Metallophytes of serpentine and calamine soils – their unique ecophysiology and potential for phytoremediation

Metallophytes of both natural and human-influenced metalliferous soils have focussed considerable attention for years due to their unique appearance and ability to colonise often extremely harsh habitats. A majority of metal-contaminated areas comprise serpentine (ultramafic, rich in Ni, Cr, and Co) and calamine (rich in Zn, Pb, and Cd) soils hosting characteristic serpentine and calamine flora, which is the focus of this review. Through microevolution, the plants inhabiting metalliferous habitats have developed a range of intriguing adaptive traits, demonstrated as characteristic morphological, behavioural, and physiological alterations that enable them to avoid and/or tolerate metal toxicity. The mechanisms responsible for protection of the plant cell from metals entering the protoplast as well as for detoxification of toxic metal ions inside the cell by chelation, vacuolar sequestration, and exclusion from the protoplast are reviewed. These mechanisms have resulted in highly specialised plants able to hyperaccumulate or avoid metals in the shoots. Potential applications of both kinds of metallophytes in rehabilitation and phytoremediation of metal-polluted sites are briefly discussed. Moreover, other beneficial applications of metal-rich plant biomass are mentioned, e.g. as a bio-ore for precious metal recovery (phytomining, agromining), a by-product for eco-catalyst production, or a natural source of micronutrients that are essential for human diet and health (biofortification). The need of active protection of metalliferous sites and conservation of metallophyte biodiversity is pointed out.