Regulation of resource partitioning coordinates nitrogen and rhizobia responses and autoregulation of nodulation in the legume Medicago truncatula

Understanding how plants respond to nitrogen in their environment is crucial for determining how they use it and how this nitrogen use impacts other processes related to plant growth and development. Under nitrogen limitation the activity and affinity of uptake systems is increased in roots and lateral root formation is regulated in order to adapt to lower nitrogen levels and scavenge from the soil. Plants in the legume family can form associations with rhizobial nitrogen fixing bacteria, and this association is tightly regulated by nitrogen levels. The effect of nitrogen on nodulation has been investigated in a number of studies but the effect of nodulation on nitrogen responses has received much less attention. We integrated molecular and phenotypic data in the legume Medicago truncatula and determined that genes controlling nitrogen influx are differently expressed depending on whether plants are mock- or rhizobia-inoculated. Our results show that a functional autoregulation of nodulation pathway is required for roots to perceive, uptake and mobilise nitrogen as well as for normal root development. Together we found that autoregulation of nodulation, root development and the location of nitrogen are processes balanced by the whole plant system as part of a resource partitioning mechanism.