In this work we aim to characterise the role of the plant hormone auxin in the development of nitrogen-fixing, indeterminate legume nodules. More specifically we want to know the timing of, and the processes responsible for, the auxin signaling induction detected upon inoculation of the root of Medicago truncatula with its symbiotic rhizobium, Sinorhizobium meliloti. Using a variety of approaches, we uncover the role of local auxin biosynthesis in the initiation of the indeterminate nodule, and indicate how local auxin biosynthesis is necessary and sufficient for lateral organ induction in M. truncatula. By employing the use of dynamic models of auxin transport, we combine our results with previous studies into a unified understanding of the interactions between auxin biosynthesis, auxin transport, and gene regulatory networks driving nodule development.