions. Initial assessment of ~300 TILLING lines against clubroot (ie in their CARP2023.22 project that is ending Mar2026) has also identified three mutants exhibiting near immunity to multiple pathotypes, pointing to potentially novel resistance sources within this still underexploited TILLING population. Systematic screening against the stem rot may uncover additional mutants with altered disease responses, which may reveal S genes functioning either specifically against S. sclerotiorum or more broadly across multiple pathosystems. Using this unique genetic resource to uncover novel S gene variants is of great value to management of clubroot and sclerotinia stem rot. Both diseases are major threats to canola production in western Canada. There are only limited number of clubroot resistance genes available, and most canola cultivars have little resistance to sclerotinia stem rot.
Discovering, characterizing and validating any S genes for the stem rot or additional S genes for clubroot would represent a major breakthrough: (i) expanding the catalog of S genes in canola, (ii) establishing proof-of-concept that S gene disruption can generate resistance to multiple economically important pathogens, and (iii) providing gene-editing targets and effective molecular markers for deployment in breeding programs. Together, these advances could shift the paradigm of disease resistance breeding in canola by integrating S gene–based strategies with traditional R-gene based breeding, accelerating the development of cultivars with broad-spectrum, durable clubroot resistance and reducing the dependence on fungicides for effective management of sclerotinia stem rot.
