The recessive mutations aux1 and axr1 of Arabidopsis confer resistance to the plant hormone auxin. The axr1 mutants display a variety of morphological defects. In contrast, the only morphological defect observed in aux1 mutants is a loss of root gravitropism. To learn more about the function of these genes in auxin response, the expression of the auxin-regulated gene SAUR-AC1 in mutant and wild-type plants has been examined. It has been found that axr1 plants display a pronounced deficiency in auxin-induced accumulation of SAUR-AC1 mRNA in seedlings as well as rosette leaves and mature roots. In contrast, the aux1 mutation has a modest effect on auxin induction of SAUR-AC1. To determine if the AUX1 and AXR1 genes interact to facilitate auxin response, plants which are homozygous for both aux1 and axr1 mutations have been constructed and characterized. The two mutations are additive in their effects on auxin response, suggesting that each mutation confers resistance by a different mechanism. However, the morphology of double mutant plants indicates that there is an inter-action between the AXR1 and AUX1 genes. In mature plants, the aux1-7 mutation acts to partially suppress the morphological defects conferred by the axr1-12 mutation. This suppression is not accompanied by an increase in auxin response, as measured by SAUR-AC1 expression, suggesting that the interaction between the AUX1 and AXR1 genes is indirect.