Fig. 4. Uptake and subsequent movement of symplastic tracers in cells of wild-type mid-torpedo embryos. (A) When embryos are removed from their seed coats, physical damage occurs in a subset of cells. As a result, small regions of cell walls and plasma membranes are broken to a sub-lethal level to provide an initial entrance site for uptake of symplastic tracers such as HPTS and F-dextran, which do not usually cross plasma membranes. Yellow jagged lines indicate the most common site of damage in our loading assay (see Results). Black jagged lines the random abraded sites on the protodermal layer, which are observed less frequently. co, cotyledon; ra, radicle; sc, seed coat. (B) A small number of cells at the base of the detached cotyledons from mid-torpedo embryos are cytoplasmically loaded with 10 kDa F-dextran (asterisks). Yet further movement to neighboring cells does not occur so that rest of the cells show only chlorophyll auto-fluorescence (red). Scale bar, 50 µm. (C) A typical example of loaded cells in a region at the edge of the protodermal layer where abrasion has occurred (marked as black jagged lines in A). Individual cells in the protodermal layer take up 10 kDa F-dextrans (green arrows) and show cytoplasmic localization of the probe. However, subsequent movement of the probe is inhibited (yellow arrows with red X). Scale bar, 5 µm. (D) A diagram showing that a partially broken cell wall and plasma membrane (jagged edge) may provide the initial entrance site for uptake (green arrow) of symplastic tracers, F-dextran or HPTS (green circles). Further symplastic transport (yellow arrows) is then determined by the SEL of plasmodesmata and the size of symplastic tracers introduced.