Relation between lipoic acid and cell redox status in wheat grown in excess copper

Cell redox status and lipoic acid contents were analysed in wheat (Triticum durum Desf. cv. Creso) plants treated with 150 μM Cu to elucidate the role of the antioxidant lipoic acid against oxidative stress. In comparison with shoots, roots suffered a higher oxidative stress showing a decrease in NADPH contents and an oxidation of glutathione and ascorbate. Shoots did not evidence a clear oxidative damage since Cu was translocated in small amounts. Lipoic acid as reduced (DHLA) or oxidised (LA) form was present in both leaves and roots of wheat. Analysis of the cell sap showed that this antioxidant was present also as free form. The analyses showed that stroma contained significant amounts of free LA and that, after acidic hydrolysis, higher amounts of LA and DHLA were released. However, lipoic acid was undetectable in both thylakoids and microsomal membranes. Cu treatment did not determine changes in the contents of total LA and DHLA in roots, they being likely involved in Cu chelation. In contrast, in leaves after 48 h of treatment the metal induced an increase in DHLA, which could in part explain the reduction in the oxidised glutathione levels. In leaves free lipoic acid was more prone to be oxidised compared to the bound form, and the reduced form disappeared in both leaves and roots after Cu treatment.