9419703 Turgeon Phloem loading is the transport process in which carbohydrate, synthesized by mesophyll cells, is transferred to the vascular tissue (phloem) of leaves in preparation for long-distance translocation to other organs. A new model of phloem loading has been put forward by Dr. Turgeion for those species that translocate stachyose in addition to sucrose. Possibly one-third of all dicotyledonous species are in this group. According to this model, sucrose diffuses from the mesophyll cells into a specialized type of companion cell (the intermediary cell) in the minor veins and is converted there into stachyose. Diffusion occurs through numerous branched plasmodesmata linking bundle sheath cells to intermediary cells. The stachyose accumulates in the intermediary cells and sieve elements because it is too large to diffuse back into the mesophyll. This award supports experimental studies into the dynamics of intermediary cell phlasmodesmata ultrastructure. These plasmodesmata are especially narrow on the intermediary cell side and this constriction is the possible site of discrimination between transport of sucrose in one direction and stachyose in the other. Cryofixation will be used to preserve plasmodesmata structure, which will be studied in mature leaves before and after a reduction in carbohydrate content. Confocal microscope investigations with membrane- specific dyes will determine whether the small vacuoles in intermediary cells, which may be involved in transport of sugar through the cell, originate from the vacuolar system or the endoplasmic reticulum. These studies will concentrate on Cucumis melo and Cucurbita pepo but other stachyose-translocation species may also be employed.
