Knock-out of BnHva22c reduces the susceptibility of Brassica napus to infection with the fungal pathogen Verticillium longisporum

Verticillium longisporum (Vl43) is a soilborne hemibiotrophic fungal pathogen causing stem striping on oilseed rape (OSR) and severe yield losses. Breeding for resistant varieties is the most promising approach to control this disease. Here, we report the identification of Hva22c as a novel susceptibility factor and its potential for improving OSR resistance. Hva22c is a member of the Hva22 gene family, originally described for barley (Hordeum vulgare). Several Hva22 members have been located at the endoplasmic reticulum. Hva22c is up-regulated in response to Vl43 in both Arabidopsis and OSR. We demonstrate that knock-out of Hva22c in OSR by CRISPR/Cas9 and its homolog in Arabidopsis by T-DNA insertion reduced plants' susceptibility to Vl43 infection and impaired the development of disease symptoms. To understand the underlying mechanism, we analysed transcriptomic data from infected and non-infected roots of hva22c knock-out and wild type plants. We identified a homozygous mutant with frame-shifts in all four BnHva22c loci displaying a vastly altered transcriptional landscape at 6 dpi. Significantly, a large set of genes was suppressed under mock conditions including genes related to the endomembrane systems. Among the up-regulated genes we found several defense-related and phytohormone-responsive genes when comparing mutant to the wild type. These results demonstrate that Hva22c is functionally required for a fully compatible plant-fungus interaction. Its loss of function reduces plant susceptibility, most likely due to endoplasmatic reticulum and Golgi dysfunction accompanied by additionally activated defense responses. These findings can help improve OSR resistance to V. longisporum infection.