Fixation of [ 13 N]N 2 and transfer of fixed nitrogen in the Anthoceros-Nostoc symbiotic association

• Published in: Planta 1985-06, Vol.164 (3), p.406-414
• Main Authors: Meeks, J.C., Enderlin, C.S., Joseph, C.M., Chapman, J.S., Lollar, M.W.L.
• Format: Article
• Language: English
• Subjects: Chemical suspensions; Cyanobacteria; Enzymes; Lichens; Nitrogen; Nitrogen fixation; Quaternary ammonium compounds; Radioactive decay; Symbiosis
• ISSN: 0032-0935; 1432-2048
• DOI: 10.1007/BF00402954

The initial product of fixation of [13N]N2 by pure cultures of the reconstituted symbiotic association between Anthoceros punctatus L. and Nostoc sp. strain ac 7801 was ammonium; it accounted for 75% of the total radioactivity recovered in methanolic extracts after 0.5 min and 14% after 10 min of incubation. Glutamine and glutamate were the primary organic products synthesized from [13N]N2 after incubation times of 0.5—10 min. The kinetics of labeling of these two amino acids were characteristic of a precursor (glutamine) and product (glutamate) relationship. Results of inhibition experiments with methionine sulfoximine (MSX) and diazo-oxonorleucine were also consistent with the assimilation of N2-derived $\mathrm{N}{\mathrm{H}}_{4}^{+}$ by Anthoceros-Nostoc through the sequential activities of glutamine synthetase (EC 6.3.1.2) and glutamate synthase (EC 1.4.7.1), with little or no assimilation by glutamate dehydrogenase (EC 1.3.1.3). Isolated symbiotic Nostoc assimilated exogenous ^{13}\mathrm{N}{\mathrm{H}}_{4}^{+}$ into glutamine and glutamate and their formation was inhibited by MSX, indicating operation of the glutamine synthetase-glutamate synthase (GS-GOGAT) pathway. However, relative to free-living cultures, isolated symbiotic Nostoc assimilated 80% less exogenous ammonium into glutamine and glutamate, implying that symbiotic Nostoc could assimilate only a fraction of N2-derived $\mathrm{N}{\mathrm{H}}_{4}^{+}$. This implication was tested by using Anthoceros associations reconstituted with wild-type or MSX-resistant strains of Nostoc incubated with [13N]N2 in the presence of MSX. The results of these experiments indicated that, in situ, symbiotic Nostoc assimilated about 10% of the N2-derived $\mathrm{N}{\mathrm{H}}_{4}^{+}$ and that $\mathrm{N}{\mathrm{H}}_{4}^{+}$ was made available to Anthoceros tissue where it was apparently assimilated by the GS-GOGAT pathway. Since less than 1% of the fixed N2 was lost to the suspension medium, it appears that transfer of $\mathrm{N}{\mathrm{H}}_{4}^{+}$ from symbiont to host tissue was very efficient in this extracellular symbiotic association.