Water status in Mesembryanthemum crystallinum under heavy metal stress

Heavy metals (HMs) are known to have negative effects on plant water status; however, the mechanisms by which plants rearrange their water relations to adapt to such conditions are poorly understood. Using the model plant Mesembryanthemum crystallinum, we studied disturbances in water status and rapid plant defence responses induced by excess copper or zinc. After a day of HM stress, reductions in root sap exudation and water deficits in leaf tissues became evident. We also observed several primary adaptive events, including a rapid decrease in the transpiration rate and progressive declines in the leaf-cell sap osmotic potential. Longer HM treatments resulted in reductions of total and relative water contents as well as proline accumulation, an increase in water retention capacity and changes in aquaporin gene expression. After 3 h of HM exposure, leaf expression of the McTIP2;2 gene, which encodes tonoplast aquaporin, was suppressed more than two-fold, thus representing one of the earliest responses to HM treatment. The expression of three additional aquaporin genes was also reduced starting at 9 h; this effect became more prominent upon longer HM exposure. These results indicate that HMs induce critical rearrangements in the water relations of M. crystallinum plants, based on the rapid suppression of transpiration flow and strong inhibition of root sap exudation. These effects then triggered an adaptive water-conserving strategy involving differential regulation of aquaporin gene expression in leaves and roots, further reductions in transpiration, and an accelerated switch to CAM photosynthesis.