Selenium is a necessary nutrient in low quantities for plants and animals, but it rapidly becomes highly toxic with increased doses. In animals, it is associated with numerous neurological, physiological, and congenital diseases. In plants, it causes reduced growth, necrosis, inefficient photosynthesis, and the accumulation of harmful free radicals. Some plants, however, like the prince's plume mustard, hyperaccumulate and sequester selenium compounds by modifying its chemical state and physical location. These organisms can tolerate levels of selenium that kill most other plants. When the mustard's cells detect higher concentration of the reactive oxygen species (ROS) selenium produces, they begin producing more ROS scavengers and enzymes that generate cysteine compounds. These enzymes incorporate sulfur into useful cysteine, but selenium competes for their activity and results in the erroneous generation of toxic selenocysteine (SeCys). But other enzymes produced by the plant tack on a methyl group to SeCys rendering it nontoxic. Transporter proteins then shuttle and sequester the methylated selenium compound at the edges of leaves where it not only poses no threat to the rest of the plant, it actually serves to repel would-be predators.
"Accumulation of Se as MeSeCys is thought to offer a safe way to sequester Se, since this amino acid does not get misincorporated into proteins and thus likely contributes to the enhanced Se tolerance and hyper-accumulation of S. pinnata." (Freeman et al. 2010:1643)
"The tolerance of S. pinnata to Se may also involve the sequestration of Se in specific epidermal locations...Se was accumulated in discrete 'hot spots' along the leaf margins...No Se was detected in vascular, spongy parenchyma, or palisade parenchyma cells...It is possible that Se accumulation in localized areas both prevents plants from Se toxicity and provides a storage mechanism for organic nontoxic Se to remobilize for future biotic defense in growing young leaves and reproductive parts...negative effects of Se on photosynthesis may be further magnified by a subsequent increase in ROS generation...Leaves of S. pinnata contained higher levels of the ROS-scavenging metabolites." (Freeman et al. 2010:1644)