Functional analysis of rice calcium-permeable cation channel OsHKT2;4
Wei Wang, Hong-Xuan Lin, Ji-Ping Gao*

National Key Laboratory of Plant Molecular Genetics, Institute of Plant Physiology and Ecology, Shanghai Institute for Biological Sciences, Chinese Academy of Sciences, Shanghai, 200032.
*Corresponding Author: Tel: (+8621)54924132;Fax: (+8621)54924015; E-mail: jpgao@sibs.ac.cn

Abstract
The view that plant High-affinity K⁺-Transporter (HKT) family members are sodium-selective uniporters or sodium-potassium symporters is widely held. Surprisingly, a subfamily 2 HKT protein, OsHKT2;4, was recently reported to function as a Ca²⁺-permeable cation channel that conducts current carried by a wide range of monovalent and divalent cations when heterologously expressed in Xenopus oocytes. However, the function of OsHKT2;4 in planta remains unknown. Here we report that heterologous expression of OsHKT2;4 in Arabidopsis altered the distribution of monovalent and divalent cations in shoot. OsHKT2;4 gene was ubiquitously expressed in all rice tissues investigated, including spikelets, leaves, leaf sheathes, internodes, nodes, the base of stems, and roots. GUS histochemical expression analysis showed that OsHKT2;4 is expressed mainly in the vascular tissue, as well as in epidermis cells of leaves and roots and mesophyll cells. The diverse expression pattern of OsHKT2;4 resembled closely that of other group 2 members. The immunogold labeling assay showed that the OsHKT2;4 protein was localized to the plasma membrane, indicating OsHKT2;4 may acts as a plasma membrane transporter to facilitate the influx and efflux of cations in plant cells. In transgenic Arabidopsis plants overexpressing OsHKT2;4, the shoot Na⁺ content was higher than that of control lines, whereas the reverse was true for Ca²⁺, Mg²⁺, and Mn²⁺ contents. These data support the hypothesis that OsHKT2;4 may function as a new type of HKT transporter and dedicate to both monovalent and divalent cation transport in rice plant.

Key words: OsHKT2;4; expression pattern; subcellular localization; ion transport;