Wx Locus Research Articles

Determination of amylose content of traditional rice varieties of Kerala using PCR-based molecular markers

The present study uses molecular approach to assess the amount of amylose in collected rice genotypes using PCR based markers. The polymorphism of Waxy allele which encodes the major enzyme catalyzing amylose synthesis was analyzed in the collected rice samples to determine the amylose content in the samples. All the rice varieties under study expressed Wxa allele corresponding to intermediate to high amylose content. This study proves that simple PCR technique can be used to detect Single nucleotide polymorphism mutation at Wx locus and provides a cost-effective alternative to predict amylose content in rice.

Relationships between starch molecular components and eating and cooking qualities of rice using single-segment substitution lines with different Wx loci

Waxy (Wx) gene in rice endosperm synthesizes amylose and amylopectin extra-long chain (ELC). Five rice lines with different Wx locus single-segment substitutions under the same genetic background were investigated for relationships of their starch molecular components and eating and cooking qualities (ECQs) including pasting, thermal, textural and sensory properties. Rice lines with different Wx loci exhibited different ECQs, and their starches had different apparent amylose contents (AACs), true amylose contents (TACs), ELC contents (ELCCs), and amylopectin chain length distributions. The AAC was correlated positively to hardness and cohesiveness of cooked rice, and negatively to adhesiveness and palatability of cooked rice and breakdown viscosity of rice flour. The TAC was correlated positively to gelatinization temperature range and negatively to gelatinization temperature and enthalpy. ELCC showed a positive relationship with pasting peak time of rice flour and chewiness of cooked rice, and could potentially be used as an important reference for rice ECQ evaluation. This study could help the applications of Wx loci in rice quality breeding.

Registration of 12 soft red winter partial waxy wheat germplasms carrying one or two null alleles at the Wx loci

AbstractPartial waxy wheat (Triticum aestivum L.) with reduced starch amylose content is potentially desirable for making bread with maintained moistness and extended shelf life and noodles with improved sensory acceptance. The USDA‐ARS developed and released 12 soft red winter (SRW) partial waxy wheat germplasms—SWQL‐K‐A‐1 (Reg. no. GP‐1077, PI 702623), SWQL‐K‐B‐1 (Reg. no. GP‐1080, PI 702626), SWQL‐K‐D‐1 (Reg. no. GP‐1082, PI 702628), SWQL‐K‐AB‐1 (Reg. no. GP‐1078, PI 702624), SWQL‐K‐AD‐1 (Reg. no. GP‐1079, PI 702625), SWQL‐K‐BD‐2 (Reg. no. GP‐1081, PI 702627), SWQL‐W‐A‐5 (Reg. no. GP‐1083, PI 702629), SWQL‐W‐B‐1 (Reg. no. GP‐1086, PI 702633), SWQL‐W‐D‐2 (Reg. no. GP‐1088, PI 702636), SWQL‐W‐AB‐4 (Reg. no. GP‐1084, PI 702630), SWQL‐W‐AD‐2 (Reg. no. GP‐1085, PI 702631), and SWQL‐W‐BD‐1 (Reg. no. GP‐1087, PI 702634)—by the introgression of one or two null alleles at the Wx loci from a soft spring waxy wheat germplasm to two SRW wheat cultivars. Twelve partial waxy wheat germplasms were selected from backcross‐four or backcross‐five derivatives of SRW wheat cultivars ‘Kristy’ and ‘Wilson’ (99ID490/5*Kristy and 99ID490/6*Wilson, respectively) by marker‐assisted selection. The kernel hardness of partial waxy wheat germplasms in Kristy and Wilson backgrounds ranged from 62.7 to 64.3 and from 28.4 to 31.7, respectively, whereas that of the corresponding wild type was 58.6 and 30.3, respectively. Amylose contents of isolated starches ranged from 23.9% to 25.8% in single‐null partial waxy wheat germplasms, from 18.4% to 24.3% in double‐null partial waxy wheat germplasms, and from 26.8% to 27.1% in parents and wild types. These partial waxy wheat germplasms would be useful for the development of SRW partial waxy wheat cultivars adapted to the eastern region of the United States and extending the use of SRW wheat in products that require a reduced starch amylose content.

