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• The Arabidopsis vacuolar CAtion e Xchangers (CAXs) play a key role in mediating cation influx into the vacuole. In Arabidopsis, there are six CAX genes. However, some members are yet to be characterized fully. • In this study, we show that CAX4 is expressed in the root apex and lateral root primordia, and that expression is increased when Ni2+ or Mn2+ levels are elevated or Ca2+ is depleted. • Transgenic plants expressing increased levels of CAX4 display symptoms consistent with increased sequestration of Ca2+ and Cd2+ into the vacuole. When CAX4 is highly expressed in an Arabidopsis cax1 mutant line with weak vacuolar Ca2+/H+ antiport activity, a 29% increase in Ca2+/H+ antiport is measured. A cax4 loss-of-function mutant and CAX4 RNA interference lines display altered root growth in response to Cd2+, Mn2+ and auxin. The DR5::GUS auxin reporter detected reduces auxin responses in the cax4 lines. • These results indicate that CAX4 is a cation/H+ antiporter that plays an important function in root growth under heavy metal stress conditions. [ABSTRACT FROM AUTHOR]

Describes the preliminary localization of Cation Exchanger1 (cax1) to the tonoplast and the molecular characterization of cax1 mutants of Arabidopsis. Properties exhibited by cax1 mutants; Perturbation of ion homeostasis in cax1 mutants; Morphological phenotypes of cax1 alleles.

In plants, high capacity tonoplast cation/H+ antiport is mediated in part by a family of CAX (cation exchanger) transporters. Functional association between CAX1 and CAX3 has previously been inferred; however, the nature of this interaction has not been established. Here we analyze the formation of "hetero-CAX" complexes and their transport properties. Co-expressing both CAX1 and CAX3 mediated lithium and salt tolerance in yeast, and these phenotypes could not be recapitulated by expression of deregulated versions of either transporter. Coincident expression of Arabidopsis CAX1 and CAX3 occurs during particular stress responses, flowering, and seedling growth. Analysis of cax1, cax3, and cax1/3 seedlings demonstrated similar stress sensitivities. When plants expressed high levels of both CAXs, alterations in transport properties were evident that could not be recapitulated by high level expression of either transporter individually. In planta coimmunoprecipitation suggested that a protein-protein interaction occurred between CAX1 and CAX3. In vivo interaction between the CAX proteins was shown using a split ubiquitin yeast two-hybrid system and gel shift assays. These findings demonstrate cation exchange plasticity through hetero-CAX interactions. [ABSTRACT FROM AUTHOR]