The gene encoding the auxin-responsive GH3 mRNA (G. Hagen, A. Kleinschmidt, TJ. Guilfoyle, Planta 162: 147-153 (1984] from soybean was cloned, and its sequence and transcription initiation site were determined. The promoter of the GH3 gene has been fused to the open reading frame of the Escherichia coli uidA gene which encodes beta-glucuronidase (GUS). This fusion gene was introduced into tobacco via Agrobacterium tumefaciens-mediated transformation, and the expression of the gene was examined by fluorometric assay and histochemical staining of young R1 tobacco seedlings and mature plants. In transgenic tobacco plants that have not been exposed to exogenous auxin, expression of the fusion gene is largely restricted to roots of young green plants and developing floral organs, including ovules, developing seeds, and pollen, of mature plants. Application of exogenous auxin to tobacco seedlings or plant organs results in a greater than 50-fold increase in expression of GUS. Auxin-induced GUS expression is greatest in vascular tissue, but not restricted to this tissue. The auxin-deduced GUS expression was characterized for kinetics, auxin specificity and dose response.