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Sources of Ammonium in Oat Leaves Treated with Tabtoxin or Methionine Sulfoximine ¹

Department of Agronomy, University of Wisconsin, Madison, Wisconsin 53706, Department of Plant Pathology, University of Wisconsin, Madison, Wisconsin 53706, United States Department of Agriculture, Agricultural Research Service, University of Wisconsin, Madison, Wisconsin 53706

Excised 7-day-old oat (Avena sativa L. cv. Jaycee) leaves were incubated in media containing 7.1 millimolar KNO₃ and 0.15 millimolar tabtoxin or 1 millimolar methionine sulfoximine (MSO) to investigate the sources of the observed ammonium accumulated. Tabtoxin and MSO are known inhibitors of glutamine synthetase, the first enzyme in the primary pathway of ammonium assimilation. During a 4- to 6-hour incubation, there was little net change in protein or total amino acid concentration. Alanine, aspartate/asparagine, and glutamate/glutamine decreased markedly under these treatments, whereas several other amino acids increased. Exogenous ¹⁵N from K¹⁵NO₃ was taken up and incorporated into the nitrate and ammonium fractions of leaves treated with tabtoxin or MSO. This result and the high in vitro activities of nitrate reductase indicated that reduction of nitrate was one source of the accumulated ammonium. Leaves incubated under 2% O₂ to reduce photorespiration accumulated only about 13% as much ammonium as did those under normal atmospheres. We conclude that most of the tabtoxin- or MSO-induced ammonium came from photo-respiration, and the remainder was from nitrate reduction.

¹ Supported by the United States Department of Agriculture, Agricultural Research Service, and by the College of Agricultural and Life Sciences, University of Wisconsin, Madison.

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				T. J. KNIGHT and P. J. LANGSTON-UNKEFER Enhancement of Symbiotic Dinitrogen Fixation by a Toxin-Releasing Plant Pathogen Science, August 19, 1988; 241(4868): 951 - 954. [Abstract] [PDF]