Virus movement proteins and other molecular probes of plasmodesmal function

Plasmodesmata exist as supramolecular complexes that interconnect plant cells to form a symplasmic continuum. Microinjection (dye‐coupling) studies confirmed the validity of the symplasmic concept by establishing that molecules of up to 1 kDa can pass from cell to cell, via plasmodesmata. Thus, cells can exchange small molecules, such as metabolites and hormones, to coordinate biochemical and physiological processes occurring within tissues. Plasmodesmata also mediate in cell‐to‐cell transport of macromolecules, including proteins and nucleic acids. This new function was elucidated by studies on virus‐plasmodesmata interaction. Plant viruses encode for movement proteins (MPs) that are essential for cell‐to‐cell spread of infectious material. When expressed in transgenic plants, these MPs are targeted to plasmodesmata and induce a considerable increase in plasmodesmal size exclusion limit (SEL). Studies on mutant MPs confirmed that this increase in SEL is essential for viral infection. Microinjection of MP alone, or together with its conjugate infectious viral transcript, indicated that both the protein and the viral RNA (or DNA) move rapidly from cell to cell. These findings are consistent with the operation of an endogenous plasmodesmal macromolecular transport pathway. Control over this transport pathway has been probed using viral MPs, and the results support the hypothesis that plasmodesmata are involved in the creation of physiological and developmental domains. Elucidation of the molecular constituents involved in the formation and functioning of plasmodesmata will provide valuable insights into the integrated functioning of higher plants, as well as potentiating the development of new strategies for viral resistance.