Metabolism of 14C-Labeled Photosynthate and Distribution of Enzymes of Glucose Metabolism in Soybean Nodules

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Abstract
The metabolism of translocated photosynthate by soybean (G. max L. Merr) nodules was investigated by 14CO2-labeling studies and analysis of nodule enzymes. Plants were exposed to 14CO2 for 30 min, followed by 12CO2 for up to 5 h. The largest amount of radioactivity in nodules was recovered in neutral sugars at all sampling times. The organic acid fraction of the cytosol was labeled rapidly. Although cyclitols and malonate were found in high concentrations in the nodules, they accumulated less than 10% of the radioactivity in the neutral and acidic fractions, respectively. Phosphate esters contained very low levels of total label, which prohibited analysis of the radioactivity in individual compounds. The whole module-labeling patterns suggested the utilization of photosynthate for the generation of organic acids (principally malate) and amino acids (principally glutamate). The radioactivity in bacteroids as a percentage of total nodule label increased slightly with time, while the percentage in the cytosol fraction declined. The labeling patterns for the cytosol were essentially the same as whole nodule-labeling patterns, and they suggest a degradation of carbohydrates for the production of organic acids and amino acids. Most of the radioactivity in bacteroids was in sugars and enzymes of glucose metabolism were then surveyed. Bacteroids from nodules formed by Rhizobium japonicum strain 110 or strain 138 lacked activity for phosphofructokinase and NADP-dependent 6-phosphogluconate dehydrogenase, key enzymes of glycolysis and the oxidative pentose-phosphate pathways. Enzymes of the glycolytic and pentose phosphate pathways were found in the cytosol fraction. In 3 experiments, bacteroids contained about 10-30% of the total radioactivity in nodules 2-5 h after pulse-labeling of plants, and 60-65% of the radioactivity in bacteroids was in the neutral sugar fraction at all sampling times. This strongly suggests some absorption and metabolism of sugars by bacteroids in spite of the lack of key enzymes. Bacteroids did possess enzymes for the formation of hexose phosphates from glucose or fructose. Radioactivity in .alpha.,.alpha.-trehalose in bacteroids increased until, after 5 h, trehalose was a major labeled compound in bacteroids. Trehalose synthesis may be a major fate of sugars entering bacteroids.