Kumar et al. (2015)''''''''s review on arsenic (As) detoxification in plants and related biotechnology provides current updates about the As metabolism and recent developments in engineering As-tolerant and low-As plants. Although we agree with most of the viewpoints expressed in the review, we would like to clarify the naming and functions of ACR2, arsenate reductase, and ACR3 arsenite efflux transporter in plants.

    In the review, the authors incorrectly referred ACR3 as arsenate reductase (see Table 1, No. 18, 19 & 24); in addition, using “ACR” to represent “arsenate reductase” is inappropriate (Fig. 1 and Section 3.1: “Detailed analysis of this QTL suggested that this encodes a novel plant ACR.”). Arsenate reductases are enzymes with catalytic activities of reducing arsenate to arsenite and one class of such enzymes is represented by ACR2 proteins.

    The name “ACR” comes from yeast Saccharomyces cerevisiae, which is a model system to study As resistance in eukaryotes. Bobrowicz et al. (1997) found a 4.2 kb region from S. cerevisiae chromosome XVI, which confers strong arsenite (AsIII) resistance. Three ACR (Arsenic Compounds Resistance) genes were found in this 4.2 kb region, namely ACR1, ACR2, and ACR3. 

    ACR1 encodes a transcriptional factor that regulates ACR2 and ACR3 transcription, possibly by directly sensing cellular As levels. Disruption of the ACR1 gene confers arsenite and arsenate (AsV) hypersensitivity (Bobrowicz et al., 1997; Ghosh et al., 1999). ACR2 encodes an arsenate reductase protein, which is required for arsenate but not arsenite resistance (Mukhopadhyay and Rosen, 1998). ACR3 encodes a plasma membrane arsenite efflux transporter that exports arsenite out of the cell into the external medium (Wysocki et al., 1997). Thus, this gene cluster provides a mechanism for sensing, reduction and efflux of As in yeast (Ghosh et al., 1999).

    In plants, ACR2 represents arsenate reductase with sequence homology to yeast ACR2 (ScACR2), like AtACR2 from Arabidopsis thaliana (also named AtASR and AtCDC25) (Landrieu et al., 2004a, 2004b), PvACR2 from Pteris vittata (Ellis et al., 2006) and OsACR2.1 and OsACR2.3 from Oryza sativa (Duan et al., 2007). All these ACR2 enzymes contain the conserved HCX₅R active site. However, not all arsenate reductases possess the conserved active site of the canonical ScACR2 and thus may not belong to the ACR2-like arsenate reductase. Rathinasabapathi et al. (2006) reported that a cTPI (cytosolic Triosephosphate Isomerase) from P. vittata increased arsenate resistance in an Escherichia coli strain defficient in arsenate reductase and suggested that this enzyme might function as an arsenate reductase. Recently, a quantitative trait locus that encodes a novel arsenate reductase critical for arsenic tolerance in plants was identified independently by two research groups. This arsenate reductase lacks HCX₅R and was termed ATQ1 (Arsenate Tolerance QTL1) (also called ARQ1 for Arsenate Reductase QTL1) and HAC1 (High Arsenic Content 1), respectively (Chao et al., 2014; Sanchez-Bermejo et al., 2014). This arsenate-reducing enzyme controls arsenic accumulation in plants and keeps arsenic low in the shoots, which provides an important new resource to engineer low-As rice (Meadows, 2014).

    Compared to plant ACR2 or plant arsenate reductase, plant arsenite efflux transporter ACR3 has rarely been reported. This is mainly because ACR3 is missing in flowering plants (angiosperms), including Arabidopsis and O. sativa. ACR3 genes are found only in the genomes of bacteria, fungi, moss, ferns and gymnosperms (Indriolo et al., 2010). In the As hyperaccumulating fern P. vittata, PvACR3 localizes to the vacuolar membrane, indicating that it likely extrudes arsenite into vacuoles for sequestration (Indriolo et al., 2010). However, in model plant Arabidopsis, PvACR3 localizes to the plasma membrane and increases arsenite efflux into external medium (Chen et al., 2013). Actually, PvACR3 is the only plant ACR3 that has been well studied. However, the subcellular localization and function of PvACR3 are still unclear and merit further study. PvACR3;1 is another reported P. vittata ACR3, but it is not characterized and its function is unknown. Still, considering its effectiveness in eliminating As from plants, we envision that ACR3 has a great potential for engineering low-As crops for food safety.