FIGURE 1.

Tissue-specific expression of OsABCC1 and OsHMA3 reduces As and Cd concentrations in the rice grain. Transporters localized at the plasma membrane and tonoplast are critical for root-to-shoot translocation and grain accumulation of As and Cd. (A) Arsenic taken up by transporters, such as OsPTs and unknown influx transporters (possibly aquaporins) located at the plasma membrane of the root epidermis, is translocated to shoots and grains, or extruded into the rhizosphere. OsABCC1, a major vacuolar PCs-As transporter, delivers As into vacuoles in phloem companion cells of node I, and inhibits As translocation into rice grains (Song et al., 2014a). Rice plants containing low concentrations of As in their grain were generated by expressing OsABCC1 and ScYCF1 specifically in the cortex, internode, and nodes using the OsRCc3 promoter (Deng et al., 2018). (B) Cd is mainly taken up by OsNramp5, which is localized at the distal side of both root exodermis and endodermis cells. In addition, OsIRT1, 2 and OsNramp1 also contribute to Cd uptake. OsHMA3 located at the tonoplast of roots are responsible for the accumulation of Cd within vacuoles and inhibit radial translocation of Cd into the stele. Low Cd-accumulating rice was generated by expressing a functional OsHMA3 transporter under the control of pOsHMA2, a rice root pericycle and nodal phloem-specific promoter (Shao et al., 2018). Epi, epidermis; Endo, endodermis; EVB, enlarged vascular bundles; DVB, diffuse vascular bundles; XTC, xylem transfer cells; PCB, parenchyma cell bridge; NVA, nodal vascular anastomoses; PPC/CC, phloem parenchyma cells and companion cells; P, phloem; V, vacuole.