Arsenic Methylation by a Genetically Engineered -legume Symbiont

Background And Aims: Arsenic (As) is one of the most widespread environmental contaminants. The aim of our study was to test a novel bioremediation system based on the symbiosis between leguminous plant and genetically engineered rhizobia.

Methods: The arsenite [As(III)] -adenosylmethionine methyltransferase gene () from the alga was inserted into the chromosome of strain R3. The As methylation ability of the recombinant was tested under free living conditions and in symbiosis with red clover plants. Arsenic speciation was determined using high-performance liquid chromatography-inductively coupled plasma mass spectrometry.

Results: Under free-living conditions, -recombinant gained the ability to methylate As(III) to methylated arsenicals, including methylarsenate [MAs(V)], dimethylarsenate [DMAs(V)] and trimethylarsine oxide [TMAs(V)O]. Red clover plants were inoculated with either control (non-recombinant) or -recombinant and exposed to 5 or 10 μM arsenite. No methylated As species were detected in red clover plants inoculated with control . In contrast, all three methylated species were detected in both the nodules and the shoots when the recombinant established symbiosis with red clover, accounting for 74.7-75.1% and 29.1-42.4% of the total As in the two plant tissues, respectively. The recombinant symbiont also volatilized small amounts of As.

Conclusions: The present study demonstrates that engineered rhizobia expressing an algal gene can methylate and volatilize As, providing a proof of concept for potential future use of legume-rhizobia symbionts for As bioremediation.