Nodule organogenesis in <i>Medicago truncatula</i> requires local stage-specific auxin biosynthesis and transport

The importance of auxin in plant organ development, including root nodule formation, is well known. The spatiotemporal distribution pattern of auxin during nodule development has been illustrated using auxin reporter constructs. However, our understanding of how this pattern is established and maintained remains elusive. Here, we studied how the auxin gradient is associated with the spatiotemporal expression patterns of known auxin biosynthesis and transport genes at different stages of nodule development in Medicago (Medicago truncatula). In addition, we examined the Medicago PIN-FORMED10 (MtPIN10) expression pattern and polar positioning on the cell membrane during nodule primordium development to investigate auxin flux. RNA interference and the application of auxin biosynthesis inhibitors were used to demonstrate the importance of auxin biosynthesis and transport at the initial stages of nodulation. Our results show that upon rhizobium inoculation before the first cell divisions, a specific subset of Medicago YUCCA (MtYUC) and MtPIN genes, as well as Medicago LIKE AUXIN RESISTANT2 (MtLAX2), are expressed in the pericycle and contribute to the creation of an auxin maximum. Overall, we demonstrate that the dynamic spatiotemporal expression of both MtYUC and MtPIN genes results in specific auxin outputs during the different stages of nodule primordia and nodule meristem formation.