Lotus japonicus LjKUP Is Induced Late During Nodule Development and Encodes a Potassium Transporter of the Plasma Membrane

The KUP family of potassium transporters in plants is large but poorly characterized. We isolated and characterized the first KUP transporter from a legume, LjKUP of Lotus japonicus. Although expressed throughout plants, LjKUP transcript levels were highest in nodules. Induction of LjKUP expression occurred late during nodule development, at a time of rapid organ expansion. A high level of LjKUP expression was maintained in mature, full-sized nodules. However, induction of LjKUP expression was independent of symbiotic nitrogen fixation (SNF), and occurred in ineffective nodules resulting from mutations in either the plant or its microsymbiont, Mesorhizobium loti. Heterologous expression of LjKUP in Escherichia coli showed that the protein is able to transport potassium. Transient expression of a GFP-LjKUP fusion protein in Arabidopsis cells indicated a plasma membrane location for the transporter. Taken together, the results indicate that LjKUP is a potassium transporter of the plasma membrane, which may play roles in cell expansion during nodule development and in ion homeostasis during SNF.