Functional structure of the indeterminate Vicia faba L. root nodule:: implications for metabolite transport

The structure of N-2-fixing nodules of faba bean (Vicia faba L.) was investigated by means of light and electron microscopy to determine possible pathways for metabolite movement. The central infected zone is enclosed by a cortex, which is composed of a loosely arranged outer part and a densely packed inner part. Both are separated by the nodule endodermis, which borders on the root endodermis and continuously envelopes the entire nodule interior, with the exception of the apical meristematic region. Vascular bundles, located peripherally within the inner cortex, are surrounded by another endodermal layer. Radial cell wails of both nodule and vascular endodermis are incrusted and form Casparian bands. Additionally all endodermal cells are coated by suberin lamellae. These modifications may prevent an apoplastic loss of solutes from the enclosed tissues, and require a symplastic metabolite passage, This is supported by high plasmodesmatal frequencies found between vascular endodermis and inner cortex. Within vascular bundles, transfer cells adjacent to xylem elements may play a role in xylem loading. The central infected tissue contains uninfected cells, possibly representing a symplastic continuity, These cells may form a preferred metabolite pathway since plasmodesmata are extremely sparse between infected cells. The low number of uninfected cells in the central tissue of L! faba nodules favors an additional apoplastic pathway.