Overexpression of Arabidopsis phytochelatin synthase (AtPCS1) does not change the maximum capacity for non-protein thiol production induced by cadmium

Phytochelatins (PCs) play an important role in heavy-metal homeostasis and detoxification. However, we previously reported that the overexpression of PC synthase in Arabidopsis does not lead to increased tolerance of cadmium but, rather, plants show higher Cd sensitivity. Here, we compared the maximum capacity for non-protein thiol (NPT) production at various concentrations of Cd in order to estimate PC synthesis indirectly for both transgenic (pcs9) and wild-type plants. The pcs9 line produced the highest level of NPT when treated with 200 μM Cd for 3 d. In comparison, the maximum productivity by the wild type was in response to 500 μM Cd. Nevertheless, the absolute amounts of NPT produced did not differ significantly between those two genotypes. Furthermore, exogenous application of 1 mM GSH did not dramatically change the capacity for either pcs9 or wild-type plants. These results suggest that Cd hypersensitivity in the transgenic pcs9 may not be caused by supraoptimal intracellular concentrations of PC, but may, instead, be due to overexpressed PC synthase itself because that enzyme can bind metals. This action, therefore, may lead to some unknown disruption in cellular metal homeostasis under Cd stress.