Analyzing the response of rice to tefuryltrione herbicide: Haplotype variation and evolutionary dynamics of the HIS1 gene.

Weeds pose multifaceted challenges in rice cultivation, leading to substantial economic losses through reduced yield and poor grain quality. Harnessing the natural genetic diversity in germplasm collections becomes crucial for identifying novel herbicide resistance loci in crops. A comprehensive analysis was conducted on 475 rice accessions from the KRICE depository, assessing their response to TFT (tefuryltrione) and probing the underlying HIS1 (HPPD INHIBITOR SENSITIVE 1) genotypic variations. The HIS1 gene, responsible for detoxifying benzobicyclon (BBC) and imparting broad-spectrum herbicide resistance, holds significant promise in rice breeding. This study explores the genetic landscape of HIS1 within Korean rice collection (KRICE), aiming to unveil genetic variations, haplotype diversity, and evolutionary relationships across diverse rice ecotypes. The indica ecotype showed the highest nucleotide diversity, while the wild and temperate japonica groups exhibited low diversity, hinting at selective sweeps and possible population expansion. Negative Tajima's D values in temperate japonica and wild groups indicate an excess of low-frequency mutations, potentially resulting from selective sweeps. In contrast, with positive Tajima's D values, admixture, indica, and aus groups suggest balancing selection. Furthermore, haplotype analysis uncovered 42 distinct haplotypes within KRICE, with four shared haplotypes between cultivated and wild accessions, four specific to cultivated accessions, and 34 specific to wild types. Phenotypic assessments of these haplotypes revealed that three haplotypes, viz., Hap_1 (predominant in japonica), Hap_2 (predominant in indica), and Hap_3 (specific to indica), displayed significant differences from aus-specific Hap_4 and indica-specific Hap_5. This study offers insights into genetic diversity, selective pressures, and ecotype-specific responses, ultimately paving the way for developing HPPD-inhibiting herbicide-resistant rice cultivars.

Harnessing γ-TMT Genetic Variations and Haplotypes for Vitamin E Diversity in the Korean Rice Collection.

Gamma-tocopherol methyltransferase (γ-TMT), a key gene in the vitamin E biosynthesis pathway, significantly influences the accumulation of tocochromanols, thereby determining rice nutritional quality. In our study, we analyzed the γ-TMT gene in 475 Korean rice accessions, uncovering 177 genetic variants, including 138 SNPs and 39 InDels. Notably, two functional SNPs, tmt-E2-28,895,665-G/A and tmt-E4-28,896,689-A/G, were identified, causing substitutions from valine to isoleucine and arginine to glycine, respectively, across 93 accessions. A positive Tajima's D value in the indica group suggests a signature of balancing selection. Haplotype analysis revealed 27 haplotypes, with two shared between cultivated and wild accessions, seven specific to cultivated accessions, and 18 unique to wild types. Further, profiling of vitamin E isomers in 240 accessions and their association with haplotypes revealed that Hap_2, distinguished by an SNP in the 3' UTR (tmt-3UTR-28,897,360-T/A) exhibited significantly lower α-tocopherol (AT), α-tocotrienol (AT3), total tocopherol, and total tocotrienol, but higher γ-tocopherol (GT) in the japonica group. Additionally, in the indica group, Hap_2 showed significantly higher AT, AT3, and total tocopherol, along with lower GT and γ-tocotrienol, compared to Hap_19, Hap_20, and Hap_21. Overall, this study highlights the genetic landscape of γ-TMT and provides a valuable genetic resource for haplotype-based breeding programs aimed at enhancing nutritional profiles.

Genomic landscape of the OsTPP7 gene in its haplotype diversity and association with anaerobic germination tolerance in rice.

