3 FAMILIES OF THIOL PEPTIDES ARE INDUCED BY CADMIUM IN MAIZE

Phytochelatins ((gamma GluCys)(n)Gly) are synthesized from glutathione by plants exposed to metals like Cd2+, Cu2+ and Zn2+. An intracellula r complex formed by phytochelatins with Cd2+ and sulfide is thought to detoxify the metal possibly by sequestration in the vacuole. It was f ound that maize seedlings exposed to Cd2+ produced phytochelatins and two additional families of cysteine-containing peptides, (gamma GluCys )(n) and (gamma GluCys)(n)Glu. All thiol peptides with n = 2 and 3 wer e purified and their structure characterized by tandem mass spectromet ry. For maize plants exposed to Cd2+ for 7 days, phytochelatins were s ynthesized preferentially in the first 24 h whereas the amounts of (ga mma GluCys)(n) and (gamma GluCys)(n)Glu were the highest thereafter. T his was probably due to an initial large pool of glutathione available in control plants compared with a dearth of gamma GluCys and no detec table gamma GluCysGlu. The (gamma GluCys)(n)Glu peptides were induced exclusively by Cd2+ as they were below the detection limit in control seedlings that contained low amounts of phytochelatins and (gamma GluC ys)(n). Since the Cys moiety of the peptides is essential for binding Cd2+, a role for accumulated (gamma GluCys)(n) and (gamma GluCys)(n)Gl u in detoxifying Cd2+ in plants must be considered.