Action of Inhibitors of Ammonia Assimilation on Amino Acid Metabolism in Hordeum vulgare L. (cv Golden Promise)

Barley (Hordeum vulgare L. cv Golden Promise) plants were grown in a continuous culture system in which the root and shoot ammonia and amino acid levels were constant over a 6-hour experimental period. Methionine sulfoximine (MSO), 1 millimolarity when added to the culture medium, caused a total inactivation of root glutamine synthetase with little effect on the shoot enzyme. Root ammonia levels increased and glutamine levels decreased, irrespective of whether the plants were grown in 1 millimolar nitrate or 1 millimolar ammonia. Levels of glutamate, aspartate, serine, threonine, and asparagine all increased. There was little alteration in the amino acid and ammonia levels in the shoot, suggesting that MSO is not rapidly transported.The addition of azaserine (25 micrograms per milliliter) to nitrate-grown plants caused a rapid increase in root ammonia, glutamine, and serine levels with a corresponding decrease in glutamate, aspartate, and alanine. Glutamine levels also increased in the shoot.The in vivo effect of MSO and azaserine was as would be predicted by their known in vitro inhibitory action if the glutamine synthetase/glutamate synthase pathway of ammonia assimilation was in operation.