Nickel hyperaccumulation mechanisms : a review on the current state of knowledge

Background: Hyperaccumulator plants are unusual plants that accumulate particular metals or metalloids, such as nickel, zinc, cadmium and arsenic, in their living tissues to concentrations that are hundreds to thousands of times greater than what is normal for most plants. The hyperaccumulation phenomenon is rare (exhibited by less than 0.2% of all angiosperms), with most of the ~500 hyperaccumulator species known globally for nickel. Scope: This review highlights the contemporary understanding of nickel hyperaccumulation processes, which include root uptake and sequestration, xylem loading and transport, leaf compartmentation and phloem translocation processes. Conclusions: Hyperaccumulator plants have evolved highly efficient physiological mechanisms for taking up nickel in their roots followed by rapid translocation and sequestration into the aerial shoots. The uptake of nickel is mainly involved with low affinity transport systems, presumably from the ZIP family. The presence of high concentrations of histidine prevents nickel sequestration in roots. Nickel is efficiently loaded into the xylem, where it mainly presents as Ni2+. The leaf is the main storage organ, which sequestrates nickel in non-active sites, e.g. vacuoles and apoplast. Recent studies show that phloem translocates high levels of nickel, which has a strong impact on nickel accumulation in young growing tissues