Chemical Form and Distribution of Selenium and Sulfur in the Selenium Hyperaccumulator Astragalus bisulcatus  

Abstract

In its natural habitat, Astragalus bisulcatus can accumulate up to 0.65% (w/w) selenium (Se) in its shoot dry weight. X-ray absorption spectroscopy has been used to examine the selenium biochemistry of A. bisulcatus. High concentrations of the nonprotein amino acid Se-methylseleno-cysteine (Cys) are present in young leaves of A. bisulcatus, but in more mature leaves, the Se-methylseleno-Cys concentration is lower, and selenate predominates. Seleno-Cys methyltransferase is the enzyme responsible for the biosynthesis of Se-methylseleno-Cys from seleno-Cys and S-methyl-methionine. Seleno-Cys methyltransferase is found to be expressed in A. bisulcatus leaves of all ages, and thus the biosynthesis of Se-methylselenoCys in older leaves is limited earlier in the metabolic pathway, probably by an inability to chemically reduce selenate. A comparative study of sulfur (S) and Se in A. bisulcatus using x-ray absorption spectroscopy indicates similar trends for oxidized and reduced Se and S species, but also indicates that the proportions of these differ significantly. These results also indicate that sulfate and selenate reduction are developmentally correlated, and they suggest important differences between S and Se biochemistries.Many selenium (Se) compounds are toxic to mammals at high concentrations, but Se is also an essential micronutrient, and low doses have been implicated in cancer prevention (Clark et al., 1996; Combs et al., 1997). Not all diets provide adequate Se, and an obvious and inexpensive way to provide Se may be to engineer food plants to accumulate higher levels of the element (Ip et al., 1994). The Se hyperaccumulator Astragalus species, such as Astragalus bisulcatus, may be an excellent source of genetic material from which to isolate genes to develop such plants. In the wild, A. bisulcatus can accumulate Se levels of up to 0.65% (w/w) dry weight in the shoots (Byers, 1936), predominantly as Se-methylseleno-Cys (Trelease et al., 1960), and similar results are readily obtained in plants grown hydroponically in the laboratory (Orser et al., 1999). Understanding Se uptake in A. bisulcatus might also allow the development of highly effective cultivars for phytoremediation (Salt et al., 1998).A critical step in the biotransformation of selenate is the initial two-electron reduction to selenite. Hyperaccumulating plants might achieve this in at least three different ways: by substituting selenate into the sulfate reduction pathway (reduction by ATP sulfurylase/adenyl sulfate (APS) reductase; Shrift, 1969;Setya et al., 1996), by substituting selenate into the nitrate uptake pathway (microbial nitrate reductases can reduce selenate; Sabaty et al., 2001), or by a specific selenate reductase. For nonhyperaccumulating plants, there is good evidence that selenate reduction occurs via substitution for sulfate in the ATP sulfurylase/APS reductase system, and that this is the ratelimiting step in selenate transformation (Shrift, 1969;Shaw and Anderson, 1974; Burnell, 1981; Pilon-Smits et al., 1999). In these spe...

Molecular Mechanisms of Selenium Tolerance and Hyperaccumulation in Stanleya pinnata

“…Species such as the Se accumulator Morinda reticulata that accumulate approximately 90% of total Se as SeCyst closely match the secondary accumulator S. albescens in their total Se accumulation when fed Se (Peterson and Butler, 1971). Moreover, SeCyst is a metabolic intermediate between SeCys and SeMet, and both amino acids can be toxic to plants when incorporated into protein (for review, see Pickering et al, 2003b). MeSeCys, on the other hand, is the result of a branching pathway that moves Se away from incorporation into protein.…”

“…Together, these findings indicate that the total Se composition in young leaves of the secondary accumulator S. albescens was 75% C-Se-C, 20% SeCys, and 5% selenate, with the free organic Se being found as SeCyst, the only C-Se-C compound detected. Since research has shown that as A. bisulcatus leaves age, Se speciation changes from mostly the C-Se-C form MeSeCys to selenate (Pickering et al, 2000(Pickering et al, , 2003b, we also analyzed the Se speciation of S. pinnata leaves of different ages. The Se speciation in old S. pinnata leaves did not change and was largely the same as it was in young leaves (Table III).…”

Cooperative Ethylene and Jasmonic Acid Signaling Regulates Selenite Resistance in Arabidopsis

“…These organisms contain selenoproteins, which require seleno-Cys (SeCys) in their active site (Stadtman, 1990(Stadtman, , 1996. To date, there is no evidence that higher plants need Se for survival, although Se has been proposed to be a beneficial element, especially for certain Se-tolerant hyperaccumulator species that grow on Se-rich soils; these include some species of Astragalus and Stanleya that are able to accumulate Se up to 1% of their dry weight (Feist and Parker, 2001;Pickering et al, 2003;Galeas et al, 2007).…”

Selenium induced selenocysteine methyltransferase gene expression and antioxidant enzyme activities in Astragalus chrysochlorus

“…It was thought that SMT is the key player in the accumulation of selenium and protection of the plants from the toxic effects (Neuhierl et al 1999). Pickering et al (2003) concluded that SMT gene expression was continuous in tissues of A. bisulcatus. In contrast, Lyi et al (2005) reported that broccoli SMT (BoSMT) expression is up-regulated by selenate treatment.…”

“…In our previous work, SMT cDNA sequences were isolated from Astragalus chrysochlorus (Çakir and Ari 2013). SMT was expressed continuously in the tissues of the Se hyperaccumulator Astragalus bisulcatus (Pickering et al 2003). While most plants cannot survive with high concentrations of Se, some of them can tolerate high concentrations and accumulate Se, like A. bisulcatus.…”