The plant hormone auxin is an important player for interkingdom communication in the rhizosphere. It is not only a crucial signaling molecule for plant biology, but it is also an ancient signaling molecule used by microorganisms. Auxin acts as a bacterial and a fungal signaling molecule, facilitating the evolution of interkingdom communication. As a consequence of the polar auxin transport in plants, auxins derived from bacteria and filamentous fungi living in the rhizosphere initiate several growth and developmental processes such as root hair initiation and tip growth, lateral root formation, and the plasticity of root system architecture.