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

In Cd-exposed oat (Avena sativa) roots Cd was found to be associated primarily with the phytochelatin (gamma-glutamylcysteinyl)(3)glutamic acid [(gamma EC)(3)G], with a peptide to Cd ratio of 1:3 (cysteine to Cd ratio of 1:1), even though both (gamma EC)(2)G and (gamma EC)(3)G were present in the roots. Phytochelatins are known to accumulate in the vacuoles of plant cells on exposure to Cd, but the mechanism is not clear. 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) protein min(-1). Transport of (gamma EC)(3)G was also energized by MgGTP and to a lesser extent MgUTP and was highly sensitive to orthovanadate, 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-dependent, vanadate-sensitive manner. Therefore, this process is a candidate for the transport of both phytochelatins, and Cd as its peptide complex, from the cytoplasm into the vacuole.