Rapid ammonia gas transport accounts for futile transmembrane cycling under NH3/NH4+ toxicity in plant roots

摘要: Futile transmembrane NH3/NH4(+) cycling in plant root cells, characterized by extremely rapid fluxes and high efflux to influx ratios, has been successfully linked toxicity. Surprisingly, the fundamental question of which species conjugate pair (NH3 or NH4(+)) participates such is unresolved. Using flux analyses with short-lived radioisotope (13)N electrophysiological, respiratory, histochemical measurements, we show that futile roots barley (Hordeum vulgare) seedlings predominately gaseous NH3 species, rather than NH4(+) ion. Influx (13)NH3/(13)NH4(+), exceeded 200 µmol g(-1) h(-1), was not commensurate membrane depolarization increases respiration, suggesting electroneutral transport. followed Michaelis-Menten kinetics for (but NH4(+)), as a function external concentration (Km = 152 µm, Vmax 205 h(-1)). Efflux (13)NH3/(13)NH4(+) responded nearly identical Km. Pharmacological characterization suggests mediation aquaporins. Our study fundamentally revises futile-cycling model demonstrating major permeating across both plasmalemma tonoplast cells under toxicity conditions.