Nitrate and Ammonium Nutrition of Plants: Physiological and Molecular Perspectives

Abstract

Nitrogen is the mineral nutrient that plants need in the greatest quantities and the one that most frequently limits plant growth and crop yields. Most plants get their nitrogen (N) from the soil as either nitrate or ammonium, with some species showing a strong preference for one ionic form over the other. The uptake of nitrate and ammonium ions by roots involves a complex set of membrane transport systems that includes both high- and low-affinity transporters; net uptake rates can also be strongly influenced by the rate at which these ions efflux from root cells. Here we review our current picture of the mechanisms responsible for the uptake and efflux of nitrate and ammonium, attempting to integrate the large body of physiological data with the recent advances in the molecular biology of nitrate and ammonium transporters in bacteria and algae as well as in higher plants. We also review what is known at the physiological and molecular levels about the regulation of the N uptake systems, a process which involves both positive signals from soil nitrate or ammonium and feedback inhibitory signals that are generated by the plant's internal N status