AUTHOR=Zhang Chi , Li Hejuan , Wang Jiayuan , Zhang Bin , Wang Wei , Lin Hongxuan , Luan Sheng , Gao Jiping , Lan Wenzhi TITLE=The Rice High-Affinity K+ Transporter OsHKT2;4 Mediates Mg2+ Homeostasis under High-Mg2+ Conditions in Transgenic Arabidopsis JOURNAL=Frontiers in Plant Science VOLUME=8 YEAR=2017 URL=https://www.frontiersin.org/journals/plant-science/articles/10.3389/fpls.2017.01823 DOI=10.3389/fpls.2017.01823 ISSN=1664-462X ABSTRACT=

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 Mg²⁺ and Ca²⁺ permeability for yeast cells and Xenopus laevis oocytes, besides K⁺ and Na⁺. However, physiological functions related to Mg²⁺in planta have not yet been identified. Here we report that OsHKT2;4 from rice (O. sativa; background Nipponbare) functions as a low-affinity Mg²⁺ transporter to mediate Mg²⁺ homeostasis in plants under high-Mg²⁺ environments. Using the functional complementation assay in Mg²⁺-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 Mg²⁺-deficient conditions and induced the Mg²⁺ currents in oocytes at millimolar range of Mg²⁺. Additionally, overexpression of OsHKT2;4 to Arabidopsis mutant lines with a knockout of AtMGT6, a gene encoding the transporter protein necessary for Mg²⁺ adaptation in Arabidopsis, caused the Mg²⁺ toxicity to the leaves under the high-Mg²⁺ stress, but not under low-Mg²⁺ environments. Moreover, this Mg²⁺ toxicity symptom resulted from the excessive Mg²⁺ translocation from roots to shoots, and was relieved by the increase in supplemental Ca²⁺. Together, our results demonstrated that OsHKT2;4 is a low-affinity Mg²⁺ transporter responsible for Mg²⁺ transport to aerials in plants under high-Mg²⁺ conditions.