Selenium in Plants

The question of whether or not selenium is a micronutrient for plants is still considered unresolved (3). Selenium has not been classified as an essential element for plants, but its role as a beneficial element in plants that are able to accumulate large amounts of it has been considered (11). Uptake and accumulation of selenium by plants is determined by the form and concentration of selenium, the presence and identity of competing ions, and affinity of a plant species to absorb and metabolize selenium (10). Variation in selenium contents of plants seems to exceed that of nearly every other element (12). Nonconcentrator or nonaccumulator plant species will accumulate >25 mg Se kg^-1 dry weight. Most crops species such as grains, grasses, fruits, vegetables, and many weed species are considered nonconcentrators (8,13). Secondary absorbers normally grow in areas with low to medium soil-selenium concentrations and can accumulate from 25 to 100 mg Se kg^-1 dry weight. They belong to a number of different genera, including Aster, Atriplex, Castelleja, Grindelia, Gutierrezia, Machaeranthera, and Mentzelia. The primary indicator or selenium-accumulator species can accumulate from 100 to 10,000 mg Se kg^-1 dry weight. This group includes species of Astragalus, Machaeranthera, Haplopappus, and Stanleya (14). These plant species are suspects for causing acute selenosis, or selenium toxicity, of range animals that consume the plants as forages (10,15). Selenium-accumulator plants can contain 100 times more selenium than nonaccumulator plants when grown on the same soil (16). Surveys of selenium concentrations in crops reveal that areas producing low-selenium crops (<0.1 mg Se kg^-1) are more common than those producing crops with toxic selenium levels (>2 mg Se kg^-1) (16).

Uptake

Selenium can be absorbed by plants as inorganic SeO₄^2- or SeO₃^2- or as organic selenium compounds such as the selenoamino acid, selenomethionine (Se-Met) (10). Selenate and organic selenium forms are taken up actively by plant roots, but there is no evidence that SeO₃^2- uptake is mediated by the same process (3). Because of the close chemical and physical similarities between selenium and sulfur, their uptake by plants is very similar. Sulfur is absorbed actively by plants, mainly as SO₄^2-. The controlling enzymes for sulfur uptake are sulfur catabolic enzymes such as aryl sulfatase, choline sulfatase, and various S permeases (3,17,18). Uptake of SO₄^2- and SeO₄^2- was shown to be controlled by the same carrier with a similar affinity for both ions (19). This action demonstrated competition between SO₄^2- and SeO₄^2- for the same binding sites on these permeases (20,21).

Many studies have demonstrated an antagonistic relationship for uptake between SeO₄^2- and SO₄^2- (10,19,22-25). When SeO₄^2- is present in high concentrations, it can competitively inhibit SO₄^2- uptake. Adding SeO₄^2- lowered SO₄^2- absorption and transport in excised barley (Hordeum vulgare L.) roots. Conversely, adding SO₄^2- lowered SeO₄^2- absorption and transport (19,26). These studies involved an SeO₄^2-/SO₄^2- ratio of 1:1. In a preliminary solution culture experiment, an SeO₄^2-/SO₄^2- ratio of 1:3 resulted in the death of onion (Allium cepa L.) plant within 6 weeks (D.A. Kopsell and W.M. Randle, University of Georgia, unpublished results, 1994). When the SeO₄^2-/SO₄^2- ratio was lowered to 1:500 or 1:125 in solution culture, Kopsell and Randle (27) reported significant increases in SO₄^2- uptake by whole onion plants. Increasing SO₄^-2 levels from 0.25 to 10 mM in solution culture inhibited SeO₄^2- uptake of broccoli (Brassica oleracea var. botrytis L.), Indian mustard (Brassica juncea Czern.), sugarbeet (Beta vulgaris L.), and rice (Oryza sativa L.) by 90% (22). Applications of gypsum (CaSO₄2H₂O) at the rates of 5.6 to 16.8 t ha^-1 reduced selenium uptake in alfalfa (Medicago sativa L.) and oats (Avena sativa L.) grown on flyash landfill soils (28).

