AXR3 and SHY2 interact to regulate root hair development

Signal transduction of the plant hormone auxin centres on the regulation of the abundance of members of the Aux/IAA family of transcriptional regulators, of which there are 29 in Arabidopsis. Auxin can influence Aux/IAA abundance by promoting the transcription of Aux/IAA genes and by reducing the half-life of Aux/IAA proteins. Stabilising mutations, which render Aux/IAA proteins resistant to auxin-mediated degradation, confer a wide range of phenotypes consistent with disruptions in auxin response. Interestingly, similar mutations in different family members can confer opposite phenotypic effects. To understand the molecular basis for this functional specificity in the Aux/IAA family, we have studied a pair of Aux/IAAs, which have contrasting roles in root hair development. We have found that stabilising mutations in AXR3/IAA17 blocks root hair initiation and elongation, whereas similar mutations in SHY2/IAA3 result in early initiation of root hair development and prolonged hair elongation, giving longer root hairs. The phenotypes resulting from double mutant combinations, the transient induction of expression of the proteins, and the pattern of transcription of the cognate genes suggest that root hair initiation is controlled by the relative abundance of SHY2 and AXR3 in a cell. These results suggest a general model for auxin signalling in which the modulation of the relative abundance of different Aux/IAA proteins can determine which down-stream responses are induced.