The SAR1 gene of Arabidopsis acts downstream of the AXR1 gene in auxin response

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Abstract
A screen for suppressors of the auxin resistant mutant axr1 in Arabidopsis thaliana has identified at least three second site suppressor loci called Suppressor of Auxin Resistance (SAR). In this study we focus on the SAR1 gene. Previous studies have documented the effects of the axr1 mutations on auxin-inhibition of root growth, auxin-induced gene expression, seedling morphology and aerial morphology. In this study, we show that the axr1 mutations also affect root hair development and epidermal cell length. The sar1-1 mutation suppresses at least partially, every aspect of the axr1 phenotype. Genetic experiments indicate that this sup- pression is gene specific. When crossed with the auxin- resistant mutant aux1-7, the suppressor has little affect on auxin response. However, the morphology of sar1-1 aux1-7 inflorescences is different from either of the single mutants indicating that both genes play a role in auxin mediated development of the inflorescence. The sar1-1 mutation also affects morphology in an AXR1 background. sar1-1 plants are shorter than wild-type, have altered leaf morphology, flower earlier than wild-type plants and appear to have reduced cell division in the primary root. In most respects sar1-1 axr1 and sar1 AXR1 plants are indistinguishable, indicating that sar1 both suppresses and is epistatic to axr1. Based on these results, we propose that SAR1 acts after AXR1 and that a major function of AXR1 is to relieve SAR1 mediated repression of auxin response.