Characterization of a novel Wx-A1b′ null allele and PCR markers for allele identification at three Wx loci

Characterization of a novel Wx-A1b′ null allele and PCR markers for allele identification at three Wx loci

Development of the PARMS Markers of the Waxy Gene and Utilization in Discriminating Wild Accessions, and Cultivated Rice (Oryza sativa L.) with Different Eating and Cooking Quality

Amylose content (AC) is the major indicator of rice eating and cooking quality (ECQ). Its synthesis in rice endosperm is mainly regulated by the protein, granule-bound starch synthase 1, which is encoded by the waxy gene (Os06g0133000, LOC_Os06g04200). The diversity of AC is largely attributable to the allelic variation at the Wx locus and the development of effective and accurate functional molecular markers to target rice variant alleles is crucial in the breeding strategy. In the present study, we developed six pairs of penta-primer amplification refractory mutation system (PARMS) markers to distinguish between Wxlv, Wxa, Wxin, Wxb, Wxmp, and Wxop,hp. These markers were successfully used to screen the genotype of large assets of genetic resources including 98 wild accessions, 55 cultivars, and 22 parental lines. Our results showed that Wxb in a low AC type was predominant in Guangxi cultivated rice as a result of cultural preference, while Wxlv in the wild accessions. Moreover, our findings surprisingly revealed the presence of Wxb in wild accession, which is a new outcome that may contribute to understanding the origin, selection and domestication processes of rice. These functional markers could be effectively used in marker-assisted breeding to improve selection efficiency of cultivars with desired AC in the early generation.

Type‐2 Diabetes and Identification of Major Genetic Determinants of Glycemic Index in Rice – A Review

AbstractRice consumers of different parts of the world always face the risk of diabetes and related health complications. Dietary intervention is the best adapted measure to maintain blood glucose levels and manage the diseases in rice consuming populations. The glycemic index of rice varieties is negatively correlated to the amylose content. Research studies have proven that the consumption of rice varieties with low glycemic index has beneficial effects in controlling diabetes. The genetic basis for the diversity observed for amylose content in different rice varieties is well elucidated. Waxy gene (Wx) is known to encode Granule–Bound Starch Synthase I (GBSS I) enzyme, which determines the amylose content in rice. Different allelic variations of Wx gene exist, which can be correlated to varying amylose content in rice. The PCR‐based markers for detecting single nucleotide polymorphism mutation at Wx locus provides a cost‐effective alternative to genotype rice varieties. This review outlines the anti‐diabetic properties of rice and strategies to identify rice varieties with a low glycemic index at the molecular level. The molecular basis of the glycemic index allows breeders to successfully identify rice germplasms with low glycemic index and engineer rice varieties with high grain yield and low glycemic index.

Registration of ten soft red winter waxy wheat germplasm lines

AbstractWaxy wheat carries null alleles at the three Wx loci encoding the granule‐bound starch synthase I and produces endosperm starch composed of mostly amylopectin with less than 5% amylose. The USDA‐ARS developed and released 10 soft red winter (SRW) waxy wheat germplasm lines, SWQL‐K‐ABD‐1, SWQL‐K‐ABD‐2, SWQL‐K‐ABD‐3, SWQL‐K‐ABD‐4, SWQL‐K‐ABD‐5, SWQL‐W‐ABD‐1, SWQL‐W‐ABD‐2, SWQL‐W‐ABD‐3, SWQL‐W‐ABD‐4, SWQL‐W‐ABD‐5 (Reg. no. GP‐1061 to Reg. no. GP‐1070, PI 698615 to PI 698624), by the introgression of the null alleles at the Wx loci from a soft spring waxy wheat germplasm to two SRW wheat cultivars adapted to the eastern region of the United States. Ten waxy wheat germplasm lines were selected from BC4 or BC5 derivatives of the SRW wheat cultivars ‘Kristy’ and ‘Wilson’ (99ID490/5*Kristy and 99ID490/6*Wilson). The kernel hardness and starch amylose content of 10 SRW waxy wheat germplasm lines ranged from 34.1 to 63.1 and 2.5–7.2%, respectively, while those of wild types and SRW wheat parents carrying the functional alleles at the three Wx loci ranged from 33.9 to 60.4 and 26.8–27.0%, respectively. The starch of the waxy wheat germplasm lines exhibited a lower pasting temperature, higher peak viscosity, and lower setback value than the corresponding wild‐type wheat starch. These waxy wheat germplasm lines would be useful for the development of SRW wheat cultivars carrying different null alleles at the Wx loci that are adapted to the eastern region of the United States.

