Expression of a cassava granule‐bound starch synthase gene in the amylose‐free potato only partially restores amylose content

Abstract

ABSTRACTGranule‐bound starch synthase I (GBSS I) is responsible for the synthesis of amylose in starch granules. A heterologous cassava GBSS I gene was tested for its ability to restore amylose synthesis in amylose‐free (amf) potato mutants. For this purpose, the cassava GBSS I was equipped with different transit peptides. In addition, a hybrid containing the potato transit peptide, the N‐terminal 89 amino acids of the mature potato GBSS I, and the C‐terminal part of cassava GBSS I was prepared. The transgenic starches were first analysed by iodine staining. Only with the hybrid could full phenotypic complementation of the amf mutation be achieved in 13% of the plants. Most transformants showed partial complementation, but interestingly the size of the blue core was similar in all granules derived from one tuber of a given plant. The amylose content was only partially restored, up to 60% of wild‐type values or potato GBSS I‐complemented plants; however, the GBSS activity in these granules was similar to that found in wild‐type ones. From this, and the observation that the hybrid protein (a partial potato GBSS I look‐alike) performs best, it was concluded that potato and cassava GBSS I have different intrinsic properties and that the cassava enzyme is not fully adapted to the potato situation.