Three families of thiol peptides are induced by cadmium in maize

Abstract

Phytochelatins ((γGluCys) n Gly) are synthesized from glutathione by plants exposed to metals like Cd 2+ , Cu 2+ and Zn 2+ . An intracellular complex formed by phytochelatins with Cd 2+ and sulfide is thought to detoxify the metal possibly by sequestration in the vacuole. It was found that maize seedlings exposed to Cd 2+ produced phytochelatins and two additional families of cysteina‐containing peptides, (γGluCys) n and (γGluCys) n Glu. All thiol peptides with n = 2 and 3 were purified and their structure characterized by tandem mass spectrometry. For maize plants exposed to Cd 2+ for 7 days, phytochelatins were synthesized preferentially in the first 24 h whereas the amounts of (γGluCys) n and (γGluCys) n Glu were the highest thereafter. This was probably due to an initial large pool of glutathione available in control plants compared with a dearth of γGluCys and no detectable γGluCysGlu. The (γGluCys) n Glu peptides were induced exclusively by Cd 2+ as they were below the detection limit in control seedlings that contained low amounts of phytochelatins and (γGluCys) n . Since the Cys moiety of the peptides is essential for binding Cd 2+ , a role for accumulated (γGluCys) n and (γGluCys) n Glu in detoxifying Cd 2+ in plants must be considered.