QTL mapping and transcriptome analysis identify novel QTLs and candidate genes in Brassica villosa for quantitative resistance against Sclerotinia sclerotiorum.

KEY MESSAGE: Novel QTLs and candidate genes for Sclerotinia-resistance were identified in B. villosa, a wild Brassica species, which represents a new genetic source for improving oilseed rape resistance to SSR. Sclerotinia stem rot (SSR), caused by Sclerotinia sclerotiorum, is one of the most destructive diseases in oilseed rape growing regions. To date, there is no effective genetic resistance against S. sclerotiorum in the B. napus germplasm and knowledge of the molecular plant-fungal interaction is also limited. To identify new resistance resources, we screened a set of wild Brassica species and identified B. villosa (BRA1896) with a high level of Sclerotinia-resistance. Two segregating F2 populations for Sclerotinia-resistance, generated by interspecific crosses between the resistant B. villosa (BRA1896) and the wild susceptible B. oleracea (BRA1909) were assessed for Sclerotinia-resistance. Genetic mapping using a 15-k Illumina Infinium SNP-array resulted in a high-density genetic map containing 1,118 SNP markers and spanning a total genetic length of 792.2 cM. QTL analysis revealed seven QTLs explaining 3.8% to 16.5% of phenotypic variance. Intriguingly, RNAseq-based transcriptome analysis identified genes and pathways specific to B. villosa, of which a cluster of five genes encoding putative receptor-like kinases (RLKs) and two pathogenesis-related (PR) proteins are co-localized within a QTL on chromosome C07. Furthermore, transcriptomic analysis revealed enhanced ethylene (ET)-activated signaling in the resistant B. villosa, which is associated with a stronger plant immune response, depressed cell death, and enhanced phytoalexin biosynthesis compared to the susceptible B. oleracea. Our data demonstrates that B. villosa represents a novel and unique genetic source for improving oilseed rape resistance against SSR.

Suppression of abscisic acid biosynthesis at the early infection stage of Verticillium longisporum in oilseed rape (Brassica napus).

Verticillium longisporum infects oilseed rape (Brassica napus) and Arabidopsis thaliana. To investigate the early response of oilseed rape to the fungal infection, we determined transcriptomic changes in oilseed rape roots at 6 days post-inoculation (dpi) by RNA-Seq analysis, in which non-infected roots served as a control. Strikingly, a subset of genes involved in abscisic acid (ABA) biosynthesis was found to be down-regulated and the ABA level was accordingly attenuated in 6 dpi oilseed rape as compared with the control. Gene expression analysis revealed that this was mainly attributed to the suppression of BnNCED3-mediated ABA biosynthesis, involving, for example, BnWRKY57. However, this down-regulation of ABA biosynthesis could not be observed in infected Arabidopsis roots. Arabidopsis ABA- defective mutants nced3 and aao3 displayed pronounced tolerance to the fungal infection with delayed and impeded symptom development, even though fungal colonization was not affected in both mutants. These data suggest that ABA appears to be required for full susceptibility of Arabidopsis to the fungal infection. Furthermore, we found that in both 6 dpi oilseed rape and the Arabidopsis nced3 mutant, the salicylic acid (SA) signalling pathway was induced while the jasmonic acid (JA)/ethylene (ET) signalling pathway was concomitantly mitigated. Following these data, we conclude that in oilseed rape the V. longisporum infection triggers a host-specific suppression of the NCED3-mediated ABA biosynthesis, consequently increasing plant tolerance to the fungal infection. We believe that this might be part of the virulence strategy of V. longisporum to initiate/establish a long-lasting compatible interaction with oilseed rape (coexistence), which appears to be different from the infection process in Arabidopsis.