Arsenic transformation and volatilization by arbuscular mycorrhizal symbiosis under axenic conditions

It is well known that arbuscular mycorrhizal (AM) fungi can enhance plant arsenic (As) resistance by influencing As uptake, translocation, and speciation; however, As transformation and volatilization by an entire plant inoculated with AM fungus remains uninvestigated. In the present study, AM symbiosis of Rhizophagus irregularis with unbroken Medicago sativa was successfully established in vitro. Afterwards, five concentrations of arsenate were applied to the culture media. The results showed that AM inoculation could methylate inorganic As into dimethylarsinic acid (DMA), dimethylarsine (DMAsH), and trimethylarsine (TMAs), which were detected in the plants, media, or air. Volatile As, accounting for a small proportion of total organic As, appeared under high arsenate exposure, accompanied by remarkable upregulation of root RiMT-11, an arsenite methyltransferase gene in R. irregularis. In addition, AM colonization significantly increased arsenite percentages in plant tissues and external media. Regardless of As species, AM inoculation tended to release the transformed As into the environment rather than transfer them to plant tissues. Our present study, for the first time, comprehensively verified As methylation, volatilization, and reduction by AM fungus associated with the entire plant under absolute axenic conditions and gained a deeper insight into As metabolism in AM symbionts.