Symplastic transport in soybean root nodules

C is unloaded from nodule phloem and may move via a symplastic or apoplastic route through adjacent pericycle cells, then symplastically through the vascular endodermis symplastically or apoplastically through the cells of the inner cortex and into the cells of the infected zone. Uninfected cells form rays and may act as preferred pathways leading to the infected cells. The same pathway options exist for the movement of nitrogenous metabolites from infected cells to the nodule xylem. Mean plasmodesmatal frequency was estimated from 1 nodule of each of 4 20-day soybean plants. All cell combinations were connected via the symplasm. Plasmodesmatal frequency was relatively low between phloem and pericycle cells. Plasmodesmatal frequency between pericycle cells was high but was low across the endodermis. Cortical cells were relatively well connected. The frequency of plasmodesmata between uninfected cell rays was high compared to infected cell contact. Plasmodesmatal frequency data were combined with estimates of cell surface areas, vascular bundle length and C and N fluxes within the nodule. It is suggested that a symplastic and apoplastic restriction to solute fluxes to and from the nodule exists at the nodule vascular endodermis. Microinjection and subsequent movement of the membrane-impermeant fluorochrome Lucifer Yellow-CH confirmed the presence of a symplastic and apoplastic restriction at the endodermis. The dye did not show a preferential “pathway” through the uninfected cell rays.