Heavy metals like Mn and Cu, though essential for normal plant growth and development, can be toxic when present in excess in the environment. For normal plant growth maintenance of metal homeostasis is important. Excess uptake of redox active elements causes oxidative destruction. Thus, uptake, transport and distribution within the plant must be strongly controlled. Regulation includes precisely targeted transport from the macro-level of the tissue to the micro-level of the cell and organelles. Membrane transport systems play very important roles in metal trafficking. This review provides a broad overview of the long distance and cellular transport as well as detoxification and homeostasis mechanisms of Mn and Cu, which are essential micronutrients but extremely toxic at elevated concentrations.

Manganese (Mn) is an essential micronutrient in all organisms but may become toxic when present in excess. To investigate whether interior and coastal varieties of Douglas fir [Pseudotsuga menziesii (Mirbel) Franco, var. glauca (DFG) and var. viridis (DFV)] differed in Mn tolerance, seedlings were exposed to excess Mn in hydroponic solutions. Root growth, biomass production, Mn concentrations in different tissues and Mn subcellular localisation were determined. Both varieties showed similar whole-plant Mn accumulation and biomass reduction in response to increases in Mn in the nutrient solution. Since excess Mn inhibited root elongation growth more strongly in DFG than in DFV, biomass allocation in DFV was shifted towards a relative increase in root biomass and a relative decrease in DFG. X-ray microanalysis showed that Mn enrichment in cell walls and vacuoles of DFV root cortex and epidermis cells was higher than in DFG. In roots, precipitates were observed in which Mn concentrations correlated with phosphorus (P) and to a minor extent with calcium. We suggest that the higher persistence of root growth under Mn stress in DFV was caused by greater entrapment in the vacuole and in the apoplast and by more efficient Mn detoxification in insoluble complexes with P than in DFG. To investigate whether P supply affected Mn uptake and toxicity, Douglas fir seedlings were exposed to excess Mn under P deficiency. In P-limited seedlings of both varieties, roots growth was less sensitive to excess Mn than under sufficient P-supply, although in planta Mn concentrations were not diminished. In DFG, which maintained P homeostasis under limited P supply, the negative effect of Mn stress was partially reversed, showing that Mn susceptibility is affected by P metabolism.

Transition metals have been suggested to play a pivotal role in the pathogenesis of Parkinson's disease. X‐ray microscopy combined with a cryogenic setup is a powerful method for elemental imaging in low concentrations and high resolution in intact cells, eliminating the need for fixation and sectioning of the specimen. Here, we performed an elemental distribution analysis in cultured primary midbrain neurons with a step size in the order of 300 nm and ~ 0.1 ppm sensitivity under cryo conditions by using X‐ray fluorescence microscopy. We report the elemental mappings on the subcellular level in primary mouse dopaminergic (DAergic) and non‐DAergic neurons after treatment with transition metals. Application of Fe²⁺ resulted in largely extracellular accumulation of iron without preference for the neuronal transmitter subtype. A quantification of different Fe oxidation states was performed using X‐ray absorption near edge structure analysis. After treatment with Mn²⁺, a cytoplasmic/paranuclear localization of Mn was observed preferentially in DAergic neurons, while no prominent signal was detectable after Mn³⁺ treatment. Immunocytochemical analysis correlated the preferential Mn uptake to increased expression of voltage‐gated calcium channels in DAergic neurons. We discuss the implications of this differential elemental distribution for the selective vulnerability of DAergic neurons and Parkinson's disease pathogenesis.