A.M. Berry1, T.M. Murphy2, K. Pawlowski3, P.A. Okubara4, K.R. Jacobsen5
*Dept Environmental Horticulture, U Calif, Davis, CA, USA 2Sect Plant Biology, U Calif, Davis, CA, USA Albrecht v. Haller Inst, Univ Goettingen, Germany 4USDA-ARS, Wash St U, Pullman, WA, USA, 5Dept Agronomy, U Calif, Davis, CA, USA
In root nodules of the actinorhizal host Datisca glomerata, glutamine synthetase (GS) is expressed at high levels in uninfected tissue, but not in infected tissue, as shown by in situ hybridization and immunolocalization. The absence of GS in the infected tissue excludes the pathway of nitrogen assimilation via ammonia excretion from the microsymbiont and plant glutamine synthesis in the infected tissue, which is common to legumes and to other actinorhizal species. We further show that arginine is highly elevated in D. glomerata nodule amino acid profiles, but not in root, xylem sap or leaf extracts. Glutamine and glutamate levels are elevated in both nodule and xylem sap, however, indicating their importance in whole-plant nitrogen transport. We hypothesize that arginine is the major storage form of nitrogen within the nodule; it is synthesized in the infected tissue and exported to the uninfected tissue, where it is further catabolized by arginase and urease to yield free ammonium. The observation that the nodules are positive for urease activity supports this hypothesis. These data explain the expression pattern of glutamine synthetase in the uninfected tissue and suggest a novel role for arginine in nodule nitrogen metabolism.
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