A new paradigm is emerging in plant biology in which proteins and ribonucleoprotein (RNP) complexes play non-cell-autonomous roles in contributing to the control over developmental and physiological processes. Plasmodesmata (PD), the intercellular organelle(s) of the plant kingdom, create the pathway for the cell-to-cell trafficking of these information macromolecules. This functional property of PD also is thought to have potentiated the development of the angiosperm phloem sieve-tube system that serves as an effective conduit for the inter-organ delivery of proteins and RNP complexes (Lucas et al., 2001; Ruiz-Medrano et al., 2001). In this review, we examine the supporting evidence that underlies the existence and function of this supracellular informational signaling pathway.