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

Abstract

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 axr1-3 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.

Biochemical analyses of indole-3-acetaldoxime-dependent auxin biosynthesis in Arabidopsis

“…IAOx was proposed as a key intermediate in IAA biosynthesis on the basis of several observations. (i) Overexpression of CYP79B2 leads to high auxin phenotypes such as elongated hypocotyls similar to known auxin overproduction lines such as yuc1D and iaaM overexpression lines (8,33). (ii) Inactivation of SUR1 or SUR2 blocks IG biosynthesis, and leads to auxin overproduction phenotypes because of the diversion of IAOx to IAA synthesis (9,10).…”

Branching in Pea (Action of Genes Rms3 and Rms4)

“…Indeed, it is possible that the substances controlled by the Rms2 and Rms3 genes are involved in the regulation of IAA level and that these regulatory processes may operate prior to, or independently of, the action of the Rms4 gene. The axrl mutant of Arabidopsis also has increased branching (Lincoln et al, 1990) and elevated IAA levels (Romano et al, 1995), but whether graft-transmissible factors are involved in the increased branching and reduced IAA response of these plants is not known.…”

Photomorphogenesis and phyfohormones

“…Taken together, these results strongly suggest involvement of a light-induced decrease in IAA level in the light inhibition of plant cell elongation. Using IAA overproducing transgenic Arabidopsis lines, Romano et al (1995) demonstrated that the regulation of hypocotyl elongation by auxin and light are independent, since IAA overproducing plants exhibited normal responses to light. The question of the physiological significance of light-regulated IAA level is thus still open.…”