Some plants "hyperaccumulate" the element selenium to extreme levels--up to 1% of the plant's dry weight. Selenium, an element with properties similar to sulfur, is an essential trace element for many organisms, but it typically is toxic at high levels, and the function behind the intriguing tendency of some plants to hyperaccumulate this element has been largely obscure. The so-called elemental-defense hypothesis proposes that hyperaccumulated elements serve a defensive function against herbivory, the predation of plants by animals.
In their new work, the researchers showed that the selenium in the hyperaccumulator plant species known as prince's plume (Stanleya pinnata) protects it from caterpillar herbivory both by deterring feeding and by causing toxicity in the caterpillar. However, the researchers also showed that in the plant's natural habitat, a newly discovered variety of the invasive diamondback moth (Plutella xylostella) has disarmed the plant's elemental defense. The new moth variety in fact thrives on plants containing highly toxic selenium levels and, in contrast to related varieties, was not deterred from either laying eggs or feeding on the plant. Furthermore, the researchers found that a selenium-tolerant wasp (Diadegma insulare) in turn parasitizes the selenium-tolerant diamondback moth.
Chemical analysis showed that the selenium-tolerant moth and its parasite both accumulate