The role of the glucosinolate‐myrosinase system in resistance to Leptosphaeria maculans was studied by monitoring changes in glucosinolate profiles in leaf tissue surrounding the site of inoculation. Susceptible Brassica napus cv. Hanna, resistant B. nigra and near isogenic lines derived from interspecific hybrids between the two species were compared. Expression of myrosinase binding protein and presence of genetic markers were also assayed. No correlation between degree of resistance and amount of sinigrin or other aliphatic glucosinolates was found. However, in time course experiments the glucosinolate profile of the L. maculans inoculated plants differed significantly from the water‐inoculated control plants in the amount of 4‐methoxy‐glucobrassicin observed. Five to eight days post‐inoculation an increased level of 4‐methoxy glucobrassicin, ranging from 30% to 47% on average, was found in the inoculated plants, whilst controls varied between 7.6% and 9.2%. This increase was seen both in susceptible and resistant material. Other changes observed could mainly be assigned as effects of wounding. Although inoculation with L. maculans elicited changes in the leaf indolyl glucosinolate profiles, in the material studied, these changes could not be correlated to resistance against the fungus.

Using a polymerase chain reaction-based strategy, we have isolated a cDNA sequence from Brassica nigra, denoted Lm1, which significantly improves blackleg resistance when expressed in transgenic oilseed rape, B. napus. Lm1 was shown to map to locations on linkage groups 3 and 8 in the Brassica B-genome previously linked to both cotyledon, adult leaf, and stem resistance. B. napus plants transformed with Lm1 under the control of a constitutive promoter exhibited broad spectrum resistance to all L. maculans isolates tested, but enhanced resistance to Alternaria brassicae and Verticillium longisporum was not observed. A transcript corresponding to the cDNA size was induced in B. nigra 12 h after fungal challenge from a level of weak constitutive expression previous to inoculation. The Lm1 sequence bears no resemblance to previously characterized plant resistance genes but has two predicted transmembrane motifs. Several sequences with high homology to Lm1 were found in the databases. Lm1 appears to be a member of a larger group of related genes present in a variety of plant species. Most of them have unknown functions, but homology between Lm1 and the nodule inception gene of Lotus japonicus suggests an intriguing link between defense-related and symbiotic pathways.

summary Leptosphaeria maculans causes blackleg disease, and resistance to this fungal pathogen is an important trait in the breeding of oilseed rape. A better comprehension of the role of the myrosinase-glucosinolate system in this context is of great value. The present study is the first to address effects on multiple components of this complex system, including concentrations of individual glucosinolates, product formation, myrosinase isoform distribution and activity, and levels of myrosinase binding proteins during the infection process. One resistant B. napus cultivar (Maluka) and one susceptible cultivar (Westar) were compared in the investigation. Our results show that the two cultivars had the same histological distribution, isoform expression, and activity of the myrosinase enzymes. The glucosinolate levels were also similar, with the exception of glucobrassicin and neoglucobrassicin, which were significantly lower in the resistant cultivar at 11 days post-infection. Growth of the fungus on the plant tissues did not alter glucosinolate levels, suggesting that L. maculans does not degrade these compounds. When the plants were starved of sulphur, and thereby depleted of glucosinolates, no increased susceptibility was observed. Hence, we suggest that the myrosinase-glucosinolate system does not determine the outcome of the interaction between B. napus and L. maculans.

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