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Environmental Sciences
Selenium (Se) is an essential trace element for humans and animals. Biofortification is a quick, efficient, and sustainable way to lower micronutrient deficiencies. The strategies of biofortification are the approaches employed to achieve biofortification. Plants and animal produce are classified as biofortified when there is an increase in the Se content in the edible portions of plants and food animals.
- selenium
- biofortification
- plants
- livestock
1. Introduction
Selenium (Se) is mainly generated as a byproduct of copper mining [1]. Since its industrial use began in the early 1900s, the global output of Se has expanded significantly. Worldwide production in 1910 was around 5000 kg [1]. According to Garside, about 3300 metric tons of Se was produced globally in 2020. China, Japan, and Germany produced the most selenium that year, producing 1120, 740 and 300 metric tons, respectively [2].
Se sources can be anthropogenic, geogenic, or both [33]. Gypsum, marlstone, volcanic eruptions, sea spray, the weathering of Se-rich rocks, soils, and animal transport are some of the natural sources of Se [34,35,36]. Atmospheric discharge is one of the most significant sources of Se in different types of soils, as natural resources volatilize Se into the atmosphere [37,38]. In clay soils, Se levels range from 0.8 to 2 mg/kg, whereas tropical soils have a range of 2 to 4.5 mg/kg [39]. In diverse soils, however, Se levels vary from 0.01 to 2 mg/kg [16]. Some studies show that Se accumulates more readily in igneous rocks than in other rock types [35,36,37,38,39,40].
Se from sediments is transported into rivers and other water bodies by fluctuations in water flow or benthic agitation (Figure 1). In certain areas, Se levels in water from wells and subsurface waters used by humans and livestock for drinking and other activities may surpass 10–20 μg/L, with some concentrations reaching hundreds of micrograms per liter [41]. These waters are not often thought of as an excellent source of Se [42]. Nonetheless, their use results in the transfer and transport of the element in the environment. Farming and industrial activities are the main anthropogenic sources and pathways of Se [43]. However, only around 5% of the overall demand for Se is used by agriculture [44], where it is used in producing fertilizers and animal feeds, among other uses. This renders industrial use the primary anthropogenic source and pathway of Se.
Figure 1. Sources and pathways of selenium in the agro-environmet.
2. Se Biofortification Strategies
2.1. Se Forms
As noted earlier, Se exists in soils in four different oxidation states that determine its behavior, specifically its mobility and bioavailability in the natural environment. Some common organic Se compounds include selenomethionine (SeMet), selenocysteine (SeCys), dimethylselenide, selenium methylselenocysteine, dimethyldiselenide, dimethylselenone, methane selenol, and dimethylselenyl sulfide [46]. Se(IV) and Se(VI) are the dominant forms of Se and are often considered the only abundant forms for plant uptake in many studies [47,48]. Se(VI) is water soluble, while Se(IV) is less water soluble and more attached to soil minerals and organic matter [47]. Metallic Se(II) and Se(0) are generally not water-soluble [48]. Se(IV) and Se(VI) are considered the most bioavailable forms for plant uptake due to their solubility [49]. Se(VI) has a higher rate of translocation from roots to shoots compared with Se(IV) [50]. This is because Se(IV) is quickly converted into organic forms like SeCys or SeMet in roots [51]. In anaerobic soils, Se(0) and organic Se(II) are the dominant forms, while Se(IV) and Se(VI) are common in aerobic soils [52]. Se(0) and metallic Se(II) are not water-soluble and, therefore, not bioavailable for plant uptake [52]. Under low redox potential conditions, Se(IV) and Se(VI) can be reduced to Se(II) and Se(0) [53]. Se(0) can also be oxidized into bioavailable inorganic Se compounds through microbial oxidation and hydrolysis [54]. The uptake and transport of Se by plants varies among species and genotypes. The mobility of Se in wheat and canola plants is in the following order: selenate > SeMet > selenite/SeCys [55].
In livestock production, Se is added to animal feed in both organic and inorganic forms [60]. Ruminants absorb and retain organic forms of Se more effectively than inorganic forms. A common way to enhance animal diets with Se is through in-feed administration of Se-enriched yeast, which has a moderate to high Se content and is a source of SeMet [61]. A safe and natural way to provide animals with Se is by offering feed with optimal Se content, as long as the level of Se in the dry matter is carefully monitored. Plants accumulate Se primarily in the inorganic form and then synthesize seleno-amino acids in SeMet, becoming a source of organic Se for animals [62].
