Auxin coreceptor IAA17/AXR3 controls cell elongation in Arabidopsis thaliana root by modulation of auxin and gibberellin perception

The nuclear TIR1/AFB - Aux/IAA auxin pathway plays a crucial role in regulating plant growth and development. Specifically, the IAA17/AXR3 protein participates in root development, and the accumulation of its mutant variant, AXR3-1, which cannot bind auxin, leads to severe root growth phenotype and agravitropism. However, the mechanism by which AXR3 regulates cell elongation is not fully understood. Here we show that the inducible expression of AXR3-1 in the Arabidopsis thaliana root triggers excessive cell elongation that is followed by growth arrest of the root. We exploited this effect to reveal the underlying molecular mechanism of AXR3 action. We show that AXR3-1 acts exclusively in the nucleus where it interferes with the nuclear auxin transcriptional pathway, while the rapid cytoplasmic auxin root growth response is not affected. The analysis of the transcriptome of the induced AXR3-1 roots revealed changes in phytohormone perception and homeostasis. We show that the accumulation of AXR3-1 disturbs auxin homeostasis which leads to excessive auxin accumulation. At the same time, the reaction of the AXR3-1 roots to gibberellin is altered. These results show that the IAA17/AXR3 maintains an optimal cell elongation rate by controlling the auxin response, auxin homeostasis and the interplay with gibberellin signaling.