Abstract
Summary The central unit of all nitrogen-fixing nodules is the symbiosome, which can be considered as a temporary but independent nitrogen-fixing endosymbiotic organelle consisting of the bacteroid, the peribacteroid space and the surrounding symbiosome membrane. Ageing and induced senescence of this membrane were investigated with respect to its protein components. No qualitative changes in the protein and nodulin composition could be detected in mature and ageing root nodules. When root nodule senescence was induced by plant decapitation, the symbiosome membrane persisted as a structure over the examined period. However, in the symbiosome membrane some nodule-specific proteins (nodulins) were extensively degraded, suggesting the presence of selective proteolytic activities against the symbiosome membrane nodulins. Enhanced proteolytic activities were observed during induced nodule senescence but not during physiological ageing. During induced senescence, activities and protein levels of glutamine synthetase (GS) and nitrogenase (N 2 ase) were significantly reduced. Glutamine synthetase activity decreased to 50% of the initial activity, while the immunodetectable amount of the respective polypeptide disappeared completely.
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