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

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

doi:10.3389/fgene.2020.00289

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.

Highlights

Starch synthases (SS) play a critical role during starch biosynthesis, elongating glucan chains by the addition of glucose from the substrate ADP-glucose (Denyer et al, 1995; Denyer et al, 1996)
A total of 35 independent transformants were obtained from hp-synthase IIa (SSIIa) lines, of which 31 (88%) harbored the hygromycin resistance gene based on marker screening by PCR
As both the hp-SSIIa-shr and hpSSIIa-op phenotypes were generated from multiple independent transgenic events, it is highly unlikely that the phenotype is due to the chance perturbation of a gene critical for the grain development

Summary

Introduction

Starch synthases (SS) play a critical role during starch biosynthesis, elongating glucan chains by the addition of glucose from the substrate ADP-glucose (Denyer et al, 1995; Denyer et al, 1996). These elongated glucan chains act as substrates for branching enzymes and debranching enzymes (Tetlow and Emes, 2014; Pfister and Zeeman, 2016). The gene for starch synthase IIa (SSIIa) codes for a major starch synthase enzyme isoform involved in the elongation of shortchain amylopectin in the cereal endosperm, which is important in distinguishing starch properties (Hannah and James, 2008; Jeon et al, 2010; Tetlow and Emes, 2017). SSIIa alleles determine the peak gelatinization temperature (GT) of rice, an essential trait in predicting cooking and eating qualities (Umemoto and Aoki, 2005; Waters et al, 2006)

Methods

Results

Conclusion