Analysis of Genotypic Diversity in the Starch Physicochemical Properties of Nonwaxy Rice: Apparent Amylose Content, Pasting Viscosity and Gel Texture

Microsatellites, single nucleotide polymorphisms and a sequence tagged site in starch-synthesizing genes in relation to starch physicochemical properties in nonwaxy rice (Oryza sativa L.)

Starch characteristics determine the quality of various products of rice, e.g., eating, cooking and processing qualities. Our previous study indicated that molecular markers inside or close to starch synthesizing genes can differentiate the starch properties of 56 waxy rices. Here we report microsatellite (or simple sequence repeat, SSR) polymorphism in the Waxy (Wx) gene, soluble starch synthase I gene (SS1) and starch branching enzyme 1 gene (SBE1), single nucleotide polymorphism (SNP) in Wx and starch branching enzyme 3 gene (SBE3), and a sequence tagged site (STS) in starch branching enzyme 1 gene (SBE1) among 499 nonwaxy rice samples and their relationships with starch physicochemical properties. The nonwaxy rice samples consist of landraces (n = 172) obtained from germplasm centers and cultivars and breeding lines (n = 327) obtained from various breeding programs. Ten (CT)( n ) microsatellite alleles, (CT)(8), (CT)(10), (CT)(11), (CT)(12, )(CT)(17), (CT)(18), (CT)(19), (CT)(20), (CT)(21), and (CT)(22), were found at the Wx locus, of which (CT)(11) was the most frequent, and (CT)(12), (CT)(21) and (CT)(22) were identified for the first time. Four (CT)( n ) microsatellite alleles were found at the SBE1 locus, (CT)(8), (CT)(9), and (CT)(10 )together with an insertion sequence of CTCTCGGGCGA, and (CT)(8) alone without the insertion, of which (CT)(9) and the insertion was a new allele identified in only one rice, IR1552. Multiple microsatellites clustered at the SS1 locus, and in addition to the three alleles previously detected (SSS-A = (AC)(2)...TCC(TC)(11)...(TC)(5)C(ACC)(11), SSS-B = (AC)(3)...TCT(TC)(6)...(TC)(4)C(ACC)(9), and SSS-C = (AC)(3)...TCT(TC)(6)...(TC)(4)C(ACC)(8)), one new allele (SSS-D = (AC)(2)...TCC(TC)(10)...(TC)(4)C(ACC)(9)) was found. Analysis of the starch physicochemical properties of the samples with different microsatellites, SNPs and STS groups indicated that these molecular markers can differentiate almost all the physicochemical properties examined, e.g., apparent amylose content (AAC), pasting viscosity characteristics, and gel textural properties. Wx SSR and Wx SNP alone explained more variations for all physicochemical properties than the other molecular markers. The total six markers could explain 92.2, 81 and 86% of total variation of AAC, gel hardness (HD), and gel cohesiveness (COH), respectively, and they could explain more than 40% of the total variation of hot paste viscosity (HPV), cool paste viscosity (CPV), breakdown viscosity (BD), setback viscosity (SB) and gel adhesiveness (ADH). However, only 29% of the total variation of peak viscosity (PV) and 37% of pasting temperature (PT) could be explained by all the molecular markers. Some of these markers can differentiate the starch physicochemical properties among the rice samples with the same Wx allele, indicating that the variation within Wx allele classes can be explained by other starch synthesizing genes. These SSRs, SNPs and STS are useful in marker-assisted breeding for the improvement of starch quality of rice.

Nucleotide diversity in starch synthase IIa and validation of single nucleotide polymorphisms in relation to starch gelatinization temperature and other physicochemical properties in rice (Oryza sativa L.)

The characteristics of starch, such as gelatinization temperature (GT), apparent amylose content (AAC), pasting temperature (PT) and other physicochemical properties, determine the quality of various products of rice, e.g., eating, cooking and processing qualities. The GT of rice flour is controlled by the alk locus, which has been co-mapped to the starch synthase IIa (SSIIa) locus. In this study, we sequenced a 2,051 bp DNA fragment spanning part of intron 6, exon 7, intron 7, exon 8 and part of 3' untranslated region of SSIIa for 30 rice varieties with diverse geographical distribution and variation in starch physicochemical properties. A total of 24 single nucleotide polymorphisms (SNPs) and one insertion/deletion (InDel) were identified, which could be classified into nine haplotypes. The mean pairwise nucleotide diversity pi was 0.00292, and Watterson's estimator theta was 0.00296 in this collection of rice germplasm. Tajima's D test for selection showed no significant deviation from the neutral expectation (D = - 0.04612, P > 0.10). However, significant associations were found between seven of the SNPs and peak GT (T (p)) at P < 0.05, of which two contiguous SNPs (GC/TT) showed a very strong association with T (p) (P < 0.0001). With some rare exception, this GC/TT polymorphism alone can differentiate rice varieties with high or intermediate GT (possessing the GC allele) from those with low GT (possessing the TT allele). In contrast, none of these SNPs or InDel was significantly associated with amylose content. A further 509 rice varieties with known physicochemical properties (e.g., AAC and PT) and known alleles of other starch synthesizing genes were genotyped for the SSIIa GC/TT alleles. Association analysis indicated that 82% of the total variation of AAC in these samples could be explained by a (CT)n simple sequence repeat (SSR) and a G/T SNP of Waxy gene (Wx), and 62.4% of the total variation of PT could be explained by the GC/TT polymorphism. An additional association analysis was performed between these molecular markers and the thermal and retrogradation properties for a subset of 245 samples from the 509 rice varieties. The SSIIa GC/TT polymorphism explained more than 60% of the total variation in thermal properties, whereas the SSR and SNP of Wx gene explained as much as the SSIIa GC/TT of the total variation in retrogradation properties. Our study provides further support for the utilization of the GC/TT polymorphism in SSIIa. As shown in our study of 509 rice varieties, the GC/TT SNP could differentiate rice with high or intermediate GT from those with low GT in about 90% of cases. Using four primers in a single PCR reaction, the GC/TT polymorphism can be surveyed on a large scale. Thus, this SNP polymorphism can be very useful in marker-assisted selection for the improvement of GT and other physicochemical properties of rice.