A comparative study of proteomic differences between pencil and storage roots of sweetpotato (Ipomoea batatas (L.) Lam.).

Fibrous roots of sweetpotato (Ipomoea batatas (L.) Lam.) usually develop into both pencil and storage roots. To understand protein function in root development, a proteomic analysis was conducted on the pencil and storage roots of the light orange-fleshed sweetpotato cultivar, Yulmi. Two-dimensional gel electrophoresis showed that expression of 30 protein spots differed between pencil and storage roots: 15 proteins were up-regulated or expressed in pencil roots and 15 in storage roots. Differentially expressed proteins spots were investigated using matrix-assisted laser desorption/ionization time of flight mass spectrometry, and 10 proteins from pencil roots were identified as binding protein isoform A, catechol oxidase, peroxidases, ascorbate peroxidase, endochitinase, flavanone 3-hydroxylase and unknown proteins. Of the proteins up-regulated in, or restricted to, storage roots, 13 proteins were identified as protein disulfide isomerase, anionic peroxidase, putative ripening protein, sporamin B, sporamin A and sporamin A precursor. An analysis of enzyme activity revealed that catechol oxidase and peroxidase as the first and last enzymes of the lignin biosynthesis pathway, and ascorbate peroxidase had higher activities in pencil than in storage roots. The total concentration of phenolic compounds was also far higher in pencil than in storage roots, and lignin accumulated only in pencil roots. These results provide important insight into sweetpotato proteomics, and imply that lignin biosynthesis and stress-related proteins are up-regulated or uniquely expressed in pencil roots. The results indicate that the reduction of carbon flow toward phenylpropanoid biosynthesis and its delivery to carbohydrate metabolism is a major event in storage root formation.