Functional analysis of CHX21:: a putative sodium transporter in Arabidopsis

The functional role of CHX21, a member of the Arabidopsis thaliana CHX cation transporter family, has been investigated in plants growing under 'ideal' conditions and in the presence of elevated NaCl levels. In public databases, AtCHX21 (At2g31910) is annotated as a putative Na+/H+ antiporter. In this study, Southern analysis was used to identify a genotype that contained a single transposon insertion within its genome; using PCR, this insertion was shown to be within the CHX21 locus. No CHX21 transcript was detectable in Atchx21 (mutant) plants using RT-PCR. In the absence of salt stress, Atchx21 showed significant quantitative differences from the wild type (AtCHX21) in development with respect to characters such as rosette width and flowering time. In the presence of 50 mM NaCl, (i) roots of Atchx21 elongated more slowly than the wild type, (ii) the leaf sap Na+ concentration was significantly lower in Atchx21 compared with the wild type, and (iii) the concentration of Na+ in the xylem was lower compared with the wild type. The concentration of Na+ exported from the leaf in the phloem was unchanged. Thus, loading of Na+ into the root xylem could explain changes in leaf concentration of Na+. This hypothesis was supported by immunolocalization which demonstrated that the AtCHX21 transporter could only be detected in root endodermal cells. Immunogold labelling of ultra-thin sections, followed by transmission electron microscopy, demonstrated the localization of the protein in the plasma membrane. The data demonstrate that the CHX21 transporter may play a role in regulation of xylem Na+ concentration and, consequently, Na+ accumulation in the leaf.