Early season flooding is a major constraint in direct-seeded rice, as rice genotypes vary in their coleoptile length during anoxia. Trehalose-6-phosphate phosphatase 7 (OsTPP7, Os09g0369400) has been identified as the genetic determinant for anaerobic germination (AG) and coleoptile elongation during flooding. We evaluated the coleoptile length of a diverse rice panel under normal and flooded conditions and investigated the Korean rice collection of 475 accessions to understand its genetic variation, population genetics, evolutionary relationships, and haplotypes in the OsTPP7 gene. Most accessions displayed enhanced flooded coleoptile lengths, with the temperate japonica ecotype exhibiting the highest average values for normal and flooded conditions. Positive Tajima's D values in indica, admixture, and tropical japonica ecotypes suggested balancing selection or population expansion. Haplotype analysis revealed 18 haplotypes, with three in cultivated accessions, 13 in the wild type, and two in both. Hap_1 was found mostly in japonica, while Hap-2 and Hap_3 were more prevalent in indica accessions. Further phenotypic performance of major haplotypes showed significant differences in flooded coleoptile length, flooding tolerance index, and shoot length between Hap_1 and Hap_2/3. These findings could be valuable for future selective rice breeding and the development of efficient haplotype-based breeding strategies for improving flood tolerance.

Multi-omics analysis reveals the genetic basis of rice fragrance mediated by betaine aldehyde dehydrogenase 2.

INTRODUCTION: Fragrance is an important economic and quality trait in rice. The trait is controlled by the recessive gene betaine aldehyde dehydrogenase 2 (BADH2) via the production of 2-acetyl-1-pyrroline (2AP). OBJECTIVES: Variation in BADH2 was evaluated at the population, genetic, transcriptional, and metabolic levels to obtain insights into fragrance regulation in rice. METHODS: Whole-genome resequencing of the Korean World Rice Collection of 475 rice accessions, including 421 breeding lines and 54 wild accessions, was performed. Transcriptome analyses of a subset of 279 accessions, proteome analyses of 64 accessions, and volatile profiling of 421 breeding lines were also performed. RESULTS: We identified over 3.1 million high-quality single nucleotide polymorphisms (SNPs) in Korean rice collection. Most SNPs were present in intergenic regions (79%), and 190,148 SNPs (6%) were located in the coding sequence, of which 53% were nonsynonymous. In total, 38 haplotypes were identified in the BADH2 coding region, including four novel haplotypes (one in cultivated and three in wild accessions). Tajima's D values suggested that BADH2 was under balancing selection in japonica rice. Furthermore, we identified 316 expression quantitative trait loci (eQTL), including 185 cis-eQTLs and 131 trans-eQTLs, involved in BADH2 regulation. A protein quantitative trait loci (pQTL) analysis revealed the presence of trans-pQTLs; 13 pQTLs were mapped 1 Mbp from the BADH2 region. Based on variable importance in projection (VIP) scores, 15 volatile compounds, including 2AP, discriminated haplotypes and were potential biomarkers for rice fragrance. CONCLUSION: We generated a catalog of haplotypes based on a resequencing analysis of a large number of rice accessions. eQTLs and pQTLs associated with BADH2 gene expression and protein accumulation are likely involved in the regulation of 2AP variation in fragrant rice. These data improve our understanding of fragrance and provide valuable information for rice breeding.

Development of an inclusive 580K SNP array and its application for genomic selection and genome-wide association studies in rice.

Rice is a globally cultivated crop and is primarily a staple food source for more than half of the world's population. Various single-nucleotide polymorphism (SNP) arrays have been developed and utilized as standard genotyping methods for rice breeding research. Considering the importance of SNP arrays with more inclusive genetic information for GWAS and genomic selection, we integrated SNPs from eight different data resources: resequencing data from the Korean World Rice Collection (KRICE) of 475 accessions, 3,000 rice genome project (3 K-RGP) data, 700 K high-density rice array, Affymetrix 44 K SNP array, QTARO, Reactome, and plastid and GMO information. The collected SNPs were filtered and selected based on the breeder's interest, covering all key traits or research areas to develop an integrated array system representing inclusive genomic polymorphisms. A total of 581,006 high-quality SNPs were synthesized with an average distance of 200 bp between adjacent SNPs, generating a 580 K Axiom Rice Genotyping Chip (580 K _ KNU chip). Further validation of this array on 4,720 genotypes revealed robust and highly efficient genotyping. This has also been demonstrated in genome-wide association studies (GWAS) and genomic selection (GS) of three traits: clum length, heading date, and panicle length. Several SNPs significantly associated with cut-off, -log10 p-value >7.0, were detected in GWAS, and the GS predictabilities for the three traits were more than 0.5, in both rrBLUP and convolutional neural network (CNN) models. The Axiom 580 K Genotyping array will provide a cost-effective genotyping platform and accelerate rice GWAS and GS studies.