Although phosphate (H2PO₄^-) is not expected to affect SeO₄^2- uptake because of the chemical dissimilarities of the two radicals, the relationship between phosphate additions and selenium levels in plants has been inconsistent (9,10,29). Hopper and Parker (29) reported that a 10-fold increase (up to 200 µM) in phosphate solution culture decreased the selenium content of ryegrass (Lolium perenne L.) shoots and roots by 30 to 50% if selenium was supplied as SeO₃. In contrast, Carter et al. (30) reported that applying up to 160 kg P ha^-1 either as H3PO₄ or concentrated superphosphate to Gooding sandy loam increased selenium concentrations in alfalfa.

Selenate can accumulate in plants to concentrations much greater than that of selenium in the surrounding medium. In contrast, SeO₃^2- did not accumulate to levels surpassing the selenium levels of the external environment (31). When broccoli, Indian mustard, and rice were grown in the presence of SeO₄^2-, SeO₃^2-, or selenomethionine (Se-Met), plants accumulated the greatest amount of shoot selenium when selenium was supplied as SeO₄^2-, followed by those provided with Se-Met (22). In the same study, sugarbeet (Beta vulgaris L.) accumulated the most shoot-Se when treated with Se-Met (22). Broccoli, swiss chard (Beta vulgaris var. cicla L.), collards (Brassica oleracea var. acephala D.C.), and cabbage (Brassica oleracea var. capitata L.) grown in soil treated with 4.5 mg SeO₃^2- kg^-1 or 4.5mg SeO₄^2- kg^-1 had a tissue concentration of Se in the range from 0.013 to 1.382 g Se kg^-1 dry weight and absorbed 10 times the amount of selenium if treated with SeO₄^2- than with SeO₃^2- (32). When roots of bean (Phaseolus vulgaris L.) were incubated in 5mmol m^-3 Na₂SeO₃ or 5mmol m^-3 Na₂SeO₄ for 3 h, there was no significant difference in selenium accumulation, but distribution within the plant was different (33). In contrast, time-dependent kinetic studies showed that Indian mustard absorbed SeO₄^2- up to 2-fold faster than SeO₃^2- (34).

Increasing levels of selenium in plants may act to suppress the tissue concentrations of nitrogen, phosphorus, and sulfur. It can also inhibit the absorption of several heavy metals, especially manganese, zinc, copper, iron, and cadmium (35). This detoxifying effect of selenium has been demonstrated as reducing cadmium effects on garlic (Allium sativum L.) cell division (36). In contrast, the application of nitrogen, phosphorus, or sulfur is known to detoxify selenium. This effect may be due to either lowering of selenium uptake by the roots or to establishment of a safe ratio of selenium to other nutrient elements (35).

Selenomethionine was readily taken up by wheat (Triticum aestivum L.) seedlings, and the uptake followed a linear pattern in response to increasing selenium solution concentrations up to 1.0 µM (37). Western wheatgrass (Pascopyrum smithii L�ve) also showed linear selenium uptake with Se-Met solution concentrations up to 0.6 mg Se L^-1 (38). Results from Ba�uelos et al. (39) showed that alfalfa accumulated selenium in plant tissues when selenium-laden mustard plant tissue was added to the soil. These studies provide evidence that organic selenium compounds in the soils may become available sources of selenium (40).

Genetic differences for selenium uptake and accumulation within species have also been reported. In 1939, Trelease and Trelease reported that cream milkvetch (cream locoweed, Astragalus racemosus Pursh.), a selenium-accumulator, produced 3.81 g dry weight in solution culture with 9 mg Na₂SeO₃ L^-1, whereas ground plum (A. crassicarpus Nutt.), a nonaccumulator, produced only 0.20 g dry weight (41). Shoots of different land races of Indian mustard grown hydroponically in the presence of 2.0 mg Na₂SeO₄ L^-1 ranged from 501 to 1092 mg Se kg^-1 dry matter, whereas shoots grown in soil culture at 2.0 mg Na₂SeO₄ kg^-1 concentration ranged from 407 to 769 mg Se kg^-1 dry matter (42). Total accumulation of selenium in onion bulb tissue ranged from 60 to 113 µg Se g^-1 dry weight among 16 different cultivars responding to 2.0 mg Na₂SeO₄ L^-1 nutrient solution (43).

