Operates as a Na(+)/H(+) antiporter that plays a role in regulation of xylem Na(+) concentration and, consequently, Na(+) accumulation in the leaf. Required for pollen tube guidance, but not for normal pollen development. May also be involved in the development or function of the female gametophyte. Specifically expressed in root endodermal cells (PubMed:16513816). Expressed in seedlings, roots, leaves, flowers, flower buds and pollen. Mutants display a reduced rosette width, a later flowering time and an altered leaf K(+) and Na(+) contents (PubMed:16513816). No defect in male fertility, due to redundancy with CHX23. Chx21 and chx23 double mutants are male sterile (PubMed:21239645). Belongs to the monovalent cation:proton antiporter 2 (CPA2) transporter (TC 2.A.37) family. CHX (TC 2.A.37.4) subfamily. The article by Evans et al was retracted by the editors due to concerns over image manipulation, which raised sufficient doubts regarding the credibility of the study.