Three Genes Define a Bacterial-Like Arsenic Tolerance Mechanism in the Arsenic Hyperaccumulating Fern Pteris vittata

Arsenic is a carcinogenic contaminant of water and food and a growing threat to human health in many regions of the world. This study focuses on the fern Pteris vittata (Pteridaceae), which is extraordinary in its ability to tolerate and hyperaccumulate very high levels of arsenic that would kill any other plant or animal outside the Pteridaceae. Here, we use RNA-seq to identify three genes (GLYCERALDEHYDE 3-PHOSPHATE DEHYDROGENASE (PvGAPC1), ORGANIC CATION TRANSPORTER 4 (PvOCT4), and GLUTATHIONE S-TRANSFERASE (PvGSTF1) that are highly upregulated by arsenic and are necessary for arsenic tolerance, as demonstrated by RNAi. The proteins encoded by these genes have unexpected properties: PvGAPC1 has an unusual active site and a much greater affinity for arsenate than phosphate; PvGSTF1 has arsenate reductase activity; and PvOCT4 localizes as puncta in the cytoplasm. Surprisingly, PvGAPC1, PvGSTF1, and arsenate localize in a similar pattern. These results are consistent with a model that describes the fate of arsenate once it enters the cell. It involves the conversion of arsenate into 1-arseno-3-phosphoglycerate by PvGAPC1. This "chemically trapped" arsenate is pumped into specific arsenic metabolizing vesicles by the PvOCT4 protein. Once inside these vesicles, 1-arseno-3-phosphoglycerate hydrolyses to release arsenate, which is then reduced by PvGSTF1 to arsenite, the form of arsenic stored in the vacuoles of this fern. This mechanism is strikingly similar to one recently described Pseudomonas aeruginosa, whose tolerance to arsenic also involves the biosynthesis and transport of 1-arseno-3-phosphoglycerate, indicating that P. vittata has evolved a simple, bacterial-like mechanism for arsenic tolerance.