INDOLE-3-ACETIC-ACID IS SYNTHESIZED FROM L-TRYPTOPHAN IN ROOTS OF ARABIDOPSIS-THALIANA

The promoter of the nit1 gene, encoding the predominantly expressed is oform of the Arabidopsis thaliana (L.) Heynh. nitrilase isoenzyme fami ly, fused to the beta-glucuronidase gene (uidA) drives beta-glucuronid ase expression in the root system of transgenic A. thaliana and tobacc o plants. This expression pattern was shown to be controlled developme ntally, suggesting that the early differentiation zone of root tips an d 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 [H-2](5)-L-tryptophan, they converted this prec usor to [H-2](5)-indole-3-acetonitrile and [H-2](5)-indole-3-acetic ac id. This latter metabolite was further metabolized into base-labile co njugates which were the predominant form of [H-2](5)-indole-3-acetic a cid extracted from roots. When [1-C-13]-indole-3-acetonitrile was fed to sterile roots: it was converted to [1-C-13]-indole-3-acetic acid wh ich was further converted to conjugates. The results prove that the A. thaliana root system is an autonomous site of indole-3-acetic acid bi osynthesis from L-tryptophan.