Control improves with age: intercellular transport in plant embryos and adults.

P lasmodesmata (PD), the intercellular channels that span plasma membranes and cell walls between adjacent plant cells, are unique structures that create cytoplasmic continuity, termed the symplast, between individual cells. Within a symplastic domain, a group of cells that are interconnected by PD are able to exchange a wide range of molecules, from ions and water to proteins and nucleic acids. Although the importance of macromolecular trafficking through PD for plant physiology is well established, the mechanism(s) by which this cell-tocell transport process is regulated remains largely unclear. PD transport can be classified into two distinct types: passive translocation by simple diffusion [also termed nontargeted movement (1)] of molecules smaller than the PD size exclusion limit (SEL), and active transport by PD dilation [also termed targeted movement (1)] of molecules larger than their SEL. The passive type of PD transport is determined solely by the PD SEL and, thus, by the PD structure and development. Thus, marker molecules, such as fluorescently labeled dextrans or GFP, that passively move between plant cells can be used as molecular tools for mapping developmental changes in PD permeability; this issue of PNAS includes a report by Kim et al. (2) that elegantly uses GFP transport to characterize changes in cell-to-cell communication during embryogenesis and early seedling development in Arabidopsis.