Glutathione is a key antioxidant metabolite to cope with mercury and cadmium stress

Glutathione (GSH) plays a dual role under heavy metal stress, as antioxidant metabolite and as precursor of phytochelatins (PCs). Studying the responses of the GSH metabolism to heavy metals is important to improve tolerance. We studied the oxidative stress signature of three gamma-glutamylcysteine synthetase (gamma ECS) Arabidopsis thaliana allele mutants (rax1-1, cad2-1, and pad2-1), first enzymatic step in the GSH synthetic pathway, when treated with 10 mu M Cd or Hg for 72 h. GSH concentration was lower in the mutants (45 % rax1-1; 30 % cad2-1; and 20 % pad2-1), which was also associated with inferior translocation of Cd or Hg to shoots, than in wild type Col-0. Glutathione reductase (GR) and NADPH-oxidase activities were inhibited in roots, phytotoxic effects consistently more pronounced in the mutants, particularly in pad2-1. Non-photochemical quenching augmented with exposure time to Cd or Hg in Col-0, but not so in the gamma ECS mutants. Mercury caused severe damage in cad2-1 and pad2-1 root proteins profile; toxic effects confirmed by GR and H+-ATPase immunodetection. PCs appeared in Col-0 roots under metal stress, and surprisingly accumulated in rax1-1. gamma ECS immunodetection revealed its overexpression in rax1-1. A minimum amount of GSH may be required for adequate metal tolerance, where gamma ECS expression could compensate GSH deficiency under stress.