The typical symptom of Paulownia witches' broom (PaWB), caused by phytoplasma infection, is excessive branching, which is mainly triggered by auxin metabolism disorder. is the early auxin-responsive gene that participates in regulating plant morphogenesis such as apical dominance, stem elongation, lateral branch development, and lateral root formation. However, no studies have investigated the response of the gene family to phytoplasma infection in . In this study, a total of 62 genes were found in the genome. Phylogenetic analysis showed that genes could be divided into eight subgroups, which were formed by tandem duplication and fragment replication. Most of them had a simple gene structure, and several members lacked one or two conserved domains. By combining the expression of genes under phytoplasma stress and SA-treated phytoplasma-infected seedlings, we found that may play a vital role in the occurrence of PaWB. Functional analysis based on homologous relationships showed a strong correlation between and branching. Protein-protein interaction prediction showed that PfARF might be the binding partner of PfAux/IAA, and the yeast two-hybrid assay and bimolecular fluorescent complementary assay confirmed the interaction of PfAux/IAA45 and PfARF13. This study provides a theoretical basis for further understanding the function of the gene family and exploring the regulatory mechanism of branching symptoms caused by PaWB.