New and old roles of plasmodesmata in immunity and parallels to tunneling nanotubes.

Effective cell-to-cell communication is critical for the survival of both unicellular and multicellular organisms. In multicellular plants, direct cell coupling across the cell wall boundaries is mediated by long membrane-lined cytoplasmic bridges, the plasmodesmata. Exciting recent discoveries suggest that the occurrence of such membrane-lined intercellular channels is not unique to plant lineages but more prevalent across biological kingdoms than previously assumed. Striking functional analogies exist among those channels, in that not only do they all facilitate the exchange of various forms of macromolecules, but also they are exploited by some opportunistic pathogens to spread infection from one host cell to another. However, host cells may have also evolved strategies to offset such exploitation of the critical cellular infrastructure by the pathogen. Indeed, recent studies support an emerging paradigm that cellular connectivity via plasmodesmata plays an important role in innate immune responses. Preliminary hypotheses are proposed as to how various regulatory mechanisms integrating plasmodesmata into immune signaling pathways may have evolved.