2. Selenium in Human Health
Selenium (Se) was discovered in 1817 by Berzelius and was considered a highly toxic and polluting element. It was not until after the 1950s that functions essential to human and animal health were attributed to Se
[9], including antioxidant activity
[10][11], hormonal regulation such as in the thyroid
[12][13], anticancer effects
[14][15], cardiovascular protection
[16][17] and antiviral properties
[18][19]. Nowadays, various review articles have shown and updated the latest findings on the role of Se in organic functioning
[1][20][21][22][23], where such functions are carried out through the synthesis of 25 known selenoproteins derived from selenocysteine (SeCys)
[24][25]. The function of most of these proteins remains unknown, but a group of these selenoproteins was shown to have oxidoreduction catalytic activity, where SeCys is located in the active site because it is more reactive than cysteine (Cys) under physiological conditions; thus, SeCys can exist as a nucleophile without electrostatic interactions and can enhance the catalytic effectiveness
[26]. The enzymes related to the oxidoreduction catalytic activity include five glutathione peroxidases, which catalyze the reduction of peroxide in the presence of glutathione; three thioredoxin reductases, which reduce thioredoxin or other proteins in the presence of NADPH; three deiodinases in the thyroid, which promote the reductive de-iodization of thyroid hormones; methionine-R-sulfoxide reductase, which reduces oxidized methionine residues within proteins; selenophosphate synthase, which catalyzes the synthesis of selenophosphate and is ATP dependent. On the other hand, there is a group of selenoproteins that do not have catalytic functions, among which are K, a simple protein with a transmembrane domain; O, the largest selenoprotein; H, related to glutathione gene expression (GSH); I, a membrane protein; T, containing a predicted redox motif. The W and V proteins are related at the C-terminal, M and Sep 15 are distant homologs proteins, and the functions of the S, R and N proteins remain even more unknown
[27][28].
3. Ionic Selenium and Nanoselenium as Biofortifiers and Stimulators of Plant Metabolism
3.1. Biofortification with Ionic Se and nSe in Crops
3.1.1. Ionic Selenium
Se biofortification in crops for human consumption has yielded encouraging results, including from the nutraceutical perspective to the increase in metabolites that enhance tolerance to adverse factors. Interesting results have been found with foliar application to crops, such as cereals, where both Na2SeO3 and Na2SeO4 work in a range between 30 and 300 g ha−1. In this range, it is possible to find an increase in the accumulation of Se in grains and stimulation of metabolism. On the other hand, to obtain the same result in vegetables, it is necessary to apply the aforementioned chemical species between 1 and 20 mg L−1 once or twice per cycle.
It has been found that the application of Se in the form of sodium selenite (Na2SeO3) between 0.86 and 5 mg L−1, as well as sodium selenate (Na2SeO4) in a range of 0.5 to 18 mg L−1, in hydroponic cultures increases the accumulation of this element and the antioxidants in the edible parts of species such as lettuce (Latuca sativa), tomato (Solanum lycopersicon L.), strawberry (Fragaria x ananassa) and basil (Ocinum basilicum).
Similar encouraging results were shown, regarding accumulation and stimulation linked to metabolism, for Se application directly to the soil with both sodium selenite (Na2SeO3) or sodium selenate in cereals from 10 to 100 g ha−1 and vegetables in the range of 20 to 940 mg L−1.
The 24 h seed imbibition technique using Na2SeO4 at concentrations ranging from 0.1 to 8.54 mg L−1 has been found to be both easy to perform and convenient due to its accumulation and stimulation of the metabolism.
3.1.2. Selenium Nanoparticles (nSe)
Recently, nanotechnology has revolutionized a wide range of areas such as pharmaceuticals, energy, communications engineering, medicine, the environment, chemical industry and plant nutrition, among others, due to the low area/surface ratio, states of aggregation and stability of nanoparticles. The synthesis and use of nSe as a nutrient and biofortifier has proven to be an interesting strategy because, additionally, it has been found to possess greater chemical stability, biocompatibility, rapid absorption and less toxicity compared to the ionic forms of this element
[29][30].
Different studies have been carried out by imbibing seeds or bare roots at nSe concentrations ranging from 1.18 to 50 mg L−1; results have shown no changes in growth, but increases in Se in the edible parts and antioxidant content. In addition, various works have been carried out where nanoparticles are applied directly to the substrate in ranges as wide as 10 to 100 ppm between horticultural species.
