Extending the elemental defence hypothesis in the light of plant chemodiversity

Some plant species tolerate and accumulate high levels of metals or metalloids in their tissues. The elemental defence hypothesis posits that metal(loid) hyperaccumulation by these plants can serve as protection against antagonists. Numerous studies support this hypothesis. In addition, as other plant species, hyperaccumulators synthesise specialised metabolites that can act as organic defences. In principle, the composition and concentration of plant-specialised metabolites vary pronouncedly not only among species, but also within species and within individuals. This variation is called chemodiversity. Surprisingly, the role of chemodiversity has received little attention in elemental defence. Thus, we advocate that the concept of the elemental defence hypothesis should be extended and linked to the multifunctionality of plant chemodiversity to better understand the eco-evolutionary dynamics and maintenance of metal(loid) hyperaccumulation. Comprehensive literature studies revealed that both metal(loid)s and specialised metabolites acting as defences are highly diverse in some hyperaccumulators and the biosynthetic pathways of these two types of defences are partly intertwined. Several edaphic-, population-, temporal- and spatial-related factors were found to influence metal(loid) diversity, which should be considered in the elemental defence hypothesis. We thus present a novel synthesis and outlook to extend the elemental defence hypothesis in the light of chemodiversity.