Association Analysis in Rice: From Application to Utilization

Genome wide association mapping of yield and various desirable agronomic traits in Rice

BACKGROUND

Rice (Oryza sativa L.) is one of the staple foods worldwide. To feed the growing population, the improvement of rice cultivars is important. To make the improvement in the rice breeding program, it is imperative to understand the similarities and differences of the existing rice accessions to find out the genetic diversity. Previous studies demonstrated the existence of abundant elite genes in rice landraces. A genome-wide association study (GWAS) was performed for yield and yield related traits to find the genetic diversity.

DESIGN

Experimental study.

METHODS AND RESULTS

A total of 204 SSRs markers were used among 17 SSRs found to be located on each chromosome in the rice genome. The diversity was analyzed using different genetic characters i.e., the total number of alleles (TNA), polymorphic information content (PIC), and gene diversity by Power markers, and the values for each genetic character per marker ranged from 2 to 9, 0.332 to 0.887 and 0.423 to 0.900 respectively across the whole genome. The results of population structure identified four main groups. MTA identified several markers associated with many agronomically important traits. These results will be very useful for the selection of potential parents, recombinants, and MTAs that govern the improvements and developments of new high yielding rice varieties.

CONCLUSIONS

Analysis of diversity in germplasm is important for the improvement of cultivars in the breeding program. In the present study, the diversity was analyzed with different methods and found that enormous diversity was present in the studied rice germplasm. The structure analysis found the presence of 4 genetic groups in the existing germplasm. A total of 129 marker-trait associations (MTAs) have been found in this study.

Genotyping-by-Sequencing Based Investigation of Population Structure and Genome Wide Association Studies for Seven Agronomically Important Traits in a Set of 346 Oryza rufipogon Accessions

Being one of the most important staple dietary constituents globally, genetic enhancement of cultivated rice for yield, agronomically important traits is of substantial importance. Even though the climatic factors and crop management practices impact complex traits like yield immensely, the contribution of variation by underlying genetic factors surpasses them all. Previous studies have highlighted the importance of utilizing exotic germplasm, landraces in enhancing the diversity of gene pool, leading to better selections and thus superior cultivars. Thus, to fully exploit the potential of progenitor of Asian cultivated rice for productivity related traits, genome wide association study (GWAS) for seven agronomically important traits was conducted on a panel of 346 O. rufipogon accessions using a set of 15,083 high-quality single nucleotide polymorphic markers. The phenotypic data analysis indicated large continuous variation for all the traits under study, with a significant negative correlation observed between grain parameters and agronomic parameters like plant height, culm thickness. The presence of 74.28% admixtures in the panel as revealed by investigating population structure indicated the panel to be very poorly genetically differentiated, with rapid LD decay. The genome-wide association analyses revealed a total of 47 strong MTAs with 19 SNPs located in/close to previously reported QTL/genic regions providing a positive analytic proof for our studies. The allelic differences of significant MTAs were found to be statistically significant at 34 genomic regions. A total of 51 O. rufipogon accessions harboured combination of superior alleles and thus serve as potential candidates for accelerating rice breeding programs. The present study identified 27 novel SNPs to be significantly associated with different traits. Allelic differences between cultivated and wild rice at significant MTAs determined superior alleles to be absent at 12 positions implying substantial scope of improvement by their targeted introgression into cultivars. Introgression of novel significant genomic regions into breeder's pool would broaden the genetic base of cultivated rice, thus making the crop more resilient.

Stable and Novel Quantitative Trait Loci (QTL) Confer Narrow Root Cone Angle in an Aerobic Rice (Oryza sativa L.) Production System

BACKGROUND

Aerobic rice production (AP) may be a solution to the looming water crisis by utilising less water compared to traditional flooded culture. As such, development of genotypes with narrow root cone angle (RCA) is considered a key AP adaptation trait as it could lead to deeper rooting and ensure water uptake at depth. Quantitative trait loci (QTL) and genes associated with rooting angle have been identified in rice, but usually in conventional transplanted systems or in upland and drought conditions. This study aimed to identify QTL associated with RCA in AP systems using a recombinant inbred line population derived from IRAT109.

