Abstract
Key messageQuantitative resistance (QR) loci discovered through genetic and genomic analyses are abundant in the Brassica napus genome, providing an opportunity for their utilization in enhancing blackleg resistance.Quantitative resistance (QR) has long been utilized to manage blackleg in Brassica napus (canola, oilseed rape), even before major resistance genes (R-genes) were extensively explored in breeding programmes. In contrast to R-gene-mediated qualitative resistance, QR reduces blackleg symptoms rather than completely eliminating the disease. As a polygenic trait, QR is controlled by numerous genes with modest effects, which exerts less pressure on the pathogen to evolve; hence, its effectiveness is more durable compared to R-gene-mediated resistance. Furthermore, combining QR with major R-genes has been shown to enhance resistance against diseases in important crops, including oilseed rape. For these reasons, there has been a renewed interest among breeders in utilizing QR in crop improvement. However, the mechanisms governing QR are largely unknown, limiting its deployment. Advances in genomics are facilitating the dissection of the genetic and molecular underpinnings of QR, resulting in the discovery of several loci and genes that can be potentially deployed to enhance blackleg resistance. Here, we summarize the efforts undertaken to identify blackleg QR loci in oilseed rape using linkage and association analysis. We update the knowledge on the possible mechanisms governing QR and the advances in searching for the underlying genes. Lastly, we lay out strategies to accelerate the genetic improvement of blackleg QR in oilseed rape using improved phenotyping approaches and genomic prediction tools.
Highlights
Brassica napus is a major source of oil for the human diet and industrial applications
Quantitative resistance was the primary form of blackleg resistance selected in most oilseed rape cultivars before resistance genes (R-genes) were explored for breeding
Quantitative resistance (QR) has been shown to afford a broad-spectrum control and its modest genetic effects render the blackleg pathogen to less likely evolve; its effectiveness is more durable compared with R-gene-mediated immunity
Summary
The study found that the degree of methylation was extended for a longer period, in the youngest leaves of the resistant cultivar, compared with the susceptible control This mechanism is proposed to inhibit further pathogen colonization, resulting in decreased stem infection, and may partly explain how seedling resistance contributes to enhancing adult plant resistance. These experiments have demonstrated that regulatory variation and the mechanisms behind it are important features of plant defence, which can be manifested throughout different stages of plant development Their direct manipulation will provide an alternative strategy to broaden phenotypic diversity for breeding purposes, including that for blackleg resistance. The genetic variance displayed by these QTL was significantly lower compared to a previous experiment utilizing the same genetic
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