Heme oxygenase-1 is involved in ascorbic acid-induced alleviation of cadmium toxicity in root tissues of Medicago sativa

This study explored molecular mechanism of ascorbic acid (AsA)-mediated enhancement of plant tolerance against cadmium (Cd) stress. Complex pharmacological, histochemical and molecular approaches were applied to analyse the effect of AsA on the alleviation of Cd stress and corresponding signalling pathway. Cd stress brought about severe oxidative damage and remarkable decrease in AsA content in alfalfa (Medicago sativa) seedling roots. Exogenous AsA not only increased AsA content in vivo, and strengthened the up-regulation of alfalfa heme oxygenase-1 (HO-1) transcript and HO activity triggered by Cd, but also significantly decreased Cd accumulation and oxidative damage, which was confirmed by the histochemical analysis. The responses of AsA were further impaired by the potent inhibitor of HO-1, zinc protoporphyrin IX (ZnPP), which were blocked further when 50 % saturation of carbon monoxide (CO) aqueous solution (in particular) or bilirubin (BR), two catalytic by-products of HO-1, was added, respectively. Molecular evidence illustrated that AsA-triggered the up-regulation of antioxidant enzyme genes, especially Mn-SOD and POD, were sensitive to ZnPP and reversed by CO. In short, above results suggested that cytoprotective roles triggered by AsA might be, at least partially, through HO-1-dependent fashion by the induction of antioxidant system and lowering Cd accumulation.