RESULTS

Four experiments conducted in glasshouse and aerobic field conditions revealed significant genotypic variation existed for RCA in the population. Single and multiple QTL models identified the presence of eight QTL distributed in chromosomes 1, 2, 3, 4, and 11. Combined, these QTL explained 36.7-51.2% of the genotypic variance in RCA present in the population. Two QTL, qRCA1.1 and qRCA1.3, were novel and may be new targets for improvement of RCA. Genotypes with higher number of favourable QTL alleles tended to have narrower RCA. qRCA4 was shown to be a major and stable QTL explaining up to 24.3% of the genotypic variation, and the presence of the target allele resulted in as much as 8.6° narrower RCA. Several genes related to abiotic stress stimulus response were found in the qRCA4 region.

CONCLUSION

Stable and novel genomic regions associated with RCA have been identified. Genotypes which had combinations of these QTL, resulted in a narrower RCA phenotype. Allele mining, gene cloning, and physiological dissection should aid in understanding the molecular function and mechanisms underlying RCA and these QTL. Ultimately, our work provides an opportunity for breeding programs to develop genotypes with narrow RCA and deep roots for improved adaptation in an AP system for sustainable rice production.

Association of molecular markers with physio-biochemical traits related to seed vigour in rice

Eighteen physio-biochemical traits influencing seed vigour were studied for their association with molecular markers using a mini core set constituted from 120 germplasm lines. High genetic variation was detected in the parameters namely chlrophyll a, Chlrophyll b, total chlorophyll, carotenoids, total anthocyanin content, gamma-oryzanols, total phenolics content, superoxide dismutase, catalase, guaicol peroxidase, total soluble sugar, total protein, seed vigour index -I and seed vigour index -II. Strong positive correlation of seed vigour index II was observed with amylose content, total anthocyanin content, catalase, total phenolic content and total flavonoid content while a negative association was observed for gamma-oryzanol content. High gene diversity (0.7169) and informative markers value (0.6789) were estimated from the investigation. Three genetic structure groups were observed in the panel population and genotypes were grouped in the subpopulations based on the seed vigour trait. Differences in the fixation indices of the three sub populations indicated existence of linkage disequilibrium in the studied panel population. Association of the traits namely total flavonoids, superoxide dismutase, catalase, chlorophyll a, Chlorophyll b, total chlorophyll, carotenoids, starch, amylose, total anthocyanin, gamma-oryzanol, total phenolics with the molecular markers were detected by Generalized Linear Model and Mixed Linear Model showing > 0.10 R² value. Association of the trait, total flavonoids with marker RM7364 located on chromosome 8 reported in earlier study was validated in this investigation. The validated markers and the novel markers detected showing higher R² value will be useful for improvement of seed vigour in rice.

Identification of QTLs for high grain yield and component traits in new plant types of rice

A panel of 60 genotypes comprising New Plant Types (NPTs) along with indica, tropical and temperate japonica genotypes was phenotypically evaluated for four seasons in irrigated situation for grain yield per se and component traits. Twenty NPT genotypes were found promising with an average grain yield varying from 5.45 to 8.8 t/ha. A total of 85 SSR markers were used in the study to identify QTLs associated with grain yield per se and related traits. Sixty-six (77.65%) markers were found to be polymorphic. The PIC values varied from 0.516 to 0.92 with an average of 0.704. A moderate level of genetic diversity (0.39) was detected among genotypes. Variation to the tune of 8% within genotypes, 68% among the genotypes within the population and 24% among the populations were observed (AMOVA). This information may help in identification of potential parents for development of transgressive segregants with very high yield. The association analysis using GLM and MLM models led to the identification of 30 and 10 SSR markers associated with 70 and 16 QTLs, respectively. Thirty novel QTLs linked with 16 SSRs were identified to be associated with eleven traits, namely tiller number (qTL-6.1, qTL-11.1, qTL-4.1), panicle length (qPL-1.1, qPL-5.1, qPL-7.1, qPL-8.1), flag leaf length (qFLL-8.1, qFLL-9.1), flag leaf width (qFLW-6.2, qFLW-5.1, qFLW-8.1, qFLW-7.1), total no. of grains (qTG-2.2, qTG-a7.1), thousand-grain weight (qTGW-a1.1, qTGW-a9.2, qTGW-5.1, qTGW-8.1), fertile grains (qFG-7.1), seed length-breadth ratio (qSlb-3.1), plant height (qPHT-6.1, qPHT-9.1), days to 50% flowering (qFD-1.1) and grain yield per se (qYLD-5.1, qYLD-6.1a, qYLD-11.1).Some of the SSRs were co-localized with more than two traits. The highest co-localization was identified with RM5709 linked to nine traits, followed by RM297 with five traits. Similarly, RM5575, RM204, RM168, RM112, RM26499 and RM22899 were also recorded to be co-localized with more than one trait and could be rated as important for marker-assisted backcross breeding programs, for pyramiding of these QTLs for important yield traits, to produce new-generation rice for prospective increment in yield potentiality and breaking yield ceiling.

