The effects of overproduction of two Agrobacterium tumefaciens T-DNA auxin biosynthetic gene products in transgen c petunia plants

Two genes of Agrobacterium tumefaciens encode enzymes that together produce indoleacetic acid (IAA). The first gene, iaaM, encodes tryptophan monooxygenase which converts tryptophan to indoleacetamide (IAM). The second gene, iaaH, encodes indoleacetamide hydrolase which converts IAM to IAA. We have engineered each of the two genes to be expressed at either high constitutive levels or in a tissue-specific manner. These chimeric genes were introduced separately into petunia plants. The transgenic plants with the Agrobacterium iaaH gene are morphologically normal but have gained the ability to use IAM as an auxin. The plants with iaaM, in contrast, are morphologically abnormal. The observed abnormalities are consistent with an overproduction of auxin and ethylene. These plants contain an approximately 10-fold excess of IAA. The consequences of this deliberate manipulation of the normal phytohormone balance in transgenic plants are described.