Many developmental processes in plants are controlled by the hormone auxin(indole-acetic acid) via regulation (both at the level of transcription and of protein degradation) of the abundance of the Aux/IAA family of transcriptional regulators. Surprisingly, similar mutations in different but highly homologous Aux/IAAs can have very different phenotypic effects, but how this specificity is achieved is unknown. On , Knox et al. describe how a gain-of-function mutation that makes AXR3 resistant to auxin-mediated degradation blocked root hair initiation and elongation in Arabidopsis, while a similar mutation in the homologous SHY2 stimulated early root hair initiation and prolonged elongation. When the mutant forms of AXR3 and SHY2 were co-expressed, aberrant root hairs were initiated but failed to grow. On the basis of their results and the known dimerisation properties of the Aux/IAAs, the researchers propose that the relative abundance of different Aux/IAAs, rather than their absolute amounts,is the key for determining which developmental process auxin induces.
