THE AXR1 AND AUX1 GENES OF ARABIDOPSIS FUNCTION IN SEPARATE AUXIN-RESPONSE PATHWAYS

The recessive mutations aux1 and axr1 of Arabidopsis confer resistance to the plant hormone auxin. The axr1 mutants display a variety of mor phological defects. In contrast, the only morphological defect observe d 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 a uxin regulated gene SAUR-AC1 in mutant and wild-type plants has been e xamined. It has been found that axr1 plants display a pronounced defic iency in auxin-induced accumulation of SAUR-AC1 mRNA in seedlings as w ell as rosette leaves and mature roots. In contrast, the aux1 mutation has a modest effect on auxin induction of SAUR-AC1. To determine if t he AUX1 and AXR1 genes interact to facilitate auxin response, plants w hich are homozygous for both aux1 and axr1 mutations have been constru cted and characterized. The two mutations are additive in their effect s on auxin response, suggesting that each mutation confers resistance by a different mechanism. However, the morphology of double mutant pla nts indicates that there is an interaction between the AXR1 and AUX1 g enes. In mature plants, the aux1-7 mutation acts to partially suppress the morphological defects conferred by the axr1-12 mutation. This sup pression is not accompanied by an increase in auxin response, as measu red by SAUR-AC1 expression, suggesting that the interaction between th e AUX1 and AXR1 genes is indirect.