Department of Biochemistry
Symbiotic nitrogen fixation in soybeans.
Rhizobium-leguminous plant symbioses are unique associations in which both the bacteria and the plant undergo complex metabolic and morphological changes to reduce atmospheric dinitrogen to organic forms of nitrogen. The nitrogen-fixation process has a high energy demand - implying that the plant's microsymbionts efficiently generate energy from the malic acid supplied by the plant. The primary energy-generating pathway of the microsymbiont is belieed to be the citric acid cycle. The developmental and kinetic regulation of several of the enzymes have been determined and a cluster of citric acid cycle genes has been isolated, cloned, and sequenced Insertional inactivation of these genes permits an evaluatio of the functional role for each of htese enzymes during symbiotic nitorgen fixation.
For the last 30 years, ammonium has been assumed to be the form of reduced nitorgen released from the microsymbiont. It was thought that the ammonium simply diffused out of the acterium to the plant where it was assimilated to amino acids and ureides. However, we have shown that alanine, not ammonium, is the sole reduced nitrogen compound released by the microsymbionts. The ammonium via nitrogen fixation is rapidly assimilated by alanine dehydrogenase into alanine, which is then efficiently transported to the plant. This demonstrated that the initial step of nitrogen assiilation occurs within the microsymbiont.
Recently, we have begun to explore the early events of rhizobium-legume interactions. At present we are attempting to identify the receptors needed for attachment on each of the two symbionts.