PLANT PHYSIOLOGY , Vol 109, Issue 1 195-202, Copyright © 1995 by American Society of Plant Biologists
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WHOLE PLANT, ENVIRONMENTAL, AND STRESS PHYSIOLOGY |
Retention of Cadmium in Roots of Maize Seedlings (Role of Complexation by Phytochelatins and Related Thiol Peptides)
W. E. Rauser and P. Meuwly
Department of Botany, University of Guelph, Guelph, Ontario, Canada N1G 2W1
Cd from roots of maize was partitioned in seedlings exposed to 3 [mu]M
CdSO4 for 1 to 7 d. Most of the root Cd (92-94%) was buffer soluble and
provided the classical metal-induced cysteine-rich, high-molecular-weight
Cd-binding complex. This complex, however, bound only part of the Cd within
the roots, from 19% after 1 d of exposure to 59% by d 7. Three families of
peptides formed the Cd-binding complex: ([gamma]-glutamic
acid-cysteine)n-glycine [([gamma]-Glu-Cys)n-Gly], or phytochelatins,
([gamma]-Glu-Cys)n, and ([gamma]-Glu-Cys)n-Glu. The monothiols
[gamma]-Glu-Cys-Gly (glutathione), [gamma]-Glu-Cys, and [gamma]-Glu-Cys-Glu
were absent from the complex. The n2 oligomers of any peptide were the
least concentrated, whereas the n3 and n4 oligomers increased in the
complex with exposure to Cd. By d 7, 75% of ([gamma]-Glu-Cys)4-Gly, 80% of
([gamma]-Glu-Cys)4, and 73% of ([gamma]-Glu- Cys)3-Glu were complexed with
Cd. The peptide thiol:Cd molar ratio for the complexes was 1.01 [plus or
minus] 0.07, as if the minimal amount of thiol was used to bind Cd.
Acid-labile sulfide occurred in the complexes from d 1 onward at the low
S2-:Cd molar ratio of 0.18 [plus or minus] 0.02.