Functional Haplotypes and Evolutionary Insight into the Granule-Bound Starch Synthase II (GBSSII) Gene in Korean Rice Accessions (KRICE_CORE).

Granule-bound starch synthase 2 (GBSSII), a paralogous isoform of GBSSI, carries out amylose biosynthesis in rice. Unlike GBSSI, it mainly functions in transient organs, such as leaves. Despite many reports on the starch gene family, little is known about the genetics and genomics of GBSSII. Haplotype analysis was conducted to unveil genetic variations (SNPs and InDels) of GBSSII (OS07G0412100) and it was also performed to gain evolutionary insight through genetic diversity, population genetic structure, and phylogenetic analyses using the KRICE_CORE set (475 rice accessions). Thirty nonsynonymous SNPs (nsSNPs) were detected across the diverse GBSSII coding regions, representing 38 haplotypes, including 13 cultivated, 21 wild, and 4 mixed (a combination of cultivated and wild) varieties. The cultivated haplotypes (C_1-C_13) contained more nsSNPs across the GBSSII genomic region than the wild varieties. Nucleotide diversity analysis highlighted the higher diversity values of the cultivated varieties (weedy = 0.0102, landrace = 0.0093, and bred = 0.0066) than the wild group (0.0045). The cultivated varieties exhibited no reduction in diversity during domestication. Diversity reduction in the japonica and the wild groups was evidenced by the negative Tajima's D values under purifying selection, suggesting the domestication signatures of GBSSII; however, balancing selection was indicated by positive Tajima's D values in indica. Principal component analysis and population genetics analyses estimated the ambiguous evolutionary relationships among the cultivated and wild rice groups, indicating highly diverse structural features of the rice accessions within the GBSSII genomic region. FST analysis differentiated most of the classified populations in a range of greater FST values. Our findings provide evolutionary insights into GBSSII and, consequently, a molecular breeding program can be implemented for select desired traits using these diverse nonsynonymous (functional) alleles.

Haplotype Variations and Evolutionary Analysis of the Granule-Bound Starch Synthase I Gene in the Korean World Rice Collection.

Granule-bound starch synthase I (GBSSI) is responsible for Waxy gene encoding the, which is involved in the amylose synthesis step of starch biosynthesis. We investigated the genotypic and haplotypic variations of GBSSI (Os06g0133000) gene, including its evolutionary relatedness in the nucleotide sequence level using single-nucleotide polymorphisms (SNPs), indels, and structural variations (SVs) from 475 Korean World Rice Collection (KRICE_CORE), which comprised 54 wild rice and 421 cultivated represented by 6 ecotypes (temperate japonica, indica, tropical japonica, aus, aromatic, and admixture) or in another way by 3 varietal types (landrace, weedy, and bred). The results revealed that 27 of 59 haplotypes indicated a total of 12 functional SNPs (fSNPs), identifying 9 novel fSNPs. According to the identified novel fSNPs, we classified the entire rice collection into three groups: cultivated, wild, and mixed (cultivated and wild) rice. Five novel fSNPs were localized in wild rice: four G/A fSNPs in exons 2, 9, and 12 and one T/C fSNP in exon 13. We also identified the three previously reported fSNPs, namely, a G/A fSNP (exon 4), an A/C fSNP (exon 6), and a C/T fSNP (exon 10), which were observed only in cultivated rice, whereas an A/G fSNP (exon 4) was observed exclusively in wild rice. All-against-all comparison of four varietal types or six ecotypes of cultivated rice with wild rice showed that the GBSSI diversity was higher only in wild rice (π = 0.0056). The diversity reduction in cultivated rice can be useful to encompass the origin of this gene GBSSI during its evolution. Significant deviations of positive (wild and indica under balancing selection) and negative (temperate and tropical japonica under purifying selection) Tajima's D values from a neutral model can be informative about the selective sweeps of GBSSI genome insights. Despite the estimation of the differences in population structure and principal component analysis (PCA) between wild and subdivided cultivated subgroups, an inbreeding effect was quantified by F ST statistic, signifying the genetic relatedness of GBSSI. Our findings of a novel wild fSNPS can be applicable for future breeding of waxy rice varieties. Furthermore, the signatures of selective sweep can also be of informative into further deeper insights during domestication.

Haplotype Analysis of BADH1 by Next-Generation Sequencing Reveals Association with Salt Tolerance in Rice during Domestication.

