Modulation of Root Nitrogen Uptake Mechanisms Mediated by Beneficial Soil Microorganisms

A diverse array of soil microorganisms exhibit plant growth-promoting (PGP) traits, many of which enhance root growth and development. These microorganisms include various taxa of bacteria, fungi, microalgae and yeasts-some of which are currently used in biofertilizers and biostimulant formulations. Recent studies have begun to unravel the complex communication between plant roots and beneficial microorganisms, revealing mechanisms that modulate root nitrogen (N) uptake beyond atmospheric N₂ fixation pathways. Root N uptake is tightly regulated by plants through multiple mechanisms. These include transcriptional and post-transcriptional control of plasma membrane-localized N transporters in the epidermis, endodermis, and xylem parenchyma. Additionally, N uptake efficiency is influenced by vacuolar N storage, assimilation of inorganic N into organic compounds, and the maintenance of electrochemical gradients across root cell membranes. Many of these processes are modulated by microbial signals. This review synthesizes current knowledge on how soil microorganisms influence root N uptake, with a focus on signaling molecules released by soil beneficial microbes. These signals include phytohormones, volatile organic compounds (VOCs), and various low-molecular-weight organic compounds that affect transporter expression, root architecture, and cellular homeostasis. Special attention is paid to the molecular and physiological pathways through which these microbial signals enhance plant N acquisition and overall nutrient use efficiency.