A two genes - for - one gene interaction between Leptosphaeria maculans and Brassica napus.

Interactions between Leptosphaeria maculans, causal agent of stem canker of oilseed rape, and its Brassica hosts are models of choice to explore the multiplicity of 'gene-for-gene' complementarities and how they diversified to increased complexity in the course of plant-pathogen co-evolution. Here, we support this postulate by investigating the AvrLm10 avirulence that induces a resistance response when recognized by the Brassica nigra resistance gene Rlm10. Using genome-assisted map-based cloning, we identified and cloned two AvrLm10 candidates as two genes in opposite transcriptional orientation located in a subtelomeric repeat-rich region of the genome.

Quantitative resistance increases the durability of qualitative resistance to Leptosphaeria maculans in Brassica napus.

It has frequently been hypothesized that quantitative resistance increases the durability of qualitative (R-gene mediated) resistance but supporting experimental evidence is rare. To test this hypothesis, near-isogenic lines with/without the R-gene Rlm6 introduced into two Brassica napus cultivars differing in quantitative resistance to Leptosphaeria maculans were used in a 5-yr field experiment. Recurrent selection of natural fungal populations was done annually on each of the four plant genotypes, using crop residues from each genotype to inoculate separately the four series of field trials for five consecutive cropping seasons.

Repeat-induced point mutation (RIP) as an alternative mechanism of evolution toward virulence in Leptosphaeria maculans.

Three avirulence genes, AvrLm1, AvrLm6, and AvrLm4-7, were recently identified in Leptosphaeria maculans and found to be localized as solo genes within large noncoding, heterochromatin-like regions mainly composed of retrotransposons, truncated and degenerated by repeat-induced point mutation (RIP). The Rlm6 resistance gene has been overcome within 3 years in outdoor experiments in France and, here, we investigate the molecular basis of evolution toward virulence at the AvrLm6 locus. A region of 235 kb was sequenced in a virulent isolate and showed the deletion of AvrLm6 and three divergent mosaics of retrotransposons.

Molecular phylogeny of the Leptosphaeria maculans-L. biglobosa species complex.

Leptosphaeria maculans (anamorph Phoma lingam), the ascomycete causing stem canker of crucifers, is a species complex that can be separated into at least seven distinct subgroups using a combination of biochemical and molecular criteria. In the present study sequences of the entire ITS region, including the 5.8S rDNA, of 38 isolates representing the seven subgroups, along with specimens from culture collections, were analysed, compared to those of closely related Leptosphaperia species, and the phylogeny inferred using parsimony and distance analyses.