Arsenic Methylation in Arabidopsis thaliana Expressing an Algal Arsenite Methyltransferase Gene Increases Arsenic Phytotoxicity

Abstract

Arsenic (As) contamination in soil can lead to elevated transfer of As to the food chain. One potential mitigation strategy is to genetically engineer plants to enable them to transform inorganic As to methylated and volatile As species. In this study, we genetically engineered two ecotypes of Arabidopsis thaliana with the arsenite (As(III)) S-adenosylmethyltransferase (arsM) gene from the eukaryotic alga Chlamydomonas reinhardtii. The transgenic A. thaliana plants gained a strong ability to methylate As, converting most of the inorganic As into dimethylarsenate [DMA(V)] in the shoots. Small amounts of volatile As were detected from the transgenic plants. However, the transgenic plants became more sensitive to As(III) in the medium, suggesting that DMA(V) is more phytotoxic than inorganic As. The study demonstrates a negative consequence of engineered As methylation in plants and points to a need for arsM genes with a strong ability to methylate As to volatile species.

Understanding arsenic dynamics in agronomic systems to predict and prevent uptake by crop plants

“…Another promising strategy is based on expressing the arsenate efflux transporter from yeast ( Saccharomyces cerevisiae ) in rice, which can reduce arsenic accumulation in brown rice by 20%. A less successful idea to methylate sodium arsenite to DMA by expressing an algal arsM gene in Arabidopsis resulted in lethal phytotoxicity 205 , suggesting that arsenic methylation in plants can only be an effective detoxification strategy if volatile arsines are the end point of the methylation.…”

Arsenic methylation by a genetically engineered Rhizobium-legume symbiont

“…Thus DMAs(V) builds up and is only slowly methylated a third time to the volatiles species TMAs(III). This interpretation is supported by the observation that Chlamydomonas reinhardtii has only limited ability to volatilize As (Tang et al 2016b). The efficiency of As methylation and volatilization varies widely among different microorganisms (Qin et al 2009; Qin et al 2006; Wang et al 2014; Zhang et al 2015), possibly due to differences in microbial growth and the catalytic ability of ArsM enzymes.…”

“…DMAs(V) is highly mobile during the root to shoot translocation (Li et al 2009; Raab et al 2007; Tang et al 2016a; Ye et al 2010), so it is possible that methylated As species produced in the nodules are efficiently transported to the above ground tissues with little retention in roots. For example, Tang et al (2016b) showed that transgenic A. thaliana expressing CrarsM contained most DMAs(V) in the shoots but very little DMAs(V) in the roots, suggesting that DMAs(V) had been efficiently transported to the shoots.…”

“…The extracted total RNA was reverse transcribed as cDNA with a TaKaRa reverse transcription kit (TaKaRa, Dalian, China). Semi-quantitative RT-PCR was performed using a primer pair (forward: 5′-ATGCCCACTGACATGCAAGAC-3′ and reverse: 5′-TCACCCGCAGCAGCGCGCCG-3′) targeting CrarsM gene as described by Tang et al (2016b). The DNA gyrase subunit B ( gyrB ) gene was used as the internal reference gene and amplified using a primer pair (forward: 5′-GCTGTCCGTCTGGTTGAA-3′ and reverse: 5′-CGCTGAGGGATGTTGTTGG-3′).…”

“…Transgenic rice expressing a bacterial arsM gene from the bacterium Rhodopseudomonas palustris showed enhanced As volatilization, although the overall efficiency of As methylation was very low (Meng et al 2011). In a recent study, the algal CrarsM gene was expressed in Arabidopsis thaliana , and the transgenic plants gained the ability to methylate and volatilize As, but appeared to suffer from DMAs(V) toxicity (Tang et al 2016b). …”