In silico analysis and expression profiling revealed Rlm1′ blackleg disease-resistant genes in Chromosome 6 of Brassica oleracea

Blackleg disease caused by Leptosphaeria maculans affects oilseeds and vegetables species of the Brassicaceae family. Several resistant genes have been reported in Brassica species in the A and B genomes, but the resistant locus has yet to be mapped in the vegetable species B. oleracea. Since both A and C genome Brassica species have high ancestral synteny, it is generally believed that functional resistance against blackleg could be present in B. oleracea. Rlm1 is a major resistant gene present in chromosome A07 of Brassica napus that interacts with the AvrLm1 avirulence gene of L. maculans for hypersensitive interaction. This study identified 15 orthologous Rlm1′ genes in the genome of B. oleracea through genome browsing. Then, the relative expression of Rlm1′ genes was investigated in two resistant lines and two susceptible cabbage lines after the inoculation of two L. maculans isolates, 03–02 s and 00–100 s, bearing avirulence gene AvrLm1. The selected Rlm1′ genes have nucleotide-binding site-toll/interleukin receptor (NBS-TIR), leucine-rich repeat (LRR), coiled-coil (CC), and pathogenesis-related domains in a 7.0-mega-base pair (Mbp) genomic segment of chromosome C06 of B. oleracea. A NBS family gene bearing a TIR domain, Bol040038, was upregulated in the resistant cabbage line ‘BN4303’ at 6, 24, and 48 h after inoculation in both isolates, indicating that this genes might offer resistance against both isolates. Three genes, namely, Bol023847, Bol040045, and Bol040066, showed differential expression in both ‘BN4303’ and ‘BN4098’ resistant cabbage lines in response to both isolates. Ten genes were upregulated in both resistant cabbage lines, and two other genes, namely, Bol039924 and Bol040069, were upregulated only in the resistant line ‘BN4098’ after the infection of the 00–100 s isolate. These results indicated that the putative Rlm1′ genes offer isolate-specific resistance. However, the mapping and functional analysis are required to determine the definitive role of the putative Rlm1′ genes.