**4. Proposed mechanisms of action of iodine, selenium, and silicon as inducers of the accumulation of antioxidants**

Iodine is considered the first inorganic antioxidant used by ancestral organisms when the concentration of atmospheric O2 increased as a result of oxygenic photosynthesis [6]. This mechanism is widely elucidated in algae, where the direct neutralization of species such as superoxide (O2 − ), hydroxyl (OH− ), singlet oxygen (<sup>1</sup> O2 ), and hydrogen peroxide (H<sup>2</sup> O2 ) [47] has been proven, mainly due to iodine oxidation–reduction power. **Figure 1** illustrates the possible mechanisms of reaction proposed by Luther et al. [48].

Subsequently, these organisms incorporated iodine as a cofactor in the reaction between the vanadium-dependent iodoperoxidase enzyme (IPO-V) and H<sup>2</sup> O2 , thus becoming an essential element against oxidative stress, but not only directly but through a specialized enzymatic mechanism. In terrestrial plants neither of these two processes is fully established, but it has been shown that it exerts a direct function as an electron donor (inorganic antioxidant) at least on the superoxide radical [34], and it has been further verified that iodine can act as a moderate prooxidant, promoting the synthesis of nonenzymatic and enzymatic antioxidants, potentiating tolerance to stress [33, 49].

The ability of silicon to reduce the uptake of elements that cause toxicity in plants is well studied in the case of salinity stress, where there is a reduction in uptake and transportation from

minum [61], cadmium [62], and chromium [63] has also been found. This beneficial effect has been attributed mainly to the reduction, by Si, of the impact that different stress factors have on the permeability of the plasma membrane, allowing it to retain selectivity in the ion flow [59]. The physiological mechanism is related to the induction of antioxidant metabolism. It has been proposed that this occurs due to a possible dual effect: decrease in ROS synthesis and increase in the activity of antioxidant enzymes [64]. Debona et al. [65], showed in wheat plants subjected to biotic stress (*Pyricularia oryzae*) a reduction in the activity of SOD, CAT, peroxidase (POD), APX, and glutathione-S transferase, explaining this phenomenon through a pos-

Regarding the synthesis of antioxidants, Kim and collaborators in 2017 [66] made an extensive compendium of the effects of Si on antioxidant metabolism, evidencing that there is more information related to the increase of enzymatic antioxidants such as SOD, CAT, and APX in plants subjected to a variety of abiotic stresses such as heavy metal toxicity [44, 67], salinity

On the contrary, Ma et al. [71] conducted an experiment on soil with stressed wheat plants with water deficit, finding an increase in the concentration of nonenzymatic antioxidants (ascorbate, glutathione, total phenolic compounds, and total flavonoid content) as well as a decrease in the lipid peroxidation. Gong et al. [72] in a similar experiment found an increase

**Figure 2** shows the proposed mechanisms in which I, Se, and Si intervene in the antioxidant

**Figure 2.** Mechanisms of action proposed for I, Se, and Si in the antioxidant metabolism of plants. In the three elements, there are two forms of action: (1) the direct form which is exemplified by the upper red arrows, where the I and Se

indirect form, which occurs by the influence of Se, I, and Si on ROS overproduction (big red ray) and consequently an increase in enzymatic and nonenzymatic antioxidants, represented by lower red arrows [34, 39, 43, 45, 47, 48, 53, 66, 71].

O2

) to water. (2) The

can reduce the superoxide radical directly to water and Se and Si directly reduce the peroxide (H<sup>2</sup>

[60]. A reduction in the absorption of heavy metals such as alu-

http://dx.doi.org/10.5772/intechopen.75069

161

The Use of Iodine, Selenium, and Silicon in Plant Nutrition for the Increase of Antioxidants…

the root to shoot of Na+

and Cl−

[68, 69], and UV radiation [70], among others.

in SOD activity, but not CAT or POD.

metabolism.

sible inhibition of Si on the fungus ability to colonize plant tissues.

