ARFs that are relieved from
Aux/IAA-mediated negative regulation in turn activate or
repress transcription of auxin-responsive genes. Of the 23
ARFs in Arabidopsis, eight are targets for miRNAs and
TAS3siRNAs: miR160 regulates ARF10, ARF16 and
ARF17, and miR167 regulates ARF6 and ARF8; whereas
miR390-dependent TAS3siRNA regulates ARF2, ARF3
and ARF4. Of these, ARF6 and ARF8 are known to be
activators of transcription, whereas other small RNA-targeted
ARFs are considered to be transcription repressors.