ENZYME DISTRIBUTION BETWEEN THE CORTEX AND THE INFECTED REGION OF SOYBEAN NODULES

Analysis of the distribution of enzymes of carbohydrate, carboxylic acid and nitrogen metabolism in nodule tissues prepared by protoplast isolation techniques has led to some confusion about their whereabouts. In this study nodule cortical and central (infected) tissue was separated manually using a scalpel blade. Protein contents of whole nodules, central region and cortex were found to be 19, 25 and 5.0 mg g-1 fresh weight, respectively. Specific activities of glutamine synthetase (GS), phosphoenolpyruvate carboxylase (PEPC), invertase (INV) and sucrose synthase (SS) were higher in the central region than in the cortex. The reverse was true for UDP glucose pyrophosphorylase (UDPGPP), phosphoglucomutase (PGM) and phosphoglucose isomerase (PGI). The central (infected) region represented a minimum of 65% of the nodule fresh weight. Calculations based on this estimate and on the activities per gram fresh weight indicate that the bulk of the potential enzyme activity is located in the central region. This was particularly so for SS, INV and GS where greater than 96% of the relevant activity was located in the central region. Even the enzymes which had greater specific activities in the cortex (UDPGPP, PGM and PGI) were, in fact, predominantly found in the central region (86, 81, and 85%, respectively). The conclusion from this data must be that most metabolism in the nodule is likely to occur in the central region, although this does not negate the importance of the cortex in carbon and nitrogen flux.