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

Abstract

The downy mildew pathogen Hyaloperonospora arabidopsidis (Hpa) 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 Hpa-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 PLASMODESMATA-LOCATED PROTEIN 1 (PDLP1) is expressed at high levels in Hpa infected cells. Unlike other plasma membrane proteins, which are often excluded from the EHM, PDLP1 is located at the EHM in Hpa-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 pdlp1,2,3 triple mutant is more susceptible to Hpa while overexpression of PDLP1 enhances plant resistance, suggesting that PDLPs enhance basal immunity against Hpa. Haustorial encasements are depleted in callose in pdlp1,2,3 mutant plants whereas PDLP1 over-expression elevates callose deposition around haustoria and across the cell surface. These data indicate that PDLPs contribute to callose encasement of Hpa haustoria and suggests that the deposition of callose at haustoria may involve similar mechanisms to callose deposition at plasmodesmata. Haustoria are specialised invasive structures that project from fungal or oomycete hyphae into host plant cells during infection, acting as sites for molecular exchange between host and pathogen. Haustoria are targets of plant defence responses, including the deposition of membranes and polysaccharides in an encasement structure that surrounds the haustorium. It is assumed that the encasement physically seals the haustorium off from the host cell. Here we have used cell biological and genetic approaches to reveal that the plasmodesmata-associated receptor-like protein PDLP1 plays a role in infection success of the Arabidopsis downy mildew pathogen, specifically in the development of the encasement. Using live cell imaging, we observed that PDLP1 relocates to the extra-haustorial membrane, and this is required for deposition of the polysaccharide callose in the encasement. This directly correlates pathogen success with the structure of the encasement, verifying the significance of the encasement in host defence. Further, our data pose the possibility that callose deposition at plasmodesmata and the haustorial encasement exploit similar mechanisms. Our findings shed light on plant defences at haustoria and how they inhibit pathogen success.