Expression polymorphism at the ARPC4 locus links the actin cytoskeleton with quantitative disease resistance to Sclerotinia sclerotiorum in Arabidopsis thaliana

Quantitative disease resistance (QDR) is a form of plant immunity widespread in nature, and the only one active against broad host range fungal pathogens. The genetic determinantsof QDR are complex and largely unknown, and are thought to rely partly on genes controlling plant morphologyand development. We used genome‐wide association mappingin Arabidopsis thaliana to identify ARPC4 as associated with QDR against the necrotrophic fungal pathogen Sclerotinia sclerotiorum. Mutants impaired in ARPC4 showed enhanced susceptibility to S. sclerotiorum, defects in the structure of the actin filaments and in their responsiveness to S. sclerotiorum. Disruption of ARPC4 also alters callosedeposition and the expression of defense‐related genes upon S. sclerotiorum infection. Analysis of ARPC4 diversity in A. thaliana identified one haplotype (ARPC4ᴿ) showing a c. 1 kbp insertion in ARPC4 regulatory region and associated with higher level of QDR. Accessions from the ARPC4ᴿ haplotype showed enhanced ARPC4 expression upon S. sclerotiorum challenge, indicating that polymorphisms in ARPC4 regulatory region are associated with enhanced QDR. This work identifies a novel actor of plant QDR against a fungal pathogen and provides a prime example of genetic mechanisms leading to the recruitment of cell morphology processes in plant immunity.