**2. Selenium metabolism**

In previous studies, it was shown that Se was absorbed by passive diffusion [30, 31]; however, it was recently shown that selenate is absorbed by the sulfur carrier, while selenite is absorbed by the phosphate carrier, and both processes are dependent on energy expenditures [32, 33]. Among the inorganic (selenate and selenite) and organic forms of selenium [selenomethionine (SeMet) and selenocysteine], selenomethionine in canola and wheat plants is the form that present the highest absorption rate and rapid translocation to the shoots [34]. Due to the chemical similarity between S and Se, both present the same route of absorption and assimilation, competing for the same carrier membrane [16]. After absorbed through the sulfur carrier (Sultr) and translocated to the shoot, selenate can be assimilated in chloroplasts and reduced to selenite in a reaction catalyzed by the enzyme ATP sulfurylase (APS) and then into selenide [16].

Non-accumulating plants can concentrate Se because APS has limited catalytic activity. On the other hand, accumulating plants overexpress APS resulting in accumulation and tolerance to high Se concentrations. Selenide may be incorporated into the S-amino acid similar to selenocysteine which can be converted to selenomethionine (SeMet) in three enzymatic steps. Incorrect insertion of the amino acid selenomethionine/selenocysteine in proteins can cause the formation of protein aggregates that promote disruption of important cellular functions [35, 28]. The incorporation of Se in proteins may occur when Se is converted to less toxic forms, because some plant species present nonprotein organic compounds containing Se such as methylselenocysteine (MeSeCys), γ-glutamyl-MeSeCys and/or selenocysteine [36]. Se can be volatilized by plants through the dimetilselenide or dimetildiselenide compounds, synthesized from selenomethionine and methylselenocysteine, respectively [16]. Se accumulating plants have the synthesis of methyl-SeCys catalyzed by the enzyme SeCys methyltransferase (SMT), accumulating methyl-SeCys, a nonprotein amino acid. Furthermore, methyl-SeCys may be converted into dimetildiselenide, a volatile compound. Expression of the enzyme SMT in non Se accumulating plants increases accumulation of Se in the form of methyl-SeCys, and its activity is related to tolerance to Se accumulation [16].

In the biochemical field, the metabolic pathways of plants are interconnected by means of some compounds. In the case of Se and nitrogen (N), the metabolism of these inorganic elements is interconnected by means of the O-acetylserine compound. Therefore, alterations to the S metabolism induced by Se interfere with that of N with respect to the metabolism of amino acids and proteins [16], considering that the amino acids methionine, phenylalanine, tyrosine and tryptophan are precursors of glucosinolate, while phenylalanine is a precursor of phenolic compounds. Thus, variations in the synthesis of these amino acids influence the synthesis of nutraceutical compounds such as glucosinolate and phenolic compounds [16].

depends on the concentration and source, as well as the plant genotype. Despite this, literature does not report Se levels in plants based on determination of critical Se concentrations with regard to leaf content and Se concentrations in the culture medium. This shortcoming complicates the adoption of one Se concentration or a narrow range of concentrations that promote plant growth at the expense of biochemical, physiological and nutritional disorders promoted

In this chapter, we sought to address the key aspects inherent to selenium and its functional relationships in the plant environment, as well as the intrinsic importance of food security regarding this nutrient, considering the main current scientific findings in the biochemical,

In previous studies, it was shown that Se was absorbed by passive diffusion [30, 31]; however, it was recently shown that selenate is absorbed by the sulfur carrier, while selenite is absorbed by the phosphate carrier, and both processes are dependent on energy expenditures [32, 33]. Among the inorganic (selenate and selenite) and organic forms of selenium [selenomethionine (SeMet) and selenocysteine], selenomethionine in canola and wheat plants is the form that present the highest absorption rate and rapid translocation to the shoots [34]. Due to the chemical similarity between S and Se, both present the same route of absorption and assimilation, competing for the same carrier membrane [16]. After absorbed through the sulfur carrier (Sultr) and translocated to the shoot, selenate can be assimilated in chloroplasts and reduced to selenite in a reaction catalyzed by the enzyme ATP sulfurylase (APS) and then into