Variability in waxy (Wx) allele, in-vitro starch digestibility, glycemic response and textural behaviour of popular Northern Himalayan rice varieties

Eight commonly cultivated and consumed rice varieties of Northern Himalayan regions and a popular high amylose rice variety were characterized at Wx locus and evaluated for resistant starch (RS), in-vitro starch digestibility, predicted glycemic index (pGI), glycemic load (GL) and textural parameters. Cytosine and thymine repeats (CT)n at waxy locus (Wx) showed high association with apparent amylose content (AAC). Both pGI and GL varied substantially within the selected varieties. The pGI was relatively lower in high and intermediate amylose Indica varieties compared to low amylose Japonica ones. However, Koshikari despite being a low amylose variety showed relatively lower pGI and GL, due to its higher RS, dietary fiber, protein and fat content. It was thus presumed that in addition to AAC, RS and other grain components also affect the glycemic response. Inherent resistance to enzymatic hydrolysis was also found to be higher in firm textured and less sticky rice varieties. The genotypes—Lalat, Basmati-1509 and Koshikari, in view of their low to moderate pGI and relatively higher RS content, can be explored in future breeding programmes to develop rice varieties whose consumption will help to prevent hyper/hypo glycemic responses in Northern Himalayan regions, where daily staple diet is rice.

The origin of Wxla provides new insights into the improvement of grain quality in rice

Appearance and taste are important factors in rice (Oryza sativa) grain quality. Here, we investigated the taste scores and related eating‐quality traits of 533 diverse cultivars to assess the relationships between—and genetic basis of—rice taste and eating‐quality. A genome‐wide association study highlighted the Wx gene as the major factor underlying variation in taste and eating quality. Notably, a novel waxy (Wx) allele, Wx la, which combined two mutations from Wx b and Wx in, exhibited a unique phenotype. Reduced GBSSI activity conferred Wx la rice with both a transparent appearance and good eating quality. Haplotype analysis revealed that Wx la was derived from intragenic recombination. In fact, the recombination rate at the Wx locus was estimated to be 3.34 kb/cM, which was about 75‐fold higher than the genome‐wide mean, indicating that intragenic recombination is a major force driving diversity at the Wx locus. Based on our results, we propose a new network for Wx evolution, noting that new Wx alleles could easily be generated by crossing genotypes with different Wx alleles. This study thus provides insights into the evolution of the Wx locus and facilitates molecular breeding for quality in rice.

Genetic dissection of eating and cooking qualities in different subpopulations of cultivated rice (Oryza sativa L.) through association mapping

BackgroundEating and cooking qualities (ECQs) of rice (Oryza sativa L.) determine consumer acceptance and the economic value of rice varieties. The starch physicochemical properties, i.e. amylose content, gel consistency, gelatinization temperature and pasting viscosity are important indices for evaluating rice ECQs. Genetic factors are required for development of rice varieties with excellent ECQs and association mapping is one of the promising approaches for discovering such associated genetic factors.ResultsA genome-wide association mapping was performed on a set of 253 non-glutinous rice accessions consisting of 83 indica and 170 japonica cultivated rice varieties through phenotyping for 11 ECQ traits in two consecutive years and genotyping with 210 polymorphic SSR and candidate-gene markers. These markers amplified 747 alleles with an average of 3.57 alleles per locus. The structure, phylogenetic relationship, and principal component analysis indicated a strong population differentiation between indica and japonica accessions and association mapping was thus undertaken within indica and japonica subpopulations. All traits showed a large phenotypic variation and highly significant phenotypic correlations were present between most of traits. A total of 33 and 30 loci were located for 11 ECQs in indica and japonica subpopulations respectively. Most of associated loci were overlapped with starch synthesis-related genes (SSRGs), and the Wx locus gathered 14 associated loci with the largest effects on amylose content, gel consistency and pasting viscosities. Eight subpopulation specific markers, RM588, Wx-(CT)n, SSI and SBE1 for indica subpopulation and RM550, Wxmp, SSIIa and SBE4 for japonica subpopulation, were identified, suggesting alleles of SSRGs showed the subspecific tendency. Nevertheless, allelic variation in SSIIa showed no tendency towards subspecies. One associated maker RM550 detected in japonica subpopulation for amylose content and pasting viscosity was verified a potential novel and stably expressed locus and could be selected for further fine mapping.ConclusionThis study illustrated the potential for dissecting genetic factors of complex traits in domesticated rice subspecies and provided highly associated markers to facilitate marker-assisted selection for breeding high-quality indica or japonica rice varieties.

