OXIDATIVE STRESS INDUCES HIGH-RATE OF GLUTATHIONE-REDUCTASE SYNTHESIS IN A DROUGHT-RESISTANT MAIZE STRAIN

Leaf discs from 7-day-old seedlings of two maize strains, one resistant (LIZA) and one sensitive (LG11) to water stress, were incubated in solutions containing different concentrations of paraquat or H2O2 in darkness and effects of these substances on the senescence parameters of the tissue and on the activities of enzymes involved in the scavenging of active oxygen species were examined. Three different parameters of senescence; chlorophyll loss, lipid peroxidation and electrolyte leakage from the cell; revealed the higher tolerance of LIZA than LG11 to these substances under this condition. Activity of glutathione reductase (GR) markedly increased with sublethal concentrations of these substances in both maize strains and was 2- to 3-fold higher in LIZA than in LG11. At higher concentrations of paraquat or H2O2, GR activity decreased, especially in LG11. When cycloheximide was added to the incubation medium containing 1 mM paraquat or H2O2, the induction of GR activity was depressed in both strains, while actinomycin D had no significant effect. These results suggest that oxidative stress caused by paraquat or H2O2 would stimulate GR de novo synthesis, probably at the level of translation by preexisting mRNA. Correlations between the stress resistance and activities of other antioxidant enzymes such as ascorbate peroxidase and superoxide dismutase were also observed.