Symplastic continuity between mesophyll and companion cells in minor veins of mature Cucurbita pepo L. leaves.

Dye-coupling studies have been undertaken to determine whether plasmodesmata between intermediary cells (companion cells) and bundle-sheath cells in the minor veins of mature Cucurbita pepo L. leaves are open to passage of low-molecular-weight compounds. The abaxial phloem of these veins was exposed by stripping the lower epidermis of the leaf and removing the spongy-mesophyll cells by abrasion. Lucifer yellow, or 6-carboxyfluorescein, were microinjected into intermediary cells by iontophoresis, and dye location was monitored by fluorescence microscopy. Dye spread from one intermediary cell to another and from intermediary cells to bundle-sheath and mesophyll cells. No movement of microinjected dye occurred in some experiments, probably because plasmodesmata closed in response to cell damage incurred during tissue preparation. Most, but not all, minor veins in tissue prepared for microinjections studies are able to accumulate exogenously supplied [(14)C]sucrose. Plasmolysis studies indicate that the solute content of intermediary cells is much higher than that of bundle-sheath cells. In C. pepo, plasmodesmata may provide a route for the selective phloem loading of export sugars.