ARS | Publication request: Root development under metal stress in Arabidopsis thaliana requires the H(+)/cation antiporter CAX4

Submitted to: New Phytologist
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: February 13, 2009
Publication Date: April 8, 2009
Citation: Mei, H., Cheng, N-H., Zhao, J., Park, S., Escareno, R.A., Pittman, J.K., Hirschi, K.D. 2009. Root development under metal stress in Arabidopsis thaliana requires the H(+)/cation antiporter CAX4. New Phytologist. 183(1):95-105.

Interpretive Summary: Transporters move nutrients and toxic heavy metal ions within and through cells. How to manipulate membrane transporters in order to improve the nutritional quality of crops requires better understanding of the physiological functions of those transporters. In this study, we characterized a particular calcium transporter, CAX4, from a model plant and addressed how and when CAX4 was expressed in plants and how this transporter functions. Using a series of biochemical assays and genetic analysis, we determined that this transporter was specifically expressed in the growing tissues, such as root tips, in plants and highly induced by heavy metal ions, suggesting a special role CAX4 may play in moving those toxic metal ions when excess. Indeed, when absent of CAX4 gene, plants were not able to transport those heavy metal ions out of cells and got sick and displayed impaired growth symptoms. Findings gained from this study provide a platform to dissect the molecular mechanisms underlying how calcium and other heavy metal ions (nutrients) in a plant are partitioned for storage and nutritional needs.

Technical Abstract: 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.