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 invest igated by means of light and electron microscopy to determine possible path ways for metabolite movement. The central infected zone is enclosed by a co rtex, which is composed of a loosely arranged outer part and a densely pack ed inner part. Both are separated by the nodule endodermis, which borders o n the root endodermis and continuously envelopes the entire nodule interior , with the exception of the apical meristematic region. Vascular bundles, l ocated peripherally within the inner cortex, are surrounded by another endo dermal layer. Radial cell wails of both nodule and vascular endodermis are incrusted and form Casparian bands. Additionally all endodermal cells are c oated by suberin lamellae. These modifications may prevent an apoplastic lo ss of solutes from the enclosed tissues, and require a symplastic metabolit e passage, This is supported by high plasmodesmatal frequencies found betwe en vascular endodermis and inner cortex. Within vascular bundles, transfer cells adjacent to xylem elements may play a role in xylem loading. The cent ral infected tissue contains uninfected cells, possibly representing a symp lastic continuity, These cells may form a preferred metabolite pathway sinc e plasmodesmata are extremely sparse between infected cells. The low number of uninfected cells in the central tissue of L! faba nodules favors an add itional apoplastic pathway.