Effects of selenium biofortification on crop nutritional quality

Abstract

Selenium (Se) at very low doses has crucial functions in humans and animals. Since plants represent the main dietary source of this element, Se-containing crops may be used as a means to deliver Se to consumers (biofortification). Several strategies have been exploited to increase plant Se content. Selenium assimilation in plants affects both sulfur (S) and nitrogen (N) metabolic pathways, which is why recent research has also focused on the effect of Se fertilization on the production of S- and N- secondary metabolites with putative health benefits. In this review we discuss the function of Se in plant and human nutrition and the progress in the genetic engineering of Se metabolism to increase the levels and bioavailability of this element in food crops. Particular attention is paid to Se biofortification and the synthesis of compounds with beneficial effects on health.

Keywords: food; nutritional quality; plant biofortification; secondary metabolites; selenium.

FIGURE 1

Selenate (and sulfate) uptake and assimilation in plants. Selenate is taken up by sulfate transporters (Sultr), and activated by ATP sulfurylase for further assimilation to selenocysteine (SeCys). SeCys can be further metabolized to selenomethionine and to volatile dimethylselenide. Non-hyperaccumulators often store selenate, because APS is a rate-limiting enzyme. Its overexpression resulted in enhanced Se accumulation and tolerance. Selenium hyperaccumulators methylate SeCys via the enzyme SeCys methyltransferase (SMT) and accumulate methyl-SeCys, a non-protein aminoacid. Methyl-SeCys may also be converted to volatile dimethyldiselenide. Expression of SMT in non-hyperaccumulators resulted in enhanced Se accumulation (as methylSeCys) and tolerance. Sulfur and nitrogen metabolic pathways interact at the level of -acetylserine. Changes in S assimilation induced by Se can in turn affect N metabolism, with respect to protein and amino acid synthesis. Amino acids methionine, phenylalanine (Phe), tyrosine (Tyr), and tryptophan (Trp) are precursors of glucosinolates (GLS) and Phe is a precursor for phenolics. Variation in the synthesis of these amino acids influence the production of nutraceutical compounds [glucosinolates (GLS) and phenolics]. In addition, Se can directly induce production of phenolics in plants.

FIGURE 2

Processes related to Se in the soil-plant system, relevant for Se biofortification. Selenate is taken up from soil and assimilated (particularly by Se hyperaccumulators) to organic forms of Se. Some Se is accumulated and some volatiled as nontoxic dimethyl(di)selenide.