Elemental imaging and speciation in plant science

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Plants are fascinating and complex organisms which are generally immobile and utilise the resources of their immediate environment to thrive. As a matter of fact, plants are at the base of the food chain, and represent the main source of food for higher terrestrial organisms, including human beings; for example, rice is a staple food for entire populations in Asia. The nutritive value of a plant itself is complemented by a plethora of trace elements that are present in its fruit and other edible plant parts; indeed, this is the main source of essential trace elements for the human population. However, plants can also take up and store nonessential elements, including those toxic or hazardous to our health. The extent to which plants take up trace elements depends largely on the environment in which the plant is growing, but not every plant reacts in the same way to the trace elements in its environment: different plants have different but specific nutritive requirements, and are more or less tolerant of toxic elements. In a world facing serious problems regarding how to feed the ever-growing population on the basis of dwindling arable land, with vast areas polluted through mining or other anthropogenic activities, it is imperative to understand how plants take up the essential elements that we rely on for a healthy living, and the mechanisms they use to cope with pollution through toxic elements. During their evolution, plants have developed strategies to acquire nutrients and essential elements in the amounts or concentrations that they require, while toxic elements may be excluded or detoxified in plants. These strategies are based on the biochemical machinery of the plant’s various cells, in the form of transporter molecules or chelators such as siderophores or phytochelatins, which enable elemental homeostasis to be achieved in a variety of different habitats. The uptake, (shortand long-range) transport and translocation of an element in a plant depend mostly on the distinct molecular occurrence of the element in the soil and the cell compartment, the actual element itself, as well as the organism and its genetic predisposition. Hence, in order to predict the distribution of elements in a plant, we need to know the molecular structures of the species of these element—their speciation—at the soil/plant interface as well as inside the different cells and cell compartments at all times. In order to understand the underlying mechanisms, it is useful to determine all of the molecular structures of all of the elements of interest in all cell compartments. Since this concept has some similarities to the proteomics approach, it is now widely termed “metallomics.” Although the current approach does not necessarily satisfy the condition that all of the species of all of the elements of interest (or one element) must be identified and quantified in a certain biological substructure, the approach in which the most important biologically active species of an element is determined may enable molecular biologists to link these species to proteins and to the encoding genes. Published in the special paper collection Elemental Imaging and Speciation in Plant Sciencewith guest editors J. Feldmann and E. Krupp.