Abstract:
Blackleg is one of the major fungal diseases in oilseed rape/canola worldwide. Most
commercial cultivars carry R gene-mediated qualitative resistances that confer a high
level of race-specific protection against Leptosphaeria maculans, the causal fungus of
blackleg disease. However, monogenic resistances of this kind can potentially be rapidly
overcome by mutations in the pathogen’s avirulence genes. To counteract pathogen
adaptation in this evolutionary arms race, there is a tremendous demand for quantitative
background resistance to enhance durability and efficacy of blackleg resistance in
oilseed rape. In this study, we characterized genomic regions contributing to quantitative
L. maculans resistance by genome-wide association studies in a multiparental mapping
population derived from six parental elite varieties exhibiting quantitative resistance,
which were all crossed to one common susceptible parental elite variety. Resistance
was screened using a fungal isolate with no corresponding avirulence (AvrLm) to
major R genes present in the parents of the mapping population. Genome-wide
association studies revealed eight significantly associated quantitative trait loci (QTL)
on chromosomes A07 and A09, with small effects explaining 3–6% of the phenotypic
variance. Unexpectedly, the qualitative blackleg resistance gene Rlm9 was found to be
located within a resistance-associated haploblock on chromosome A07. Furthermore,
long-range sequence data spanning this haploblock revealed high levels of singlenucleotide
and structural variants within the Rlm9 coding sequence among the parents
of the mapping population. The results suggest that novel variants of Rlm9 could play a
previously unknown role in expression of quantitative disease resistance in oilseed rape.
