Histological comparisons of plant and Rhizobium induced ineffective nodules in alfalfa

Abstract

Root nodule tissues from an effective association and two ineffective associations between Medicago sativa (alfalfa) and Rhizobium meliloti were compared histologically. In nodule tissue formed by ineffective R. meliloti strain 1021726, rhizobia released into host cells sometimes became enveloped in large masses of an apparent polysaccaride-like material and did not develop into bacteroids. Bacteroids produced by this strain were smaller and showed fewer pleomorphic shapes than bacteroids in effective nodules. Nodules induced by this strain senesced much more rapidly than effective nodules. MnPL-480 is an alfalfa genotype that produces ineffective nodules with most effective strains of R. meliloti. These nodules were strikingly different than either effective or ineffective nodules induced by wild type R. meliloti strains. MnPl-480 nodules were tumour-like and most cells were filled with starch. In contrast to either effective or bacterial induced ineffective associations on other lines of alfalfa, MnPL-480 nodules had few infected cells and little proliferation of infection threads. Plant (MnPL-480) induced ineffectiveness appears to be expressed differently than bacterial (strain 1021726) induced ineffectiveness. In either association ineffectiveness may be expressed at more than one level within the nodule.