Background/Question/Methods The element selenium (Se) is essential for many organisms but toxic at higher levels. Hyperaccumulator plants like Astragalus and Stanleya can accumulate Se up to 1% of their dry weight. We address the hypothesis that hyperaccumulation serves a defensive function against herbivory, using a combination of controlled laboratory experiments, field surveys and manipulative field experiments.
Results/Conclusions Our results indicate that Se hyperaccumulation can protect plants against herbivory, due to both deterrence and toxicity. Many invertebrate herbivores (aphids, spidermites, thrips, grasshoppers, crickets, caterpillars) are deterred by Se-rich plants and suffer toxicity when they feed on them; Se also deters prairie dog herbivory. Hyperaccumulators contain most Se in leaf hairs and epidermis of young leaves and reproductive tissues, typical for defense compounds. Moreover, hyperaccumulators harbored fewer invertebrates than non-hyperaccumulators in the field, and fewer invertebrate species. Some of the herbivores that were found feeding on hyperaccumulators in the field appear to be specialists. They are Se-tolerant and accumulate levels of Se that likely affect higher trophic levels. Biochemical studies on Se-tolerant herbivores suggest their tolerance mechanism is exclusion of Se from proteins, similar to their plant host. One Se-tolerant herbivore, a diamondback moth, was found to harbor a Se-tolerant parasitic wasp, suggesting a higher trophic level adaptation. In conclusion, while Se accumulation protects plants from generalist herbivores, some specialist herbivores have evolved resistance, as have some of their parasites, potentially forming a portal for Se into the local ecosystem. These results give insight into the functional significance of Se hyperaccumulation, which is of significance since little is known about ecological implications of plant hyperaccumulation of any element, not just Se. The findings from this project may have broad implications, since the effects of hyperaccumulation of Se on herbivores and other ecological partners are probably similar for other toxic metals or metalloids. Furthermore, understanding trophic movements of hyperaccumulated elements will be critical to the development and practice of phytoremediation and of cultivating Se-fortified foods.