Body of Abstract: As food demands rise, rational crop trait engineering will be vital for achieving higher agricultural productivity to meet global demands. The auxin signaling network regulates growth and development of most plant tissues involved in crop yield and heartiness, which makes it an ideal target for engineering. PB1 binding domains play a critical role in auxin response, however using protein-protein interactions between PB1 domains of auxin signaling components has never been tested as a way to predictably tune pathway function. The primary reason for this is that auxin genes are composed of large gene families with conserved redundancy, which causes functional overlap and promiscuous interactions between family members. To understand and overcome these complexities, we swapped binding domains of auxin-responsive transcription factors with a panel of synthetic binding domains to evaluate our ability to control auxin pathway function by modulating complexity, identity, and affinity of PB1-mediated-protein-protein interactions. We then used a synthetic assay to measure the impact of these synthetic binding domains on pathway function. Finally, we expressed synthetic interaction domains in protoplasts to show that they function in the context of the native auxin response network without any interference from gene family members, which we term orthogonality. Our data indicates that auxin pathway function can be tuned by modifying PB1 interactions using synthetic interaction domains. We also show predictable and orthogonal function of an auxin pathway incorporating synthetic interaction domains in protoplasts alongside the native auxin network. Together, these findings show that PB1-mediated-protein-protein interactions are a tuning knob for engineering auxin signaling.