Pathogen-defence gene prp1-1 from potato encodes an auxin-responsive glutathione S-transferase

Genetic studies have previously implicated the prp1 gene family in the defence of potato against infection with the late blight fungus Phytophthora infestans. Here, we show that the concentrations of PRP1 mRNA as well as protein rapidly increase in potato leaves after fungal infection and stay at high levels during an extended period of the infection cycle. After separation of subcellular components by differential centrifugation, PRP1 protein was identified in the cytosolic fraction. Expression studies with chimeric promoter/beta-glucuronidase gene constructs in transgenic potato plants provided evidence that transcription of the prp1-1 gene, representing one member of the prp1 gene family, is at least partly responsible for the accumulation of PRP1 mRNA and protein upon fungal infection. After expression of the prp1-1-coding sequence in Escherichia coli, the corresponding 26-kDa protein exhibited glutathione S-transferase activity with Km values of 9.8 mM and 0.11 mM for the artificial standard substrate 1-chloro-2,4-dinitrobenzene and glutathione, respectively. Photoaffinity labeling of the protein with tritiated 5-azido-indole-3-acetic acid suggested that the phytohormone indole-3-acetic acid or a structurally related compound serve as a regulator or substrate of the prp1-1 encoded glutathione S-transferase. This assumption was further supported by the inhibitory effect of the phytohormone on the enzyme activity in vitro. The implications of these findings for a potential involvement of indole-3-acetic acid in the control of defence reactions are discussed.