Subcellular localization determines the availability of non-targeted proteins to plasmodesmatal transport

Background: Individual plant cells are encased in a cell wall. To enable ce ll-to-cell communication, plants have evolved channels, termed plasmodesmat a, to span thick walls and Interconnect the cytoplasm between adjacent cell s. How macromolecules pass through these channels is now beginning to be un derstood. Results: Using two green fluorescent protein (GFP) reporters and a noninvas ive transfection system, we assayed for intercellular macromolecular traffi c In leaf epidermal cells. Plasmodesmata were found in different states of dilation. We could distinquish two forms of protein movement across plasmod esmata, non-targeted and targeted. Although leaves have generally been cons idered closed to non-specific transport of macromolecules, we found that 23 % of the cells had plasmodesmatal channels In a dilated state, allowing GFP that was not targeted to plasmodesmata to move Into neighboring cells. GFP fusions that were targeted to the cytoskeleton or to the endoplasmic retic ulum did not move between cells, whereas those that were localized to the c ytoplasm or nucleus diffused to neighboring cells in a size-dependent manne r. Superimposed upon this non-specific exchange, proteins that were targete d to the plasmodesmata could transit efficiently between 62% of transfected cells. Conclusions: A significant population of leaf cells contain plasmodesmata I n a dilated state, allowing macromolecular transport between cells. Protein movement potential is regulated by subcellular address and size. These par ameters of protein movement illustrate how gradients of signaling macromole cules could be formed and regulated, and suggest that non-cell-autonomous d evelopment in plants may be more significant than previously assumed.