Regulation of calcium signalling and gene expression by glutathione

The glutathione redox couple is an information-rich redox buffer that interacts with numerous cellular components. To explore the role of glutathione in redox signalling, leaf contents were increased either chemically, by feeding reduced glutathione (GSH), or genetically, by over-expressing the first enzyme of the GSH biosynthetic pathway, gamma-glutamylcysteine synthetase (gamma-ECS). Leaf discs were also fed glutathione disulphide (GSSG), leading to increases in both GSH and GSSG. The effects of increases in GSH were compared with non-specific changes in leaf thiol status induced by feeding dithiothreitol (DTT) or the monothiol beta-mercaptoethanol (beta-ME). Photosynthesis measurements showed that none of the feeding treatments greatly disrupted leaf physiology. Transgenic plants expressing aequorin were used to analyse calcium signatures during the feeding treatments. Calcium release occurred soon after the onset of GSH or GSSG feeding, but was unaffected by DTT or beta-ME. Pathogenesis-related protein 1 (PR-1) was induced both in the gamma-ECS overexpressors and by feeding GSH, but not GSSG. Feeding DTT also induced PR-1. Key transcripts encoding antioxidative enzymes were much less affected, although glutathione synthetase was suppressed by feeding thiols or GSSG. It is concluded that modulation of glutathione contents transmits information through diverse signalling mechanisms, including (i) the establishment of an appropriate redox potential for thiol/disulphide exchange and (ii) the release of calcium to the cytosol.