Introduction

The growth of plants in nature depends on their ability to respond to their environment. For the metabolism of metals, plants require a balance between the uptake of sufficient essential metal ions to maintain growth and development and the ability to protect sensitive cellular activity from excessive concentrations of essential and non-essential metals. Although phytotoxic amounts of metal occur more frequently from industrial and agricultural pollution than in soils under natural conditions, nevertheless, survival mechanisms are required to detect not only external/internal concentrations of metals, but also essential from non-essential metal ions. Plants thus have the ability to ‘sense’ metal ions since it is central to normal metal metabolism, protection from metal toxicity, and adaptation to metal tolerance. Such recognition can be envisaged to occur by a number of physiological processes, but at the molecular level it is likely to be the binding of metal ions to a protein, which directly or indirectly changes the pattern of cellular activity, usually by changing gene expression. Evidence is now emerging that this molecular recognition is ‘programmed’: evolution has fashioned proteins either to have rigid binding sites which accept some ions while rejecting others, or to have flexible binding sites in which the stereochemistry of the ion determines the final shape of the protein. In either case, evolution has given the organism the capability to distinguish metals and partition them in different ways.