Transition metals such as zinc and iron are essential for virtually every biological process. For example, current estimates assume that around 10% of all eukaryotic proteins are zinc-dependent. Our understanding of acquisition and distribution pathways for these micronutrients, however, is still limited. Plants have to acquire essential microelements from soil solutions that can vary in concentrations of the respective ions by orders of magnitude. Not only deficiency is a threat but also toxicity. An excess of zinc or other metal ions can inhibit growth due to their tendency to interact strongly with various cellular components. A homeostatic system comprising metal transporters, metal ligands and regulatory proteins maintains the concentrations of essential elements within rather narrow physiological ranges inside plant tissues.

Relevant as environmental factors for plants are not only macro- and microelements, but also potentially highly toxic elements without biological function, for example cadmium and arsenic. Nonessential toxic elements are present in the environment either because of natural causes or because of anthropogenic pollution.

Human well-being depends in many ways on the ionome of plants, i.e. the concentrations of essential and nonessential elements especially in edible tissues. Plant-derived food is a major source of micronutrients and an estimated three billion people around the world are threatened by zinc or iron deficiency. Furthermore, most of the human cadmium intake and a large fraction of the arsenic intake are due to the consumption of plants. Thus, we need to better understand the pathways determining metal accumulation in plants, the storage sites and mechanisms as well as the chemical environment, which greatly influences bioavailability. This will enable the generation of crops with elevated micronutrient concentrations and much less accumulation of non-essential, toxic elements.

We are pursuing different approaches to dissect metal accumulation in plants. One of them focuses on the ability of certain plant species to hyperaccumulate zinc and cadmium up to levels more than 1000-fold higher than in non-hyperaccumulating plants. Our model is Arabidopsis halleri, a close relative of A. thaliana, and growing in old mining areas in Central Europe. A second approach is aiming at identifying components of metal tolerance in A. thaliana. Finally, we are studying the monocot models rice and barley.