Genome-wide Association Mapping Identifies a New Arsenate Reductase Enzyme Critical for Limiting Arsenic Accumulation in Plants

<div><p>Inorganic arsenic is a carcinogen, and its ingestion through foods such as rice presents a significant risk to human health. Plants chemically reduce arsenate to arsenite. Using genome-wide association (GWA) mapping of loci controlling natural variation in arsenic accumulation in <i>Arabidopsis thaliana</i> allowed us to identify the arsenate reductase required for this reduction, which we named High Arsenic Content 1 (HAC1). Complementation verified the identity of <i>HAC1</i>, and expression in <i>Escherichia coli</i> lacking a functional arsenate reductase confirmed the arsenate reductase activity of HAC1. The HAC1 protein accumulates in the epidermis, the outer cell layer of the root, and also in the pericycle cells surrounding the central vascular tissue. Plants lacking <i>HAC1</i> lose their ability to efflux arsenite from roots, leading to both increased transport of arsenic into the central vascular tissue and on into the shoot. HAC1 therefore functions to reduce arsenate to arsenite in the outer cell layer of the root, facilitating efflux of arsenic as arsenite back into the soil to limit both its accumulation in the root and transport to the shoot. Arsenate reduction by HAC1 in the pericycle may play a role in limiting arsenic loading into the xylem. Loss of <i>HAC1</i>-encoded arsenic reduction leads to a significant increase in arsenic accumulation in shoots, causing an increased sensitivity to arsenate toxicity. We also confirmed the previous observation that the ACR2 arsenate reductase in <i>A. thaliana</i> plays no detectable role in arsenic metabolism. Furthermore, <i>ACR2</i> does not interact epistatically with <i>HAC1</i>, since arsenic metabolism in the <i>acr2 hac1</i> double mutant is disrupted in an identical manner to that described for the <i>hac1</i> single mutant. Our identification of <i>HAC1</i> and its associated natural variation provides an important new resource for the development of low arsenic-containing food such as rice.</p>