Molecular studies of Arabidopsis and Brassica with focus on resistance to Leptosphaeria maculans

Blackleg caused by Leptosphaeria maculans is a widespread fungal disease on B~assica napus (oilseed rape). In contrast, Arabidopsis thaliana and B. nigra are in general highly resistant. This study presents results from genomic interaction between the A. thaliana and B. napus genome with focus on L. maculans resistance. Identification and partial characterization of A. thaliana resistance in accessions, L. maculans susceptible mutants, and signaling pathways were also performed. Finally, a resistance gene to L. maculans from B. nigra was cloned and transferred to B. napus. Chromosome counts and RFLP analyses of A. thaliana DNA content in A. thaliana (+) B. napus back-crossed progeny were performed. The results showed that in BC,, originating from symmetric hybrids, the frequency of retained A. thaliana loci was reduced to 42%. The average chromosome number decreased from 48 in BC1 to 39 in BC2. These results can be compared with the asymmetric hybrid derived BC,, that had 16% loci present and an average chromosome number of 38. Clearly, symmetric hybrid offspring retained most DNA as complete chromosomes whereas asymmetric hybrid offspring contained mostly DNA fragments. Pathogen screening of the hybrid offspring revealed both cotyledon and adult leaf resistance to L. maculans. The adult leaf resistance was localized to chromosome 3 from A. thaliana, on two areas on each side of the centromere. A. thaliana resistance was examined in 171 accessions from 27 countries. Only four accessions displayed any susceptibility. To further explore the underlying causes of the resistance, a set of L. maculans disease susceptible mutants (lms) were isolated. Two of the mutants, lmsl and lms5 have been mapped to chromosome 2 and 1, respectively. The resistance was further analyzed with respect to defense signaling and effector molecules. The results indicated that resistance in A . thaliana against L . maculans depended on camalexin and is independent on salicylic acid, jasmonic acid or ethylene response. In contrast, lmsl produced wild type level of camalexin and higher expression levels than wild type of P R l , and PDFl.2. The results suggest the possibility of at least two independent resistance factors in A. thaliana. A gene conferring resistance to L. maculans, L m l , was cloned from B. nigra. Sequence analysis revealed a novel protein with two putative trans-membrane motifs and homology to the nin protein of Lotus japonicus and to A. thaliana sequences of unknown function. The knowledge gained in A. thaliana and from Lml will promote a further understanding of the mechanisms underlying resistance to L. macula