The functional characterization of LjNRT2.4 indicates a novel, positive role of nitrate for an efficient nodule N2- fixation activity

N2-fixing nodules are formed on the roots of legume plants as result of the symbiotic interaction with rhizobia. Nodule functioning requires high amounts of carbon and energy andtherefore legumes have developed finely tuned mechanisms to cope with the changing external environmental conditions, including nutrient availability and flooding. Theinvestigation of the role of nitrate as regulator of the Symbiotic Nitrogen Fixation has been limited to the inhibitory effects exerted by high external concentrations on nodule formation,development and functioning. We describe a nitrate dependent route acting at low external concentrations that become crucial in hydroponic conditions to ensure an efficient nodule functionality. Combinedgenetic, biochemical and molecular studies are used to unravel the novel function of the LjNRT2.4 gene. Two independent null mutants are affected in the nitrate content of nodules, consistently with the LjNRT2.4 temporal and spatial profiles of expression. The reduced nodular nitrate contentis associated to a strong reduction of nitrogenase activity and a severe N-starvation phenotype observed under hydroponic conditions. We also report the effects of the mutations on thenodular NO production and content.We discuss the involvement of LjNRT2.4 in a nitrate-NO respiratory chain taking place in the N -fixing nodules.