Nodule-Specific Polypeptides from Effective Alfalfa Root Nodules and from Ineffective Nodules Lacking Nitrogenase

Abstract

In addition to leghemoglobin, at least 9 nodule-specific polypeptides from the alfalfa Rhizobium meliloti symbiosis were identified by immune assay. Some of these polypeptides may be subunits of large proteins but none appeared to be subunits of the same multimeric protein. All 9 of the nodule-specific polypeptides were localized to within the plant cytosol; they were not found in extracts of bacteroids or in the peribacteroid space. At least 1 of these nodule-specific polypeptides was antigenically related to nodule-specific polypeptides in pea and/or soybean. Apparently ineffective nodules elicited by R. meliloti strains containing mutations in 4 different genes required for nitrogenase synthesis contained reduced concentrations of leghemoglobin and of several of the nodule-specific polypeptides. Other nodule-specific polypeptides were unaltered or actually enriched in the ineffective nodules. Many of the differences between the ineffective and effective nodules were apparent in nodules harvested shortly after the nodules became visible. These differences were greatly amplified in older nodules. When the 4 ineffective nodule types were compared to one another, there were clear quantitative differences in the concentrations of several of the nodule-specific polypeptides. Lack of a functional nitrogenase [Pisum sativum and Glycine max, respectively] does not have a single direct effect on nodule development.