We have recovered eight new auxin-resistant lines of Arabidopsis that carry mutations in the AXR7 gene. These eight lines, together with the 12 lines described in a previous report, define at least five different axr7 alleles. All of the mutant lines have a similar phenotype. Defects include decreases in plant height, root gravitropism, hypocotyl elongation, and fertility. Mutant line axr7-3 is less resistant to auxin than the other mutant lines and has less severe morphological abnormalities. This correlation suggests that the morphological defects are a consequence of a defect in auxin action. To determine whether the altered morphology of mutant plants is associated with changes in cell size or tissue organization, tissue sections were examined using scanning electron microscopy. No clear differences in cell size were observed between wild-type and mutant tissues. However, the vascular bundles of mutant stems were found to be less well differentiated than those in wild-type stems. The auxin sensitivity of rosette-stage plants was determined by spraying plants with auxin solutions. Mutant rosettes were found to be significantly less sensitive to exogenously applied auxin than wild-type rosettes, indicating that the AXR7 gene functions in aerial portions of the plant. Our studies suggest that the AXR7 gene is required for auxin action in most, if not all, tissues of the plant and plays an important role in plant development. Linkage studies indicate that the gene is located on chromosome 1 approximately 2 centiMorgans from the closest restriction fragment length polymorphism
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