2.2. Se Biofortification Strategies in Plants
2.2.1. Foliar Application
Foliar application appears to be the most popular method of applying selenium among all methods because of its simplicity and preferable outcomes. The danger of environmental contamination also seems to be lower. Studies have demonstrated that foliar spray entails minimal use of Se salts [63,64]. This technique entails spraying a crop’s leaf surface with a Se-containing solution. Selenium enters the plant through the leaf cuticles. Particles can also enter plants through trichomes, stomata, stigma, and hydathodes [65]. In this respect, soil chemistry and microbiological processes have less of an impact on Se, resulting in a higher absorption rate with modest quantities of administered Se solution. With this strategy, there are changes in plant-specific parameters that must be taken into account, including the quantity of Se applied, leaf area and surface structure, and leaf structure.
2.2.2. Soil Application
This technique involves amending the soil with Se to raise the amount of overall or bioaccessible Se, enhance the rhizosphere conditions for soil crops, and raise the Se content of produce. With this approach, Se is applied either as Se salts, Se solution, or Se-containing fertilizers. Soil chemistry and microbial activities affect whether Se administered with this technique will result in a desired effect. This strategy is said to have been employed by the Finnish government to boost the population’s daily consumption of selenium [70,71]. Soil Se application has been shown to have a favorable influence on various plant physiological systems. Plants absorb Se in the form of organic Se (SeCys and SeMet), Se(IV), and Se(VI) [72,73]. Although plant roots cannot absorb Se(II), they may do so for organic Se species such as SeCys and SeMet and inorganic Se species such as Se(0), Se(IV), and Se(VI) [74,75]. Se (VI) has been shown to enter plants through the sulfate transporters SULTR1; 2 and SULTR1 [72], while Se(IV) enters plants via phosphate transporter transport [76,77]. OsPT2, a phosphate transporter, has been demonstrated to be involved in plant uptake of Se(IV) [78].
2.2.3. Microbial-Assisted Biofortification
Agronomic biofortification strategies are not always successful due to a number of factors including impromptu rainfall in the case of foliar applications and pH, and heavy metals in soil applications. Plant growth characteristics and yield have been documented to be influenced by microorganisms located in the rhizosphere via a range of processes. These processes include the release of hormones, nutrient transformations, and stress mitigation [82,83]. The roles played by microorganisms in this respect may be species and/or Se species and bioavailability dependent. Bacterial species such as Bacillus, Entrobacter, Paenibacillus, and Pseudomonas have been shown to be capable of Se transformations via methylation and oxidation reduction processes [52,84]. Previous studies found that inoculating wheat with Se-tolerant bacteria derived from Se-deficient soils increased tissue Se accumulation [85]. Researchers demonstrated in 2015 that various bacterial consortia increased Se concentrations in Indian mustard growing in seleniferous soil [86].
2.2.4. Genetic Biofortification
Genetic engineering has the capability to enhance the capacity of plants to accumulate selenium as an alternative to agronomic approaches. Nevertheless, due to the stringent limitations on the usage of transgenics that are still present in several nations, genetic engineering is still not as prevalent and recognized as agronomic biofortification [94]. However, various chromosomal loci linked to elevated Se accumulation in a number of crops have been reported [74,95,96]. Marker-assisted breeding can be employed to transfer high-Se chromosomal loci from high-yielding, low-Se edible plant varieties into the breeding population [75].
2.2.5. Crop Breeding
Some researchers believe that conventional crop breeding may be a sustainable and long-term approach to crop biofortification with Se [101]. However, compared to genetic biofortification crop breeding is a slower and less accurate method as this procedure is generally executed by hand. For instance, one study lasted for about five years [102]. Additionally, establishing appropriate and viable genotypic variation may be difficult [103]. Nevertheless, it can be utilized to create new plant types with enhanced features. Crop breeding for Se biofortification uses the conventional procedure of cross-pollinating two separate plants to develop a new hybrid plant with a mix of features from both parents to promote Se absorption and translocation to edible portions of the crop. The researchers’ goal in the above-mentioned study was to breed Se-rich red glutinous rice and evaluate the concentration of Se and protein in various parts of the rice.
2.3. Se Biofortification Strategies in Livestock
In livestock, biofortification comprises employing agronomic or biotechnological techniques to increase the quantity of vital nutrients in edible sections of animals [104]. Se fertilization of farmlands, dietary supplementation via feed concentrate rations, and direct administration, including injections, are viable supplementation techniques. High Se feed concentrations are expected to raise Se concentrations in livestock. Animals can generate a variety of selenoproteins, including glutathione peroxidase, selenoprotein P, selenoprotein W, thioredoxin reductase, and other iodothyronine deiodinases, using absorbed selenium forms [105]. However, excessive Se ingestion in livestock (5–50 mg per kg of mass) may cause alkali disease, characterized by hoof deformities, a lack of vitality, anemia, and stiffness [106].
This entry is adapted from the peer-reviewed paper 10.3390/agriculture13020416
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