The Plasmodesmal Protein PDLP1 Localises to Haustoria-Associated Membranes during Downy Mildew Infection and Regulates Callose Deposition

<div><p>The downy mildew pathogen <i>Hyaloperonospora arabidopsidis</i> (<i>Hpa</i>) is a filamentous oomycete that invades plant cells via sophisticated but poorly understood structures called haustoria. Haustoria are separated from the host cell cytoplasm and surrounded by an extrahaustorial membrane (EHM) of unknown origin. In some interactions, including <i>Hpa</i>-Arabidopsis, haustoria are progressively encased by host-derived, callose-rich materials but the molecular mechanisms by which callose accumulates around haustoria remain unclear. Here, we report that <i>PLASMODESMATA-LOCATED PROTEIN 1</i> (<i>PDLP1</i>) is expressed at high levels in <i>Hpa</i> infected cells. Unlike other plasma membrane proteins, which are often excluded from the EHM, PDLP1 is located at the EHM in <i>Hpa</i>-infected cells prior to encasement. The transmembrane domain and cytoplasmic tail of PDLP1 are sufficient to convey this localization. PDLP1 also associates with the developing encasement but this association is lost when encasements are fully mature. We found that the <i>pdlp1,2,3</i> triple mutant is more susceptible to <i>Hpa</i> while overexpression of <i>PDLP1</i> enhances plant resistance, suggesting that PDLPs enhance basal immunity against <i>Hpa.</i> Haustorial encasements are depleted in callose in <i>pdlp1,2,3</i> mutant plants whereas <i>PDLP1</i> over-expression elevates callose deposition around haustoria and across the cell surface. These data indicate that PDLPs contribute to callose encasement of <i>Hpa</i> haustoria and suggests that the deposition of callose at haustoria may involve similar mechanisms to callose deposition at plasmodesmata.</p>