Developmental change in starch granules in sweetpotato callus

Developmental change in starch granules formed in sweetpotato callus was examined. The starch content increased from nearly 0 to 10.6% of the dried calli over one week. The chain-length distribution of the debranched starches determined by gel-permeation chromatography revealed that no amylose was present in the one-day sample. During development, amylose fraction increased continuously for seven days to 12.3% of total starch. On the other hand, the ratio of two unit-chain fractions from amylopectin was nearly identical, and the short-chain distribution (DP=6–35) was also similar throughout the culture period. Furthermore, all starch granules showed an A-type crystallite. These results suggest that the amylopectin molecules were completed as a granule as early as the first day before the appearance of the amylose fraction. With respect to the structural change of starch granules, the transcript of the GBSSI gene was detected in a constant level during the culture for one month. These results indicate that in spite of the occurrence of the GBSSI transcript at the start of starch accumulation, the structural change from waxy to normal feature of starch granule occurred during the culture of sweetpotato calli.