Increase of amylose content of sweetpotato starch by RNA interference of the starch branching enzyme II gene (IbSBEII)

In the storage roots of sweetpotato (Ipomoea batatas (L.) Lam. cv. Kokei 14), 10 to 20% of the starch is essentially unbranched linear amylose and the other major component is branched amylopectin. The starch branching enzymes, which are responsible for production of amylopectin to form a-1,6-linkages in the glucan can be divided into two classes, class A (e.g. potato and maize SBEII, pea SBEI) and class B (e.g. potato and maize SBEI, pea SBEII). On the bases of the registered cDNA of sweetpotato SBEII (IbSBEII) encoding class A branching enzyme, we constructed double-stranded RNA (dsRNA) interference vectors and introduced them into sweetpotato genome via Agrobacterium-mediated gene transformation. We obtained eight independent transgenic plants by using two kinds of RNA interference (RNAi) constructs, encoding GBSSI 1st intron-spliced RNA or a GUS fragment-spliced RNA, respectively. All transgenic plants were confirmed not to express IbSBEII by RT-PCR and to have the starch with a higher amylose content than the non-transgenic control (up to 25% compared to 10% in the control). Both constructs induced the same level of silencing of IbSBEII in all transgenic plants. The morphological characters showed no significant differences between the transgenic and control plants. Starch yield of transgenic tubers was slightly lower than that of non-transgenic tubers. The starch granules of the transgenic plants were similar to those of typical sweetpotato starchs in shape and the distribution in granule size, but slightly different in grain structure.