Genetic diversity analysis of specialty glutinous and low-amylose rice (Oryza sativa L.) landraces of Assam based on Wx locus and microsatellite diversity

The sticky rice of Assam is traditionally classified as bora (glutinous) and chokuwa (semi-glutinous) based on their stickiness after cooking. The Waxy (Wx) gene encodes for granule-bound starch synthase (GBSS) that controls the synthesis of amylose, which is a key determinant of rice end-use quality attributes. In this report, we analysed the level of variation in grain quality traits in a collection of bora and chokuwa cultivars, and examined the nucleotide diversity at the Wx locus of selected rice accessions to identify the possible cause of low-amylose in these rice cultivar groups. The Wx gene sequencing from 24 bora and chokuwa cultivars revealed several nucleotide variations that can explain the variation in the amylose phenotypes. The nucleotide polymorphisms in the downstream intron regions were similar to those reported in Bangladeshi Beruin cultivars. Among the Wx polymorphisms, the CTn microsatellite in exon 1 and G/T SNP in intron 1 (G/T-Int1) should be considered for marker assisted breeding involving bora cultivars. The Wx gene tree, classified the bora accessions possessing the G/T-Int1 SNP as japonicas. However, cluster analysis using microsatellite markers classified the bora and chokuwa cultivars as indica, and intermediate of indica-aus. The findings of this study supplemented our understanding on the evolution of the Wx gene under human selection. The results will assist plant breeders to effectively improve the bora and chokuwa landraces.

Allelic variations of the Wx locus in cultivated rice and their use in the development of hybrid rice in China.

To make better use of global germplasm resources for improving the eating quality of hybrid rice, using the resequencing data from the 3,000 rice genomes project (3K RGP), the allelic variations of the rice Wx locus were analysed. With the exception of five rare alleles discovered for the first time in our study, most of these alleles were known alleles of Wx. Furthermore, a set of Kompetitive allele-specific PCR (KASP) markers based on these Wx alleles have been developed, and thirty-six main parents of hybrid rice from 1976 to 2018 were selected for Wx genotyping. The results showed that only three Wx alleles existed in the main parents of hybrids, and the allelic combination of the hybrids changed from Wxa/Wxb and Wxlv/Wxb to Wxb/Wxb with the development of hybrid rice. Wxb is widely used in the male parents of hybrid rice. Wxa and Wxlv were used in the female parents of early hybrid rice, and they were gradually replaced by Wxb. In the future, more favourable Wx alleles from cultivated rice should be identified, introduced, and effectively used to improve hybrid rice quality.

Single nucleotide polymorphisms in GBBSI and SSIIa genes in relation to starch physicochemical properties in selected rice (Oryza sativa L.) varieties

Single nucleotide polymorphisms in GBBSI and SSIIa genes in relation to starch physicochemical properties in selected rice (Oryza sativa L.) varieties

Genetic Analysis for Cooking and Eating Quality of Super Rice and Fine Mapping of a Novel Locus qGC10 for Gel Consistency.

