FIGURE 2.

C-type ATP-binding cassette transporters mediate non-protein thiol-dependent As and Cd vacuolar sequestration. Glutathione synthesized by GSH1 is translocated from plastids to the cytosol through CLT1, a putative GSH transporter located at the plastid envelope, while phytochelatins (PCs) are synthesized by phytochelatin synthetases (PCS) in the cytosol. Inorganic arsenic and cadmium form complexes with GSH and PCs. The GS₃-As and GS₂-Cd complexes might be sequestrated into the vacuole by unknown ABC transporters; PC-As can be delivered to the vacuole by AtABCC1, AtABCC2, and OsABCC1; and PC-Cd can be translocated into the vacuole by SpHMT1, AtABCC1, AtABCC2, and AtABCC3. Loss-of-function mutants for Arabidopsis PCS1 and rice CLT1 exhibited hypersensitivity to As and increased As translocation from roots to shoots due to the limited availability of a cytosolic As chelator in roots (Yang et al., 2016; Hayashi et al., 2017). Rice abcc1 and pcs1 mutants accumulated high levels of As in the grain due to the defect in vacuolar As sequestration in node I (Song et al., 2014a; Hayashi et al., 2017).