Transgene-mediated auxin overproduction in Arabidopsis: hypocotyl elongation phenotype and interactions with the hy6-1 hypocotyl elongation and axr1 auxin-resistant mutants

Transgenic Arabidopsis thaliana plants constitutively expressing Agrobacterium tumefaciens tryptophan monooxygenase (iaaM) were obtained and characterized. Arabidopsis plants expressing iaaM have up to 4-fold higher levels of free indole-3-acetic acid (IAA) and display increased hypocotyl elongation in the light. This result clearly demonstrates that excess endogenous auxin can promote cell elongation in a whole plant. Interactions of the auxin-overproducing transgenic plants with the phytochrome-deficient hy6-1 and auxin-resistant axrl-3 mutations were also studied. The effects of auxin overproduction on hypocotyl elongation were not additive to the effects of phytochrome deficiency in the hy6-1 mutant, indicating that excess auxin does not counteract factors that limit hypocotyl elongation in hy6-1 seedlings. Auxin-overproducing seedlings are also qualitatively indistinguishable from wild-type controls in their response to red, far-red, and blue light treatments, demonstrating that the effect of excess auxin on hypocotyl elongation is independent of red and blue light-mediated effects. All phenotypic effects of iaaM-mediated auxin overproduction (i.e. increased hypocotyl elongation in the light, severe rosette leaf epinasty, and increased apical dominance) are suppressed by the auxin-resistant axr1-3 mutation. The axr1-3 mutation apparently blocks auxin signal transduction since it does not reduce auxin levels when combined with the auxin-overproducing transgene.