ULTRASTRUCTURAL IMMUNOLOCALIZATION OF UBIQUITIN CONJUGATED PROTEINS IN LEGUME ROOT-NODULES

The immunolocalization of ubiquitin and ubiquitinated proteins was det ermined in Rhizobium-induced root nodules of Phaseolus vulgaris and Vi cia hirsuta. Mature nodules of either species displayed the signal for ubiquitin in the cell cytosol and nuclei. Plants submitted to dark, p resented in the nodules symptoms of senescence at the structural and u ltrastructural levels and increased proteinase activity of nodule extr acts. Immunoblot experiments utilizing a monoclonal antibody raised ag ainst ubiquitin showed a modified pattern and an increase in the inten sity of the lower molecular mass bands of nodule extract derived from dark treated plants. Our results indicate that the ubiquitin-mediated proteolytic pathway is present in legume root nodules formation and th at it is involved in the early stages of senescence.