Transgressive segregation reveals two Arabidopsis TIR-NB-LRR resistance genes effective against Leptosphaeria maculans, causal agent of blackleg disease

In a cross between the two resistant accessions Col-0 and Ler-0, a 15:1 segregation was found in F-2, suggesting the presence of unlinked resistance loci to Leptosphaeria maculans. One hundred Col-4 x Ler-0, and 50 Ler-2 x Cvi-1 recombinant inbred lines, and seven susceptible Ler-0 x Ws-0 F-2 progenies were examined to identify the two loci. Resistance in Col-4, Ws-0 and Cvi-1 (RLM1) was mapped to the marker m305 on chromosome 1. Col-4 x Ler-0 and Ler-2 x Cvi-1 mapping populations located RLM2(Ler) on the same arm of chromosome 4. A tight physical location of RLM2 was established through near-isogenic lines. This region was found to correspond to an ancient duplication event between the RLM1 and RLM2 loci. Two independent T-DNA mutants in a TIR-NB-LRR R gene (At1g64070) displayed susceptibility, and L. maculans susceptible mutant phenotypes were confirmed to be allelic for rlm1 in F-1 after crosses with susceptible rlm1(Ler)rlm2(Col) plants. Complementation of rlm1(Ler)rlm2(Col) with the genomic Col-0 sequence of At1g64070 conferred resistance. In addition, two T-DNA mutants in a neighbouring homologous TIR-NB-LRR gene (At1g63880) displayed moderate susceptibility to L. maculans. Sequence analysis revealed that At1g64070 was truncated by a premature stop codon, and that At1g63880 was absent in Ler-0. RNA interference confirmed that Ler-0 resistance is dependent on genes structurally related to RLM1. Camalexin was identified as a quantitative co-dominant resistance factor of Col-0 origin, but independent of RLM1. RLM1/RLM2 resistance was, however, found to require RAR1 and partially HSP90.1.