THE AXR2-1 MUTATION OF ARABIDOPSIS-THALIANA IS A GAIN-OF-FUNCTION MUTATION THAT DISRUPTS AN EARLY STEP IN AUXIN RESPONSE

The dominant axr2-1 mutation of Arabidopsis thaliana confers resistance to the plant hormones auxin, ethylene, and abscisic acid. In addition, axr2-1 has pleiotropic effects on plant morphology which include gravitropic defects in roots, hypocotyls and inflorescences of axr2-1 plants. Two genetic screens were conducted to isolate new mutations at the AXR2 locus. First, axr2-1 pollen was gamma-irradiated, crossed onto wild-type plants, and the M(1) progeny screened for loss of the axr2-1 phenotype. Large deletions of the axr2-1 region on chromosome 3 resulted; however, none of these deletions appeared to be heritable. In the second, M(2) seed obtained from axr2-1 gl-l plants was screened for reversion of the axr2-1 phenotype. One revertant line, axr2-r3, has a distinctive phenotype caused by a second mutation at the axr2 locus. To learn more about the nature of the axr2-1 mutation, the effects of varying the ratio of wild-type to mutant copies of the AXR2 gene were examined by comparing plants of the following genotypes: +/+, +/+/+, axr2-1/axr2-1, axr2-1/+ and axr2-1/+/+. Additionally, accumulation of transcripts from the auxin-inducible SAUR-AC1 gene was examined to determine the response of wild-type and mutant plants to auxin. Wild-type seedlings and mature plants accumulate transcripts with auxin treatment. In contrast, axr2-1 tissue does not accumulate SAUR-AC1 transcripts in response to auxin. Taken together, these results indicate that axr2-1 is a neomorphic or hypermorphic mutation that disrupts an early step in an auxin response pathway.