Increase in ascorbate-glutathione metabolism as local and precocious systemic responses induced by cadmium in durum wheat plants

Durum wheat plants (Triticum durum cv Creso) were grown in the presence of cadmium (0-40 mu M) and analysed after 3 and 7 d for their growth, oxidative stress markers, phytochelatins, and enzymes and metabolites of the ascorbate (ASC)glutathione (GSH) cycle. Cd exposure produced a dose-dependent inhibition of growth in both roots and leaves. Lipid peroxidation, protein oxidation and the decrease in the ascorbate redox state indicate the presence of oxidative stress in the roots, where H2O2 overproduction and phytochelatin synthesis also occurred. The activity of the ASCGSH cycle enzymes significantly increased in roots. Consistently, a dose-dependent accumulation of Cd was evident in these organs. On the other hand, no oxidative stress symptoms or phytochelatin synthesis occurred in the leaves; where, at least during the time of our analysis, the levels of Cd remained irrelevant. In spite of this, enzymes of the ASCGSH cycle significantly increased their activity in the leaves. When ASC biosynthesis was enhanced, by feeding plants with its last precursor, L-galactono-gamma-lactone (GL), Cd uptake was not affected. On the other hand, the oxidative stress induced in the roots by the heavy metal was alleviated. GL treatment also inhibited the Cd-dependent phytochelatin biosynthesis. These results suggest that different strategies can successfully cope with heavy metal toxicity. The changes that occurred in the ASCGSH cycle enzymes of the leaves also suggest that the whole plant improved its antioxidant defense, even in those parts which had not yet been reached by Cd. This precocious increase in the enzymes of the ASCGSH cycle further highlight the tight regulation and the relevance of this cycle in the defense against heavy metals.