A dose-dependent bimodal switch by homologous Aux/IAA transcriptional repressors

Combinatorial interactions between different regulators diversify and enrich the chance of transcriptional regulation in eukaryotic cells.However,a dose-dependent functional switch of homologous transcriptional repressors has rarely been reported.Here,we show that SHY2,an auxin/indole-3-acetic acid(Aux/IAA)repressor,exhibits a dose-dependent bimodal role in auxin-sensitive root-hair growth and gene transcrip-tion in Arabidopsis,whereas other Aux/lAA homologs consistently repress the auxin responses.The co-repressor(TOPLESS[TPL])-binding affinity of a bimodal Aux/IAA was lower than that of a consistently re-pressing Aux/IAA.The switch of a single amino acid residue in the TPL-binding motif between the bimodal form and the consistently repressing form switched their TPL-binding affinity and transcriptional and bio-logical roles in auxin responses.Based on these data,we propose a model whereby competition between homologous repressors with different co-repressor-binding affinities could generate a bimodal output at thetranscriptional anddevelopmental levels.