Emerging mechanisms for heavy metal transport in plants

Heavy metal ions such as Cu2+, Zn2+, Mn2+, Fe2+, Ni2+ and Co2+ are essentia l micronutrients for plant metabolism but when present in excess, these, an d non-essential metals such as Cd2+, Hg2+ and Pb2+, can become extremely to xic. Thus mechanisms must exist to satisfy the requirements of cellular met abolism but also to protect cells from toxic effects. The mechanisms deploy ed in the acquisition of essential heavy metal micronutrients have not been clearly defined although a number of genes have now been identified which encode potential transporters. This review concentrates on three classes of membrane transporters that have been implicated in the transport of heavy metals in a variety of organisms and could serve such a role in plants: the heavy metal (CPx-type) ATPases, the natural resistance-associated macropha ge protein (Nramp) family and members of the cation diffusion facilitator ( CDF) family. We aim to give an overview of the main features of these trans porters in plants in terms of structure, function and regulation drawing on information from studies in a wide variety of organisms. (C) 2000 Elsevier Science B.V. All rights reserved.