A Cluster of Major Specific Resistance Genes to Leptosphaeria maculans in Brassica napus

Abstract

Two types of genetic resistance to Leptosphaeria maculans usually are distinguished in Brassica napus: qualitative, total resistance expressed at the seedling stage and quantitative, partial resistance expressed at the adult plant stage. The latter is under the control of many genetic factors that have been mapped through quantitative trait loci (QTL) studies using 'Darmor' resistance. The former usually is ascribed to race-specific resistance controlled by single resistance to L. maculans (Rlm) genes. Three B. napus-originating specific Rlm genes (Rlm1, Rlm2, and Rlm4) previously were characterized. Here, we report on the genetic identification of two novel resistance genes, Rlm3 and Rlm7, corresponding to the avirulence genes AvrLm3 and AvrLm7. The identification of a novel L. maculans- B. napus specific interaction allowed the detection of another putative new specific resistance gene, Rlm9. The resistance genes were mapped in two genomic regions on LG10 and LG16 linkage groups. A cluster of five resistance genes (Rlm1, Rlm3, Rlm4, Rlm7, and Rlm9) was strongly suggested on LG10. The relation between all these specific resistance genes and their potential role in adult-plant field resistance is discussed. These two Rlm-carrying regions do not correspond to major QTL for Darmor quantitative resistance.

Complexities of Chromosome Landing in a Highly Duplicated Genome: Toward Map-Based Cloning of a Gene Controlling Blackleg Resistance in Brassica napus

“…Classical and molecular genetic studies on a wide array of plant-pathogen systems have demonstrated that R loci can be single genes with multiple alleles (like the 13 alleles for the L locus in flax) or, more commonly, clusters of genes that can affect resistance to different pathogens (reviewed in Michelmore and Meyers 1998). A similar close relationship has been suggested in earlier reports between cotyledon resistance genes from Shiralee and Maluka (Mayerhofer et al 1997), between those from Maluka and RB87-62 (Rimmer et al 1999), and between the resistance genes Rlm3, Rlm4, Rlm7, and Rlm9 present in the B. napus lines Maxol, Quinta, 23.1.1., and Darmor, respectively (Delourme et al 2004).…”

“…Closely linked loci for adult and seedling resistance were identified in the canola cultivar Major and in the B. napus breeding accessions RB87-62 and DH88-752 (Ferreira et al 1995;Zhu and Rimmer 2003). Similarly, the race-specific cotyledon resistance gene Rlm1 has been shown to explain 70% of the variation for adult plant resistance in the French cultivar Maxol (Delourme et al 2004). In fact, the strong correlation between seedling and field resistance in the Australian canola cultivar Maluka was exploited to expedite the development of the cultivar Quantum for Western Canadian field conditions (Stringam et al 1995).…”

“…A cluster of five cotyledon resistance genes was suggested on LG10. Due to the absence of common molecular markers this linkage group cannot be associated directly with the previously cited mapping studies but on the basis of comparisons of resistance gene and avirulence gene specificities Delourme et al (2004) suggest that LG10 corresponds to LG6 of Ferreira et al (1995) and hence to N7. A concentration of blackleg resistance loci appears to be on N7; however, little is known about the relationship of these genes and much less is known about their role in the defense response.…”

“…Interestingly, a single major locus controlling field disease resistance in the cultivar Cresor has also been mapped to the same region of N7 (Dion et al 1995). More recently, race-specific resistance genes identified in the cultivars Maxol, Quinta, Glacier, Samourai, and Darmor have been mapped to two genomic regions on linkage groups LG10 and LG16 of a consensus linkage map (Delourme et al 2004). A cluster of five cotyledon resistance genes was suggested on LG10.…”

Genome wide identification of NBS-LRR genes in Brassica and their association with disease resistance in Brassica napus

“…Avr gene clustering in L. maculans and a counterpart clustering of R genes in B. napus (Delourme et al, 2004).…”