Betaine aldehyde dehydrogenase 1 (BADH1), a paralog of the fragrance gene BADH2, is known to be associated with salt stress through the accumulation of synthesized glycine betaine (GB), which is involved in the response to abiotic stresses. Despite the unclear association between BADH1 and salt stress, we observed the responses of eight phenotypic characteristics (germination percentage (GP), germination energy (GE), germination index (GI), mean germination time (MGT), germination rate (GR), shoot length (SL), root length (RL), and total dry weight (TDW)) to salt stress during the germination stage of 475 rice accessions to investigate their association with BADH1 haplotypes. We found a total of 116 SNPs and 77 InDels in the whole BADH1 gene region, representing 39 haplotypes. Twenty-nine haplotypes representing 27 mutated alleles (two InDels and 25 SNPs) were highly (p < 0.05) associated with salt stress, including the five SNPs that have been previously reported to be associated with salt tolerance. We observed three predominant haplotypes associated with salt tolerance, Hap_2, Hap_18, and Hap_23, which were Indica specific, indicating a comparatively high number of rice accessions among the associated haplotypes. Eight plant parameters (phenotypes) also showed clear responses to salt stress, and except for MGT (mean germination time), all were positively correlated with each other. Different signatures of domestication for BADH1 were detected in cultivated rice by identifying the highest and lowest Tajima's D values of two major cultivated ecotypes (Temperate Japonica and Indica). Our findings on these significant associations and BADH1 evolution to plant traits can be useful for future research development related to its gene expression.

Functional Haplotype and eQTL Analyses of Genes Affecting Cadmium Content in Cultivated Rice.

BACKGROUND: Rice is a major food resource for Asian countries including Korea. However, most Asian countries are facing food safety problems due to cropland contamination by heavy metals. Thus, this study was conducted to investigate genetic factors affecting the expression of cadmium (Cd) gene, and to confirm differences in Cd translocation among cultivars because the current molecular understanding of Cd uptake-transport mechanisms remains insufficient. Associations between genotypes and gene expression level of Cd-related genes such as NRAMP, MTP, and HMA gene families in the rice core collection were analyzed at the genomic level. RESULTS: Os01g0956700, Os05g0128400 and Os11g0485200 showed strong associations between expression level and genotype in the rice core collection, the regulatory factors that associated with these genes in cis and trans were founded. The association between the expression level and genotype of the candidate gene (Os01g0611300: metal tolerance protein) predicted to affect Cd content in rice by a previous genome-wide association study (GWAS) was also analyzed. Furthermore, as a result of the phylogeny and haplotype analyses of the candidate gene, high-Cd tolerance cultivars were selected. The correlations between Cd and other inorganic components (Mg, Mn, Fe, Cu and Zn) in the roots, stems, leaves and unpolished grain of selected rice cultivars were analyzed. CONCLUSION: Therefore, these results may be useful for understanding the uptake-transport mechanisms of Cd and other inorganic components via molecular genetics and may help rice breeders develop new low-Cd cultivars in the near future.

Genome-Wide Association Study for Squalene Contents and Functional Haplotype Analysis in Rice.

Squalene is an isoprenoid compound that acts as the intermediate metabolite in cholesterol synthesis. Squalene is not very susceptible to peroxidation, and it quenches singlet oxygen in the skin, which is caused by UV exposure and other ionizing radiation sources. Squalene is a precursor to phytosterol synthesis, and it has been widely studied for its ability to reduce oxidation, cancer activity, and cholesterol levels. We performed a genome-wide association study for squalene in rice using 1.6 million high-quality SNPs extracted from 295 accessions' resequencing data. The candidate gene locus Os09g0319800-an orthologue of terpene synthase in Arabidopsis-showed up as the most likely candidate gene amongst the identified loci. Nucleotide variations in the promoter were associated with squalene content variations within the japonica group. The results of this study can provide clues for understanding the mechanisms of squalene biosynthesis in rice.

Signatures of differential selection in chloroplast genome between japonica and indica.

