Membrane Contacts in Plasmodesmata: Structural Components and Their Functions

Plasmodesmata (PD) are intercellular channels in plant tissues providing continuity of the cytoplasm, the plasma membrane (PM) and the endoplasmic reticulum (ER) of neighboring cells. These channels allow the active transport of macromolecules such as proteins or RNAs. Thus, PD are believed to play a critical role in the functional unity of plant tissues and the transport of signals required for plant development and responses to external stimuli. Recent findings indicate that the PD channel contains a specialized type of ER-PM membrane contact sites (MCSs), structural links formed between ER and PM with tethering proteins. As shown for animal cells, MCSs are essential for lipid and protein trafficking between ER and PM membranes as well as for stress responses or the maintenance of ER structural integrity. On the other hand, our knowledge of the PD-specific MCSs is still scarce, and experimentally supported models of organization of their structural elements are only starting to emerge. Here, we review the structural and functional properties of proteins that can take part in establishing MCSs in PD. We also discuss the significance of cytoskeleton, lipid membrane microdomains and cell wall components for the maintenance and remodeling of PD-specific MCS in response to various biotic and abiotic stresses.