Herbivory enhances cadmium foraging in the invasive hyperaccumulating plant Phytolacca americana

Biological invasions and heavy metal pollution are two pervasive environmental challenges threatening global ecosystems. While the elemental defence hypothesis posits that certain plants accumulate heavy metals to deter herbivores, it remains unclear whether invasive hyperaccumulators actively forage for these toxic elements and whether such below‐ground behaviour is dynamically induced by above‐ground biotic stress. Here, we investigated the foraging behaviour of the invasive hyperaccumulator Phytolacca americana in a rhizobox experiment simulating a dual‐stress environment of heterogeneous cadmium (Cd) distribution and Spodoptera litura herbivory. We considered root morphological responses and employed integrated transcriptomic and metabolomic analyses to elucidate the underlying mechanisms. We found that P. americana preferentially allocated root biomass to Cd‐enriched soil patches when exposed to cadmium. Notably, above‐ground insect herbivory significantly amplified this below‐ground Cd foraging response. The interaction between Cd stress and herbivory markedly altered root morphology. Cd accumulated primarily in fine roots, where plants optimized their specific root length and tissue density to efficiently exploit Cd‐rich zones. Integrated molecular analyses revealed that Cd treatment significantly suppressed cytokinin levels in the roots. This hormonal shift likely promotes lateral root formation and accelerates heavy metal sequestration. Our results demonstrate that above‐ground biotic interactions can actively drive below‐ground heavy metal foraging strategies in invasive plants, manifesting as a complex morphological plasticity mediated by cytokinin signalling. More broadly, our study highlights how synergistic responses to multiple environmental stressors can enhance the adaptive capacity and ecological success of invasive species in contaminated habitats. Read the free Plain Language Summary for this article on the Journal blog.