Assimilation of ^{13}\mathrm{N}{\mathrm{H}}_{4}^{+}$ by Anthoceros grown with and without symbiotic Nostoc

• Published in: Planta 1983-01, Vol.158 (5), p.384-391
• Main Authors: Meeks, John C., Enderlin, Carol S., Wycoff, Keith L., Chapman, John S., Joseph, Cecillia M.
• Format: Article
• Language: English
• Subjects: Amino acids; Dehydrogenases; Enzymes; Nitrogen; Physiological assimilation; Plant biochemistry; Plants; Quaternary ammonium compounds; Radioactive decay; Symbiosis
• ISSN: 0032-0935; 1432-2048

The pathways of assimilation of ammonium by pure cultures of symbiont-free Anthoceros punctatus L. and the reconstituted Anthoceros-Nostoc symbiotic association were determined from time-course (5—300 s) and inhibitor experiments using ^{13}\mathrm{N}{\mathrm{H}}_{4}^{+}$. The major product of assimilation after all incubation times was glutamine, whether the tissues were cultured with excess ammonium or no combined nitrogen. The 13N in glutamine was predominantly in the amide-nitrogen position. Formation of glutamine and glutamate by Anthoceros-Nostoc was strongly inhibited by either 1 mM methionine sulfoximine (MSX) or 1 mM exogenous ammonium. These data are consistent with the assimilation of ^{13}\mathrm{N}{\mathrm{H}}_{4}^{+}$ and formation of glutamate by the glutamine synthetase (EC 6.3.1.2)-glutamate synthase (EC 1.4.7.1) pathway in dinitrogen-grown Anthoceros-Nostoc. However, in symbiont-free Anthoceros, grown with 2.5 mM ammonium, formation of glutamine, but not glutamate, was decreased by either MSX or exogenous ammonium. These results indicate that during short incubation times ammonium is assimilated in nitrogen-replete Anthoceros by the activities of both glutamine synthetase and glutamate dehydrogenase (EC 1.4.1.2). In-vitro activities of glutamine synthetase were similar in nitrogen-replete Anthoceros and Anthoceros-Nostoc, indicating that the differences in the routes of glutamate formation were not based upon regulation of synthesis of the initial enzyme of the glutamine synthetase-glutamate synthase pathway. When symbiont-free Anthoceros was cultured for 2 d in the absence of combined nitrogen, total ^{13}\mathrm{N}{\mathrm{H}}_{4}^{+}$ assimilation, and glutamine and glutamate formation in the presence of inhibitors, were similar to dinitrogen-grown Anthoceros-Nostoc. The routes of immediate (within 2 min) glutamate formation and ammonium assimilation in Anthoceros were apparently determined by the intracellular levels of ammonium; at low levels the glutamine synthetase-glutamate synthase pathway was predominant, while at high levels independent activities of both glutamine synthetase and glutamate dehydrogenase were expressed.