Metabolism

The incorporation of SeO₄^2- into organic compounds in plants occurs in the leaves (44). In a similar manner, SO₄^2- is reduced to sulfide (S^2-) in the leaves before being assimilated into the S-containing amino acid, cysteine (45). After SO₄^2- enters the cell it can be bound covalently in different secondary metabolites or immediately reduced and assimilated (46). Selenate is assimilated in the same metabolic pathways as SO₄^2-. Discrimination between SO₄^2- and SeO₄^2- was noted to occur at the level of amino acid incorporation into proteins. Uptake ratios between SO₄^2- and SeO₄^2- remained constant over a 60-h period for excised barley roots, but the ratio of S/Se decreased for free amino acid content and increased for proteins during assimilation (24).

In a series of solution-culture experiments with corn (Zea mays L.), Gissel-Neilsen (47) reported immediate selenium uptake and translocation to the leaves. Xylem sap contained 80 to 90% of ⁷⁵Se supplied as SeO₃ in amino-acid form, whereas 90% of ⁷⁵Se supplied as SeO₄ was recovered unchanged (47). In the leaves, selenate is converted into adenosine phosphoselenate (APSe) by ATP sulfurylase (Figure 18.1). In a similar fashion, SO₄^2- is first activated by ATP sulfurylase to form adenosine phosphosulfate (48). It has been suggested that ATP sulfurylase is not only the rate-limiting enzyme controlling the reduction of SO₄^2- (46), but it also appears to be the rate-limiting step in reduction of SeO₄^2- to SeO₃^2- (34,49). Overexpression of ATP sulfurylase in Indian mustard increased reduction of supplied SeO₄^2- (49). Following reduction of SeO₄^2-, APSe is converted into SeO₃^2-. Selenite is coupled to reduced glutathione (GSH), a sulfur-containing tripeptide to form a selenotrisulfide. Selenotrisulfide is reduced first to selenoglutathione and then to Se^2-. Selenide reacts with O-acetylserine to form selenocysteine (Se-Cys), which is further converted into Se-Met via selenocystathionine and selenohomocysteine (40). Ng and Anderson (50) reported that cysteine synthase enzymes extracted from selenium accumulator and nonaccumulator plants utilize Se^2- as an alternative substrate to S^2- to form Se-Cys in lieu of cysteine and that the affinity for Se^2- was substantially greater than for S^2-.

Volatilization

Biological methylation of selenium to produce volatile compounds occurs in plants, animals, fungi, bacteria, and microorganisms (9). The predominant volatile selenium species is dimethylselenide, which is less toxic (1/500 to 1/700) than the inorganic selenium species (51). Plant species differ in their rates of selenium volatilization, and these rates are correlated with tissue selenium concentrations (52). The ability of plants to accumulate selenium is a good indicator of their potential volatilization rate. It was reported that selenium was more readily transported to the shoots of an accumulator plant (Astragalus bisulcatus A. Gray), whereas a barrier to selenium movement to the shoots was seen in the nonaccumulator plant, western wheatgrass (Pascopyrum smithii A. Löve) (38). However, in broccoli, the roots were shown to be the primary site for selenium volatilization (53). In an earlier experiment with broccoli, Zayed and Terry (54) revealed that a decrease in selenium volatilization was observed with increased application of SO₄^2- fertilizer.

FIGURE 18.1 Proposed pathway for formation of the two Se-amino acids, Se-cysteine and Se-methionine in plants. (Abbreviations: APSe, adenosine 5'-selenophosphate; GSH, reduced glutathione; GSSeSG, selenotrisulphide; GSSeH, selenoglutathione; O-AS, acetylserine.) From A. L�uchli. Bot. Acta 106:455- 468, 1993.

