Broad‐spectrum resistance of <scp>A</scp>rabidopsis <scp>C</scp>24 to downy mildew is mediated by different combinations of isolate‐specific loci

<jats:title>Summary</jats:title><jats:p> <jats:list list-type="bullet"> <jats:list-item><jats:p>Most natural <jats:italic><jats:styled-content style="fixed-case">A</jats:styled-content>rabidopsis thaliana</jats:italic> accessions are susceptible to one or more isolates of the downy mildew pathogen <jats:italic><jats:styled-content style="fixed-case">H</jats:styled-content>yaloperonospora arabidopsidis</jats:italic> (<jats:italic><jats:styled-content style="fixed-case">H</jats:styled-content>pa</jats:italic>). However, <jats:styled-content style="fixed-case">A</jats:styled-content>rabidopsis <jats:styled-content style="fixed-case">C</jats:styled-content>24 has proved resistant to all <jats:italic><jats:styled-content style="fixed-case">H</jats:styled-content>pa</jats:italic> isolates tested so far. Here we describe the complex genetic basis of broad‐spectrum resistance in <jats:styled-content style="fixed-case">C</jats:styled-content>24.</jats:p></jats:list-item> <jats:list-item><jats:p>The genetics of <jats:styled-content style="fixed-case">C</jats:styled-content>24 resistance to three <jats:italic><jats:styled-content style="fixed-case">H</jats:styled-content>pa</jats:italic> isolates was analyzed by segregation analysis and quantitative trait locus (<jats:styled-content style="fixed-case">QTL</jats:styled-content>) mapping on recombinant inbred and introgression lines.</jats:p></jats:list-item> <jats:list-item><jats:p>Resistance of <jats:styled-content style="fixed-case">C</jats:styled-content>24 to downy mildew was found to be a multigenic trait with complex inheritance. Many identified resistance loci were isolate‐specific and located on different chromosomes. Among the <jats:styled-content style="fixed-case">C</jats:styled-content>24 resistance <jats:styled-content style="fixed-case">QTLs</jats:styled-content>, we found dominant, codominant and recessive loci. Interestingly, none of the identified loci significantly contributed to resistance against all three tested <jats:italic><jats:styled-content style="fixed-case">H</jats:styled-content>pa</jats:italic> isolates.</jats:p></jats:list-item> <jats:list-item><jats:p>Our study demonstrates that broad‐spectrum resistance of <jats:styled-content style="fixed-case">A</jats:styled-content>rabidopsis <jats:styled-content style="fixed-case">C</jats:styled-content>24 to <jats:italic><jats:styled-content style="fixed-case">H</jats:styled-content>pa</jats:italic> is based on different combinations of multiple isolate‐specific loci. The identified quantitative resistance loci are particularly promising as they provide an important basis for the cloning of susceptibility‐ and immunity‐related genes.</jats:p></jats:list-item> </jats:list> </jats:p>