Plasmodesmata and the cell-to-cell transport of proteins and nucleoprotein complexes

Plasmodesmata serve an important role in establishing cytoplasmic continuity between neighbouring cells within plant tissues and organs. In this symplasmic system, the microchannels formed within the plasmodesmal cytoplasmic annulus function to allow the exchange of small metabolites and ions. Recent studies have demonstrated that plasmodesmata also have the capacity to mediate the cell-to-cell transport of proteins and nucleic acids. Initial insight into this unique function was gained through studies on plant viruses. Numerous viruses have been shown to encode a non-structural protein(s), termed the movement protein, that functions in the transport of viral infectious material through plasmodesmata. These findings led to the discovery that plant proteins, including transcription factors, are also able to use this plasmodesmal highway to traffic through cells of a particular tissue. Similar to viral movement proteins, a number of these endogenous proteins also have the capacity to mediate the cell-to-cell transport of their own mRNA. This special property of plasmodesmata provided the basis for the hypothesis that higher plants function as supracellular organisms, in contrast to the multicellular mode adopted by animals. Current experiments are aimed at identifying the cellular components involved in both mediating and regulating this cell-to-cell transport of macromolecules. Such regulation may account, in part, for the division of the symplasm into developmental and physiological domains.