By histochemical GUS staining, we demonstrate that transcription from a short promoter fragment of the potato gst1 gene is locally induced after infection of a host plant with various types of pathogenic or symbiotic organisms. This regulatory unit is not active in noninfected tissues, except root apices and senescing leaves. Measuring the expression of a fusion between the promoter fragment and the gus gene in transgenic plants, therefore, allows comparison of the induction of defense reactions in different types of plant-microbe interactions, in one and the same plant.