REGULATION OF THIOL CONTENTS IN MAIZE ROOTS BY INTERMEDIATES AND EFFECTORS OF GLUTATHIONE SYNTHESIS

The regulation of the synthesis and the levels of GSH and other acid-soluble thiols was studied in maize roots by applying GSH, GSH monoethylester, OTC (L-2-oxothiazolidine-4-carboxylic acid), the GSH precursors cysteine and gamma-EC (gamma-glutamylcysteine), the gamma-EC synthetase (EC 6.3.2.2) inhibitor BSO (buthionine S,R-sulfoximine), and the herbicide safener benoxacor (4-dichloroacetyl-3,4-dihydro-3-methyl-2H-1,4-benzooxazine). OTC, GSH and GSH monoethylester did not affect the GSH content, whereas exogenous cysteine and gamma-EC augmented it 1.6- and 2.5-fold, respectively. Both cysteine and gamma-EC strongly increased their endogenous level. Benoxacor increased the activity of adenosine 5'-phosphosulfate sulfotransferase and gamma-EC synthetase within 2 and 4 h, respectively, but did not affecte GSH synthetase activity (EC 6.3.2.3). It rapidly induced a high level of gamma-EC and doubled the cysteine and the GSH contents. When benoxacor and gamma-EC were applied together an additional increase in GSH was detected. BSO alone or together with benoxacor resulted in decreased GSH levels. Both substances combined caused an accumulation of gamma-EC. These results are discussed on the basis of the localization of the GSH synthesizing enzymes in the proplastids and the cytoplasm and the intracellular site of action of benoxacor and BSO.