**5. Limits between selenium biofortification and toxicity**

Plant growth is affected by excess Se, and the sensitivity of plants to toxicity of this mineral element is dependent on its concentration and source. However, in literature, the limits between Se biofortification and toxicity are not evident and based on critical toxicity levels which consider 10% reduction in plant growth [65, 72]. For example, Ríos et al. [18] observed that the concentration of 40 μmol L−1 selenate, in contrast to concentrations of selenite, was optimal for lettuce growth and biofortification. However, these authors reported a reduction of plant growth when the selenate concentration was >40 μmol L−1.

It should be noted that the above results reported by the limits between Se biofortification and toxicity are not evident and based on critical toxicity levels which consider 10% reduction in plant growth [65, 72]. For example, Ríos et al. [18] contradict those presented by Hawrylak-Nowak [67], which places the limit of lettuce biofortification at concentrations 15 and 20 μmol L−1 for the sources of selenite and selenate, respectively, by the fact that no negative impact was observed on growth and the content of photosynthetic pigments at these Se concentrations. To investigate the effect of Se in lettuce, Ríos et al. [73] observed that the provision of selenate was not toxic up to the concentration of 80 μmol L−1. On the other hand, selenite concentrations >5 μmol L−1 showed symptoms of toxicity related to reduced growth. These authors attributed growth reduction in plants to the nutritional imbalance of macro- and micronutrients, as well as the oxidative stress caused by toxic levels of selenite used in the study.

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

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

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

Plant growth is affected by excess Se, and the sensitivity of plants to toxicity of this mineral element is dependent on its concentration and source. However, in literature, the limits between Se biofortification and toxicity are not evident and based on critical toxicity levels which consider 10% reduction in plant growth [65, 72]. For example, Ríos et al. [18] observed that the concentration of 40 μmol L−1 selenate, in contrast to concentrations of selenite, was optimal for lettuce growth and biofortification. However, these authors reported a reduction

It should be noted that the above results reported by the limits between Se biofortification and toxicity are not evident and based on critical toxicity levels which consider 10% reduction in plant growth [65, 72]. For example, Ríos et al. [18] contradict those presented by

[18] and cucumber [64] grown under sources and increasing Se concentrations.

**5. Limits between selenium biofortification and toxicity**

of plant growth when the selenate concentration was >40 μmol L−1.

reduction in reactions catalyzed by glutathione peroxidase (**Figure 2**).

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

of plants [70].

In cucumbers, the application of selenate and selenite showed toxic effects with in concentrations >80 and 20 μmol L−1, respectively [64]. In this study, leaf area, leaf and root fresh weight were severely reduced at selenite concentrations >20 μmol L−1. These authors showed that the concentration of chlorophyllian pigments and fluorescence of chlorophyll were more negatively affected by selenite compared to selenate.

It is known that the human health and plant nutrition are closely linked to soil fertility, and diseases associated with Se deficiency are documented in areas where the content of this element in soil is low, as in China, Denmark, Finland, New Zealand and central and eastern Siberia [74, 75]. However, there is no record of human intoxication by Se due to consumption of plant species of food interest (*e.g*., corn, rice, beans, wheat, barley, etc.), presumably due to the fact that these plants are not Se accumulators. An interesting fact was recorded on a farm in the state of South Dakota, United States, where the owners recorded low hatchability of chicken eggs or chicks born with deformities and soon died. When investigating the fact, the United States Department of Agriculture identified high levels of Se in wheat grains which were used to feed the chickens [76]. Although literature relates the intake of foods rich in Se to the lower risk of emergence of diseases such as colon, stomach, prostate and lung cancer [51], there are studies that relate Se supplementation to increased risk of death due to prostate cancer [77]. However, the population along the Tapajos River in the Brazilian state of Pará presents high levels of Se in their blood in function of the considerable consumption of nuts from *Bethollethia excelsa* and *Lecythis usi‐ tata*, which have high Se levels. In these nuts, the Se concentration is quite variable, within the range from 0.03 to 512 mg g−1 [59], well above the daily intake recommended by the Scientific Committee on Foods of the European Commission, which recommends a daily intake of approximately 55 μg day−1 [27]. Nevertheless, in this population, the consumption of nuts rich in Se appears to protect against the emergence of diseases resulting from advancing age such as cataract and motor dysfunction [78]. The recommended daily Se intake (**Table 1**) varies in function of age, gender, pregnancy and lactation. Among the sexes, there is no difference with regard to ingestion demands, except when women are pregnant or lactating, where intake should be increased by 9 and 27%, respectively [79]. However, the maximum tolerable daily Se limit at which there occurs initial toxicity symptoms (**Table 2**) characterized by hair loss, and the appearance of brittle nails is similarly dependent on sex, age, pregnancy and lactation [79].


**Table 1.** Recommended dietary allowances for selenium.


Source: [79].

\*Breast milk, formula, and food should be the only sources of selenium for infants.

**Table 2.** Tolerable upper intake levels for selenium.

#### **6. Conclusions and future perspectives**

This chapter addressed the main issues referring to the mineral element selenium, its history of discovery, its presence in the soil-plant system and its main functions in mammals and humans, always based on recent scientific findings that can guide students, teachers and researchers in their studies. Also assessed were the mechanisms of absorption, transportation and assimilation of selenium in organic compounds, as well as its biofortifying and toxic effect in plants. Moreover, its essentiality in mammals has been addressed in order to emphasize its importance in food and human health due to its function in the human metabolism. It was therefore sought to present the importance of this nutrient in the plant-human system, and therefore provide information for future studies seeking to clarify other functional aspects of selenium in plants.
