Solution structure of the PsIAA4 oligomerization domain reveals interaction modes for transcription factors in early auxin response

Indole-3-acetic acid (IAA or auxin) is indispensable during the entire plant life cycle and regulates diverse processes and transitions via hierarchical gene expression. The hormone triggers rapid destruction of AUXIN/INDOLE-3-ACETIC ACID (AUX/IAA) repressors that control AUXIN RESPONSE FACTOR (ARF) activators in auxin-regulated gene transcription. AUX/IAA and ARF proteins interact physically via their C-terminal Phox and Bem1p (PB1) domain, which is shared between both families. This work reports the solution structure of a wild-type AUX/IAA PB1 domain and identifies amino acid residues that engage in directional interaction of monomers, largely via hydrogen bonds between conserved basic and acidic surface patches. Our study provides a framework for unraveling molecular determinants that confer specificity to complex interactions between AUX/IAA and ARF transcription factors.