PLASMODESMAL CELL-TO-CELL TRANSPORT OF PROTEINS AND NUCLEIC-ACIDS

The complexity associated with post-translational processing, in terms of protein sorting and delivery is now well understood. Although such studies have been focused almost exclusively on the fate of proteins within the cell in which they are synthesized, recent studies indicate that it is time to broaden this focus to incorporate the concept of i ntercellular targeting of proteins. Direct evidence is now available t hat viral and endogenous proteins can be synthesized in a particular c ell and subsequently transported into neighboring (or more distant) ce lls. Plasmodesmata, plasma membrane-lined cytoplasmic pores, are thoug ht to establish the intercellular pathway responsible for this cell-to -cell trafficking of macromolecules (proteins and nucleic acids). Thes e recent findings establish a new paradigm for understanding the manne r in which higher plants exert control over developmental processes. W e discuss the concept that programming of plant development involves s upracellular control achieved by plasmodesmal trafficking of informati onal molecules, herein defined as supracellular control proteins (SCPs ). This novel concept may explain why, in plants, cell fate is determi ned by position rather than cell lineage. Finally, the circulation of long-distance SCPs, within the phloem, may provide the mechanism by wh ich the plant signals to the shoot apical meristem that it is time to switch to the reproductive phase of its development.