Naming and functions of ACR2, arsenate reductase, and ACR3 arsenite efflux transporter in plants (correspondence on: Kumar, S., Dubey, R.S., Tripathi, R.D., Chakrabarty, D., Trivedi, P.K., 2015. Omics and biotechnology of arsenic stress and detoxification in plants: current updates and prospective. Environ Int. 74:221-230.).

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 reductasewith 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 deficient in arsenate reductase and suggested that this enzyme might