LIGHT-DEPENDENT MODULATION OF FOLIAR GLUTATHIONE SYNTHESIS AND ASSOCIATED AMINO-ACID-METABOLISM IN POPLAR OVEREXPRESSING GAMMA-GLUTAMYLCYSTEINE SYNTHETASE

Glutathione (GSH), gamma-glutamylcysteine (gamma-EC) and major free am ino acids were measured in darkened and illuminated leaves from untran sformed poplars (Populus tremula x P. alba) and poplars expressing Esc herichia coli genes for gamma-glutamylcysteine synthetase (gamma-ECS; EC 3.2.3.3) and glutathione reductase (GR; EC 1.6.4.2). In poplars ove rexpressing gamma-ECS, foliar gamma-EC contents and GSH contents were markedly enhanced compared to poplars lacking the bacterial gene for t he enzyme. However, the quantitative relationship between the foliar p ools of gamma-EC and GSH in these transformants was markedly dependent on light. In the dark, GSH content was relatively low and gamma-EC co ntent high, the latter being higher than the foliar GSH contents of un transformed poplars in all conditions. Hence, this transformation appe ars to elevate gamma-EC from the ranks of a trace metabolite to one of major quantitative importance. On illumination, however, gamma-EC con tent decreased fourfold whereas GSH content doubled. Glutathione was a lso higher in the light in untransformed poplars and in those overexpr essing GR. In these plants, gamma-EC was negligible in the light but i ncreased in the dark. Cysteine content was little affected by light in ally of the poplar types. No light-dependent changes in the extractab le activities of gamma-ECS, glutathione synthetase (EC 3.2.3.2) or GR were observed. In contrast, both the activation state and the maximum extractable activity of nitrate reductase (EC 1.6.6.1) were increased by illumination. In all poplar types, glutamate and aspartate were the major amino acids. The most marked light-induced increases in individ ual amino acids were observed in the glutamine, asparagine, serine and glycine pools. Illumination of leaves from poplars overexpressing gam ma-ECS at elevated CO2 or low O-2 largely abolished the inverse light- dependent changes in gamma-EC and GSH. Low O-2 did not affect foliar c ontents of cysteine or glutamate but prevented the light-induced incre ase in the glycine pool. It is concluded that light-dependent glycine formation through the photorespiratory pathway is required to support maximal rates of GSH synthesis, particularly under conditions where th e capacity for gamma-EC synthesis is augmented.