A glutathione S-transferase gene was amplified from cDNA of Nicotiana tabacum roots infected with Phytophthora parasitica var. nicotianae. The gene was cloned in sense and anti-sense orientation to an RNAi vector for induced gene silencing, and reduced expression of the gene was detected by RT-PCR. A statistically significant increase in resistance of N. tabacum to infection following gene silencing was found for glutathione S-transferase-silenced plants compared with control plants. Some defense genes were up-regulated in glutathione S-transferase-silenced plants during the interaction with the pathogen. This is the first evidence of the role of glutathione S-transferase as negative regulator of defense response. (C) 2009 Elsevier Inc. All rights reserved.