Auxin as Long-Distance Signal Controlling Root Architecture in Response to Nitrogen

Abstract

Plants show extensive root plasticity in response to nitrogen availability. Lateral root initiation and emergence are affected by nitrogen concentration and distribution to coordinate the ability to capture maximum nitrogen while minimizing carbon expenditure. Legumes and actinorhizal plants have additionally evolved symbioses with nitrogen-fixing bacteria that leads to the formation of nodules. Nodule numbers are controlled by systemic autoregulation of nodulation (AON) signaling through a receptor-like kinase acting in the shoot. The AON genes also control lateral root formation in response to nitrogen to varying extents in different legumes. Auxin transport control from the shoot to the root is one of the signals affecting nodule and lateral root development, and this is under the control of the AON gene in the legume Medicago truncatula. Nitrogen availability modulates long-distance auxin transport and this partly requires the function of the AON gene. Thus we propose a model in which nitrogen availability is perceived in the shoot, is processed by the AON gene, and feeds back on root architecture via control of shoot-to-root auxin transport.