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Chemical warfare between the host and the pathogen plays a crucial role in plant-necrotrophic pathogen interactions, but examples of its involvement in quantitative disease resistance in plants are poorly documented. In the Daucus carota-Alternaria dauci pathosystem, the novel toxin aldaulactone has been identified as a key factor in both fungal pathogenicity and the carrot’s partial resistance to the pathogen. Bioinformatic analyses have pinpointed a secondary metabolism gene cluster that harbors two polyketide synthase genes, AdPKS7 and AdPKS8 , that are likely responsible for the biosynthesis of aldaulactone. Here, we present the functional validation of AdPKS7 and AdPKS8 as genes responsible for aldaulactone production in A. dauci . We generated A. dauci knock-out mutants for AdPKS7 and AdPKS8 by replacing essential domains with a hygromycin resistance gene, marking the first reported case of genetic manipulation in A. dauci . Following transformation, the mutants were analyzed for toxin production via HPLC-UV and assessed for pathogenicity in planta . Aldaulactone production was abolished in all PKS mutants, which also exhibited significantly reduced pathogenicity on H1-susceptible carrot leaves. These findings confirm the roles of AdPKS7 and AdPKS8 in aldaulactone biosynthesis and their contribution to fungal pathogenicity.

Background Multidrug resistance has been identified in the fungal pathogen responsible for Septoria leaf blotch, Zymoseptoria tritici, since 2011. It has been linked to the overexpression of the gene encoding the MFS1 transporter due to inserts in the promoter region of MFS1 (PMFS1), namely types I-III. Recently, two new inserts were discovered in PMFS1 that were not linked to MDR, interrogating about whether PMFS1 inserts are the only drivers of MDR in Z. tritici. The goal of our study was to gain a more complete view of MDR in Z. tritici by examining the genotypic diversity associated with the MDR phenotype in a large sample of the modern population. Results We isolated 384 potential MDR strains between 2020 and 2021 in northern Europe for PMFS1 genotype and MDR assessment. We discovered six new inserts in PMFS1, bringing the total count to 13 including one insertion-deletion in the 5' UTR region. Of these, 11 display similarities with transposable elements, and 3 are not linked to MDR. Some field strains were significantly more resistant than their respective reference of the same PMFS1 genotype and some strains without insert displayed MDR phenotype. Conclusion We described the landscape of the MDR in modern Z. tritici population and postulate that PMFS1 is a hotspot for insertions involving transposition events. Our study shows that MDR cannot be solely explained by inserts found in PMFS1, and that additional mechanisms might be at work.Competing Interest StatementThe authors have declared no competing interest.