MGATP-DEPENDENT TRANSPORT OF PHYTOCHELATINS ACROSS THE TONOPLAST OF OAT ROOTS

In Cd-exposed oat (Avena sativa) roots Cd was found to be associated p rimarily with the phytochelatin (gamma-glutamylcysteinyl)(3)glutamic a cid [(gamma EC)(3)G], with a peptide to Cd ratio of 1:3 (cysteine to C d ratio of 1:1), even though both (gamma EC)(2)G and (gamma EC)(3)G we re present in the roots. Phytochelatins are known to accumulate in the vacuoles of plant cells on exposure to Cd, but the mechanism is not c lear. Here we present evidence for the transport of the phytochelatins (gamma EC)(3)G and (gamma EC)(2)G as well as the Cd complex Cd-(gamma EC)(3)G across the tonoplast of oat roots. Transport of (gamma EC)(3) G had a K-m for MgATP of 0.18 mM and a V-max of 0.7 to 1 nmol mg(-1) p rotein min(-1). Transport of (gamma EC)(3)G was also energized by MgGT P and to a lesser extent MgUTP and was highly sensitive to orthovanada te, with 50%-inhibitory concentration of 0.9 mu M. The Cd complex Cd-( gamma EC)(3)G and (gamma EC)(2)G were also transported in a MgATP-depe ndent, vanadate-sensitive manner. Therefore, this process is a candida te for the transport of both phytochelatins, and Cd as its peptide com plex, from the cytoplasm into the vacuole.