Futile transmembrane NH(4)(+) cycling: A cellular hypothesis to explain ammonium toxicity in plants

Most higher plants develop severe toxicity symptoms when grown on ammonium (NH [Formula: see text]) as the sole nitrogen source. Recently, NH [Formula: see text] toxicity has been implicated as a cause of forest decline and even species extinction. Although mechanisms underlying NH [Formula: see text] toxicity have been extensively sought, the primary events conferring it at the cellular level are not understood. Using a high-precision positron tracing technique, we here present a cell-physiological characterization of NH [Formula: see text] acquisition in two major cereals, barley (Hordeum vulgare), known to be susceptible to toxicity, and rice (Oryza sativa), known for its exceptional tolerance to even high levels of NH [Formula: see text]. We show that, at high external NH [Formula: see text] concentration ([NH [Formula: see text]](o)), barley root cells experience a breakdown in the regulation of NH [Formula: see text] influx, leading to the accumulation of excessive amounts of NH [Formula: see text] in the cytosol. Measurements of NH [Formula: see text] efflux, combined with a thermodynamic analysis of the transmembrane electrochemical potential for NH [Formula: see text] , reveal that, at elevated [NH [Formula: see text]](o), barley cells engage a high-capacity NH [Formula: see text]-efflux system that supports outward NH [Formula: see text] fluxes against a sizable gradient. Ammonium efflux is shown to constitute as much as 80% of primary influx, resulting in a never-before-documented futile cycling of nitrogen across the plasma membrane of root cells. This futile cycling carries a high energetic cost (we record a 40% increase in root respiration) that is independent of N metabolism and is accompanied by a decline in growth. In rice, by contrast, a cellular defense strategy has evolved that is characterized by an energetically neutral, near-Nernstian, equilibration of NH [Formula: see text] at high [NH [Formula: see text]](o). Thus our study has characterized the primary events in NH [Formula: see text] nutrition at the cellular level that may constitute the fundamental cause of NH [Formula: see text] toxicity in plants.