Differential responses of sweetpotato peroxidases to heavy metals

Oxidative stress is one of the major causes of damage in plants exposed to different types of environmental stress, including heavy metals. Accumulation of heavy metals in plants can disrupt many cellular functions and plant growth. To assess the contribution of oxidative stress to heavy metal toxicity in plants, young sweetpotato plants (Ipomoea batatas) were treated with increasing concentrations of Cd, Cu and Zn, and grown in half Murashige and Skoog nutrient solution culture. Plant growth was significantly inhibited and internal metal content was increased in a dose-dependent manner for each metal. The generation of H2O2 in leaves and fibrous roots correlated positively with metal dose. The specific activity of peroxidases (PODs) in fibrous roots was markedly enhanced by metal treatment, whereas in leaves, activity was low and only slightly affected by metal treatment. Analysis of 13 POD genes revealed differential expression of PODs in response to heavy metals. Several genes for acidic PODs (swpa2, swpa3 and swpa4) and basic PODs (swpb1, swpb3 and swpab4) were strongly expressed under all metal treatment conditions in leaves or fibrous roots. The expression of swpa1 was increased in leaves and fibrous roots by Cd and Cu treatment, whereas swpb5 expression was reduced by all metals in fibrous roots. These results indicate that increased H2O2 levels in response to heavy metal stress are closely linked to an improved antioxidant defense capability mediated by POD.