Studies on the Root of Zea mays L.-Structure of the Adventitious Roots with Respect to Phloem Unloading

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Abstract
The adventitious roots of Zea mays L. were studied by transmission electron microscopy to determine whether a symplastic pathway of phloem unloading in this tissue is feasible. In addition to cytological aspects of the various cell types of the root, the structure, distribution, and frequency of plasmodesmata between cell types were determined. The protoplasts of contiguous cells of the root are joined by various numbers of cytoplasmic connections. Except for the connections associated with secondary and tertiary state endodermal cells, the cytoplasmic connections are similar to those between similar cell types in other species, i.e., plasmodesmata between parenchymatic elements, and pore-plasmodesmata between sieve-tube members and parenchyma cells (companion, vascular parenchyma, or pericycle cells in the Zea root). The plasmodesmata of secondary and tertiary state endodermal cells are conspicuously constricted in the region of the suberin lamella. Also, the plasmodesmata between cortical and endodermal cells and between endodermal and pericycle cells are characterized on the endodermal cell side of the wall by an electron-dense area surrounding their neck regions. The highest frequency of cytoplasmic connections occurs between sieve-tube members and their associated companion cells. Results indicate the structure of the Zea root is compatible with a symplastic pathway of phloem unloading and transport to cells of the cortex and pith.