Technical Abstract: The plant hormone indoleacetic acid IAA transcriptionally activates early genes in plants. The Aux/IAA family of early genes encodes proteins that are short-lived and nuclear-localized. They also contain a putative prokaryotic beta-alpha-alpha DNA binding motif whose formation requires protein dimerization. Here, we show that the pea PS-IAA4 and Arabidopsis IAA1 and IAA2 proteins perform homo- and heterotypic interactions in yeast using the two-hybrid system. Gel-filtration chromatography and chemical cross-linking experiments demonstrate that the PS-IAA4 and LAA1 proteins interact to form homodimers in vitro. Deletion analysis of PS-IAA4 indicates that the beta-alpha-alpha containing acidic C terminus of the protein is necessary for homotypic interactions in the yeast two-hybrid system. Screening an Arabidopsis lambda-ACT cDNA library using LAA1 as a bait reveals heterotypic interactions of LAA1 with known and newly discovered members of the Arabidopsis Aux/IAA gene family. The new member IAA24 has similarity to ARF1, a transcription factor that binds to an auxin response element. Combinatorial interactions among the various members of the Aux/IAA gene family may regulate a variety of late genes as well as serve as autoregulators of early auxin-regulated gene expression. These interactions provide a molecular basis for the developmental and tissue-specific manner of auxin action.