ARTIFICIALLY INDUCED SENESCENCE OF SOYBEAN ROOT-NODULES AFFECTS DIFFERENT POLYPEPTIDES AND NODULINS IN THE SYMBIOSOME MEMBRANE COMPARED TO PHYSIOLOGICAL AGING

The central unit of all nitrogen-fixing nodules is the symbiosome, whi ch can be considered as a temporary but independent nitrogen-fixing en dosymbiotic organelle consisting of the bacteroid, the peribacteroid s pace and the surrounding symbiosome membrane. Ageing and induced senes cence of this membrane were investigated with respect to its protein c omponents. No qualitative changes in the protein and nodulin compositi on could be detected in mature and ageing root nodules. When root nodu le senescence was induced by plant decapitation, the symbiosome membra ne persisted as a structure over the examined period. However, in the symbiosome membrane some nodule-specific proteins (nodulins) were exte nsively degraded, suggesting the presence of selective proteolytic act ivities against the symbiosome membrane nodulins. Enhanced proteolytic activities were observed during induced nodule senescence but not dur ing physiological ageing. During induced senescence, activities and pr otein levels of glutamine synthetase (GS) and nitrogenase (N(2)ase) we re significantly reduced. Glutamine synthetase activity decreased to 5 0% of the initial activity, while the immunodetectable amount of the r espective polypeptide disappeared completely.