Effects of Short-Term N2 Deficiency on N Metabolism in Legume Nodules

Publisher Website
Google Scholar
Abstract
The study aimed to test the hypothesis that ammonia production by Rhizobium bacteroids provides not only a source of N for growth but has a central regulatory role in maintaining the metabolic activity and functional integrity of the legume nodule. Production of ammonia in intact, attached nodules was interrupted by short-term (up to 3 days) exposure of the nodulated root systems of cowpea (Vigna unguiculata L. Walp cv. Vita 3: Rhizobium CB 756) and lupin (Lupinus albus L. cv. Ultra: Rhizobium WU 425) to atmospheres of Ar:O2 (80:20; vol/vol). Treatment did not affect nodule growth, levels of plant cell and bacteroid protein, leghemoglobin content, or nitrogenase (EC 1.18.2.1) activity (acetylene reduction) but severely reduced (by 90%) synthesis and export of the major nitrogenous solutes produced by the 2 symbioses (ureides in cowpea, amides in lupin). Glutamine synthetase (EC 6.3.1.2) and NAD:glutamate oxidoreductase (EC 1.4.1.2) were more or less stable to Ar:O2 treatment, but activities of the glutamine-utilizing enzymes, glutamate synthase (EC 1.4.1.13 [EC 2.6.1.53]), asparagine synthetase (EC 6.3.5.4) (lupin only) and de novo purine synthesis (cowpea only), were all markedly reduced. Production and export of nitrogenase solutes by both symbioses resumed within 2 h after transferring Ar:O2-treated plants back to air. In each case the major exported product of fixation after transfer was initially glutamine, reflecting the relative stability of glutamine synthetase activity. Subsequently, glutamine declined and products of its assimilation became predominant consistent with resurgence of enzymes for the synthesis of asparagine in lupin and ureides in cowpea. Enzymes not directly involved with either ammonia or glutamine assimilation (purine synthesis, purine oxidation, and C metabolism of both bacteriods and plant cells) also showed transient changes in activity following interruption by N2 supply. These data have been interpreted to indicate a far-reaching effect of the production of ammonia by bacteroids on a wide range of enzymes, possibly through control of protein turnover, rather than a highly specific effect of ammonia, or some product of its assimilation, on a few enzyme species.