Soybean (Glycine max) nodule bacteroids contain high concentrations of poly-,B-hydroxybutyrate and possess a depolymerase system that catalyzes the hydrolysis of the polymer.Changes in poly-p-hydroxybutyrate content Evidence for the metabolic pathway of utilization of BOHB was provided by Sierra and Gibbons (30), who demonstrated that BOHB was oxidized to acetoacetate by crude extracts from M. halodenitrificans. Acetoacetate was further metabolized when ATP, Mg2+, coenzyme A, and oxaloacetate were added to the extracts, suggesting that acetyl-CoA was an intert mediate of the reaction and that utilization occurred through the tricarboxylic acid cycle.The catalysis of nitrogen reduction by cell-free extracts of nitrogen-fixing microogranisms and nodule bacteroids requires a supply of ATP (12,23,25) and an appropriate reductant (4,8, 12,14,25). It has been estimated (3, 10) that 3 to 19 mg of carbohydrate are consumed by legume root nodules for each milligram of N2 fixed. Moreover, two or more carbon atoms are required for the export of each fixed nitrogen atom, in the forms of amino acids and amides, from the nodules to the host (33).It has been established that up to 50% of the dry weight of Rhizobium japonicum bacteroids consists of PHB (14), and that /3-hydroxybutyrate oxidation in in vitro experiments is capable of supplying the electrons for the support of bacteroid nitrogenase activity (14). The purpose of this investigation, therefore, was to study the utilization of PHB in soybean root nodules and to assess the possible role of the polymer as a source of energy for maintenance of nitrogenase activity in nodule bacteroids.
MATERIALS AND METHODS
CHEMICALSReagent grade chemicals or those of the highest grade available were obtained from commercial sources. NAD, sodium DL-3-hydroxybutyrate, DL-isocitrate lactone (hydrolyzed as recommended by the manufacturer), 2-mercaptoethanol, EDTA, 750 www.plantphysiol.org on May 9, 2018 -Published by Downloaded from