Rice phytochelatin synthases Os PCS 1 and Os PCS 2 make different contributions to cadmium and arsenic tolerance

Cadmium (Cd) and arsenic (As) pollution in paddy soil and their accumulation in rice ( Oryza sativa ) pose serious threats to human health. Rice internally detoxifies these toxic metal and metalloid to some extent, resulting in their accumulation within the edible parts. However, the mechanisms of Cd and As detoxification in rice have been poorly elucidated. Plants synthesize thiol‐rich metal‐chelating peptides, termed phytochelatins ( PC s). We characterized rice PC synthase ( PCS ) and investigated its contribution to Cd and As tolerance in rice. We identified two PCS homolog genes, Os PCS 1 and Os PCS 2 , in the rice genome. The expression of Os PCS 1 was upregulated by As( III ) stress in the roots but that of Os PCS 2 was not significantly affected. The expression level of Os PCS 2 was higher than that of Os PCS 1 in the shoots and roots. Recombinant Os PCS 1 and Os PCS 2 proteins differed in their metal activation. Os PCS 1 was more strongly activated by As( III ) than by Cd; however, Os PCS 2 was more strongly activated by Cd than by As( III ). Genetically engineered plants having their Os PCS 2 expression silenced via RNA interference ( Os PCS 2 RNA i) contained less PC s and more glutathione ( GSH ), a substrate of PC synthesis, than wild‐type plants, although there was no significant difference in Os PCS 1 RNA i plants. Os PCS 2 RNA i plants were sensitive to As( III ) stress, but Cd tolerance was little affected. On the other hand, treatment with buthionine sulfoximine, an inhibitor of GSH biosynthesis, significantly decreased Cd and As tolerance of rice seedlings. These findings indicate that Os PCS 2 is a major isozyme controlling PC synthesis, and that PC s are important for As tolerance in rice. However, PC synthesis may make a smaller contribution to Cd tolerance in rice, and GSH plays crucial roles, not only as a substrate of PC synthesis.