This is an open access article under the terms of the Creat ive Commo ns Attri bution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.

Brassica napus is highly susceptible towards Verticillium longisporum (Vl43) with no effective genetic resistance. It is believed that the fungus reprogrammes plant physiological processes by up-regulation of so-called susceptibility factors to establish a compatible interaction. By transcriptome analysis, we identified genes, which were activated/up-regulated in rapeseed after Vl43 infection. To test whether one of these genes is functionally involved in the infection process and loss of function would lead to decreased susceptibility, we firstly challenged KO lines of corresponding Arabidopsis orthologs with Vl43 and compared them with wild-type plants. Here, we report that the KO of AtCRT1a results in drastically reduced susceptibility of plants to Vl43. To prove crt1a mutation also decreases susceptibility in B. napus, we identified 10 mutations in a TILLING population. Three T3 mutants displayed increased resistance as compared to the wild type. To validate the results, we generated CRISPR/Cas-induced BnCRT1a mutants, challenged T2 plants with Vl43 and observed an overall reduced susceptibility in 3 out of 4 independent lines. Genotyping by allele-specific sequencing suggests a major effect of mutations in the CRT1a A-genome copy, while the C-genome copy appears to have no significant impact on plant susceptibility when challenged with Vl43. As revealed by transcript analysis, the loss of function of CRT1a results in activation of the ethylene signalling pathway, which may contribute to reduced susceptibility. Furthermore, this study demonstrates a novel strategy with great potential to improve plant disease resistance.

Summary 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.

No abstract

No abstract