Effect of simultaneous inhibition of starch branching enzymes I and IIb on the crystalline structure of rice starches with different amylose contents.

Mutating or inhibiting genes encoding starch branching enzymes (SBEs) can increase the amylose content (AC) of cereals. We analyzed endosperm starches from three rice cultivars with different ACs and from transgenic lines derived from them. The transgenic lines had simultaneously inhibited SBE I and IIb genes. Compared with the starch from their wild-type parents, the starch from transgenic lines showed significantly increased apparent ACs and lamella size and decreased relative crystallinity, double helix content, and lamellar peak scattering intensity, and altered short-range ordered structure in the external region. These changes were more prominent in the line derived from the high-AC cultivar than in those derived from waxy and low-AC cultivars. Inhibiting both SBE I and IIb changed the crystalline structure of starch from A-type to CA-type in lines derived from waxy and low-AC cultivars, and from A-type to C-type in that derived from the high-AC cultivar.