Rice (Oryza sativa L.) is an important cereal that provides food for more than half of the world’s population. Besides grain yield, improving grain quality is also essential to rice breeders. Amylose content (AC), gelatinization temperature (GT) and gel consistency (GC) are considered to be three indicators for cooking and eating quality in rice. Using a genetic map of RILs derived from the super rice Liang-You-Pei-Jiu with high-density SNPs, we detected 3 QTLs for AC, 3 QTLs for GT, and 8 QTLs for GC on chromosomes 3, 4, 5, 6, 10, and 12. Wx locus, an important determinator for AC and GC, resided in one QTL cluster for AC and GC, qAC6 and qGC6 here. And a novel major QTL qGC10 on chromosome 10 was identified in both Lingshui and Hangzhou. With the BC4F2 population derived from a CSSL harboring the segment for qGC10 from 93-11 in PA64s background, it was fine mapped between two molecular markers within 181 kb region with 27 annotated genes. Quantitative real-time PCR results showed that eight genes were differentially expressed in endosperm of two parents. After DNA sequencing, only LOC_Os10g04900, which encodes a F-box domain containing protein, has 2 bp deletion in the exon of PA64s, resulting in a premature stop codon. Therefore, LOC_Os10g04900 is considered to be the most likely candidate gene for qGC10 associated with gel consistency. Identification of qGC10 provides a new genetic resource for improvement of rice quality.

Conversion of a normal maize hybrid into a waxy version using in vivo CRISPR/Cas9 targeted mutation activity

Waxy maize is a specialty maize that produces mainly amylopectin starch with special food or industrial values. The objective of this study was to overcome the limitations of wx mutant allele acquisition and breeding efficiency by conversion of parental lines from normal to waxy maize. The intended mutation activity was achieved by in vivo CRISPR/Cas9 machinery involving desired-target mutation of the Wx locus in the ZC01 background, abbreviated as ZC01-DTMwx. Triple selection was applied to segregants to obtain high genome background recovery with transgene-free wx mutations. The targeted mutation was identified, yielding six types of mutations among progeny crossed with ZC01-DTMwx. The amylopectin contents of the endosperm starch in mutant lines and hybrids averaged 94.9%, while those of the wild-type controls were significantly (P < 0.01) lower, with an average of 76.9%. Double selection in transgene-free lines was applied using the Bar strip test and Cas9 PCR screening. The genome background recovery ratios of the lines were determined using genome-wide SNP data. That of lines used as male parents was as high as 98.19% and that of lines used as female parents was as high as 86.78%. Conversion hybrids and both parental lines showed agronomic performance similar to that of their wild-type counterparts. This study provides a practical example of the efficient extension of CRISPR/Cas9 targeted mutation to industrial hybrids for transformation of a recalcitrant species.

Identification and Verification of Quantitative Trait Loci Affecting Milling Yield of Rice

Rice is generally consumed in the form of milled rice. The yield of total milled rice and head mill rice is affected by both the paddy rice yield and milling efficiency. In this study, three recombinant inbred line (RIL) populations and one F4:5 population derived from a residual heterozygous (RH) plant were used to determine quantitative trait loci (QTLs) affecting milling yield of rice. Seven traits were analyzed, including recovery of brown rice (BR), milled rice (MR) and head rice (HR); grain yield (GY); and the yield of brown rice (BRY), milled rice (MRY) and head rice (HRY). A total of 77 QTLs distributed on 35 regions was detected in the three RIL populations. Four regions, where qBR5, qBR7, qBR10, and qBR12 were located, were validated in the RH-derived F4:5 population. In the three RIL populations, all the 11 QTLs for GY detected were accompanied with QTLs for two or all the three milling yield traits. Not only the allele direction for milling yield traits was unchanged, but also the effects were consistent with GY. In the RH-derived F4:5 population, regions controlling GY also affected all three milling yield traits. Results indicated that variations of BRY and MRY were mainly ascribed to GY, but HRY was determined by both GY and HR. Results also showed that the regions covering GW5–Chalk5 and Wx loci had major effects on milling quality and milling yield of rice. These two regions, which have been known to affect multiple traits determining grain quality and yield of rice, provide good candidates for milled yield improvement.

Identification and verification of quantitative trait loci for eating and cooking quality of rice (Oryza sativa)

AbstractThis study was conducted to identify new quantitative trait loci (QTLs) that have stable effects for eating and cooking quality (ECQ) of rice. Three recombinant inbred line populations of indica rice were each planted in two years. Three traits for ECQ, amylose content (AC), gel consistency (GC) and alkali spreading value (ASV), were measured for QTL analysis. A total of 13 QTLs were detected, including four for AC, six for ASV and three for GC. Two QTLs, qGC4 in the interval RM16252–RM335 on the short arm of chromosome 4 and qGC6.2 in the Alk region, were validated in a population derived from a residual heterozygote that was homozygous at the major locus Wx. In the absence of segregation at the Wx locus, qGC4 and qGC6.2 had additive effects of 2.46 and 8.18 mm, respectively, offering a potential for improving GC property of rice varieties. Comparison between qGC4 and previous results suggests that qGC4 is likely a new QTL for GC, providing a candidate for gene cloning and functional characterization.