Selenium participates in antioxidant metabolism with different mechanisms, both directly and indirectly. An example of the direct effect is observed with the application of Se at low concentrations (≤ 2 μM) in plants subjected to different stresses such as heavy metal toxicity [50, 51], low temperature [52], high temperature [53], or UV radiation [54], where a direct neutralization of the radicals O<sup>2</sup> .− and H2 O2 occurs. Also among the direct mechanisms is the function of Se as a cofactor in the activity of the enzyme glutathione peroxidase [55]. The indirect relationship occurs with the overproduction of reactive oxygen species due to an excess of selenium (≥ 6 μM). This process is attributed to the assimilation of Se and is dependent on the chemical species. An example of this was demonstrated by Paciolla et al. [56], in cinerary leaves, where the application of Na2 SeO<sup>3</sup> showed an increase in the concentration of H2 O2 , while Na2 SeO4 did not show the same effect. The difference was probably due to the reduction to which Se4 must to be subjected to L-selenomethionine for its subsequent transport through the plant; instead, Se6 is transported directly to the shoot of the plants, as has been shown in rice and broccoli [57]. The use of the reducing potential to assimilate Se4 causes an increase in the formation of ROS, which triggers a higher synthesis of antioxidants such as ascorbate, tocopherol, and glutathione as well as enzymatic antioxidants such as superoxide dismutase (SOD), catalase (CAT), and ascorbate peroxidase (APX) [58].

The mechanisms through which Si reduces oxidative stress can be divided into three modalities: structural, reducing the absorption of heavy metals, and physiological mechanisms.

The structural mechanism is of a mechanical nature, attributed to deposition of stable Si in the form of biosilica (SiO<sup>2</sup> ) in the cell walls, giving it rigidity and resistance [59].

$$\begin{array}{rcl} \text{1)} & \text{l} + \text{H}\_{2}\text{O}\_{2} \longrightarrow & [\text{l} \cdot \text{H}\_{2}\text{O}\_{2}]^{\cdot \cdot} \longrightarrow & \text{HO}\text{l} + \text{OH}^{\cdot} \\\\ \text{2)} & \text{l} + \text{H}\_{2}\text{O}\_{2} \longrightarrow & [\text{l} \cdot \text{H}\_{2}\text{O}\_{2}]^{\cdot \cdot} \longrightarrow & 2\text{OH}^{\cdot} + \text{l}^{\*} \\\\ \text{3)} & \text{l}^{\*} + \text{OH}^{\cdot} \longrightarrow & \text{HO}\text{l} \\\\ \text{4)} & \text{l}^{\*} + ^{1}\text{O}\_{2} \longrightarrow & [\text{l} \cdot \text{O}\_{2}]^{\cdot \cdot} \longrightarrow & \text{HO}\_{2} + \text{HO}\text{l} \end{array}$$

**Figure 1.** Mechanism of reaction between iodide (I− ) and reactive oxygen species such as hydrogen peroxide (1 and 2), hydroxyl (3), and singlet oxygen (4). Figure designed from data obtained from Medrano-Macías et al. [34].

The ability of silicon to reduce the uptake of elements that cause toxicity in plants is well studied in the case of salinity stress, where there is a reduction in uptake and transportation from the root to shoot of Na+ and Cl− [60]. A reduction in the absorption of heavy metals such as aluminum [61], cadmium [62], and chromium [63] has also been found. This beneficial effect has been attributed mainly to the reduction, by Si, of the impact that different stress factors have on the permeability of the plasma membrane, allowing it to retain selectivity in the ion flow [59].

Subsequently, these organisms incorporated iodine as a cofactor in the reaction between the

element against oxidative stress, but not only directly but through a specialized enzymatic mechanism. In terrestrial plants neither of these two processes is fully established, but it has been shown that it exerts a direct function as an electron donor (inorganic antioxidant) at least on the superoxide radical [34], and it has been further verified that iodine can act as a moderate prooxidant, promoting the synthesis of nonenzymatic and enzymatic antioxidants,

