Cell wall development in early symbiotic interactions between Rhizobium leguminosarum and legumes

Abstract

To better understand the events which lead to the formation of Rhizobium-induced root nodules, this research examined the developmental changes in the plant cell wall during the early interactions between Rhizobium and peas (Pisum sativum L) and vetch (Vicia villosa Roth). The secretory pathway of cell wall polysaccharides was analyzed in freeze-substituted Vicia root hairs. Methyl-esterified pectin (MePGA), non-esterified pectin (PGA), and xyloglucan were immunolocalized using monoclonal antibodies. Pectin was found to be synthesized in Golgi bodies, secreted in a highly methyl-esterified form, and de-esterified in muro. In addition, the same Golgi bodies contained xyloglucan and MePGA, demonstrating that Golgi bodies concurrently synthesize at least two cell wall components. After inoculation of roots with Rhizobium, the deposition of cell wall components is re-directed to the infection thread, a tube of plant material through which rhizobia invade the host plant. The polysaccharide composition of the infection threads resembles the host plant cell wall. Both contain MePGA, PGA, and xyloglucan. Also, the distribution of the components within the infection thread is similar to that of the cell type in which thread is contained. Unlike the polysaccharide composition, the protein components of infection threads differ from host cell walls. The infection thread matrix contains proline-rich protein(s) (PRPs), whereas the fibrillar wall of the thread and the root hair cell wall do not. In roots and nodules, PRPs are components of xylem tracheary elements, phloem fibers, and the Casparian strip. In nodules, parenchyma intercellular spaces also contained PRPs. The PRPs in the nodule parenchyma and in infection threads are potentially symbiosis-specific nodulins. Characterization of the PRPs in nodules is in progress. Vegetative and seed lectins were immunolocalized using polyclonal antibodies. Lectin was not detected unequivocally on the surface of uninoculated root hairs or in infection structures. Both antibodies detected lectin in protein bodies in mature seed tissue. The hypothesis that lectin mediates specificity of Rhizobium-legume interactions is not supported by these data.