Diverse antioxidant enzyme levels in different sweetpotato root types during storage root formation

Sweetpotato forms different types of root during storage root production, including fibrous roots (FR), thick roots (TR), and storage roots (SR). To understand the functions that antioxidant enzymes play in the development of these different roots, we investigated the activities of superoxide dismutase (SOD), ascorbate peroxidase (APX), catalase (CAT), peroxidase (POD), and polyphenol oxidase (PPO). Significantly higher activity levels were observed in FR than in both TR and SR for SOD, APX and CAT. Both POD and PPO activity were significantly higher in FR and TR than in SR. Quantitative RT-PCR analysis was used to investigate antioxidant gene expression patterns in the different stage and root types of sweetpotato. Some genes displayed root-type or stage specific responses depending on the root part. CuZnSOD, APX, swpa7, swpa8, swb6, and swpb7 were expressed at significantly higher levels in the FR and SR than in the TR. CAT, swpa9, swpa4, and swpb7 exhibited significantly higher expression levels in the FR and TR than in the SR. In addition, they also showed higher levels of expression in the lower parts of the FR and TR at the early stage, whereas SR was expressed in the upper part. These results indicate that antioxidant enzymes are differentially regulated during the early stages of sweetpotato root development.