Molecular mechanisms of potassium and sodium uptake in plants

Potassium (K+) is an essential nutrient and the most abundant cation in plants, whereas the closely related ion sodium (Na+) is toxic to most plants at high millimolar concentrations. K+ deficiency and Na+ toxicity are both major constraints to crop production worldwide. K+ counteracts Na+ stress, while Na+, in turn, can to a certain degree alleviate K+ deficiency. Elucidation of the molecular mechanisms of K+ and Na+ transport is pivotal to the understanding – and eventually engineering – of plant K+ nutrition and Na+ sensitivity. Here we provide an overview on plant K+ transporters with particular emphasis on root K+ and Na+ uptake. Plant K+-permeable cation transporters comprise seven families: Shaker-type K+ channels, 'two-pore' K+ channels, cyclic-nucleotidegated channels, putative K+/H+ antiporters, KUP/HAK/KT transporters, HKT transporters, and LCT1. Candidate genes for Na+ transport are the KUP/HAK/KTs, HKTs, CNGCs, and LCT1. Expression in heterologous systems, localization in plants, and genetic disruption in plants will provide insight into the roles of transporter genes in K+ nutrition and Na+ toxicity.