Fig. 1.

Schematic representation of the As cycle in the environment and plants. Equilibrium between arsenate [As(V)] and arsenite [As(III)] in soil solutions is mainly dependent on the redox conditions. Arsenate is taken up by roots by phosphate transporters (P), and arsenite is taken up by a subclass of aquaporins (NIP), some of them also transporting silicon (Si). Methylated forms of As (MetAs) are also taken up by NIP and Si transporters. Inside plants, these types of transporters are also involved in the distribution of As between organs and tissues. As(V) is enzymatically reduced into As(III) in plant cells by arsenate reductase (AR), leading to the conversion of glutathione (GSH) to its oxidized form (GSSG). Arsenite can be effluxed to the environment by a root Si transporter or methylated. A cascade of methylation can then transform As into a gaseous form that is volatilized. Another pathway of detoxification occurs by the synthesis of phytochelatins (PCs) corresponding to a three enzymatic step condensation of three amino acids: cysteine (Cys), glutamate (Glu), and glycine (Gly). PC synthesis and their complexation to As(III) are coordinated to the transport of the PC–As(III) complex to the vacuole subcellular compartment through the activity of two members of a subclass of ATP binding cassette (ABC) transporters: ABCC1 and -2.