Plasmodesmata: Dynamics, Domains and Patterning

Although it has long been known that plasmodesmata establish cytoplasmic continuity between most cells within the body of the plant, it is only recently that these special structures have been viewed as dynamic intercellular organelles (Lucas and Wolf, Trends in Cell Biology 3 : 308–315, 1993) involved in the transport of macromolecules. Ultrastructural studies have provided important information on the formation of plasmodesmata during cytokinesis (primary plasmodesmata) and as post-cytokinetic events, where the new cytoplasmic bridges are inserted within the existing wall (secondary plasmodesmata). Modifications to plasmodesmal frequency, and presumably composition probably reflect developmental and physiological requirements for symplasmic continuity/communication. Pioneering studies on viral movement proteins provided the first direct experimental evidence that proteins and protein-nucleic acid complexes could traffic cell to cell, via plasmodesmata. This knowledge paved the way for the identification and characterization of endogenous proteins that also possess a similar capacity for cell-to-cell transport. Such proteins range from plant transcription factors, involved in orchestration of plant development, to proteins present in the phloem sap of angiosperms which are probably involved in maintenance of the enucleate sieve tube system. Information from viral infection studies and plant development support the concept that plasmodesmata play an essential role in the formation of developmental and physiological domains in which regulated trafficking of macromolecules establishes a non-cell-autonomous control network. Finally, the role of plasmodesmal trafficking of informational molecules is discussed in terms of providing a novel means for controlling cell differentiation. This concept is illustrated using current knowledge on root hair and trichome pattern formation.