The promoter of the nit1 gene, encoding the predominantly expressed isoform of the Arabidopsis thaliana (L.) Heynh. nitrilase isoenzyme family, fused to the beta-glucuronidase gene (uidA) drives beta-glucuronidase expression in the root system of transgenic A. thaliana and tobacco plants. This expression pattern was shown to be controlled developmentally, suggesting that the early differentiation zone of root tips and the tissue surrounding the zone of lateral root primordia formation may constitute sites of auxin biosynthesis in plants. The root system of A. thaliana was shown to express functional nitrilase enzyme. When sterile roots were fed [2H]5-L-tryptophan, they converted this precursor to [2H]5-indole-3-acetonitrile and [2H]5-indole-3-acetic acid. This latter metabolite was further metabolized into base-labile conjugates which were the predominant form of [2H]5-indole-3-acetic acid extracted from roots. When [1-13C]-indole-3-acetonitrile was fed to sterile roots, it was converted to [1-13C]-indole-3-acetic acid which was further converted to conjugates. The results prove that the A. thaliana root system is an autonomous site of indole-3-acetic acid biosynthesis from L-tryptophan.