The rib1 mutant is resistant to indole-3-butyric acid, an endogenous auxinin arabidopsis

The presence of indole-3-butyric acid (IBA) as an endogenous auxin in Arabi dopsis has been recently demonstrated. However, the in vivo role of IBA rem ains to be elucidated. We present the characterization of a semi-dominant m utant that is affected in its response to IBA, but shows a wild-type respon se to indole-3-acetic acid (IAA), the predominant and most studied form of auxin. We have named this mutant rib1 for resistant to IBA. Root elongation assays show that rib1 is specifically resistant to IBA, to the synthetic a uxin 2,4-dichlorophenoxyacetic acid, and to auxin transport inhibitors. rib 1 does not display increased resistance to IAA, to the synthetic auxin naph thalene acetic acid, or to other classes of plant hormones. rib1 individual s also have other root specific phenotypes including a shortened primary ro ot, an increased number of lateral roots, and a more variable response than wild type to a change in gravitational vector. Adult rib1 plants are morph ologically indistinguishable from wild-type plants. These phenotypes sugges t that rib1 alters IBA activity in the root, thereby affecting root develop ment and response to environmental stimuli. We propose models in which RIB1 has a function in either IBA transport or response. Our experiments also s uggest that IBA does not use the same mechanism to exit cells as does IAA a nd we propose a model for IBA transport.