Non-accumulating plants can concentrate Se because APS has limited catalytic activity. On the other hand, accumulating plants overexpress APS resulting in accumulation and tolerance to high Se concentrations. Selenide may be incorporated into the S-amino acid similar to selenocysteine which can be converted to selenomethionine (SeMet) in three enzymatic steps. Incorrect insertion of the amino acid selenomethionine/selenocysteine in proteins can cause the formation of protein aggregates that promote disruption of important cellular functions [35, 28]. The incorporation of Se in proteins may occur when Se is converted to less toxic forms, because some plant species present nonprotein organic compounds containing Se such as methylselenocysteine (MeSeCys), γ-glutamyl-MeSeCys and/or selenocysteine [36]. Se can be volatilized by plants through the dimetilselenide or dimetildiselenide compounds, synthesized from selenomethionine and methylselenocysteine, respectively [16]. Se accumulating plants have the synthesis of methyl-SeCys catalyzed by the enzyme SeCys methyltransferase (SMT), accumulating methyl-SeCys, a nonprotein amino acid. Furthermore, methyl-SeCys may be converted into dimetildiselenide, a volatile compound. Expression of the enzyme SMT in non Se accumulating plants increases accumulation of Se in the form of methyl-SeCys, and

by toxic Se levels.

selenide [16].

physiological and nutritional fields of plants.

224 Superfood and Functional Food - An Overview of Their Processing and Utilization

its activity is related to tolerance to Se accumulation [16].

**2. Selenium metabolism**

Several studies report the positive impact of Se on the plant metabolism, particularly due to its abiotic stress mitigating effect. In this sense, Se plays an important role in increasing the activity of antioxidant enzymes to contribute to the detoxification of reactive oxygen species, considering that this mineral element participates in the active site of these enzymes. These enzymes, called glutathione peroxidases, appear quite active in plants subjected to various abiotic stresses such as drought stress [37, 38], salinity [39] and heavy metal toxicity [40], conferring stress tolerance to plants. This effect of Se is evident, because when supplied at concentrations of 10 and 50 μM of selenate beneficial effects were observed in wheat plants grown under appropriate and reduced N availability. In this study, Se promoted a better response of the parameters fluorescence and gas exchange, with a positive impact on the growth of wheat plants [41].

On the other hand, Se in toxic concentrations may compromise energy synthesis by redox reactions (*i.e*., photosynthesis and respiration) due to substitution of S for Se in the cysteine amino acid residue. Cysteine constitutes an important site for binding and stabilization of Fe-S metal centers, heme groups and ions participating in the flow of electrons in the mitochondria and chloroplasts. In this regard, it is speculated that substitution of the cysteine amino acid residue by selenocysteine in proteins rich in Fe-S metal centers disturbs the flow of electrons in the mitochondria and chloroplasts [28]. This fact implies the reduction of energy synthesis, and consequently reduced plant growth.

Moreover, the reduction of selenite to selenate via the S metabolic pathway demands a great glutathione input, a biochemical component involved in important redox reactions in cellular homeostasis [42]. This fact explains the decrease in root growth of plants induced by selenate when it is assimilated into organic compounds, since there is a depletion in the cellular glutathione content [28]. This was observed in brassica plants of *Stanleya albescens* sensitive to Se toxicity, which when exposed to toxic concentrations of this element had their growth compromised due to oxidative stress caused by the increased leaf accumulation of hydrogen peroxide (H2O2) and superoxide anion (O2 − ) in contrast to the more tolerant genotype *Stanleya pinnata*, which has a high glutathione content [43].

The metabolisms of nitrogen (N) and S are interconnected by means of the compound Oacetylserine [16]. Thus, the supply of Se to plants may interfere with the N metabolism. It was indicated that the supply of Se to barley plants reduced the nitrate assimilatory process because of reduced activity of the nitrate and nitrite reductase enzymes in leaf and root tissues. However, intensity of the reduction was greater when Se was supplied in the form of selenite [44].

It was recently demonstrated that toxic concentrations of Se promote reduction of plant growth in *Arabidopsis thaliana* due to incomplete mobilization of starch reserves overnight, reduced expression of genes encoding the synthesis of endotransglucosilase/endohidrolase enzymes and expansins, as well as nutritional disorders [45]. Therefore, despite the benefits of low Se concentrations, it has a large negative impact on the plant metabolism when provided in high concentrations on plant growth by affecting metabolic processes of energy acquisition, cell expansion, and absorption and assimilation of essential nutrients.