Volatilization of selenium is also influenced by the chemical form of selenium in the growing medium. The rate of selenium volatilization of a hybrid poplar (Populus tremula x alba) was 230- fold higher in sand culture if 20 µM Se was supplied as Se-Met than as SeO₃^2-, and volatilization from SeO₃^2- was 1.5-fold that from SeO₄^2- (49). Selenium volatilization by shoots of broccoli, Indian mustard, sugarbeet, or rice supplied with Se-Met was also many folds higher than that from plants supplied with SeO₃^2- (22). In Indian mustard, Se-volatilization rates were doubled or tripled in sand culture amended with 20 µM SeO₃^2- relative to rates with 20 µM SeO₄^2- (34). These data indicate that selenium volatilization from SeO₄^2- is limited by the rate of SeO₄^2- reduction as well as by the form of selenium available (22,34).

Phytoremediation

An increasing problem with irrigation agriculture in arid and semi-arid regions is the appearance of selenium in soils, ground water, and drainage effluents (12,55,56). The greatest concerns for selenium contamination come in areas where water systems drain seleniferous soils. One area of the United States that has come under close investigation because of elevated levels of selenium in the water is the San Joaquin Valley in California (57,58). Selenium enters the groundwater as soluble selenites and selenates and as suspended particles of sparingly soluble and organic forms of the element (8). The mobility of selenium in groundwater is related to its speciation in the aqueous solution, sorption properties of the substrate, and solubility of the solid phases (59). The ability of certain plants to take up, accumulate, and volatilize selenium has an important application in phytoremediation of selenium from the environment (3). Phytoremediation of selenium from contaminated soils is more practical and economical than its physical removal (60). Bioaccumulation of selenium in wetland habitats is also a problem and results in selenium toxicity to wildlife (61). There is a danger of selenium re-entering the local ecosystem if plant tissues that have accumulated selenium are consumed by wildlife or allowed to degrade (62).

The search for germplasm with the potential for effective phytoremediation has begun (63). The most ideal plant species for selenium phytoremediation should have the ability for rapid establishment and growth, ability to accumulate or volatilize large amounts of selenium, tolerate salinity and elevated soil boron, and develop large amounts of biomass on high-selenium soils (3,62–64). Indian mustard was more efficient at accumulating selenium than milkvetch (Astragalus incanus L.), Australian saltbush (Atriplex semibaccata R. Br.), old man saltbush (Atriplex nummularia Lindl.), or tall fescue (Festuca arundinacea Schreb.) when grown in potting soil amended with 3.5 mg Se6> kg^-1 or 3.5 mg Se4> kg^-1 as selenate or selenite (60).

Two of the options available once selenium is phytoextracted from contaminated soils are volatilization of methylated Se forms or harvest and removal of selenium-enriched plant biomass. Plant species with a high affinity for phytovolatilization could remove selenium from the environment by releasing it into the atmosphere, where it is dispersed and diluted by air currents (3,11,62). Most of the selenium in the air comes from windblown dusts, volcanic activity, and discharges from human activities such as the combustion of fossil fuels, smelting and refining of nonferrous metals, and the manufacturing of glass and ceramics (8). The large particulate and aerosol forms of selenium generally are not readily available for intake by plants or animals. When 15 crop species were grown in solution culture with 20µM SeO₄^2-, rice, broccoli, or cabbage volatized 200 to 350µg Se m^-2 leaf area day^-1, whereas sugar beet, bean, lettuce (Lactuca sativa L.), or onion volatized less than 15µg Se m^-2 leaf area day^-1 (52). One of the proposed disposal schemes for selenized plants from phytoremediation is as a source of forage for selenium-deficient livestock (3,60) Accurate determination of selenium levels as well as other trace elements in plant tissues and the use of other forages in a blended mixture would be needed to ensure proper dietary selenium levels in animal feeds (60,62).