Proline metabolism in N2-fixing root nodules: energy transfer and regulation of purine synthesis

Abstract

N2-fixing root nodules of soybean (Glycine max L. Merr.) convert atmospheric N2 to ammonia(um) in an energy-intensive enzymatic reaction. These nodules synthesize large quantities of purines because nitrogen fixed by bacteria contained within this tissue is transferred to the shoots in the form of ureides, which are degradation products of purines. In animal systems, it has been proposed that proline biosynthesis by pyrroline-5-carboxylate reductase (P5CR) is used to generate the NADP+ required for the synthesis of the purine precursor ribose 5-phosphate. We have examined the levels, properties, and location of P5CR and proline dehydrogenase (ProDH) in soybean nodules. Nodule P5CR was found in the plant cytosol. Its activity was substantially higher than that reported for other animal and plant tissues and is 4-fold higher than in pea (Pisum sativum) nodules (which export amides). The Km for NADPH was lower by a factor of 25 than the Km for NADH, while the Vmax with NADPH was one-third of that with NADH. P5CR activity was diminished by NADP+ but not by proline. These characteristics are consistent with a role for P5CR in supporting nodule purine biosynthesis rather than in producing proline for incorporation into protein. ProDH activity was divided between the bacteroids and plant cytosol, but less than 2% was in the mitochondria-rich fractions. The specific activity of ProDH in soybean nodule bacteroids was comparable to that in rat liver mitochondria. In addition, we propose that some of the proline synthesized in the plant cytosol by P5CR is catabolized within the bacteroids by ProDH and that this represents a novel mechanism for transferring energy from the plant to its endosymbiont.