Effect of methionine sulphoximine on vacuolar storage of amino acids in Lemna minor L

Abstract

The effect of inhibition of glutamine synthetase by methionine sulphoximine (MSO) on the tissue content of amino acids was compared with its effect on their storage in the vacuole, in illuminated Lemna minor plantlets. MSO resulted in a considerable decline in the tissue contents of glutamate, glutamine, aspartate and asparagine. 14C-Labelling studies revealed enhanced conversion of glutamate and asparagine to water-soluble compounds other than amino acids, and CO 2. Plantlets were cultured on medium containing tritiated water for 48 h, followed by unlabelled medium for 48 h. The tritium content of particular amino acids was determined at time intervals after supplying MSO. Tritium per unit of that particular amino acid rose in the case of aspartate, glutamate and glutamine but remained constant in the case of asparagine. Tritium per dry weight of tissue remained essentially unchanged for aspartate, glutamate and glutamine but declined dramatically for asparagine. These results are interpreted as indicating that MSO resulted in the exit of asparagine from the vacuole, but no substantial change in the vacuolar pools of aspartate, glutamate and glutamine.