Cloning and characterization of sweetpotato isoamylase gene (IbIsa1) isolated from tuberous root

The cDNA clone of the sweetpotato isoamylase gene, encoding starch-debranching enzyme (IbIsa1) in sweetpotato (Ipomoea batatas (L.), cv. Kokei 14), was isolated and sequenced. The amino acid sequence of IbIsa1 is 70% and 79% identical to that of AtIsa1 from Arabidopsis and StIsa1 from potato, respectively. DNA gel-blot analysis demonstrated that at least two copies IbIsa1 are present in the sweetpotato genome. IbIsa1 was found to be strongly expressed in tuberous root. The transcript level of IbIsa1 in the root of rooted single-leaf cuttings was extremely low during the first 15 to 40 days after planting. The transcript levels continuously increased up to 50 days, at which time the tuberous root was almost completely developed. This indicates that IbIsa1 may work in concert with AGPase large subunit, GBSSI and SBEII during the primary phase of starch granule formation.