Formation of semi-compound C-type starch granule in high-amylose rice developed by antisense RNA inhibition of starch-branching enzyme.

Cereal starch granules with high-amylose and resistant starch (RS) always show irregular morphology and special crystalline structure, but their formation during grain development is not yet clear. In our previous studies, we had generated a transgenic rice line (TRS) enriched with amylose and RS, which contained semi-compound starch showing a C-type crystalline structure. In this study, the formation of semi-compound C-type starch granule during TRS endosperm development was carefully investigated with light, scanning electron, and transmission electron microscopes and X-ray powder diffraction. The results showed that the TRS starch subgranules, each with a central hilum, were individually initiated in amyloplast and showed an A-type crystal at the early stage of starch granule development, which was similar to that in its wild type. However, with the endosperm development, the amylose content in TRS endosperm starch increased and the B-type starch crystal was deposited in the periphery of subgranules; then, the adjacent subgranules fused together and finally formed a continuous outer layer band surrounding the entire circumference of the starch granule. Accordingly, a mechanistic model for the formation of semi-compound C-type starch granules is proposed.