Cation transport is a critical process in all organisms and is essential for mineral nutrition, ion stress tolerance, and signal transduction. Transporters that are members of the Ca²⁺/cation antiporter (CaCA) superfamily are involved in the transport of Ca²⁺ and/or other cations using the counter exchange of another ion such as H⁺ or Na⁺. The CaCA superfamily has been previously divided into five transporter families: the YRBG, Na⁺/Ca²⁺ exchanger (NCX), Na⁺/Ca²⁺, K⁺ exchanger (NCKX), H⁺/cation exchanger (CAX), and cation/Ca²⁺ exchanger (CCX) families, which include the well-characterized NCX and CAX transporters. To examine the evolution of CaCA transporters within higher plants and the green plant lineage, CaCA genes were identified from the genomes of sequenced flowering plants, a bryophyte, lycophyte, and freshwater and marine algae, and compared with those from non-plant species. We found evidence of the expansion and increased diversity of flowering plant genes within the CAX and CCX families. Genes related to the NCX family are present in land plant though they encode distinct MHX homologs which probably have an altered transport function. In contrast, the NCX and NCKX genes which are absent in land plants have been retained in many species of algae, especially the marine algae, indicating that these organisms may share “animal-like” characteristics of Ca²⁺ homeostasis and signaling. A group of genes encoding novel CAX-like proteins containing an EF-hand domain were identified from plants and selected algae but appeared to be lacking in any other species. Lack of functional data for most of the CaCA proteins make it impossible to reliably predict substrate specificity and function for many of the groups or individual proteins. The abundance and diversity of CaCA genes throughout all branches of life indicates the importance of this class of cation transporter, and that many transporters with novel functions are waiting to be discovered.

Metal transport from the cytosol to the vacuole is thought to be an important component of ion tolerance and of a plant's potential for use in phytoremediation. The Arabidopsis antiporter CAX2(calcium exchanger 2) may be a key mediator of this process. CAX2 expression in yeast suppressed both Ca²⁺ and Mn²⁺ growth defects. A peptide-specific antibody to the antiporter reacted with a 39-kD protein from plant vacuolar membranes. Tobacco (Nicotiana tabacum) plants expressingCAX2 accumulated more Ca²⁺, Cd²⁺, and Mn²⁺ and were more tolerant to elevated Mn²⁺ levels. Expression of CAX2 in tobacco increased Cd²⁺ and Mn²⁺ transport in isolated root tonoplast vesicles. These results suggest that CAX2 has a broad substrate range and modulation of this transporter may be an important component of future strategies to improve plant ion tolerance.

The regulation of ions within cells is an indispensable component of growth and adaptation. The plant SOS2 protein kinase and its associated Ca²⁺ sensor, SOS3, have been demonstrated to modulate the plasma membrane H⁺/Na⁺ antiporter SOS1; however, how these regulators modulate Ca²⁺ levels within cells is poorly understood. Here we demonstrate that SOS2 regulates the vacuolar H⁺/Ca²⁺ antiporter CAX1. Using a yeast growth assay, co-expression of SOS2 specifically activated CAX1, whereas SOS3 did not. CAX1-like chimeric transporters were activated by SOS2 if the chimeric proteins contained the N terminus of CAX1. Vacuolar membranes from CAX1-expressing cells were made to be H⁺/Ca²⁺-competent by the addition of SOS2 protein in a dose-dependent manner. Using a yeast two-hybrid assay, SOS2 interacted with the N terminus of CAX1. In each of these yeast assays, the activation of CAX1 by SOS2 was SOS3-independent. In planta, the high level of expression of a deregulated version of CAX1 caused salt sensitivity. These findings suggest multiple functions for SOS2 and provide a mechanistic link between Ca²⁺ and Na⁺ homeostasis in plants.

Calcium (Ca²⁺) efflux from the cytosol modulates Ca²⁺ concentrations in the cytosol, loads Ca²⁺ into intracellular compartments, and supplies Ca²⁺ to organelles to support biochemical functions. The Ca²⁺/H⁺ antiporter CAX1 (for CALCIUM EXCHANGER 1) of Arabidopsis is thought to be a key mediator of these processes. To clarify the regulation of CAX1, we examined CAX1 RNA expression in response to various stimuli. CAX1 was highly expressed in response to exogenous Ca²⁺. Transgenic tobacco plants expressing CAX1 displayed symptoms of Ca²⁺ deficiencies, including hypersensitivity to ion imbalances, such as increased magnesium and potassium concentrations, and to cold shock, but increasing the Ca²⁺ in the media abrogated these sensitivities. Tobacco plants expressing CAX1 also demonstrated increased Ca²⁺ accumulation and altered activity of the tonoplast-enriched Ca²⁺/H⁺ antiporter. These results emphasize that regulated expression of Ca²⁺/H⁺ antiport activity is critical for normal growth and adaptation to certain stresses.

Plant-based foods offer an array of nutrients that are essential for human nutrition and promote good health. However, the major staple crops of the world are often deficient in some of these nutrients. Traditional agricultural approaches can marginally enhance the nutritional value of some foods, but the advances in molecular biology are rapidly being exploited to engineer crops with enhanced key nutrients. Nutritional targets include elevated mineral content, improved fatty acid composition, increased amino acid levels, and heightened antioxidant levels. Unfortunately, in many cases the benefits of these “biofortified” crops to human nutrition have not been demonstrated.