Development of Root Nodules in Phaseolus vulgaris Inoculated with Rhizobium and Mycorrhizal Fungi

Bean (Phaseolus vulgaris) was inoculated at planting with vesicular arbuscular mycorrhizal fungi (VAM) and Rhizobium phaseoli strain 127K44. Plants inoculated with both symbionts formed nodules in large clusters, as opposed to the single or linear groups of nodules that formed on plants inoculated only with Rhizobium. Light microscopic examination of root and nodule sections indicated that by 5 wk after inoculation, hyphal infection of the nodule cortex was abundant. Vesicles were observed in root and nodule tissue. Structures similar to degenerating arbuscules were seen in the central region of the nodule and in the nodule cortex. Intact arbuscules typical of VAM were not observed. Hyphae were also present in the central region of the nodule, particularly in regions where senescence of bacteria-filled cells had begun.