Cytosolic metal handling in plants: determinants for zinc specificity in metal transporters and metallothioneins

The handling of trace elements by plants plays a fundamental role in food security and safety. Therefore, there is a need to understand the mechanisms that govern metal ion trafficking in plants. The past decade has seen an immense expansion of knowledge on metal ion transport in a variety of biological systems including plants, but as for other organisms, the mechanisms for intracellular zinc trafficking remain enigmatic. The current report highlights recent advances in understanding zinc transport in plants and in identifying the biomolecules involved in this process. A particular focus is put on pinpointing determinants for zinc specificity, and we also highlight areas in need of development. Reliable experimental speciation data are a first step towards systems biology approaches to mineral nutrition in plants—but there is also a need to understand molecular details. One intriguing question, in particular in the context of predicting protein function, concerns how discrimination between metal ions in a biological system functions. Metal ion trafficking in plants is mediated by several transporter families. Determinants for specificity appear to reside mainly in cytosolic domains and often operate through metal-mediated protein–protein interactions.