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

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

JOURNAL=Frontiers in Plant Science

VOLUME=9

YEAR=2018

URL=https://www.frontiersin.org/journals/plant-science/articles/10.3389/fpls.2018.01591

DOI=10.3389/fpls.2018.01591

ISSN=1664-462X

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