Adaptive Significance of Nickel Hyperaccumulation by Plants

ABSTRACT Nickel (Ni) hyperaccumulation in plants is an unusual attribute that imposes significant physiological costs. Persistence of Ni hyperaccumulation across a range of taxa suggests a significant evolutionary advantage to this trait. This review examines seven non‐mutually exclusive hypotheses to explain the evolution of Ni hyperaccumulation in plants. The hypotheses include elemental defense, elemental allelopathy, drought tolerance, inadvertent uptake, enhanced reproductive fitness, incremental advantage, and metal tolerance and disposal. While our main focus is on Ni hyperaccumulation, we also include selected information about the hyperaccumulation of other metals where relevant to the seven hypotheses. Additionally, we discuss common experimental techniques used when studying Ni hyperaccumulation and recognize recent advances in technology available to study hyperaccumulation. We also identify current gaps in research that should be prioritized to help us better understand the evolutionary significance of metal hyperaccumulation.