Information regarding nSe application in biofortification and improvement in plant metabolism (e.g., the appropriate concentrations in hydroponic or soil cultures) is still lacking.
In the same way, the mechanism by which these nanoparticles are absorbed is still not fully elucidated. One of the widely accepted options is that absorption happens both intra- and extracellularly through the tissues, until reaching the xylem; the way in which nanoparticles pass through the casparian strip is not yet clear, but it could be through the meristematic zone. The cell wall acts as a physical barrier; however, it contains pores with diameters between 5 and 20 nm, and nanoparticles smaller than this will enter freely
[31]. It is also possible that nanoparticles greater than 20 nm enlarge the pores, inducing the formation of cavities to enter via endocytosis or even through transmembrane proteins or ionic channels
[32].
As described above, to date, encouraging results have been obtained in the field of biofortification with ionic selenium in horticultural species widely consumed by humans, such as lettuce and tomato, as well as in the stimulation of redox metabolism, which leads to an increase in tolerance to adverse factors. However, a promising alternative is the use of selenium nanoparticles, where a reduction in application complexity may be achieved, and this leads to important results in the potentiation of antioxidant metabolism, the promotion of agronomic sustainability and a reduction in waste. Therefore, more research should be carried out on the plant cell level as well as interactions within the entire trophic chain and environment
[33].
3.2. Stimulation of Plant Metabolism
It has been shown that the application of selenium in plants, via foliar, hydroponic cultures, directly to the soil or by imbibition, stimulates plant metabolism, resulting in improved growth, improved synthesis of molecules involved in defense
[34] and an increase in stress tolerance
[35]. However, the exact mechanics under which these processes occur are unclear. The main pathway of selenium’s impact will be pointed out below, when it is applied in ionic form and in nanoparticles.
3.2.1. Ionic Selenium
Several research works have been carried out to demonstrate the effect of selenium on plant metabolism, more specifically the impact on phytochemicals, and the results have been quite varied. In a relatively constant way, it has been established that low concentrations of Se can act as an antioxidant and high concentrations as a pro-oxidant
[36]. However, establishing the values of “high” or “low” concentrations is difficult. Each form of application tolerates different selenium concentration ranges, taking into account the following pattern: imbibition > hydroponics > foliar > soil.
The application of Se and its impact on antioxidant metabolism in plants have been linked to both primary and secondary metabolites.
Primary Metabolites
Regarding primary metabolites, there is an increase in the activity of glutathione peroxidase (GPX), an enzyme that involves selenic acid (PSeOH) in its catalytic cycle. This reacts with glutathione, a tripeptide that functions as a coenzyme in this reaction and contains a sulfhydryl functional group (-SH), to form a selenyl-sulfide adduct. At this point, a second GSH molecule intervenes, and here selenol (PSeH) is formed in the active site where peroxide reduction takes place
[37]. In addition, it has been reported that the application of selenium promotes an increase in the activity of enzymes with an antioxidant capacity, such as ascorbate peroxidase, catalase, superoxide dismutase, dehydroascorbate reductase, glutathione reductase and monodehydroascorbate reductase
[38].
Although not as thoroughly studied, it has also been related to the increase in other non-catalytic antioxidant proteins, such as thioredoxin (TrxR) and P protein, the latter containing more than 10 Se atoms
[39].
In a study carried out in rapeseed (
Brassica napus), important effects on primary metabolism were elucidated, including a higher concentration of glucose, coupled with higher ATP production; increased superoxide dismutase activity in the mitochondria and potentiation in the pentose pathway phosphate, which supplies a large number of non-enzymatic antioxidants; as well as a reduction in the tricarboxylic acid (TCA) cycle
[40].
An increase has also been found in the synthesis of sulfur amino acids (Cys and Met) and selenoamino acids such as SeCys and Semet, which are incorporated into proteins. However, there is also evidence of other non-protein amino acids being synthesized, such as γ-glutamyl methyl seleniocysteine (γ-gluMetSeCys), methyl-SeCys and methyl-Semet, mainly in hyperaccumulator families of S, such as
Brassica and
Allium. In addition, these metabolites have shown powerful anticancer activities
[41][42]. Within this same group of plants, volatile species such as dimethyl-diselenide (DMDS) are produced, which partly controls selenium accumulation. In non-accumulating plant species (those that accumulate < 100 mg Se per kg DW), the methylated species of selenium, dimethyl selenide (DMSe), is synthesized
[43].