BACKGROUND: The domestication process of Asian rice (Oryza sativa L.) is complicated. It's well established that Oryza rufipogon is the ancestor of Asian rice, although the number of domestication events still controversial. Recently, numerous types of studies based on rice nuclear genome have been conducted, but the results are quite different. Chloroplasts (cp) are also part of the rice genome and have a conserved cyclic structure that is valuable for plant genetics and evolutionary studies. Therefore, we conducted chloroplast-based studies, aiming to provide more evidence for the domestication of Asian rice. RESULTS: A total of 1389 variants were detected from the chloroplast genomes of 412 accessions obtained through the world. Oryza sativa L. ssp. japonica exhibited slightly less diversity (π) than Oryza sativa L. indica and wild rice. The fixation index values (FST) revealed that indica and japonica exhibited farther genetic distances compared with wild rice. Across cp genome, Tajima's D test demonstrated that different selection sites occurred in Asian rice. Principal component analyses (PCA) and multidimensional scaling (MDS) clearly classify the Asian rice into different groups. Furthermore, introgression patterns identified that indica and japonica shared no introgression events in cp level, and phylogenetic studies showed cultivated rice were well separated from different type of wild rice. CONCLUSIONS: Here, we focus on the domestication of Asian rice (indica and japonica). Diversity and phylogenetic analyses revealed some selection characteristics in the chloroplast genome that potentially occurred in different Asian rice during the domestication. The results shown that Asian rice had been domesticated at least twice. In additional, japonica may experience a strong positive selection or bottleneck event during the domestication.

Genome-wide association study of vitamin E using genotyping by sequencing in sesame (Sesamum indicum).

BACKGROUND: At least eight structurally related forms of vitamin E occur in nature, four tocopherols and four tocotrienols, all of which are potent membrane-soluble antioxidants. In this study, we detected two major isoforms in sesame (Sesamum indicum L.) seed: γ-tocopherol and ß-tocotrienol. The objective of this study is to investigate the genetic basis of these vitamin E isoforms. METHODS: We  conducted a genome-wide association study (GWAS) using 5962 genome-wide markers, acquired from 96 core sesame accessions. The GWAS was performed using generalized linear (GLM) and mixed linear (MLM) models. RESULTS: LG08_6621957, on chromosome 8, was detected as having a significant association with γ-tocopherol in both models. It explained 20.9% of γ-tocopherol variation in sesame. For ß-tocotrienol, no significant loci were detected according to the two models, but one locus, SLG03_13104062, explained 17.8% of the phenotypic variation. Based on structure and phylogenetic studies, the 96 accessions were clearly clustered into two subpopulations. CONCLUSION: This study on sesame demonstrates and provides an evidence that genotyping by sequencing (GBS) based GWAS can be used to identifying important loci for small growing crops. The significant SNPs or genes could be useful for improving the vitamin E content in sesame breeding programs.

Isolation and characterization of a dominant dwarf gene, d-h, in rice.

Plant height is an important agronomic trait that affects grain yield. Previously, we reported a novel semi-dominant dwarf mutant, HD1, derived from chemical mutagenesis using N-methyl-N-nitrosourea (MNU) on a japonica rice cultivar, Hwacheong. In this study, we cloned the gene responsible for the dwarf mutant using a map-based approach. Fine mapping revealed that the mutant gene was located on the short arm of chromosome 1 in a 48 kb region. Sequencing of the candidate genes and rapid amplification of cDNA ends-polymerase chain reaction (RACE-PCR) analysis identified the gene, d-h, which encodes a protein of unknown function but whose sequence is conserved in other cereal crops. Real-time (RT)-PCR analysis and promoter activity assays showed that the d-h gene was primarily expressed in the nodes and the panicle. In the HD1 plant, the d-h gene was found to carry a 63-bp deletion in the ORF region that was subsequently confirmed by transgenic experiments to be directly responsible for the gain-of-function phenotype observed in the mutant. Since the mutant plants exhibit a defect in GA response, but not in the GA synthetic pathway, it appears that the d-h gene may be involved in a GA signaling pathway.

Analysis of segregation distortion and its relationship to hybrid barriers in rice.

