Reduction and coordination of arsenic in Indian mustard

Abstract

The bioaccumulation of arsenic by plants may provide a means of removing this element from contaminated soils and waters. However, to optimize this process it is important to understand the biological mechanisms involved. Using a combination of techniques, including x-ray absorption spectroscopy, we have established the biochemical fate of arsenic taken up by Indian mustard (Brassica juncea). After arsenate uptake by the roots, possibly via the phosphate transport mechanism, a small fraction is exported to the shoot via the xylem as the oxyanions arsenate and arsenite. Once in the shoot, the arsenic is stored as an As(III)-tris-thiolate complex. The majority of the arsenic remains in the roots as an As(III)-tris-thiolate complex, which is indistinguishable from that found in the shoots and from As(III)-tris-glutathione. The thiolate donors are thus probably either glutathione or phytochelatins. The addition of the dithiol arsenic chelator dimercaptosuccinate to the hydroponic culture medium caused a 5-fold-increased arsenic level in the leaves, although the total arsenic accumulation was only marginally increased. This suggests that the addition of dimercaptosuccinate to arsenic-contaminated soils may provide a way to promote arsenic bioaccumulation in plant shoots, a process that will be essential for the development of an efficient phytoremediation strategy for this element.

Figure 1

Edge fitting of selected arsenic K near-edge spectra from Indian mustard. Sample spectra were fitted with combinations of spectra from various model compounds. The results of these fits are presented as percentages. A, Roots from plants treated with 250 μm arsenate for 5 d fitted with 97% As^III-tris-glutathione at pH 5.5 (a) and 3% arsenate at pH 9.0 (b). B, Xylem sap exudate from plants treated with 25 μm arsenate for 5 d fitted with 41% arsenate at pH 9.0 (a) and 59% arsenite at pH 5.5 (b). C, Leaves from plants treated with 250 μm arsenate and 250 μm dimercaptosuccinate for 2 d fitted with 33% As^III-tris-glutathione at pH 5.5 (a) and 67% As^III-dimercaptosuccinate at pH 5.5 (b). In all panels the dots, solid line, and dotted line below show the data, the best fit, and the residual, respectively.

Figure 2

Arsenic K near-edge spectra of aqueous solutions of arsenite and arsenate. Arsenite is shown at pH 5.5 (solid line) and pH 12 (dashed line), and arsenate at pH 4.5 (solid line) and pH 9 (dashed line).

Figure 3

Arsenic K near-edge spectra of As^III-tris-glutathione at pH 5.5, arsenite at pH 5.5, dimethylarsinate at pH 9.0, and arsenate at pH 9.0.

Figure 4

Arsenic K near-edge spectra of root and leaf of Indian mustard treated with 25 μm arsenate for 5 d. The spectra are compared with those of As^III-tris-glutathione at pH 5.5 [As^III(SG)₃] and arsenate at pH 9.0.

Figure 5

Arsenic K-edge EXAFS (A) and corresponding Fourier transforms (B) of Indian mustard and arsenic model compounds. Top to bottom: Root of Indian mustard treated with 25 μm arsenate for 5 d; leaves of Indian mustard treated with 250 μm arsenate for 5 d (note that leaves from the 25 μm arsenate treatment were too weak for EXAFS); As^III-tris-glutathione at pH 5.5 [As^III(SG)₃]; dimethylarsinate at pH 9.0 (DMA); arsenite at pH 5.5; and arsenate at pH 9.0. Solid lines show the data and the dashed lines the best fit (Table IV). Fourier transforms have been phase-corrected for the first shell (As-S or As-O). In A, the ordinate zero is indicated for each spectrum by a dotted line. Thus, the spectra are offset vertically but are plotted with the same relative scale.

Figure 6

Arsenic K near-edge spectra of root and leaf of Indian mustard treated with both 250 μm arsenate and 250 μm dimercaptosuccinate for 2 d compared with those of As^III-tris-glutathione at pH 5.5 and As^III-dimercaptosuccinate at pH 5.5. The inset shows the region around 11,880 eV on an expanded scale.