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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]