title: Characterization and regulation of ammonium transport systems in Citrus plants.
abstract: We have investigated both the kinetics and regulation of 15NH4+ influx in roots of 3-month-old hydroponically grown Citrus (Citrus sinensis L. Osbeck x Poncirus trifoliata Blanco) seedlings. The 15NH4+ influx is saturable below an external ammonium concentration of 1 mM, indicating the action of a high-affinity transport system (HATS). The HATS is under feedback repression by the N status of the plant, being down-regulated in plants adequately supplied with N during growth, and up-regulated by N-starvation. When assayed between 1 and 50 mM [15NH4+]0, the 15NH4+ influx showed a linear response typical of a low-affinity transport system (LATS). The activity of the LATS increased in plants supplied with NH4+ as compared with plants grown on an N-free medium. Transfer of the plants to N-free solution resulted in a marked decrease in the LATS-mediated 15NH4+ influx. Accordingly, resupply of NH4+ after N-starvation triggered a dramatic stimulation of the activity of the LATS. These data provide evidence that in Citrus plants, the LATS or at least one of its components is inducible by NH4+. Even when up-regulated, both the HATS and the LATS displayed a limited capacity, as compared with that usually found in herbaceous species. The use of various metabolic uncouplers or inhibitors indicated that 15NH4+ influx mediated by the HATS is strongly dependent on energy metabolism and H+ transmembrane electrochemical gradient. By contrast, the LATS is not affected by protonophores or inhibitors of the H(+)-ATPase, suggesting that its activity is mostly driven by the NH4+/NH3 transmembrane gradient. In agreement with these hypotheses, the HATS-mediated 15NH4+ influx was strongly inhibited when the solution pH was raised from 4 to 7, whereas influx mediated by the LATS was slightly stimulated.
authors.affiliation: Unidad de Fisiología Vegetal, Departamento de Ciencias Experimentales, Escuela Superior de Tecnologia y Ciencias Experimentales, Universitat Jaume I, 12071 Castellón, Spain.
authors.forename: M
authors.initials: M
authors.lastname: Cerezo
authors.forename: P
authors.initials: P
authors.lastname: Tillard
authors.forename: A
authors.initials: A
authors.lastname: Gojon
authors.forename: E
authors.initials: E
authors.lastname: Primo-Millo
authors.forename: P
authors.initials: P
authors.lastname: García-Agustín
mesh_terms: D001692:Biological Transport / Q000502:physiology; D002957:Citrus / Q000502:physiology*; D015536:Down-Regulation; D006863:Hydrogen-Ion Concentration; D018527:Hydroponics; D007476:Ionophores; D007700:Kinetics; D009587:Nitrogen Isotopes; D018517:Plant Roots / Q000502:physiology*; D006180:Proton-Translocating ATPases / Q000037:antagonists & inhibitors; D000644:Quaternary Ammonium Compounds / Q000008:administration & dosage / Q000037:antagonists & inhibitors / Q000378:metabolism*; D015854:Up-Regulation
chemical_list: D007476:Ionophores; D009587:Nitrogen Isotopes; D000644:Quaternary Ammonium Compounds; D006180:Proton-Translocating ATPases