BACKGROUND: Segregation distortion (SD) is a frequently observed occurrence in mapping populations generated from crosses involving divergent genotypes. In the present study, ten genetic linkage maps constructed from reciprocal F2 and BC1F1 mapping populations derived from the parents Dasanbyeo (indica) and Ilpumbyeo (japonica) were used to identify the distribution, effect, and magnitude of the genetic factors underlying the mechanisms of SD between the two subspecies. RESULTS: SD loci detected in the present study were affected by male function, female function, and zygotic selection. The most pronounced SD loci were mapped to chromosome 3 (transmitted through male gametes), chromosome 5 (transmitted through male gametes), and chromosome 6 (transmitted through female gametes). The level of SD in BC1F1 populations which defined by chi-square value independence multiple tests was relatively low in comparison to F2 populations. Dasanbyeo alleles were transmitted at a higher frequency in both F2 and BC1F1 populations, suggesting that indica alleles are strongly favored in inter-subspecific crosses in rice. SD loci in the present study corresponded to previously reported loci for reproductive barriers. In addition, new SD loci were detected on chromosomes 2 and 12. CONCLUSION: The identification of the distribution of SD and the effect of genetic factors causing SD in genetic mapping populations provides an opportunity to survey the whole genome for new SD loci and their relationships to reproductive barriers. This provides a basis for future research on the elucidation of the genetic mechanisms underlying SD in rice, and will be useful in molecular breeding programs.

Single nucleotide polymorphisms and haplotype diversity in rice sucrose synthase 3.

Rice sucrose synthase 3 (RSUS3) is expressed predominantly in rice seed endosperm and is thought to play an important role in starch filling during the milky stage of rice seed ripening. Because the genetic diversity of this locus is not known yet, the full sequence of RSUS3 from 43 rice varieties was amplified to examine the distribution of DNA polymorphisms. A total of 254 sequence variants, including SNPs and insertion/deletions, were successfully identified in the 7733 bp sequence that comprises the promoter, exons and introns, and 3' downstream nontranscribed region (NTR). Eleven haplotypes were distinguished among the 43 rice varieties based on nucleotide variation in the 3 defined regions (5' NTR, transcript, and 3' NTR). The promoter region showed evidence of a base change on a cis-element that might influence the functional role of the motif in seed-specific expression. The genetic diversity of the RSUS3 gene sequences in the rice germplasm used in this study appears to be the result of nonrandom processes. Analysis of polymorphism sites indicated that at least 11 recombinations have occurred, primarily in the transcribed region. This finding provides insight into the development of a cladistic approach for establishing future genetic association studies of the RSUS3 locus.

Map-based cloning of the ERECT PANICLE 3 gene in rice.

Panicle architecture in rice can have a strong influence on yield. Using N-methyl-N-nitrosourea mutagenesis, we isolated an erect panicle mutant, Hep, from Hwasunchalbyeo, a glutinous japonica rice cultivar. Genetic analysis revealed that the erect panicle phenotype was controlled by a single recessive mutation designated erect panicle 3 (ep3). Genetic mapping revealed that the ep3 mutation was located on the short arm of chromosome 2 in a 0.1 cM region delimited by the STS markers STS5803-5 and STS5803-7. The ep3 locus corresponded to 46.8 kb region and contained six candidate genes. Comparison of the DNA sequences of the candidate genes from wild-type and erect panicle plants revealed a single base-pair change in the second exon of LOC_Os02g15950, which is predicted to result in a nonsense mutation. LOC_Os02g15950 encodes a putative F-box protein containing 515 amino acids and is expressed throughout the plant during all growth stages. A line carrying a T-DNA insertion in LOC_ Os02g15950 was obtained and shown to have the same phenotype as the ep3 mutant, thus confirming the identification of LOC_Os02g15950 as the ERECT PANICLE 3 (EP3) gene. The ep3 mutation causes a significant increase in the number of small vascular bundles as well as the thickness of parenchyma in the peduncle, which results in the erect panicle phenotype.

High-resolution mapping of two rice brown planthopper resistance genes, Bph20(t) and Bph21(t), originating from Oryza minuta.

Brown planthopper (BPH) is one of the most destructive insect pests of rice. Wild species of rice are a valuable source of resistance genes for developing resistant cultivars. A molecular marker-based genetic analysis of BPH resistance was conducted using an F(2) population derived from a cross between an introgression line, 'IR71033-121-15', from Oryza minuta (Accession number 101141) and a susceptible Korean japonica variety, 'Junambyeo'. Resistance to BPH (biotype 1) was evaluated using 190 F(3) families. Two major quantitative trait loci (QTLs) and two significant digenic epistatic interactions between marker intervals were identified for BPH resistance. One QTL was mapped to 193.4-kb region located on the short arm of chromosome 4, and the other QTL was mapped to a 194.0-kb region on the long arm of chromosome 12. The two QTLs additively increased the resistance to BPH. Markers co-segregating with the two resistance QTLs were developed at each locus. Comparing the physical map positions of the two QTLs with previously reported BPH resistance genes, we conclude that these major QTLs are new BPH resistance loci and have designated them as Bph20(t) on chromosome 4 and Bph21(t) on chromosome 12. This is the first report of BPH resistance genes from the wild species O. minuta. These two new genes and markers reported here will be useful to rice breeding programs interested in new sources of BPH resistance.

