Rice Soluble Starch Synthase I: Allelic Variation, Expression, Function, and Interaction With Waxy.

Qianfeng Li,Li Jiang,Xinyan Liu,Qiaoquan Liu,Min Zhong,Changquan Zhang,Meiyan Jiang,Xiaolei Fan,Minghong Gu

doi:10.3389/fpls.2018.01591

Abstract

Starch, which is composed of amylose and amylopectin, is the key determinant of rice quality. Amylose is regulated by the Waxy (Wx) gene, whereas amylopectin is coordinated by various enzymes including eight soluble starch synthases (SSSs), of which SSSI accounts for ∼70% of the total SSS activity in cereal endosperm. Although great progress has been made in understanding SSSI gene expression and function, allelic variation and its effects on gene expression, rice physicochemical properties and qualities, and interactions with the Wx gene remain unclear. Herein, SSSI nucleotide polymorphisms were analyzed in 165 rice varieties using five distinct molecular markers, three of which reside in an SSSI promoter and might account for a higher expression of the SSSIi allele in indica ssp. than of the SSSIj allele in japonica ssp. The results of SSSI promoter-Beta-Glucuronidase (β-GUS) analysis were consistent with the expression results. Moreover, analysis of near isogenic lines (NILs) in the Nipponbare (Nip) background showed that Nip (SSSIi) and Nip (SSSIj) differed in their thermal properties, gel consistency (GC), and granule crystal structure. Knockdown of SSSI expression using the SSSI-RNA interference (RNAi) construct in both japonica and indica backgrounds caused consistent changes in most tested physicochemical characteristics except GC. Moreover, taste value analysis (TVA) showed that introduction of the SSSI allele in indica or knockdown of SSSI expression in japonica cultivars significantly reduced the comprehensive taste value, which was consistent with the superior taste of japonica against indica. Furthermore, to test the potential interaction between SSSI and different Wx alleles, three NILs within the Wx locus were generated in the indica cv. Longtefu (LTF) background, which were designated as LTF (Wxa), LTF (Wxb), and LTF (wx). The SSSI-RNAi construct was also introduced into these three NILs, and physiochemical analysis confirmed that the knockdown of SSSI significantly increased the rice apparent amylose content (AAC) only in the Wxa and Wxb background and caused different changes in GC in the NILs. Therefore, the effect of SSSI variation on rice quality also depends on its crosstalk with other factors, especially the Wx gene. These findings provide fundamental knowledge for future breeding of rice with premium eating and cooking qualities.

Highlights

Rice (Oryza sativa L.) is the most important carbohydrate and energy source for human nutrition
Polymorphism of the SSSI gene was identified by the alignment of SSSI sequences from 12 rice cultivars for which genomes are available online or were determined by us
In exon 6, there is an A/G single nucleotide polymorphisms (SNPs), which is closely correlated to an MspI recognition site that is only present in the japonica cultivar

Summary

Introduction

Rice (Oryza sativa L.) is the most important carbohydrate and energy source for human nutrition. Starch is the major component of rice, accounting for ∼90% of milled rice endosperm. Starch composition and structure are closely correlated with rice eating and cooking qualities (ECQs), especially taste properties of cooked rice (Tian et al, 2009). Great efforts have been made to improve rice quality, and a number of ECQ-related genes have been identified and characterized (Aluko et al, 2004; Jeon et al, 2010; Phing et al, 2016), the underlying genetic mechanisms remain ambiguous as grain quality traits are complex. Starch, which is the main determinant of rice ECQs, consists of two glucose homopolymers, amylose and amylopectin. Amylopectin is the major component of the total starch weight, and it contributes greatly to its semicrystalline nature. Differences in the apparent amylose content (AAC) in rice varieties are closely correlated with natural allelic variation at the Wx locus. A number of novel, natural Wx alleles have been reported, including Wxmp, Wxmq, and Wxop (Sato et al, 2002; Liu et al, 2009; Tian et al, 2009; Yang et al, 2013)

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Conclusion