Metal Hyperaccumulators

A metal hyperaccumulator is defined as a plant species of which any one individual, when grown in its natural habitat in the wild, contains an extraordinarily high level of a metal or metalloid in above-ground tissues of > 10,000 µg g^-1 Mn, > 3,000 µg g^-1 Zn, > 1,000 µg g^-1 Ni, Se, As, Co, Pb, Cu, Th, or Sb, or > 100 µg g^-1 Cd in dry biomass. These concentrations are more than an order of magnitude above the critical toxicity thresholds in ordinary plants. Thus, metal hyperaccumulation requires extraordinarily efficient and specific mechanisms of metal extraction from the soil, root-to-shoot metal partitioning, and internal metal detoxification. Metal hyperaccumulation and associated hypertolerance in plants bear great promise for the development of phytoremediation and phytomining technologies, in addition to serving as extreme model traits in molecular physiology, evolution and ecology. About 50% of the ~500 known metal hyperaccumulator taxa are in the Brassicaceae family, of which two species – the Zn/Cd/Ni hyperaccumulator Noccaea caerulescens and the Zn/Cd hyperaccumulator Arabidopsis halleri – have emerged as new model species. Various genome-wide comparative approaches including closely related non-hyperaccumulator species have yielded important insights into both molecular mechanisms and the evolution of metal hyperaccumulation. Through reverse genetics, we have demonstrated the functions of key metal hyperaccumulation genes. We are presently assessing species-wide phenotypic diversity in A. halleri, and taking first steps towards its genetic analysis. The current status of this work will be presented in relation to traits of interest for the development of phytoremediation technologies.