Evolution of selenium hyperaccumulator plants and their herbivores

Background/Question/Methods

Selenium (Se), at elevated levels, is toxic and responsible for the loss of $330 million worth of livestock annually in the United States.  Certain plants, Se hyperaccumulators, have evolved to contain up to 10,000 mg Se kg^-1 DW. Most studies examining the functional significance of Se hyperaccumulation examine the role elevated Se plays as a defense against herbivores and pathogens, termed the elemental defense hypothesis. Lab studies have shown that elevated levels of Se protect plants from a variety of Lepidoptera larvae, aphids and fungal infections. Additional field studies and mammalian herbivory studies are required to explore the link between hyeraccumulation and protection. The aim of this study is to explore the functional significance and ecological interactions between Se hyperaccumulators and their associated herbivores. The first study presented shows that Se hyperaccumulators are protected from prairie dog herbivory in their natural environment. The second study describes the discovery of a Se tolerant herbivore and gives insight into the Se tolerance mechanism involved.

Results/Conclusions

In the first study, two field surveys showed that the Se hyperaccumulator Astragalus bisulcatus (two-grooved milkvetch), growing naturally on prairie dog colonies, were more protected from prairie dog herbivory than other plants on the same sites. Follow-up manipulative field studies showed that plants pretreated with Se suffer less prairie dog herbivory than plants of the same species treated without Se. During the second study, observations revealed a population of the invasive diamondback moth (Plutella xylostella) in the field thriving on Se hyperacccumulating plants, which were toxic to other herbivores. Laboratory studies revealed that this population of diamondback moth did not suffer Se toxicity and had no feeding or oviposition preference between plants treated with and without Se. In contrast, a population of diamondback moth collected from a Se free environment was sensitive to Se and preferred to feed and oviposit on plants without Se. The tolerance mechanism was discovered using x-ray absorption spectroscopy, which revealed that the Se tolerant population accumulates methylselenocysteine compared to the Se sensitive population, which accumulates the more toxic selenocysteine.
These results lend support to the elemental defense hypothesis as well as provide insight into the natural Se fluxes in Se-enriched systems.