Cadmium has contrasting effects on polyethylene glycol-sensitive and resistant cell lines in the Mediterranean halophyte species Atriplex halimus L.

Beside a direct toxicity, cadmium impact on plants involves both a secondary-induced water stress and an oxidative stress. Proliferating cell lines of Atriplex halimus were selected for their sensitivity or resistance to polyethylene glycol (PEG 10,000, 20%) and then exposed to 100 microM CdCl2 in the simultaneous presence or absence of PEG 20% or 150 mM NaCl. The PEG resistant cell line exhibited a higher growth in the presence of Cd than the sensitive line, although Cd acccumulation was higher in the former than in the latter. Exogenous PEG induced an increase in Cd concentration in the sensitive but not in the resistant cell line while NaCl induced a decrease in Cd accumulation in both cell lines. In the presence of Cd alone, the water content (WC) was higher and the osmotic potential was lower in PEG-sensitive than in PEG resistant line. The presence of PEG in the Cd-containing medium increased the WC and decreased the osmotic potential in PEG-resistant line comparatively to Cd stress alone, while an inverse trend was observed for the sensitive line. The PEG-resistant cell line displayed a higher ability to cope with oxidative stress in relation to an increase of endogenous antioxidants (glutathione and ascorbic acid), a high constitutive superoxide dismutase (EC 1.15.1.1) activity and an efficient Cd-induced increase in glutathione reductase (GR) (EC 1.6.4.1) and ascorbate peroxidase (APX) (EC 1.11.1.11). Cadmium tolerance in PEG-resistant line is thus not related to any strategy of Cd exclusion or osmotic adjustment but to tolerance mechanisms allowing the tissue to restrict the deleterious impact of accumulated Cd.