Roots of maize seedlings retain most of their cadmium through two complexes

The buffer-soluble cadmium (Cd) from roots of two cultivars of maize exposed to 3 mu mol . L-1 Cd for 5 and 7 days was partitioned into low molecular weight (LMW) and high molecular weight (HMW) Cd-binding complexes. The complexes contained 3 families of cysteine-rich peptides: (gamma-glutamylcysteine)(n)-glycine, (gamma-glutamylcysteine)(n) and (gamma-glutamylcysteine)(n)-glutamic acid. The proportions of mono- and dithiol peptides were higher in the LMW complex than in the HMW form, both complexes had similar proportions of tri- to pentathiols and molar ratios of acid-labile sulfide to peptide thiol. The vacuolar HMW complex contained more Cd per peptide thiol. The cytosolic LMW complex contained considerable acidlabile sulfide, addition of more Cd gave the vacuolar HMW complex. The buffer extracts used to measure the complexes represented 93 to 96% of the root Cd. The HMW complex was more abundant than the LMW form. The 2 complexes together bound 75 to 88% of the Cd in the maize roots.