Functional Genomic Validation of the Roles of Soluble Starch Synthase IIa in Japonica Rice Endosperm

Vito M Butardo Jr; Jixun Luo; Zhongyi Li; Michael J Gidley; Anthony R Bird; Ian J Tetlow; Melissa Fitzgerald; Stephen A Jobling; Sadequr Rahman

doi:10.3389/fgene.2020.00289

Functional Genomic Validation of the Roles of Soluble Starch Synthase IIa in Japonica Rice Endosperm

Front Genet. 2020 Apr 2:11:289. doi: 10.3389/fgene.2020.00289. eCollection 2020.

Authors

Vito M Butardo Jr^{1

2}, Jixun Luo¹, Zhongyi Li¹, Michael J Gidley³, Anthony R Bird⁴, Ian J Tetlow⁵, Melissa Fitzgerald⁶, Stephen A Jobling¹, Sadequr Rahman^{1

7}

Affiliations

¹ CSIRO Agriculture and Food, Canberra, ACT, Australia.
² Department of Chemistry and Biotechnology, Faculty of Science, Engineering and Technology, Swinburne University of Technology, Hawthorn, VIC, Australia.
³ Centre for Nutrition and Food Sciences, The University of Queensland, St Lucia, QLD, Australia.
⁴ Nutrition and Health, CSIRO, Adelaide, SA, Australia.
⁵ Department of Molecular and Cellular Biology, College of Biological Science, University of Guelph, Guelph, ON, Canada.
⁶ School of Agriculture and Food Sciences, Faculty of Science, University of Queensland, St Lucia, QLD, Australia.
⁷ School of Science and the Tropical Medicine and Biology Platform, Monash University, Bandar Sunway, Malaysia.

Abstract

The enzyme starch synthase IIa (SSIIa) in cereals has catalytic and regulatory roles during the synthesis of amylopectin that influences the functional properties of the grain. Rice endosperm SSIIa is more active in indica accessions compared to japonica lines due to functional SNP variations in the coding region of the structural gene. In this study, downregulating the expression of japonica-type SSIIa in Nipponbare endosperm resulted in either shrunken or opaque grains with an elevated proportion of A-type starch granules. Shrunken seeds had severely reduced starch content and could not be maintained in succeeding generations. In comparison, the opaque grain morphology was the result of weaker down-regulation of SSIIa which led to an elevated proportion of short-chain amylopectin (DP 6-12) and a concomitant reduction in the proportion of medium-chain amylopectin (DP 13-36). The peak gelatinization temperature of starch and the estimated glycemic score of cooked grain as measured by the starch hydrolysis index were significantly reduced. These results highlight the important role of medium-chain amylopectin in influencing the functional properties of rice grains, including its digestibility. The structural, regulatory and nutritional implications of down-regulated japonica-type SSIIa in rice endosperm are discussed.

Keywords: RNA silencing; amylopectin; amylose; rice; starch synthase.