PHOTOSYSTEM-II AND OXYGEN REGULATION IN SESBANIA-ROSTRATA STEM NODULES

The tropical wetland legume Sesbania rostrata Brem, produces nitrogen- fixing stem nodules which are green and contain chlorophyll, the chlor oplasts being concentrated in a band in the inner and mid-cortex close to the nitrogen-fixing cells, The photosystem II thylakoid membrane p roteins D1, D2 and PsbO, which are essential for photosynthetic O-2 ev olution, were shown by immunoblotting to be present in extracts of lea ves and stem nodules, Immunogold labelling confirmed their presence on stem nodule thylakoids and showed that labelling was most intense in well-developed chloroplasts in the mid-cortex and least intense in the smaller, less-abundant chloroplasts adjacent to the nitrogen-fixing c ells, Concentrations of the oxygen-carrying protein leghaemoglobin (Lb ) did not differ between stem and S. rostrata root nodules, and Lb was localized in bacteroid-containing cells, including those immediately adjacent to the cortex, in both nodule types. Moreover, nitrogenase co mponent 2 was localized in bacteroids within the outermost layers of i nfected cells, suggesting that a low pO(2) was maintained, despite the nearby chloroplasts, Nodule extracts examined by ELISA and immunoblot s, using the monoclonal antibody MAC265, showed greatly enhanced expre ssion of a 139 kDa glycoprotein in stem compared to root nodules, Immu nogold labelling showed that material containing the MAC265 antigen oc cluded intercellular spaces, and was present in cell walls, throughout the dorter of stem nodules (particularly in the chloroplast-rich inne r and mid-cortex), but was considerably less evident in root nodules.