Selenium participates in antioxidant metabolism with different mechanisms, both directly and indirectly. An example of the direct effect is observed with the application of Se at low concentrations (≤ 2 μM) in plants subjected to different stresses such as heavy metal toxicity [50, 51], low temperature [52], high temperature [53], or UV radiation [54], where a direct

the function of Se as a cofactor in the activity of the enzyme glutathione peroxidase [55]. The indirect relationship occurs with the overproduction of reactive oxygen species due to an excess of selenium (≥ 6 μM). This process is attributed to the assimilation of Se and is dependent on the chemical species. An example of this was demonstrated by Paciolla et al.

the plants, as has been shown in rice and broccoli [57]. The use of the reducing potential to

of antioxidants such as ascorbate, tocopherol, and glutathione as well as enzymatic antioxidants such as superoxide dismutase (SOD), catalase (CAT), and ascorbate peroxidase

The mechanisms through which Si reduces oxidative stress can be divided into three modalities: structural, reducing the absorption of heavy metals, and physiological mechanisms.

The structural mechanism is of a mechanical nature, attributed to deposition of stable Si in the

hydroxyl (3), and singlet oxygen (4). Figure designed from data obtained from Medrano-Macías et al. [34].

) in the cell walls, giving it rigidity and resistance [59].

SeO<sup>3</sup>

causes an increase in the formation of ROS, which triggers a higher synthesis

did not show the same effect. The difference was prob-

must to be subjected to L-selenomethionine for its

) and reactive oxygen species such as hydrogen peroxide (1 and 2),

.− and H2

[56], in cinerary leaves, where the application of Na2

subsequent transport through the plant; instead, Se6

SeO4

, while Na2

O2

O2 occurs. Also among the direct mechanisms is

showed an increase in the con-

is transported directly to the shoot of

, thus becoming an essential

vanadium-dependent iodoperoxidase enzyme (IPO-V) and H<sup>2</sup>

potentiating tolerance to stress [33, 49].

160 Antioxidants in Foods and Its Applications

neutralization of the radicals O<sup>2</sup>

O2

ably due to the reduction to which Se4

**Figure 1.** Mechanism of reaction between iodide (I−

centration of H2

assimilate Se4

(APX) [58].

form of biosilica (SiO<sup>2</sup>

The physiological mechanism is related to the induction of antioxidant metabolism. It has been proposed that this occurs due to a possible dual effect: decrease in ROS synthesis and increase in the activity of antioxidant enzymes [64]. Debona et al. [65], showed in wheat plants subjected to biotic stress (*Pyricularia oryzae*) a reduction in the activity of SOD, CAT, peroxidase (POD), APX, and glutathione-S transferase, explaining this phenomenon through a possible inhibition of Si on the fungus ability to colonize plant tissues.

Regarding the synthesis of antioxidants, Kim and collaborators in 2017 [66] made an extensive compendium of the effects of Si on antioxidant metabolism, evidencing that there is more information related to the increase of enzymatic antioxidants such as SOD, CAT, and APX in plants subjected to a variety of abiotic stresses such as heavy metal toxicity [44, 67], salinity [68, 69], and UV radiation [70], among others.

On the contrary, Ma et al. [71] conducted an experiment on soil with stressed wheat plants with water deficit, finding an increase in the concentration of nonenzymatic antioxidants (ascorbate, glutathione, total phenolic compounds, and total flavonoid content) as well as a decrease in the lipid peroxidation. Gong et al. [72] in a similar experiment found an increase in SOD activity, but not CAT or POD.

**Figure 2** shows the proposed mechanisms in which I, Se, and Si intervene in the antioxidant metabolism.

**Figure 2.** Mechanisms of action proposed for I, Se, and Si in the antioxidant metabolism of plants. In the three elements, there are two forms of action: (1) the direct form which is exemplified by the upper red arrows, where the I and Se can reduce the superoxide radical directly to water and Se and Si directly reduce the peroxide (H<sup>2</sup> O2 ) to water. (2) The indirect form, which occurs by the influence of Se, I, and Si on ROS overproduction (big red ray) and consequently an increase in enzymatic and nonenzymatic antioxidants, represented by lower red arrows [34, 39, 43, 45, 47, 48, 53, 66, 71].
