As in the cases of the embryo and many lateral organs, lateral root (LR) development is triggered by the phytohormone auxin. LR development is also known to be inhibited by abscisic acid (ABA). Here we show that an auxin response factor ARF3, which participates in pattern formation of aerial lateral organs, such as leaf or flower, is upregulated by auxin during LR development in Arabidopsis. This study demonstrates that ARF3 expression is regulated at the transcriptional level and by posttranscriptional gene silencing (PTGS), which is mediated by a group of transacting small-interfering RNA (tasiRNA-ARF). The tasiRNA-ARF pathway and ARF3 expression are sensitive to auxin and ABA treatment, suggesting that tasiRNA-ARF-dependent PTGS of ARF3 is linked to auxin promotion of LR development and to ABA inhibition.