Selenium metabolism in plants

Selenium (Se) is not an essential element for plants, although it can benefit their growth and survival in some envionments. Excess tissue Se concentrations are toxic. The ability to sequester Se in vacuoles, synthesise non-toxic Se metabolites, or volatilise Se compounds determines maximum tissue Se concentrations and the ability to colonise seleniferous soils. This review first classifies plant species on their abilities to accumulate Se in their tissues and to colonise seleniferous soils. It then presents our knowledge of Se uptake by roots and its movement within the plant, the primary and secondary metabolism of Se in plants, effects of Se on sulfur and nitrogen metabolism, and the detoxification of excessive Se by plants. Finally, it presents a current hypothesis for the evolution of seleniferous flora. Selenium and sulfur share the same primary metabolism. When grown in the same environment, most plant species have similar tissue Se/S quotients. However, Se-hyperaccumulator species, which can have tissue Se concentrations >1 mg g−1 dry matter, have larger Se/S quotients than other species. Secondary Se metabolism determines differences in tissue Se concentration among plant species. Among non-hyperaccumulator species, alliums and brassicas have particularly large tissue Se concentrations. Selenium hyperaccumulation results from the effective metabolic detoxification of Se in tissues. Differences in Se metabolism determine the maximum Se concentrations in plant tissues, which is important for the delivery of Se to diets of herbivores and for the evolution of plant species to colonise seleniferous soils.