AXR2 Encodes a Member of the Aux/IAA Protein Family

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Abstract
The dominant gain-of-function axr2-1 mutation of Arabidopsis causes agravitropic root and shoot growth, a short hypocotyl and stem, and auxin-resistant root growth. We have cloned theAXR2 gene using a map-based approach, and find that it is the same as IAA7, a member of the IAA(indole-3-acetic acid) family of auxin-inducible genes. Theaxr2-1 mutation changes a single amino acid in conserved domain II of AXR2/IAA7. We isolated loss-of-function mutations inAXR2/IAA7 as intragenic suppressors ofaxr2-1 or in a screen for insertion mutations inIAA genes. A null mutant has a slightly longer hypocotyl than wild-type plants, indicating that AXR2/IAA7 controls development in light-grown seedlings, perhaps in concert with other gene products. Dark-grown axr2-1 mutant plants have short hypocotyls and make leaves, suggesting that activation of AXR2/IAA7 is sufficient to induce morphological responses normally elicited by light. Previously described semidominant mutations in two other Arabidopsis IAA genes cause some of the same phenotypes as axr2-1, but also cause distinct phenotypes. These results illustrate functional differences among members of the Arabidopsis IAA gene family.