The Cd-tolerant rice mutant cadH-5 is a high Cd accumulator and shows enhanced antioxidant activity

To investigate the mechanism of cadmium (Cd) detoxification in rice (Oryza sativa L.), a Cd-tolerant mutant cadH-5, obtained by an Agrobacterium tumefaciens-based gene-delivery system, was used for a Cd-tolerance and accumulation study. After 15 d of exposure to 0.75 mM CdCl2, significant phenotypic differences were observed between the wild type (WT) and cadH-5. When exposed to 0.5 mM CdCl2, higher Cd levels were accumulated in cadH-5 root cell wall, root cytosol, and membranes than those in WT. However, Cd concentrations in root tissues varied in both WT and cadH5. No significant difference of hydrogen peroxide (H2O2) concentrations was observed between WT and cadH-5, while contents of cell-wall polysaccharides and phytochelatins (PCs) in the mutant were higher compared to WT. The ratios of reduced glutathione to oxidized glutathione (GSH : GSSG) and ascorbate to dehydroascorbate (ASC : DHA) were lower in WT than in cadH-5, while the NADPH : NADP+ ratio was different to the ratios of GSH : GSSG and ASC : DHA; the ascorbate peroxidase (APX, EC 1.11.1.11), glutathione peroxidase (GR, EC 1.6.4.2), dehydroascorbate reductase (DHAR, EC 1.8.5.1), and monodehydroascorbate reductase (MDHAR, EC 1.6.5.4) activities were lower in WT compared to cadH-5. Our results indicate that under long-term Cd stress, cadH-5 plants can accumulate more Cd with more PC. Also, the redox status of ASC-GSH cycle was more inhibited in WT than in cadH-5 plants, rendering WT less able to scavenge reactive oxygen species (ROS). The cadH-5 mutant maintains relatively high ASC, GSH, and NADPH concentrations, ratios of ASC : DHA, GSH : GSSG, and NADPH : NADP+, as well as antioxidative enzymatic activities and PC concentrations. Thus, it is tolerant of relatively high Cd accumulation.