Genome-wide association study of important agronomic traits within a core collection of rice (Oryza sativa L.)

BACKGROUND

Cultivated rice (Oryza sativa L.) is one of the staple food for over half of the world's population. Thus, improvement of cultivated rice is important for the development of the world. It has been shown that abundant elite genes exist in rice landraces in previous studies.

RESULTS

A genome-wide association study (GWAS) performed with EMMAX for 12 agronomic traits measured in both Guangzhou and Hangzhou was carried out using 150 accessions of Ting's core collection selected based on 48 phenotypic traits from 2262 accessions of Ting's collection, the GWAS included more than 3.8 million SNPs. Within Ting's core collection, which has a simple population structure, low relatedness, and rapid linkage disequilibrium (LD) decay, we found 32 peaks located closely to previously cloned genes such as Hd1, SD1, Ghd7, GW8, and GL7 or mapped QTL, and these loci might be natural variations in the cloned genes or QTL which influence potentially agronomic traits. Furthermore, we also detected 32 regions where new genes might be located, and some peaks of these new candidate genes such as the signal on chromosome 11 for heading days were even higher than that of Hd1. Detailed annotation of these significant loci were shown in this study. Moreover, according to the estimated LD decay distance of 100 to 350 kb on the 12 chromosomes in this study, we found 13 identical significant regions in the two locations.

CONCLUSIONS

This research provided important information for further mining these elite genes within Ting's core collection and using them for rice breeding.

Association Study Reveals Genetic Loci Responsible for Arsenic, Cadmium and Lead Accumulation in Rice Grain in Contaminated Farmlands

Accumulation of toxic heavy metals and metalloids (THMMs) in crop grain remarkably affects food safety and human health. Reducing the content of THMMs in grain requires the identification and manipulation of the genes regulating their accumulation. This study aimed to determine the genetic variations affecting grain THMM accumulation in rice by using association mapping. We used 276 accessions with 416 K single nucleotide polymorphisms (SNPs) and performed genome-wide association analysis of grain THMM concentrations in rice grown in heavily multi-contaminated farmlands. We detected 22, 17, and 21 quantitative trait loci (QTLs) for grain arsenic, cadmium, and lead concentrations, respectively. Both inter- and intra-subpopulation variants accounted for these QTLs. Most QTLs contained no known THMM-related genes and represented unidentified novel genes. We examined the candidate genes in qGAS1, a QTL for grain arsenic concentration with the best P-value detected for the entire population. We speculated that a transport protein of the multidrug and toxin extrusion family could be the candidate gene for this QTL. Our study suggested that the genetic regulation of grain THMM accumulation is very complex and largely unknown. The QTLs and SNPs identified in this study might help in the identification of new genes regulating THMM accumulation and aid in marker-assisted breeding of rice with low grain THMM content.

Genetic diversity, linkage disequilibrium, and population structure in a panel of Brazilian rice accessions

Narrowing of genetic diversity and the quantitative nature of most agronomic traits is a challenge for rice breeding. Genome-wide association studies have a great potential to identify important variation in loci underlying quantitative and complex traits; however, before performing the analysis, it is important to assess parameters of the genotypic data and population under study, to improve the accuracy of the genotype-phenotype associations. The aim of this study was to access the genetic diversity, linkage disequilibrium, and population structure of a working panel of Brazilian and several introduced rice accessions, which are currently being phenotyped for a vast number of traits to undergo association mapping. Ninety-four accessions were genotyped with 7098 SNPs, and after filtering for higher call rates and removing rare variants, 93 accessions and 4973 high-quality SNPs remained for subsequent analyses and association studies. The overall mean of the polymorphic information content, heterozygosity, and gene diversity of the SNPs was comparable to other rice panels. The r² measure of linkage disequilibrium decayed to 0.25 in approximately 150 kb, a slow decay, explained by the autogamous nature of rice and the small size of the panel. Regarding population structure, eight groups were formed according to Bayesian clustering. Principle components and neighbor-joining analyses were able to distinguish part of the groups formed, mainly regarding the sub-species indica and japonica. Our results demonstrate that the population and SNPs are of high quality for association mapping.

