title: High-affinity potassium and sodium transport systems in plants.
abstract: All living cells have an absolute requirement for K+, which must be taken up from the external medium. In contrast to marine organisms, which live in a medium with an inexhaustible supply of K+, terrestrial life evolved in oligotrophic environments where the low supply of K+ limited the growth of colonizing plants. In these limiting conditions Na+ could substitute for K+ in some cellular functions, but in others it is toxic. In the vacuole, Na+ is not toxic and can undertake osmotic functions, reducing the total K+ requirements and improving growth when the lack of K+ is a limiting factor. Because of these physiological requirements, the terrestrial life of plants depends on high-affinity K+ uptake systems and benefits from high-affinity Na+ uptake systems. In plants, both systems have received extensive attention during recent years and a clear insight of their functions is emerging. Some plant HAK transporters mediate high-affinity K+ uptake in yeast, mimicking K+ uptake in roots, while other members of the same family may be K+ transporters in the tonoplast. In parallel with the HAK transporters, some HKT transporters mediate high-affinity Na+ uptake without cotransporting K+. HKT transporters have two functions: (i) to take up Na+ from the soil solution to reduce K+ requirements when K+ is a limiting factor, and (ii) to reduce Na+ accumulation in leaves by both removing Na+ from the xylem sap and loading Na+ into the phloem sap.
authors.affiliation: Laboratorio de Microbiología, Departamento de Biotecnología, Escuela Técnica Superíor de Ingenieros Agrónomos, Universidad Politécnica de Madrid, E-28040 Madrid, Spain. alonso.rodriguez@upm.es
authors.forename: Alonso
authors.initials: A
authors.lastname: Rodríguez-Navarro
authors.forename: Francisco
authors.initials: F
authors.lastname: Rubio
mesh_terms: D000251:Adenosine Triphosphatases / Q000378:metabolism; D017360:Arabidopsis / Q000235:genetics / Q000378:metabolism; D017136:Ion Transport; D008954:Models, Biological; D010940:Plant Proteins / Q000145:classification / Q000235:genetics / Q000502:physiology; D010944:Plants / Q000235:genetics / Q000378:metabolism*; D011188:Potassium / Q000378:metabolism*; D015221:Potassium Channels / Q000145:classification / Q000235:genetics / Q000502:physiology; D012964:Sodium / Q000378:metabolism*; D015222:Sodium Channels / Q000145:classification / Q000235:genetics / Q000502:physiology
chemical_list: D010940:Plant Proteins; D015221:Potassium Channels; D015222:Sodium Channels; D012964:Sodium; D000251:Adenosine Triphosphatases; D011188:Potassium