(A) qPCR showing the expression levels of auxin responsive genes in WT, arfb2 arfb4Δ (arfb2,4), iaa1b iaa2Δ (iaa1b,2), and arfb2 arfb4 iaa1b iaa2Δ (iaa1b,2 arfb2,4), mock- or 10 μM IAA-treated for five hours. Error bars represent s.e.m. *p<0.05 (t-test comparing the arfb2 arfb4Δ, and arfb2 arfb4 iaa1b iaa2Δ lines to either WT or iaa1b iaa2Δ, respectively), n=3. (B) Three independent arfb1-4 RNAi and control lines mock- or 10 μM IAA-treated. Each line is presented by two images (DsRED fluorescence (red), chlorophyll auto-florescence (blue). Each image is from projection stacks of multiple confocal sections. Scale bar: 100 μm. (C) WT and an ARFa8 overexpression line (ARFa8OE) grown for one month on BCDAT without auxin or with 12.5 μM NAA. Scale bar: 0.5 cm. (D) qPCR showing the expression levels of auxin responsive genes in WT and ARFa8OE mock- or 10 μM IAA-treated for five hours. Error bars represent s.e.m. *p<0.05 (t-test comparing the ARFa8OE line to WT), n=3. (E) Model for auxin regulation of transcription. The expression level of an auxin responsive gene is determined by the interplay between the Aux/IAAs, the repressing ARFs, and the activating ARFs. At low auxin levels the Aux/IAAs provide stringent repression. At high auxin levels, the Aux/IAAs are degraded resulting in increased transcription through the action of the activating ARFs. The repressing ARFs act to buffer gene expression by attenuating the activity of the activating ARFs. The interplay between the three protein groups results in a wide range of gene expression levels that contribute to a dynamic and context specific auxin response.