Evolutionary and Ecological Aspects of Plant Selenium Hyperaccumulation

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SummarySelenium hyperaccumulation is the intriguing phenomenon that certain plant species accumulate extraordinary concentrations of this toxic element (up to 0.6% of dry weight), even from low-Se soils. The goal of this project is to obtain insight into why plant Se hyperaccumulation may have evolved, and the consequences of this trait for other species in hyperaccumulator habitats. The overall hypotheses are: (1) Se significantly affects interactions between hyperaccumulator plants and herbivores. Se protects plants from general invertebrate and vertebrate herbivory due to Se-mediated deterrence and toxicity (based on preliminary results). On the other hand, specialist Se-tolerant herbivores may have evolved that in turn accumulate Se, affecting higher trophic levels. (2) Selenium is essential for the capacity of hyperaccumulator plants to compete with nonaccumulator species, since hyperaccumulators are only found on Se-rich soils. These hypotheses will be tested using the two hyperaccumulator species, Astragalus bisulcatus and Stanleya pinnata, in comparison with related local nonaccumulator species, using a combination of manipulative field experiments, field surveys, and laboratory studies. The specific objectives are to determine: (1) whether Se protects hyperaccumulator plants from invertebrate and vertebrate herbivory in the field; (2) the relative effectiveness of different Se forms in protecting plants from invertebrate herbivory; (3) the plasticity of Se hyperaccumulation in response to herbivory and Se; (4) Se tolerance and accumulation in invertebrate herbivores and how herbivore Se accumulation affects the next trophic level; (5) how hyperaccumulator plants affect growth of neighboring nonaccumulator plant species, and their competitive interactions in soil with or without Se. The emerging research area of metal hyperaccumulation ecology promises to reveal exciting biological phenomena. This study will shed light on the evolution and ecology of Se hyperaccumulation in plants, a fascinating physiological process that is still poorly understood. This study will extend our knowledge of hyperaccumulation across the biological hierarchy from molecules to ecosystems: from Se-related gene expression in response to herbivory and the effectiveness of different selenocompounds on herbivory to the study of hyperaccumulator-herbivore-predator interactions. Se accumulation in plants also has substantial economic consequences and applications. Hyperaccumulating "locoweeds" cause significant losses in livestock and may also affect endangered wildlife. Selenium is an environmental pollutant, and phytoremediation offers an attractive cleanup strategy for the Se problem. On the other hand, Se is an essential nutrient and powerful anti-carcinogen, and Se-enriched diets have been shown to improve human and animal health. A better understanding of the factors controlling plant Se accumulation may help develop plants with altered Se accumulation or tolerance. Such plants may be useful for Se phytoremediation or as fortified foods. Knowledge of how plant Se accumulation affects growth and survival of invertebrates, and identification of Se-specialist feeders may be used to control both populations of invertebrate pests and of Se-accumulating "locoweeds". The project will train graduate and undergraduate students as well as high school students.