PCR marker-based evaluation of the eating quality of japonica rice ( Oryza sativa L.).

Evaluation of eating quality in early breeding generations of rice is critical to developing varieties with better palatability. This paper reports DNA markers associated with eating quality of temperate japonica rice and an evaluation method aided by multiple regression analysis. A total of 30 markers comprising STSs, SNPs, and SSRs were tested for their association with palatability using 22 temperate japonica varieties with different palatability values. Eating quality-related traits of the 22 varieties were also measured. Of the 30 markers, 18 were found to be significantly associated with palatability and, consequently, a model regression equation with an R(2) value of 0.99 was formulated to estimate the palatability by the marker data set. Validation of the model equation using selected breeding lines indicated that the marker set and the equation are highly applicable to evaluation of the palatability of cooked rice in temperate japonica varieties.

Comparison of molecular linkage maps and QTLs for morphological traits in two reciprocal backcross populations of rice.

Comparison of maps and QTLs between populations may provide us with a better understanding of molecular maps and the inheritance of traits. We developed and used two reciprocal BC1F1 populations, IP/DS//IP and IP/DS//DS, for QTL analysis. DS (Dasanbyeo) is a Korean tongil-type cultivar (derived from an indica x japonica cross and similar to indica in its genetic make-up) and IP (Ilpumbyeo) is a Korean japonica cultivar. We constructed two molecular linkage maps corresponding to each backcross population using 196 markers for each map. The length of each chromosome was longer in the IP/DS//IP population than in the IP/DS//DS population, indicating that more recombinants were produced in the IP/DS//IP population. Distorted segregation was observed for 44 and 19 marker loci for the IP/DS//IP and IP/DS//DS populations, respectively; these were mostly skewed in favor of the indica alleles. A total of 36 main effect QTLs (M-QTLs) and 15 digenic epistatic interactions (E-QTLs) were detected for the seven traits investigated. The phenotypic variation explained (PVE) by M-QTLs ranged from 3.4% to 88.2%. Total PVE of the M-QTLs for each trait was significantly higher than that of the E-QTLs. The total number of M-QTLs identified in the IP/DS//IP population was higher than in the IP/DS//DS population. However, the total PVE by the M-QTLs and E-QTLs together for each trait was similar in the two populations, suggesting that the two BC1F1 populations are equally useful for QTL analysis. Maps and QTLs in the two populations were compared. Eleven new QTLs were identified for SN, SF, GL, and GW in this study, and they will be valuable in marker-assisted selection, particularly for improving grain traits in tongil-type varieties.

Fine mapping and candidate gene analysis of hwh1 and hwh2, a set of complementary genes controlling hybrid breakdown in rice.

Hybrid breakdown (HB), a phenomenon of reduced viability or fertility accompanied with retarded growth in hybrid progenies, often arises in the offspring of intersubspecific hybrids between indica and japonica in rice. We detected HB plants in F8 recombinant inbred lines derived from the cross between an indica variety, Milyang 23, and a japonica variety, Tong 88-7. HB plants showed retarded growth, with fewer tillers and spikelets. Genetic analysis revealed that HB was controlled by the complementary action of two recessive genes, hwh1 and hwh2, originating from each of both parents, which were fine-mapped on the short arm of chromosome 2 and on the near centromere region of the long arm of chromosome 11, respectively. A comparison of the sequences of candidate genes among both parents and HB plants revealed that hwh1 encoded a putative glucose-methanol-choline oxidoreductase with one amino acid change compared to Hwh1 and that hwh2 probably encoded a putative hexose transporter with a six amino acid insertion compared to Hwh2. Investigation of the distribution of these alleles among 54 japonica and indica cultivars using candidate gene-based markers suggested that the two loci might be involved in developing reproductive barriers between two subspecies.