Plasmodesmata are intercellular organelles in plants that establish cytoplasmic continuity between neighboring cells. Microinjection studies showed that plasmodesmata facilitate the cell-to-cell transport of a plant-encoded transcription factor, KNOTTED1 (KN1). KN1 can also mediate the selective plasmodesmal trafficking of kn1 sense RNA. The emerging picture of plant development suggests that cell fate is determined at least in part by supracellular controls responding to cellular position as well as lineage. One of the mechanisms that enables the necessary intercellular communication appears to involve transfer of informational molecules (proteins and RNA) through plasmodesmata.