Heavy metal resistance in higher plants: biochemical and genetic aspects

Abstract

Copper, zinc, cadmium and aluminium resistance in higher plants is discussed with regard to genetic and biochemical aspects. In general, metal resistance is genetically determined and metal-specific. Due to the lack of a proper measure of metal resistance, it is still not known whether the resistance is controlled by major genes or whether it is polygenic. However, by applying a more accurate method evidence is brought forward that copper resistance is controlled by a major gene.Resistance in higher plants is mainly based on tolerance, which implies uptake of heavy metals and the ability to “tolerate” excess internal metal concentrations. A complete exclusion of heavy metals is as yet not demonstrated.Upon exposure to toxic concentrations of cadmium, zinc and copper metal tolerant and non tolerant plants synthesize metal-lothiopeptides or phytochelatins. However, there is no conclusive evidence that these compounds play an essential role in the tolerance mechanism. Other potential chelators such as organic acids could be of importance, but evidence for their role is lacking. Compartmentation of excess metals in subcellular bodies (vacuoles) or organs (leaves) seems an effective strategy to avoid toxic effects. For copper tolerance structural modifications at the level of the plasmamembrane could be of significance. Results of biochemical studies, carried out in cell suspension lines, have to be confirmed with studies at the cellular and higher integration levels in intact plants.