It is known that the presence of Se promotes increases of sulfur transporters sultr 1 and 2 as well as ATP sulfurylase at the transcriptomic level, which leads to greater absorption of sulfur and, therefore, the synthesis of both primary and secondary metabolites that contain these elements
[44]. However, it is important to highlight the balance between Se and S concentrations. It has been found that high Se and low S concentrations promote competition, avoiding adequate absorption of sulfur and, in this case, a reduction in the synthesis of the mentioned metabolites
[45].
In a study carried out in rice (
Oryza sativum), a three-fold increase in fatty acids (oleic, linoleic and linolenic) was found after the application of selenite and selenite; however, the metabolic pathway that was affected is not clear
[46].
Secondary Metabolites
After the application of ionic species of selenium, both positive and negative effects have been reported on the concentration of secondary sulfur metabolites such as glutathione and glucosinolates. For the first, examples of increased concentrations were evidenced in radish (
Raphanus sativum L.)
[47] and plum trees (
Prunus domestica)
[48]. On the other hand, the reduction in GSH found in strawberry plants was associated with an increase in the concentration of its oxidized form: glutathione disulfide (GSSH)
[49].
Various S hyperaccumulating plants, such as broccoli (
Brassica oleracea), show increased synthesis of glucosinolates with exogenous application of Se, which participate in defense against herbivores and are synthesized mainly from methionine and phenylalanine
[50].
There is a close relationship between the metabolism of sulfur and nitrogen; the proportion of these in most plant species is preserved between 1:45 and 1:30, respectively. About 80% of S and N assimilation is directed to the production of protein amino acids. After modifications in the absorption of S due to the aforementioned effect on transporters with the application of Se
[51], the content of metabolites dependent on these elements, such as glucosinolates, is also affected.
It has also been found that selenium modifies the phenylpropanoid pathway, increasing the enzymatic activity of phenylalanine ammonium lyase (PAL), which is why several investigations have found a positive correlation with phenolic compounds, which act as non-enzymatic antioxidants
[52]. Similarly, an increase in the synthesis of ascorbic acid has been found, which functions as a direct scavenger of reactive oxygen species and a cofactor of enzymes with antioxidant activity, such as APX
[53].
In summary, selenium stimulates metabolism in two ways: (1) via antioxidants, where it participates in the catalytic cycle of enzymes such as APX, and (2) via pro-oxidants, where selenite and selenate probably mimic moderate oxidative stress and the detection of synthesized reactive species will trigger signaling to achieve the formation of all antioxidant machinery
[54].
3.2.2. Selenium Nanoparticles (nSe)
Unlike readily available research on Se applications in its ionic form, research on its nanoparticulate (nSe) form is much more recent and scarcer. Although results have been favorable, even more than that observed in the ionic form, there is still a long way to go. It is generally known that nSe have a strong impact on antioxidant metabolism, which is why they have probably been successfully tested in various species to cope with different types of stress. Examples of this are reported in sorghum (
Sorghum bicolor) subjected to high temperatures
[55]; strawberry, tomato, coriander, basil and barley (
Hordeum vulgare) under stress by salinity
[29][56][57][58][59]; increase in tolerance to stress caused by pathogens such as
Alternaria solani [30],
Meloidogyne incognita [31] and
Botrytis cinearea [60]. The reason why Se can trigger these beneficial effects could be related to a change in the redox status of the cell, causing a greater stimulation in the synthesis of non-enzymatic antioxidants such as lycopene and carotenoids. It could also be related to an increase in the activity of the main enzymes involved in the antioxidant pathway such as glutathione peroxidase, where selenium acts as a cofactor, and it is possible to obtain ionic selenium from nanoparticles
[61]. A reduction in the loss of photosynthetic pigments such as chlorophyll a and b has also been noted during adverse conditions, which leads to an improvement in the photosynthetic rate, maintenance of homeostasis and improvement in growth
[57]. This can be explained, at least partially, by nSe being stored in the thylakoid membranes, providing them with stability. A reduction in lipoperoxidation has also been proposed as protecting and stabilizing some enzymes that catalyze the synthesis of these photosynthetic pigments
[6].