Light regulation of native and Escherichia coli glutathione reductase in transgenic tobacco

Abstract

Summary In plants, defence against oxygen free radicals relies on the Asada-Halliwell pathway, a series of enzyme-catalysed redox reactions that reduce toxic oxygen species to water at the expense of reductant. The rate limiting step in this cycle is believed to be the reduction of glutathione, by glutathione reductase (GR, EC 1.6.4.2), using NADPH. As a tool to understanding the plant antioxidant system, tobacco ( Nicotiana tabacum cv. Samsun) containing the Escherichia coli GR gene ( gor ) has been engineered by Agrobacterium tumefaciens -mediated transformation. The response of these transformants to light has been examined and data indicates that both plant and E. coli glutathione reductases are subject to diurnal variation. Total GR activity (GRA) in control and transgenic plants increases following exposure to light, but the increases are greater in the transgenic plants. Separation of native and E. coli GR isoforms indicates that this is due to increases in both tobacco and E. coli GRA. Rises in E. coli GRA can be ascribed primarily to a light-mediated post-translational activation. Although E. coli GR transcript and protein levels vary over the photoperiod, the nature and magnitude of the fluctuations do not account for the changes in E. coli GRA, suggesting that E. coli GR is subjected to a light-mediated post-translational activation. While tobacco GR levels could not be assessed, it is likely that tobacco GR is subject to the same post-translational activation.