• Arabidopsis;
  • cation/H+ antiporter;
  • CAX;
  • metal homeostasis;
  • root development


  • • 
    The Arabidopsis vacuolar CAtion eXchangers (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.