Arsenic biotransformation and volatilization in transgenic rice

Abstract

• Biotransformation of arsenic includes oxidation, reduction, methylation, and conversion to more complex organic arsenicals. Members of the class of arsenite (As(III)) S-adenosylmethyltransferase enzymes catalyze As(III) methylation to a variety of mono-, di-, and trimethylated species, some of which are less toxic than As(III) itself. However, no methyltransferase gene has been identified in plants. • Here, an arsM gene from the soil bacterium Rhodopseudomonas palustris was expressed in Japonica rice (Oryza sativa) cv Nipponbare, and the transgenic rice produced methylated arsenic species, which were measured by inductively coupled plasma mass spectrometry (ICP-MS) and high-performance liquid chromatography-inductively coupled plasma mass spectrometry (HPLC-ICP-MS). • Both monomethylarsenate (MAs(V)) and dimethylarsenate (DMAs(V)) were detected in the roots and shoots of transgenic rice. After 12 d exposure to As(III), the transgenic rice gave off 10-fold greater volatile arsenicals. • The present study demonstrates that expression of an arsM gene in rice induces arsenic methylation and volatilization, theoretically providing a potential stratagem for phytoremediation.

Fig. 1

Identification of transgenic rice. (a) Northern blot analysis of T0 generation rice. WT, wild type, the nontransgenic control; p35S, transgenic rice with p35S vector; lanes 1, 2, 3 and 5, independent positive transgenic rice lines expressing arsM at the transcriptional level; lanes 4 and 6, negative transgenic rice lines expressing no arsM at the transcriptional level. (b) Semi-quantitative PCR analysis of T3 generation rice with actin1 gene as a constitutive internal standard. WT, wild type, the nontransgenic control; p35S, transgenic rice with p35S vector; lanes 1–10, individual positive transgenic rice plants with p35S-arsM.

Fig. 2

HPLC-ICP-MS chromatograms of arsenic speciation in the root of transgenic rice, with the relative amounts of arsenic expressed as CPS. The standards (solid line) contain 10 μg l⁻¹ of mixed As(III), DMAs(V), MAs(V) and As(V). The control (dotted line) contains 1% HNO₃ only. With 24-h exposure to 10 μM As(III) (dashed line) or 50 μM As(V) (dash-dot line), DMAs(V) and MAs(V) were detected in the root of the transgenic rice. No DMAs(V) or MAs(V) was detected in the root of the nontransgenic control (dash-dot-dot line).

Fig. 3

Quantification of volatile arsenicals in eluates of chemo-traps from transgenic rice with 12-d exposure to 10 μM As(III). Data are means ± SE (n = 2 for blank, n = 4 for WT, n = 10 for arsM). Means followed by the same letter are not significantly different at P < 0.05 using One-Way ANOVA in SPSS. Open bars, blank (chemo-trap control with no rice); hatched bars, WT (wild type, the nontransgenic control); cross-hatched bars, arsM (transgenic rice); SE, standard error.