Metal toxicity is an important factor limiting the growth of plants in many environments. Some metals, such as copper and zinc, are micronutrients at low concentrations and become toxic at higher levels, whereas others (e.g. aluminium and lead) are only known for their toxicity. In some cases, soils are naturally very high in metals. An example is the serpentine soils, which have elevated levels of chromium, cobalt, iron, magnesium and nickel, but are very low in calcium. Low soil pH makes most metals more available for plant uptake and can indirectly lead to metal toxicity (e.g. through acid rain)1. Some acidic soils have high aluminium availability, and this can be an important limiting factor for agriculture. Localized pollution caused by human activity (e.g. mining or smelting) often leads to toxicity problems. For example, the spoil heaps at Old Gang mine in Swaledale in North Yorkshire, UK, are still very high in plant available lead (Figure 1). Sewage sludge that is released on to agricultural land often contains high levels of toxic metals. But what happens to a plant when it is growing in an environment with toxic levels of metals, and how do some plants tolerate these environments? This brief article considers some of the recent trends in the study of metal toxicity and tolerance in plants.

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