Commentary: Toward a more physiologically and evolutionarily relevant definition of metal hyperaccumulation in plants

More than 500 plant species have demonstrated the ability to absorb metals or metalloids from soils and concentrate them at extremely high levels in leaf tissues (van der Ent et al., 2013; Cappa and Pilon-Smits, 2014). This phenomenon, known as hyperaccumulation, has profound ecological implications including herbivory deterrence, trophic transfer of metals, and modifications of soil chemistry (Boyd, 2004; Rascio and Navari-Izzo, 2011; Fones and Preston, 2013). Over the past few decades, many attempts have been made to develop an unambiguous, standardized definition of metal hyperaccumulation in plants (Baker and Whiting, 2002; van der Ent et al., 2013; Pollard et al., 2014). Several general recommendations and guidelines for standardizing hyperaccumulation research have been put forward recently, not all of which are compatible with the examination of hyperaccumulation from physiological, genetic, and evolutionary perspectives. Here we highlight several key issues with previous guidelines, and propose a refined definition that is more reflective of both the genetic and physiological mechanisms underlying hyperaccumulation and the evolutionary history of this phenomenon.