Superior adaptation of aerobic rice under drought stress in Iran and validation test of linked SSR markers to major QTLs by MLM analysis across two years

Drought is one of the biggest challenges for rice (Oryza sativa L.) production in rainfed areas. Developing "aerobic rice" cultivars could be a valuable alternative to irrigated/rainfed areas. During 2010-2013, 115 rice genotypes, including non-local cultivars and aerobic rice genotypes, were evaluated and 31 rice genotypes were screened, while 21 Iranian lowland rice cultivars (52 genotypes) were investigated under non-stress and drought conditions at the University of Guilan, Rasht, Iran, in 2014 and 2017. The results revealed the superiority of high yielding genotypes, namely Neda (6.202 t ha^- 1), IR82639-B-B-140-1 (6.020 t ha^- 1), and IR82635-B-B-82-2 (5.75 t ha^- 1) under non-stress, Panda (4.512 t ha^- 1), and IR82639-B-B-140-1 (4.08 t ha^- 1), under drought stress conditions. Based on the molecular markers evaluation using identified SSR markers linked to major QTLs different important traits specially drought stress, IR 82639-B-B-140-1 showed the highest genetic distance with high-quality Iranian lowland cultivars, which could be considered as a donor for the development of new cultivars. Moreover, the assignment of rice genotypes based on Jaccard distance clustering was in agreement with the grouping of structure analysis. The validation test using MLM analysis in this natural population revealed the most important significant associations that were identified under drought conditions. These are: the associations between RM306, RM319, RM511, RM28166, and RM11943 with different grain yield (GY)-related traits simultaneously and stable across both years. These markers, which were verified in a natural population across 2 years, could be considered as the potential markers for use in marker-assisted breeding and to improve the grain yield of rice.

Genome-wide association study of rice grain width variation

Seed size is variable within many plant species, and understanding the underlying genetic factors can provide insights into mechanisms of local environmental adaptation. Here we make use of the abundant genomic and germplasm resources available for rice (Oryza sativa) to perform a large-scale genome-wide association study (GWAS) of grain width. Grain width varies widely within the crop and is also known to show climate-associated variation across populations of its wild progenitor. Using a filtered dataset of >1.9 million genome-wide SNPs in a sample of 570 cultivated and wild rice accessions, we performed GWAS with two complementary models, GLM and MLM. The models yielded 10 and 33 significant associations, respectively, and jointly yielded seven candidate locus regions, two of which have been previously identified. Analyses of nucleotide diversity and haplotype distributions at these loci revealed signatures of selection and patterns consistent with adaptive introgression of grain width alleles across rice variety groups. The results provide a 50% increase in the total number of rice grain width loci mapped to date and support a polygenic model whereby grain width is shaped by gene-by-environment interactions. These loci can potentially serve as candidates for studies of adaptive seed size variation in wild grass species.

Genome-wide association mapping for root cone angle in rice

BACKGROUND

Plant root systems play a major role in anchoring and in water and nutrient uptake from the soil. The root cone angle is an important parameter of the root system architecture because, combined with root depth, it helps to determine the volume of soil explored by the plant. Two genes, DRO1 and SOR1, and several QTLs for root cone angle have been discovered in the last 5 years.

RESULTS

To find other QTLs linked to root cone angle, a genome-wide association mapping study was conducted on two panels of 162 indica and 169 japonica rice accessions genotyped with two sets of SNP markers (genotyping-by-sequencing set with approximately 16,000 markers and high-density-rice-array set with approximately 300,000 markers). The root cone angle of all accessions was measured using a screen protractor on images taken after 1 month of plant growth in the Rhizoscope phenotyping system. The distribution of the root cone angle in the indica panel was Gaussian, but several accessions of the japonica panel (all the bulus from Indonesia and three temperate japonicas from Nepal or India) appeared as outliers with a very wide root cone angle. The data were submitted to association mapping using a mixed model with control of structure and kinship. A total of 15 QTLs for the indica panel and 40 QTLs for the japonica panel were detected. Genes underlying these QTLs (+/-50 kb from the significant markers) were analyzed. We focused our analysis on auxin-related genes, kinases, and genes involved in root developmental processes and identified 8 particularly interesting genes.

CONCLUSIONS

The present study identifies new sources of wide root cone angle in rice, proposes ways to bypass some drawbacks of association mapping to further understand the genetics of the trait and identifies candidate genes deserving further investigation.

