Three-dimensional organization of uninfected tissue in soybean root nodules and its relation to cell specialization in the central region

Abstract

The three-dimensional structure of uninfected tissue in the central infected region of soybean nodules has been studied using several methods. Transverse and longitudinal sections have been examined. A view of the surface of the central region has been obtained by partially clearing nodules and removing their cortex. A three-dimensional reconstruction of the central region has been assembled using transverse sections. It was found that some of the uninfected cells participate in forming characteristic structural aggregates. A centrally located spherical region with a diameter half that of the entire infected region is filled with interconnected aggregates of uninfected cells. A set of branching interconnected planes of uninfected cells emanate from the sphere and extend out to the surface of the infected region. The planes are oriented generally in a longitudinal direction but are sometimes at other angles. These planes divide the nodule into compartments of various sizes and shapes. The planes are perforated by irregularly shaped groups of infected cells. Uninfected cells also often occur arranged in lines oriented approximately radially in the central region. Irregularly shaped fine aggregates of uninfected cells occur outside the central sphere formation and are interconnected by narrow lines of uninfected cells. All of these types of formations could provide uninfected paths all or part of the way from the center of the central region to the cortex. The planes and lines often contain uninfected cells elongated perpendicular to the surface of the central region, suggesting that the routes may serve in the transport of substances from the inside to the surface of the central region. The distribution of plasmodesmata also appears to favor transport from one uninfected cell to another.