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The rib1 mutant is resistant to indole-3-butyric acid, an endogenous auxin in Arabidopsis thaliana.    C. S. Waddell and J. Poupart.  Dept. of Biology, McGill University, Montreal, Canada.

The presence of indole-3-butyric acid (IBA) as an endogenous auxin in Arabidopsis has been recently demonstrated.  However, the in vivo role of IBA remains to be elucidated.  We have characterized a semi-dominant mutant that is affected in its response to IBA, but shows a wild-type response to indole-3-acetic acid (IAA), the predominant and most studied form of auxin.  This specificity of auxin response is unique amongst published auxin resistant mutants; it demonstrates that rib1 can discriminate between the two known endogenous auxins of Arabidopsis.  We have named this mutant rib1 for resistant to IBA.  Root elongation assays show that rib1 is specifically resistant to IBA, to the synthetic auxin 2,4‑dichlorophenoxyacetic acid (2,4-D), and to auxin transport inhibitors.  rib1 does not display increased resistance to IAA or to the synthetic auxin naphthalene acetic acid (NAA), nor to other classes of plant hormones.  rib1 individuals also have other root specific phenotypes, including a shorter primary root, an increased number of lateral roots, and a more variable response than wild type to a change in gravitational vector. Adult rib1 plants are morphologically indistinguishable from wild-type plants.  These phenotypes suggest that IBA is a physiologically active auxin in roots with specific roles in root development and response to environmental stimuli.  We propose that RIB1 could function in either IBA transport or response.  This work was supported by a grant from the Natural Sciences and Engineering Council of Canada.