**4. Toxicity of selenium**

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,

Selenium is an essential inorganic element for humans and animals, and one of the organic forms of Se, methylselenocysteine, appears to be an effective food source of Se [16]. Se is incorporated into a range of selenoproteins involved in several important metabolic activities such as synthesis of thyroid hormones and antioxidative activity [46, 47]. Selenium is an important inorganic component for the antioxidant metabolism of enzymes, making up part of the active site of enzymes from the group of glutathione peroxidases (GSH-Px) which plays an important role in detoxification of free radicals. The GSH-Px catalyze the reduction of hydroperoxide radicals (H2O2 for instance) by the oxidation of glutathione (GSH), a nonenzy-

It is noted that the accumulation of Se in foods is closely related to the content of this nutrient in the soil. However, consumption of foods poor in Se or low ingestion of foods containing Se is associated with the emergence of numerous diseases such as cancer, type II diabetes, heart disease, pulmonary dysfunction, seizures in children, impaired development and cerebral

Currently, Se deficiency affects about 1 billion people worldwide due to soils lacking this mineral nutrient in some countries [52]. This edaphic characteristic was registered in countries such as Sweden, Finland, USA and China [16, 53, 54]. Because there is a close relationship between plant mineral nutrition and human health, food fortification with Se, via a strategy known as biofortification, is an effective way to add Se to human food and prevent the emergence of diseases related to deficient Se intake. This strategy proved to be effective due to the fact that Se presents chemical similarity to S, and both have the same carrier membranes

Because vegetables are considered an important source of bioactive compounds that contain polyunsaturated fatty acids, phytochemicals such as flavonoids and glucosinolates, many of which can inhibit cell proliferation, induce apoptosis and act synergistically when combined in foods [55], some vegetable groups are more suitable for biofortification because they are natural accumulators of Se such as brassicas [16]. This group of plants has significant levels of glucosinolates, a substance of great nutraceutical interest. In this context, recent studies confirm the biofortifying effect of Se in edible plants in the group of brassicas. For example, in a major study [21] showed the positive impact of Se biofortification in broccoli. This study

cell expansion, and absorption and assimilation of essential nutrients.

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**3. Selenium biofortification**

matic component of the antioxidative metabolism [48].

functions, as well as pregnancy and conception [49–51].

and biochemical pathway of assimilation [28, 33].

From the point of view of plant nutrition, the effects of Se are twofold, since at low concentrations it can promote biofortification but at high concentrations it triggers toxicity in plants [64]. This duality of effects on plant growth is related to the narrow range between biofortification and toxicity, which in literature is not well established based on determination of critical Se levels. It is understood that the critical toxicity level in the soil or nutrient solution is that which implies 10% reduction in plant growth, since the benefit of fertilization reaches its maximum between 90 and 95% of relative growth of the culture [65]. In plants, the effect of Se toxicity is dependent on the Se concentration and source, as well as plant genotype. However, for the same Se concentration, selenite appears to be more toxic than selenate. This fact is justified by the rapid incorporation of selenite into organic compounds still in the root system [28, 66]. In general, symptoms of Se toxicity in plants are characterized by reduced growth, as observed in lettuce [67], spinach [68], cucumber [64] and pea [29]. Reduced plant growth, as a symptom of Se toxicity, is based on two biochemical mechanisms related to Se absorption and assimilation in organic compounds.

**Figure 2.** Model describing the two distinct mechanisms of Se toxicity in plants. Inorganic Se contributes to oxidative stress, while its reduction to selenocysteine may inadvertently substitute the amino acid cysteine and create malformed selenoproteins. The possible targets and ramifications of oxidative stress induced by Se and nonspecific selenoproteins are proposed. Adapted with permission from [28]. These symptoms were observed in the study of toxicity to Se sources and concentrations in the lettuce cultivar Vera, conducted under hydroponic conditions in a study of Ref. [71]. These symptoms were most intense in the source selenite at Se concentrations >8 μM (**Figure 3**). In this study (unpublished data), two response patterns were evident for lettuce plants with respect to Se sources and concentrations in the shoot, root and leaf area growth.

**Figure 3.** Visual aspect of applying increasing concentrations of selenite and sodium selenate to lettuce plants. Sources [71].

symptom of Se toxicity, is based on two biochemical mechanisms related to Se absorption and

**Figure 2.** Model describing the two distinct mechanisms of Se toxicity in plants. Inorganic Se contributes to oxidative stress, while its reduction to selenocysteine may inadvertently substitute the amino acid cysteine and create malformed selenoproteins. The possible targets and ramifications of oxidative stress induced by Se and nonspecific selenoproteins are proposed. Adapted with permission from [28]. These symptoms were observed in the study of toxicity to Se sources and concentrations in the lettuce cultivar Vera, conducted under hydroponic conditions in a study of Ref. [71]. These symptoms were most intense in the source selenite at Se concentrations >8 μM (**Figure 3**). In this study (unpublished data), two response patterns were evident for lettuce plants with respect to Se sources and concentrations in the

**Figure 3.** Visual aspect of applying increasing concentrations of selenite and sodium selenate to lettuce plants. Sources

assimilation in organic compounds.

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

shoot, root and leaf area growth.

[71].

**Figure 4.** Leaf area (A), shoot (B) and root dry mass (C) of plants and the lettuce cultivar Vera submitted to selenium sources and concentrations in hydroponic cultivation. Selenite (\_\_°\_\_); Selenate (--•--). Source: [71].

The first mechanism is related to substitution of S for Se in the amino acids cysteine and methionine. This exchange of S for Se implies substitution of the amino acids cysteine and methionine for their analogs selenomethionine and selenocysteine during protein synthesis. This substitution is more deleterious in relation to cysteine, because this amino acid residue is of great importance in the structure and function of proteins and formation of disulfide bonds, enzymatic catalysis, metal bonding sites and redox state regulation. It is also suggested that iron as a metal cofactor complexed to selenocysteine can interrupt the flow of electrons in chloroplasts and mitochondria (**Figure 2**), since these cytoplasmic organelles have electron transport systems with supramolecular organization [28]. Therefore, any replacement of S by Se impedes electron flow due to changes in the protein conformational structure, and hence energy synthesis coordinated through the electron transport chain. The second mechanism is related to the participation of glutathione (GSH), a tripeptide active in cellular redox homeostasis regulation [69] and in the selenite reduction stage [28]. The participation of GSH in this step is crucial, since under Se toxicity conditions there may be a functional imbalance of GSH, since there is increased GSH demand for selenite reduction compared to hydroperoxide reduction in reactions catalyzed by glutathione peroxidase (**Figure 2**).

This imbalance in favor of selenite reduction triggers an oxidative burst that results in reduced plant growth [28]. However, plants considered accumulators or hyperaccumulators have biochemical mechanisms that prevent Se incorporation in proteins, by adding the methyl radical to the intermediate compound selenocysteine which is volatilized [16, 28]. From a symptomatology point of view, Se toxicity is characterized by reduced growth and chlorosis of plants [70].

In the lower concentrations of selenite or selenate, there was greater shoot, root and leaf area growth (**Figure 4A**–**C**). However, concentrations >14 and 16 μM of Se (selenite and selenate) for the shoots, and 9.7 and 30 μM (selenite and selenate) for the root resulted in growth reduction in these organs and more intense chlorosis in plants grown in the presence of selenite (**Figure 4A**–**C**). These responses were similar to those observed in lettuce [18] and cucumber [64] grown under sources and increasing Se concentrations.
