Involvement of a Metallothionein-Like Copper Complex in the Mechanism of Copper Tolerance in Mimulus guttatus

The possible involvement of a metallothionein-like copper complex in the mechanism of copper tolerance in Mimulus guttatus has been examined. In the roots of copper-tolerant plants, after exposure to copper, a constant proportion of the total copper content is bound to this complex. The complex is present in the roots of both copper-tolerant and non-tolerant plants, and is also induced in the roots of both by increased exposure to copper. Differences in the amount of the complex in the roots of copper-tolerant and non-tolerant plants suggest that it may be involved in the mechanism of copper tolerance. However, only a small proportion (6%) of the copper in the roots of the copper-tolerant clone is bound to the complex after growth in a high concentration (16 μ M) of copper; this result suggests that the mechanism of tolerance does not simply involve the sequestration of all excess copper by the complex. The complex may have a transient function in the mechanism of copper tolerance.