Contributions of cell wall and metal-binding peptide to Cd- and Cu-tolerances in suspension-cultured cells of tomato

Possible roles of cell wall and cytoplasmic peptides in the tolerance of cells to Cu²⁺ and Cd²⁺ ions were studied in suspension-cultured cells of tomato (Lycopersicon esculentum L. cv. Palace). Cu²⁺ and Cd²⁺ ions inhibited growth of wild type cells at concentrations more than 100 and 200 μM, respectively. Tomato cells readily developed tolerance to Cd²⁺ ions up to 1 mM but not to Cu²⁺ ions, after repeated subculturings in the presence of the respective ions. Such a metal-specific adaptation of cells was not due to the difference in the total uptakes between Cd²⁺ and Cu²⁺ ions by cells. Wild-type cells accumulated Cd²⁺ preferentially into the cytoplasmic peptide fraction and Cu²⁺ into the cell-wall fraction, when grown under the subtoxic metal conditions. Under excess metal conditions, Cd-tolerant cells produced greater amounts of Cd-binding peptides in the cytoplasm and retained lesser amounts of Cd²⁺ ions in the cell wall than did wild-type cells. In contrast, tomato cells grown in the presence of Cu²⁺ ions synthesized no detectable amounts of Cu-binding peptides in the cytoplasm and retained most of the Cu²⁺ in the cell-wall fraction, irrespective of cell lines. These results suggested that the cytoplasmic peptides rather than cell wall properties have a primary role in the response of tomato cells to excess metal environments.