Defense Reactions of Infected Plants: Roles of Glutathione and Glutathione S-Transferase Enzymes G. GULLNER and T. KÔMÍVES Plant Protection Institute, Hungarian Academy of Sciences, H-1525 Budapest, P.O. Box 102, Hungary (Received: 15 December 2005; accepted: 20 January 2006) In recent years, the importance of the endogenous plant tripeptide glutathione (GSH) has been increas- ingly recognized in plant-pathogen interactions due to its contribution to various signaling and defense mech- anisms. In this paper the recent developments in the research for possible roles of GSH in infected plants are summarized. GSH participates not only in antioxidative and detoxification reactions but also in redox regula- tion of the expression of protective genes in infected cells. Several lines of evidence suggest that glutathione S-transferase (GST) isoenzymes also have an important role in plant disease resistance, but their exact func- tions have remained elusive. Keywords: glutathione, glutathione S-transferase, lipid hydroperoxides, plant defense reactions, redox regulation, signaling. Abbreviations: AA, ascorbic acid; AP, ascorbate peroxidase; DA, dehydroascorbate; DR, dehydro- ascorbate reductase; GPOX, glutathione peroxidase; GR, glutathione reductase; GSH, reduced glutathione; GSSG, oxidized glutathione disulfide; GST, glutathione S-transferase; HR, hypersensitive reaction; JA, jas- monate; MA, monodehydroascorbate; MR, monodehydroascorbate reductase; PPV, plum pox virus; PR-1, pathogenesis related protein 1; PrSH, protein thiol; PrSSG, protein thiol-glutathione mixed disulfide; ROS, reactive oxygen species; SA, salicylic acid; SAR, systemic acquired resistance; SOD, superoxide dismutase; ZYMV, zucchini yellow mosaic virus. Plants respond to microbial infections with a battery of defense reactions. In case of a successful defense (disease resistance) the early recognition of pathogens results in the elicitation of a complex network of signaling pathways, which rapidly and efficiently activates a wide array of defense responses. Glutathione (GSH, γ-L-glutamyl-L-cys- teinyl-glycine), which is the most important non-protein thiol compound in plants, plays important roles in both signaling and defense reactions in infected plants (Foyer and Rennenberg, 2000; Gullner and Kômíves, 2001; Wagner et al., 2002). The roles traditionally attributed to GSH in infected plants are the participation in detoxification of reactive oxygen species (ROS), reinforcement of cell walls, formation of phytoalexins, and degradation of various toxic substances in reactions catalyzed by the Acta Phytopathologica et Entomologica Hungarica 41 (1–2), pp. 3–10 (2006) DOI: 10.1556/APhyt.41.2006.1-2.2 0238–1249/$ 20.00 © 2006 Akadémiai Kiadó, Budapest