OsARM1, an R2R3 MYB Transcription Factor, Is Involved in Regulation of the Response to Arsenic Stress in Rice

Bioaccumulation of arsenic (As) in rice (<i>Oryza sativa</i>) increases human exposure to this toxic, carcinogenic element. Recent studies identified several As transporters, but the regulation of these transporters remains unclear. Here, we show that the rice R2R3 MYB transcription factor OsARM1 (ARSENITE-RESPONSIVE MYB1) regulates As-associated transporters genes. Treatment with As(III) induced <i>OsARM1</i> transcript accumulation and an OsARM1-GFP fusion localized to the nucleus. Histochemical analysis of <i>OsARM1pro::GUS</i> lines indicated that <i>OsARM1</i> was expressed in the phloem of vascular bundles in basal and upper nodes. Knockout of <i>OsARM1</i> (<i>OsARM1-KO</i> CRISPR/Cas9-generated mutants) improved tolerance to As(III) and overexpression of <i>OsARM1</i> (<i>OsARM1-OE</i> lines) increased sensitivity to As(III). Measurement of As in As(III)-treated plants showed that under low As(III) conditions (2 μM), more As was transported from the roots to the shoots in <i>OsARM1-KOs</i>. By contrast, more As accumulated in the roots in <i>OsARM1-OEs</i> in response to high As(III) exposure (25 μM). In particular, the As(III) levels in node I were significantly higher in <i>OsARM1-KOs</i>, but significantly lower in <i>OsARM1-OEs</i>, compared to wild-type plants, implying that OsARM1 is important for the regulation of root-to-shoot translocation of As. Moreover, <i>OsLsi1, OsLsi2</i>, and <i>OsLsi6</i>, which encode key As transporters, were significantly downregulated in <i>OsARM1-OEs</i> and upregulated in <i>OsARM1-KOs</i> compared to wild type. Chromatin immunoprecipitation-quantitative PCR of <i>OsARM1-OEs</i> indicated that OsARM1 binds to the conserved MYB-binding sites in the promoters or genomic regions of <i>OsLsi1, OsLsi2</i>, and <i>OsLsi6</i> in rice. Our findings suggest that the OsARM1 transcription factor has essential functions in regulating As uptake and root-to-shoot translocation in rice.