Abstract
The present investigation was performed to examine effects of changing the multimeric structure and composition of isoamylase (ISA) on starch synthesis in rice endosperm by reducing the expression of ISA1 or ISA2 gene or over-expression of ISA2 gene. Although suppression of ISA2 gene expression had no effects on the starch phenotypes, over-expression of the ISA2 gene showed a drastic decrease in the synthesis of starch with modified granules devoid of the thermal and crystalline properties and an accumulation of water-soluble highly branched malto-dextrins in the endosperm. In the over-expressed line all the ISA1 activity was accounted for by the form of ISA1-ISA2 whereas in the wild-type endosperm both the ISA1 and ISA1-ISA2 were present. The results indicate that the ISA1 is solely sufficient, but not the ISA1-ISA2, for the starch production in rice endosperm, sharply contrasting with the recent results that the ISA1-ISA2 is the only form for ISA activity playing a crucial role in the normal amylopectin biosynthesis in potato tuber and Arabidopsis leaf. Thus this study shows a functional diversity of ISA oligomeric structure between monocots and dicots.
