Defence responses in Brassica nigra and B. napus to the fungal pathogen Leptosphaeria maculans

The fungal pathogen Leptosphaeria maculans causes blackleg disease on oilseed rape (Brassica napus). It attacks leaves, cotyledons, stems and pods, and is one of the main threats to oilseed rape production world wide. Field resistant cultivars are available, but the rapid evolution of new and more virulent L. maculans isolates demands a more efficient use of resistance sources. Brassica species containing the B-genome (i.e. B. nigra, B. juncea and B. carinata) are resistant to blackleg, and it is of interest to gain more knowledge about the underlying defence mechanisms. This thesis focuses on different aspects of B-genome derived resistance to L. maculans. At the genomic level, the location and organisation of loci conferring resistance to L. maculans in the B-genome was studied. At the metabolic level, the role of the glucosinolate-myrosinase system was studied, and at the sequence level a potential resistance gene was cloned and characterised. The results from the mapping study showed that loci conferring blackleg resistance were maintained in a triplicated region, earlier designated B, on linkage groups 2, 5, and 8 of the B-genome. Cotyledon and adult-leaf resistance segregated as different loci in all three B-genome species, and may be governed by different mechanisms. The glucosinolate-myrosinase system is found mainly in cruciferous plants, and has been proposed as a mediator of defence against L. maculans. We studied resistance levels and glucosinolate profiles of 23 near isogenic lines derived from hybrids between B. napus and B. nigra, and found no correlation between resistance and glucosinolate content. The levels of indolylic glucosinolates were affected 5-8 days after inoculation with L. maculans, but it was observed in both resistant and susceptible plants. This response may represent a defence mechanism that the Brassica specialist L. maculans has evolved to overcome. Other components of the glucosinolate-myrosinase system were also studied in detail, and there were no indications that this system is responsible for blackleg resistance. Finally, a gene conferring resistance to L. maculans (Lm1) was cloned from B. nigra, using a PCR-based strategy. B. napus plants transformed with Lm1 exhibited increased resistance to four different isolates of L. maculans in both cotyledons and adult leaves. Lm1 encodes a novel protein with two putative transmembrane motifs. Related genes of unknown function were found in Arabidopsis thaliana and rice (Oryza sativa), and there was similarity to the symbiotic nin gene from Lotus japonicus. The exact function of Lm1 remains to be elucidated