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Comparative Study
.2002 Feb;128(2):625-33.
doi: 10.1104/pp.010686.

Selenium assimilation and volatilization from selenocyanate-treated Indian mustard and muskgrass

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Comparative Study

Selenium assimilation and volatilization from selenocyanate-treated Indian mustard and muskgrass

Mark P de Souza et al. Plant Physiol.2002 Feb.

Abstract

Selenocyanate (SeCN(-)) is a major contaminant in the effluents from some oil refineries, power plants, and in mine drainage water. In this study, we determined the potential of Indian mustard (Brassica juncea) and muskgrass (a macroalga, Chara canescens) for SeCN(-) phytoremediation in upland and wetland situations, respectively. The tolerance of Indian mustard to toxic levels of SeCN(-) was similar to or higher than other toxic forms of Se. Indian mustard treated with 20 microM SeCN(-) removed 30% (w/v) of the Se supplied in 5 d, accumulating 554 and 86 microg of Se g(-1) dry weight in roots and shoots, respectively. Under similar conditions, muskgrass removed approximately 9% (w/v) of the Se supplied as SeCN(-) and accumulated 27 microg of Se g(-1) dry weight. A biochemical pathway for SeCN(-) degradation was proposed for Indian mustard. Indian mustard and muskgrass efficiently degraded SeCN(-) as none of the Se accumulated by either organism remained in this form. Indian mustard accumulated predominantly organic Se, whereas muskgrass contained Se mainly as selenite and organic Se forms. Indian mustard produced volatile Se from SeCN(-) in the form of less toxic dimethylselenide. Se volatilization by Indian mustard accounted for only 0.7% (w/v) of the SeCN(-) removed, likely because the biochemical steps in the production of dimethylselenide from organic Se were rate limiting. Indian mustard is promising for the phytoremediation of SeCN(-) -contaminated soil and water because of its remarkable abilities to phytoextract SeCN(-) and degrade all the accumulated SeCN(-) to other Se forms.

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Figures

Figure 1
Figure 1
Tolerance of Indian mustard seedlings to selenate,selenite, and SeCN−. Seedlings were germinatedin one-half-strength Murashige and Skoog agar containing thedifferent chemical forms of Se at concentrations of 0, 20, or 200μm. Vertical bars indicate 1se from themean,n = 24. Bars showing the same letter code withineach graph are not significantly different (P >0.05).
Figure 2
Figure 2
Se concentrations in tissues of Indian mustard andmuskgrass supplied with selenate (A), selenite (B), orSeCN− (C). All Se species were supplied at 20μm in hydroponic solution. Vertical bars indicate 1se from the mean,n = 4. Bars showing thesame letter code in all parts of the figure are not significantlydifferent from each other (P > 0.05). The biomass(fresh weight) of the plants did not change over the 5-d exposure toSe. The dry weights at the end of the 5-d period for selenate,selenite, and SeCN−-supplied Indian mustard were0.58 ± 0.21, 0.62 ± 0.14, and 0.60 ± 0.10,respectively, and for muskgrass were 0.40 ± 0.028, 0.37 ±0.028, 0.40 ± 0.046, respectively.
Figure 3
Figure 3
Se volatilization by Indian mustard (A) andmuskgrass (B) supplied with selenate, selenite, orSeCN−. All Se species were supplied at 20μm in hydroponic solution. Vertical bars indicate 1se from the mean,n = 4. The rates of Sevolatilization and the statistical differences between the differentlines are presented in Table I. The dry weights of the plants are shownin the legend to Figure 2.
Figure 4
Figure 4
Se K near-edge x-ray absorption spectra ofselenium accumulated by Indian mustard and muskgrass supplied with 20μm SeCN− (top) compared with thespectra for aqueous solutions of selenate, selenite,l-selenocystine (seleno-Cys dimer), and Se-Met, which wereused as Se standards.
Figure 5
Figure 5
Proposed pathway of SeCN−assimilation by Indian mustard to the volatile Se forms DMSe andH2Se. Also shown is a simplified version of theselenate assimilation pathway for Indian mustard (Terry et al., 2000).The numbers by the arrows represent the enzymes involved. The two majorrate-limiting enzymes for selenate assimilation to DMSe are ATPsulfurylase (1) and Met methyltransferase (MMT, 2). Enzyme 3 is a novelthiol methyltransferase (Attieh et al., 2000) that methylatesSeCN− to CH3SeCN. Enzyme 4is unknown or a plant homolog for the bacterial thiocyanate hydrolase.This enzyme degrades SeCN− to volatileH2Se or the Se2− thatenters the Se assimilation pathway for DMSe production.OCN− is detoxified by the enzyme cyanase (5),which has been cloned from Arabidopsis (Aichi et al., 1998).
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References

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