**1. Introduction**

The discovery of selenium (Se) by Swedish chemist Jons Jacob Berzelius about 200 years ago initiated the start of several studies with this chemical element in living organisms [1, 2]. However, it was [3] who identified Se as an essential nutrient for the growth and development of bacteria, mammals and birds. It is currently considered as an essential micronutrient for humans and animals, as well as for certain lower organisms such as algae, fungi and bacteria [4].

© 2017 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2017 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Se is a trace element widespread in various geochemical environments [5], and its average content in the earth's crust is estimated at 0.05 mg kg−1. However, concentrations exceeding 0.5 mg kg−1 can be found. Selenium levels are slightly higher in mafic rocks; however, they rarely exceed 0.1 mg kg−1. In sedimentary rocks, Se is associated with the clay fraction, and thus, its abundance is higher in clayey sediments compared to sandstone and limestone. Selenium exists in four valence states, of which the state −2 predominates in organic compounds. The forms selenite (Se4+) and selenate (Se6+) do not form stable compounds in geochemical environments and are preferably adsorbed by minerals, particularly clay minerals, and oxides and hydroxides of Fe and Mn [6]. In the soil-plant system, the plant has the important role of recycling and releasing Se forms from the soil to the food chain. In this sense, the Se concentration in agricultural products and forage depends on the Se content in the soil and its availability [7–9]. However, Se availability in the soil is restricted, and its content is relatively low due to weathering and acidity [6, 7, 9, 10].

Because of the chemical similarity between Se and sulfur (S), the metabolism of Se in higher plants is closely related to S. Therefore, its absorption by the roots of plants is governed by the solubility of adsorbed forms and the transformation of organic Se forms (**Figure 1**). Due to the ability of plants to absorb Se, they can be classified according to Se accumulation in their tissues, including the classes of hyperaccumulators (1000–15,000 mg Se kg−1 dry weight), accumulators (>100 mg Se kg−1 dry weight) and non-accumulators (<100 mg Se kg−1 dry weight) [11]. Many species of the genera *Astragulus, Xylorrhyza* and *Stanleya* are typically Se accumulators because they are capable of growing in selenium-rich soil without showing any negative effect of Se toxicity, accumulating Se contents in the range of 20–40 mg Se g−1 DM while presenting no toxic effect [12, 13].

Because the range of Se concentrations in the soil varies, for example, from 8000 mg kg−1 in soils of the Russian city of Tuva to 0.005 mg kg−1 in soils of Finland and China [15], there is close relationship between Se deficiency in soils and the appearance of disease symptoms related to low Se intake in humans and mammals. Various efforts have thus been made to enrich agricultural feed crops with Se via fertilization or genetic breeding in a strategy called biofortification [16, 17]. According to this strategy, the supply of Se to the plant promotes a positive response in human health, preventing the onset of diseases related to low intake of this nutrient. In this sense, several studies report the positive effect of using Se in biofortification of lettuce [18], tomato [19], broccoli [20, 21], cucumber [22] and carrot crops [23].

In this context, biofortification of edible plants appears to be important from a nutritional point of view because selenium participates in constitution of selenoproteins, for example, the active site of antioxidant enzymes from the group of glutathione peroxidases (GSH-Px), which are reactive oxygen species detoxifying enzymes [24, 25]. There are several isoenzymes of the glutathione peroxidase family (with 4 g atom per mole of protein): cellular glutathione peroxidase (GSH-Px 1); glutathione peroxidase of the intestinal epithelium (GSH-Px 2); plasma glutathione peroxidase (GSH-Px 3); hydroperoxide phospholipid glutathione peroxidase (GSH-Px 4); and glutathione peroxidase present in the sperm (GSH-Px 5). Of these, GSH-Px 1 is the most abundant in mammalian selenoproteins [26]. Additionally, Se is of great importance

to human health because of its presence in chemical constitution of the iodothyronine deiodinase enzyme, which is involved in metabolism of the thyroid hormone [27].

