Outer Membrane Protein Changes During Bacteroid Development Are Independent of Nitrogen Fixation and Differ Between Indeterminate and Determinate Nodulating Host Plants of Rhizobium leguminosarum. Henk P. Roest. Leiden University, Institute of Molecular Plant Sciences, Clusius Laboratory, Wassenaarseweg 64, 2333 AL Leiden, The Netherlands. Leentje Goosen-de Roo, Carel A. Wijffelman, Ruud A. de Maagd, and Ben J. J. Lugtenberg. Leiden University, Institute of Molecular Plant Sciences, Clusius Laboratory, Wassenaarseweg 64, 2333 AL Leiden, The Netherlands. MPMI 8:14-22. Accepted 14 September 1994. Copyright 1995 The American Phytopathological Society.

The outer membrane of bacteroids contains largely decreased levels of protein antigen groups II and III in comparison with that of free-living rhizobia (R. A. de Maagd, R. de Rijk, I. H. M. Mulders, and B. J. J. Lugtenberg, J. Bacteriol. 171:1136-1142, 1989). Since we intend to study the molecular basis of the development of bacterium to bacteroid, we wanted to know whether these outer membrane protein differences are conserved in various plant-Rhizobium combinations. For this purpose we developed a faster assay in which cell Iysates instead of isolated cell envelopes were used to analyze these outer membrane changes. With this method the previously described low levels of antigen groups II and III in isolated bacteroid cell envelopes were confirmed. Moreover the described decrease in antigen groups II and III was also found in bacteroids of Rhizobium leguminosarum bv. viciae with a mutated nifA or nifK gene as well as in the non-fixing pea mutant FN1 inoculated with the wild-type strain 248. This indicates (hat the decrease in the antigen levels is not restricted to effective nodules. The results also showed that the decrease in antigen group II not only occurs in bacteroids from pea, but also in bacteroids from vetch, broadbean, white clover, and common bean. Antigen group III, however, remained present in bacteroids from common bean. It is concluded that the changes in antigen group II are not restricted to a specific cross-inoculation group but represent a general phenomenon in the rhizobial bacteroid differentiation process. Of the tested plants, the decrease in antigen group III was not found in bacteroids from common bean and appeared to be restricted to bacteroids from indeterminate nodules. Therefore one should expect that at least two molecular mechanisms are responsible for these outer membrane protein changes and that elucidation of these mechanisms will contribute to our understanding of bacteroid development.