Root development under metal stress in Arabidopsis thaliana requires the H+/cation antiporter CAX4

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. New Phytologist (2009) 183: 95-105doi: 10.1111/j.1469-8137.2009.02831.x.