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

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.

Genetic Basis of Variation in Rice Seed Storage Protein (Albumin, Globulin, Prolamin, and Glutelin) Content Revealed by Genome-Wide Association Analysis.

Rice seed storage protein (SSP) is an important source of nutrition and energy. Understanding the genetic basis of SSP content and mining favorable alleles that control it will be helpful for breeding new improved cultivars. An association analysis for SSP content was performed to identify underlying genes using 527 diverse Oryza sativa accessions grown in two environments. We identified more than 107 associations for five different traits, including the contents of albumin (Alb), globulin (Glo), prolamin (Pro), glutelin (Glu), and total SSP (Total). A total of 28 associations were located at previously reported QTLs or intervals. A lead SNP sf0709447538, associated for Glu content in the indica subpopulation in 2015, was further validated in near isogenic lines NIL(Zhenshan97) and NIL(Delong208), and the Glu phenotype had significantly difference between two NILs. The association region could be target for map-based cloning of the candidate genes. There were 13 associations in regions close to grain-quality-related genes; five lead single nucleotide polymorphisms (SNPs) were located less than 20 kb upstream from grain-quality-related genes (PG5a, Wx, AGPS2a, RP6, and, RM1). Several starch-metabolism-related genes (AGPS2a, OsACS6, PUL, GBSSII, and ISA2) were also associated with SSP content. We identified favorable alleles of functional candidate genes, such as RP6, RM1, Wx, and other four candidate genes by haplotype analysis and expression pattern. Genotypes of RP6 and RM1 with higher Pro were not identified in japonica and exhibited much higher expression levels in indica group. The lead SNP sf0601764762, repeatedly detected for Alb content in 2 years in the whole association population, was located in the Wx locus that controls the synthesis of amylose. And Alb content was significantly and negatively correlated with amylose content and the level of 2.3 kb Wx pre-mRNA examined in this study. The associations or candidate genes identified would provide new insights into the genetic basis of SSP content that will help in developing rice cultivars with improved grain nutritional quality through marker-assisted breeding.

Effects of different Wx alleles on amylopectin molecular structure and enzymatic hydrolysis properties of rice starch

ABSTRACTAllelic variation at the Wx locus plays a vital role in regulating apparent amylose content (AAC) as well as other important starch properties in rice grain. However, the extent of Wx allelic variation affecting amylopectin fine structure and enzymatic hydrolysis properties of rice starch remains unknown. In this paper, a series of rice single-segment substitution lines with five different Wx alleles (wx, Wxt, Wxg1, Wxg2 and Wxg3) were used for comparative studies. The results showed that the five Wx alleles exhibited a wide variation in amylopectin molecular structure, hydrolysis and in vitro digestion properties of rice starch. Amylopectin chain length distribution as evaluated by fluorophore-assisted capillary electrophoresis was significantly different for the five Wx alleles, which might result in different functional properties. The hydrolysis degree of the five Wx genotype starches by α-amylase or amyloglucocidase was in the order wx > Wxt > Wxg1 > Wxg3 > Wxg2. The rapidly digestible starch (RDS) content of the tested rice starches followed the order wx = Wxg3 > Wxt = Wxg1 > Wxg2. The extent decrease of slowly digestible starch (SDS) content was wx = Wxt > Wxg1 > Wxg2 > Wxg3, while the trend in resistant starch (RS) content was Wxg2 > Wxg3 > Wxg1 > Wxt > wx. Correlation analysis indicated that AAC was significantly positively correlated with RS but significantly negatively correlated with SDS and hydrolysis. The RDS showed a significant positive correlation with amylopectin short chains (DP 6–12) but no significant correlations with other structural properties. The present results could provide important information for the application of rice starches with different Wx genotypes.