Se is a trace element widespread in various geochemical environments [5], and its average content in the earth's crust is estimated at 0.05 mg kg−1. However, concentrations exceeding 0.5 mg kg−1 can be found. Selenium levels are slightly higher in mafic rocks; however, they rarely exceed 0.1 mg kg−1. In sedimentary rocks, Se is associated with the clay fraction, and thus, its abundance is higher in clayey sediments compared to sandstone and limestone. Selenium exists in four valence states, of which the state −2 predominates in organic compounds. The forms selenite (Se4+) and selenate (Se6+) do not form stable compounds in geochemical environments and are preferably adsorbed by minerals, particularly clay minerals, and oxides and hydroxides of Fe and Mn [6]. In the soil-plant system, the plant has the important role of recycling and releasing Se forms from the soil to the food chain. In this sense, the Se concentration in agricultural products and forage depends on the Se content in the soil and its availability [7–9]. However, Se availability in the soil is restricted, and its content is relatively

Because of the chemical similarity between Se and sulfur (S), the metabolism of Se in higher plants is closely related to S. Therefore, its absorption by the roots of plants is governed by the solubility of adsorbed forms and the transformation of organic Se forms (**Figure 1**). Due to the ability of plants to absorb Se, they can be classified according to Se accumulation in their tissues, including the classes of hyperaccumulators (1000–15,000 mg Se kg−1 dry weight), accumulators (>100 mg Se kg−1 dry weight) and non-accumulators (<100 mg Se kg−1 dry weight) [11]. Many species of the genera *Astragulus, Xylorrhyza* and *Stanleya* are typically Se accumulators because they are capable of growing in selenium-rich soil without showing any negative effect of Se toxicity, accumulating Se contents in the range of 20–40 mg Se g−1 DM while presenting no toxic

Because the range of Se concentrations in the soil varies, for example, from 8000 mg kg−1 in soils of the Russian city of Tuva to 0.005 mg kg−1 in soils of Finland and China [15], there is close relationship between Se deficiency in soils and the appearance of disease symptoms related to low Se intake in humans and mammals. Various efforts have thus been made to enrich agricultural feed crops with Se via fertilization or genetic breeding in a strategy called biofortification [16, 17]. According to this strategy, the supply of Se to the plant promotes a positive response in human health, preventing the onset of diseases related to low intake of this nutrient. In this sense, several studies report the positive effect of using Se in biofortifica-

tion of lettuce [18], tomato [19], broccoli [20, 21], cucumber [22] and carrot crops [23].

In this context, biofortification of edible plants appears to be important from a nutritional point of view because selenium participates in constitution of selenoproteins, for example, the active site of antioxidant enzymes from the group of glutathione peroxidases (GSH-Px), which are reactive oxygen species detoxifying enzymes [24, 25]. There are several isoenzymes of the glutathione peroxidase family (with 4 g atom per mole of protein): cellular glutathione peroxidase (GSH-Px 1); glutathione peroxidase of the intestinal epithelium (GSH-Px 2); plasma glutathione peroxidase (GSH-Px 3); hydroperoxide phospholipid glutathione peroxidase (GSH-Px 4); and glutathione peroxidase present in the sperm (GSH-Px 5). Of these, GSH-Px 1 is the most abundant in mammalian selenoproteins [26]. Additionally, Se is of great importance

low due to weathering and acidity [6, 7, 9, 10].

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

effect [12, 13].

**Figure 1.** Selenium in the soil-plant-atmosphere system. Plants absorb selenate or selenite from the soil solution. Se concentration in the soil solution depends on the solubility of Se forms present and the biological transformation of organic Se forms. Adapted with permission from [14].

Despite the biofortifying character of Se in plants, it also has a toxic effect when provided to plants in high concentrations. This toxicity results from the substitution of S for Se in cysteine and methionine amino acids, reducing the number of disulfide bonds and this altering the structure and functionality of proteins, causing a negative impact on plant growth. Furthermore, another toxic effect of Se during its assimilation into organic compounds results in depletion of the nonenzymatic antioxidant, glutathione. Therefore, there is an imbalance between detoxification and formation of free radical species, which results in a significant oxidative burst and consequent reduction in plant growth [28].

It should be emphasized that biofortification/toxicity of Se is well reported in literature [18, 21–23, 29]; however, the transition between biofortification and toxicity by Se is narrow and 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 by toxic Se levels.

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, physiological and nutritional fields of plants.
