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

Rice ( ; background Nipponbare) contains nine (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 and Ca permeability for yeast cells and oocytes, besides K and Na . However, physiological functions related to Mg have not yet been identified. Here we report that OsHKT2;4 from rice ( ; background Nipponbare) functions as a low-affinity Mg transporter to mediate Mg homeostasis in plants under high-Mg environments. Using the functional complementation assay in Mg -uptake deficient strains MM281 and electrophysiological analysis in oocytes, we found that OsHKT2;4 could rescue the growth of MM281 in Mg -deficient conditions and induced the Mg currents in oocytes at millimolar range of Mg . Additionally, overexpression of OsHKT2;4 to mutant lines with a knockout of , a gene encoding the transporter protein necessary for Mg adaptation in , caused the Mg toxicity to the leaves under the high-Mg stress, but not under low-Mg environments. Moreover, this Mg toxicity symptom resulted from the excessive Mg translocation from roots to shoots, and was relieved by the increase in supplemental Ca . Together, our results demonstrated that OsHKT2;4 is a low-affinity Mg transporter responsible for Mg transport to aerials in plants under high-Mg conditions.