Modulates cell-to-cell trafficking (PubMed:18215111). Required for systemic acquired resistance (SAR) which is mediated by the signaling molecules azelaic acid (AzA), glycerol-3-phosphate (G3P), and salicylic acid (SA). Required for the proper localization and stability of AZI1 which is involved in SAR (PubMed:27078071). Mediates callose deposition during downy mildew fungal infection around haustoria. Haustoria are unicellular protrusions from hyphae and function as the site of molecular exchange of nutrients and effectors between host and pathogen (PubMed:25393742). Interacts with AZI1 (PubMed:27078071). Interacts with PDLP5 (PubMed:27078071). Does not interact with DIR1 (PubMed:27078071). (Microbial infection) Interacts with Grapevine fanleaf virus (GFLV) 2B-MP (PubMed:20886105). Interacts with Cauliflower mosaic virus (CaMV) movement protein (PubMed:20886105). Co-localizes with the Grapevine fanleaf virus 2B-MP at the base of tubules within modified plasmodesmata. Co-localizes with Cauliflower mosaic virus movement protein. Utilizes the secretory pathway for delivery to plasmodesmata. Highly expressed in cell suspension. Expressed in epidermal and spongy mesophyll cells, and the cell wall interface at the base of the leaf trichome (at protein level) (PubMed:19704520, PubMed:18215111). Expressed in haustoria-containing cells (PubMed:25393742). By downy mildew fungal infection. pdlp1 and pdlp3 double mutant shows altered protein diffusion (measured using GFP). In pdlp1, pdlp2 and pdlp3 triple mutant there is inhibition of GFLV 2BMP tubule formation. Virus cell-to-cell movement is negatively affected. There is a 22% reduction in mean surface area of infection foci by GFLV and an approximately 12 h delay in long distance movement in comparison to wild-type plants. There is also a systemic delay in systemic Cauliflower mosaic virus (CaMV) spread. Overexpression shows a reduced-growth phenotype that correlates with transgene copy number and cell-to-cell trafficking of GFP, used to measure protein transport, is significantly impaired. Belongs to the cysteine-rich repeat secretory protein family. Plasmodesmata-located proteins (PDLD) subfamily. PDLPs were initially named Cysteine-rich secretory proteins based on a classification work that failed to predict the transmembrane region at the C-terminus (PubMed:11402176). However, it was later shown that PDLPs are membrane proteins (PubMed:18215111, PubMed:20886105).