In Arabidopsis thaliana, the plant hormone auxin plays critical roles at almost all developmental stages by regulating expression of various auxin response genes. It has been known that auxin controls transcription of auxin-response genes through a core SCFTIR1/AFB-Aux/IAA-ARF nuclear signaling module. However, little is known about whether other transcription factors also act in modulating auxin responsive gene expression through direct interactions with these core auxin-signaling components. Our previous work showed that a group of bHLH transcription factors named TEOSINTE BRANCHED1, CYCLODEA, PROLIFERATING CELL FACTORS (TCPs) play an important role in auxin/temperature-regulated cell growth in Arabidopsis seedlings. In this thesis, we checked whether TCPs function in controlling auxin signaling through direct interaction with Aux/IAAs and ARFs, key transcription factors in auxin regulated gene expression. Using yeast two-hybrid assays, we identified interactions between several TCPs and Aux/IAAs and ARFs. We then confirmed some of these interactions with pull-down experiments. Further, we located the domains that mediate the interactions between TCPs and Aux/IAAs and those between TCPs and ARFs using yeast two-hybrid assays with truncated forms of these TFs. In conclusion, our work has identified interactions between TCPs and key TFs that mediate auxin-regulated transcription, and thus provides novel insights into the molecular mechanisms understanding the auxin signaling and crosstalk between auxin signaling and other signaling pathways.