Genome wide association analysis of cold tolerance at germination in temperate japonica rice (Oryza sativa L.) varieties

A pool of 200 traditional, landraces and modern elite and old cultivars of rice, mainly japonica varieties adapted to temperate regions, have been used to perform a genome wide association study to detect chromosome regions associated to low temperature germination (LTG) regulation using a panel of 1672 SNP markers. Phenotyping was performed by determining growth rates when seeds were germinated at 25° and 15°C in order to separate the germination vigorousness from cold tolerance effects. As expected, the ability to produce viable seedlings varied widely among rice cultivars and also depended greatly on temperature. Furthermore, we observed a differential response during seed germination and in coleoptile elongation. Faster development at 15°C was observed in seeds from varieties traditionally used as cold tolerant parents by breeders, along with other potentially useful cultivars, mainly of Italian origin. When phenotypic data were combined with the panel of SNPs for japonica rice cultivars, significant associations were detected for 31 markers: 7 were related to growth rate at 25°C and 24 to growth rates at 15°. Among the latter, some chromosome regions were associated to LTG while others were related to coleoptile elongation. Individual effects of the associated markers were low, but by combining favourable alleles in a linear regression model we estimated that 27 loci significantly explained the observed phenotypic variation. From these, a core panel of 13 markers was selected and, furthermore, two wide regions of chromosomes 3 and 6 were consistently associated to rice LTG. Varieties with higher numbers of favourable alleles for the panels of associated markers significantly correlated with increased phenotypic values at both temperatures, thus corroborating the utility of the tagged markers for marker assisted selection (MAS) when breeding japonica rice for LTG.

Footprints of domestication revealed by RAD-tag resequencing in loquat: SNP data reveals a non-significant domestication bottleneck and a single domestication event

Background

The process of crop domestication has long been a major area of research to gain insights into the history of human civilization and to understand the process of evolution. Loquat (Eriobotrya japonica Lindl.) is one of the typical subtropical fruit trees, which was domesticated in China at least 2000 years ago. In the present study, we re-sequenced the genome of nine wild loquat accessions collected from wide geographical range and 10 representative cultivated loquat cultivars by using RAD-tag tacit to exploit the molecular footprints of domestication.

Results

We obtained 26.4 Gb clean sequencing data from 19 loquat accessions, with an average of 32.64 M reads per genotype. We identified more than 80,000 SNPs distributed throughout the loquat genome. The SNP density and numbers were slightly higher in the wild loquat populations than that in the cultivated populations. All cultivars were clustered together by structure, phylogenetic and PCA analyses.

Conclusion

The modern loquat cultivars have experienced a non-significant genetic bottleneck during domestication, and originated from a single domesticated event. Moreover, our study revealed that Hubei province of China is probably the origin center of cultivated loquat.

Electronic supplementary material

The online version of this article (doi:10.1186/s12864-017-3738-y) contains supplementary material, which is available to authorized users.

Association Analysis of Markers Derived from Starch Biosynthesis Related Genes with Starch Physicochemical Properties in the USDA Rice Mini-Core Collection

Rice eating and cooking quality is largely determined by starch physicochemical properties. The diverse accessions in the USDA rice mini-core collection (URMC) facilitate extensive association analysis of starch physicochemical properties with molecular markers specific to starch biosynthesis related genes. To identify significant trait-marker associations that can be utilized in rice breeding programs for improved starch quality, we conducted two association analyses between 26 molecular markers derived from starch biosynthesis related genes and 18 parameters measured of starch physicochemical properties in two sets of the mini-core accessions successfully grown in two environments in China. Many significant trait-marker associations (P < 0.001) were detected in both association analyses. Five markers of Waxy gene, including the (CT)n repeats, the G/T SNP of intron 1, the 23 bp sequence duplication (InDel) of exon 2, the A/C SNP of exon 6, and the C/T SNP of exon 10, were found to be primarily associated with starch traits related to apparent amylose content (AAC), and two markers targeting the 4,329-4,330 bp GC/TT SNPs and 4,198 bp G/A SNP of SSIIa gene were mainly associated with traits related to gelatinization temperature (GT). Two new haplotypes were found in the mini-core collection based on the combinations of the 23 bp InDel and three SNPs (G/T of intron 1, A/C of exon 6, and C/T of exon 10) of Waxy gene. Furthermore, our analyses indicated that the (CT)n polymorphisms of Waxy gene had a non-negligible effect on AAC related traits, as evidenced by significant variation in AAC related traits among rice accessions with the same Waxy SNPs but different (CT) n repeats. As the five Waxy markers and the two SSIIa markers showed consistent major effects on starch quality traits across studies, these markers should have priority for utilization in marker-assisted breeding.