The Rice High-Affinity K(+) Transporter OsHKT2;4 Mediates Mg(2+) Homeostasis under High-Mg(2+) Conditions in Transgenic Arabidopsis

Rice (Oryza sativa; background Nipponbare) contains nine HKT (high-affinity K(+) transport)-like genes encoding membrane proteins belonging to the superfamily of Ktr/TRK/HKT. OsHKTs have been proposed to include four selectivity filter-pore-forming domains homologous to the bacterial K(+) channel KcsA, and are separated into OsHKT1s with Na(+)-selective activity and OsHKT2s with Na(+)-K(+) symport activity. As a member of the OsHKT2 subfamily, OsHKT2;4 renders Mg(2+) and Ca(2+) permeability for yeast cells and Xenopus laevis oocytes, besides K(+) and Na(+). However, physiological functions related to Mg(2+) in planta have not yet been identified. Here we report that OsHKT2;4 from rice (O. sativa; background Nipponbare) functions as a low-affinity Mg(2+) transporter to mediate Mg(2+) homeostasis in plants under high-Mg(2+) environments. Using the functional complementation assay in Mg(2+)-uptake deficient Salmonella typhimurium strains MM281 and electrophysiological analysis in X. laevis oocytes, we found that OsHKT2;4 could rescue the growth of MM281 in Mg(2+)-deficient conditions and induced the Mg(2+) currents in oocytes at millimolar range of Mg(2+). Additionally, overexpression of OsHKT2;4 to Arabidopsis mutant lines with a knockout of AtMGT6, a gene encoding the transporter protein necessary for Mg(2+) adaptation in Arabidopsis, caused the Mg(2+) toxicity to the leaves under the high-Mg(2+) stress, but not under low-Mg(2+) environments. Moreover, this Mg(2+) toxicity symptom resulted from the excessive Mg(2+) translocation from roots to shoots, and was relieved by the increase in supplemental Ca(2+). Together, our results demonstrated that OsHKT2;4 is a low-affinity Mg(2+) transporter responsible for Mg(2+) transport to aerials in plants under high-Mg(2+) conditions.