The contents of cysteine, gamma-glutamylcysteine and glutathione, as well as the activity of enzymes of assimilatory sulfate reduction and glutathione synthesis in second leaves and root tips of maize seedlings (Zea mays L. cv. LG 11) cultivated at low temperatures, were compared with controls grown at 25 degrees C. In vitro nitrate reductase (EC 1.6.6.1) activity was measured for comparison. Compared with the controls, adenosine 5'-phosphosulfate sulfotransferase activity was 3.5- and 5.5-fold higher at 12 degrees C in second leaves and root tips, respectively. After 3 days of growth, the activity of gamma-glutamylcysteine synthetase (EC 6.3.2.2) and glutathione synthetase (EC 6.3.2.3) in the second leaves was also significantly higher at 12 degrees C compared with 25 degrees C. Consistent with these enzyme activities, there was an increase in cysteine and glutathione contents both in second leaves and root tips after 3 days of growth at 12 degrees C. Nitrate reductase activity of second leaves and root tips was not affected by growth at 12 degrees C, indicating that the increase in adenosine 5'-phosphosulfate sulfotransferase activity and of the enzymes of glutathione synthesis was specific. Our results demonstrate that the increased activity of a key enzyme of assimilatory sulfate reduction and of the enzymes of glutathione synthesis contribute to the increased glutathione levels measured at 12 degrees C.
