AtNHX8, a member of the monovalent cation:proton antiporter-1 family in Arabidopsis thaliana, encodes a putative Li⁺/H⁺ antiporter

The Arabidopsis monovalent cation:proton antiporter-1 (CPA1) family includes eight members, AtNHX1-8. AtNHX1 and AtNHX7/SOS1 have been well characterized as tonoplast and plasma membrane Na⁺/H⁺ antiporters, respectively. The proteins AtNHX2-6 have been phylogenetically linked to AtNHX1, while AtNHX8 appears to be related to AtNHX7/SOS1. Here we report functional characterization of AtNHX8. AtNHX8 T-DNA insertion mutants are hypersensitive to lithium ions (Li⁺) relative to wild-type plants, but not to the other metal ions such as sodium (Na⁺), potassium (K⁺) and caesium (Cs⁺). AtNHX8 overexpression in a triple-deletion yeast mutant AXT3 that exhibits defective Na⁺/Li ⁺ transport specifically suppresses sensitivity to Li⁺, but does not affect Na⁺ sensitivity. Likewise, AtNHX8 overexpression complemented sensitivity to Li⁺, but not Na⁺, in sos1-1 mutant seedlings, and increased Li⁺ tolerance of both the sos1-1 mutant and wild-type seedlings. Results of Li⁺ and K⁺ measurement of loss-of-function and gain-of-function mutants indicate that AtNHX8 may be responsible for Li⁺ extrusion, and may be able to maintain K⁺ acquisition and intracellular ion homeostasis. Subcellular localization of the AtNHX8-enhanced green fluorescent protein (EGFP) fusion protein suggested that AtNHX8 protein is targeted to the plasma membrane. Taken together, our findings suggest that AtNHX8 encodes a putative plasma membrane Li⁺/H⁺ antiporter that functions in Li detoxification and ion homeostasis in Arabidopsis.