Unravelling the genetic basis and regulation networks related to fibre quality improvement using chromosome segment substitution lines in cotton.

The elucidation of genetic architecture and molecular regulatory networks underlying complex traits remains a significant challenge in life science, largely due to the substantial background effects that arise from epistasis and gene-environment interactions. The chromosome segment substitution line (CSSL) is an ideal material for genetic and molecular dissection of complex traits due to its near-isogenic properties; yet a comprehensive analysis, from the basic identification of substitution segments to advanced regulatory network, is still insufficient. Here, we developed two cotton CSSL populations on the Gossypium hirsutum background, representing wide adaptation and high lint yield, with introgression from G. barbadense, representing superior fibre quality. We sequenced 99 CSSLs that demonstrated significant differences from G. hirsutum in fibre, and characterized 836 dynamic fibre transcriptomes in three crucial developmental stages. We developed a workflow for precise resolution of chromosomal substitution segments; the genome sequencing revealed substitutions collectively representing 87.25% of the G. barbadense genome. Together, the genomic and transcriptomic survey identified 18 novel fibre-quality-related quantitative trait loci with high genetic contributions and the comprehensive landscape of fibre development regulation. Furthermore, analysis determined unique cis-expression patterns in CSSLs to be the driving force for fibre quality alteration; building upon this, the co-expression regulatory network revealed biological relationships among the noted pathways and accurately described the molecular interactions of GhHOX3, GhRDL1 and GhEXPA1 during fibre elongation, along with reliable predictions for their interactions with GhTBA8A5. Our study will enhance more strategic employment of CSSL in crop molecular biology and breeding programmes.

Identification of hub genes involved in gibberellin-regulated elongation of coleoptiles of rice seeds germinating under submerged conditions.

Rapid elongation of coleoptiles from rice seeds to reach the water surface enables plants to survive submergence stress and therefore plays a crucial role in allowing direct seeding in rice cultivation. Gibberellin (GA) positively influences growth in rice, but the molecular mechanisms underlying its regulation of coleoptile elongation under submerged conditions remain unclear. In this study, we performed a weighted gene co-expression network analysis to conduct a preliminarily examination of the mechanisms. Four key modules were identified with high correlations to the GA regulation of submergence tolerance. The genes within these modules were mainly involved in the Golgi apparatus and carbohydrate metabolic pathways, suggesting their involvement in enhancing submergence tolerance. Further analysis of natural variation revealed that the specific hub genes Os03g0337900, Os03g0355600, and Os07g0638400 exhibited strong correlations with subspecies divergence of the coleoptile elongation phenotype. Consistent with this analysis, mutation of Os07g0638400 resulted in a lower germination potential and a stronger inhibition of coleoptile elongation under submerged conditions. The hub genes identified in this study provide new insights into the molecular mechanisms underlying GA-dependent tolerance to submergence stress in rice, and a potential basis for future modification of rice germplasm to allow for direct seeding.

Mapping and candidate gene analysis of QTLs for grain shape in a rice chromosome segment substitution line Z485 and breeding of SSSLs.

Grain shape is one of the most important factors that affects rice yield. Cloning novel grain shape genes and analyzing their genetic mechanisms are crucial for high yield breeding. In this study, a slender grain CSSL-Z485 with 3-segments substitution in the genetic background of Nipponbare was constructed in rice. Cytological analysis showed that the longer grain length of Z485 was related to the increase in glume cell numbers, while the narrower grain width was associated with the decrease in cell width. Three grain shape-related quantitative trait locus (QTLs), including qGL12, qGW12, and qRLW12, were identified through the F2 population constructed from a cross between Nipponbare and Z485. Furthermore, four single segment substitution lines (SSSLs, S1-S4) carrying the target QTLs were dissected from Z485 by MAS. Finally, three candidate genes of qGL12 for grain length and qGW12 for grain width located in S3 were confirmed by DNA sequencing, RT-qPCR, and protein structure prediction. Specifically, candidate gene 1 encodes a ubiquitin family protein, while candidate genes 2 and 3 encode zinc finger proteins. The results provide valuable germplasm resources for cloning novel grain shape genes and molecular breeding by design. Supplementary information: The online version contains supplementary material available at 10.1007/s11032-024-01480-x.

Comparative proteomic analysis of chromosome segment substitution lines of Thai jasmine rice KDML105 under short-term salinity stress.

MAIN CONCLUSIONS: Heat shock proteins, ROS detoxifying enzymes, and ion homeostasis proteins, together with proteins in carbohydrate metabolism, cell structure, brassinosteroids, and carotenoid biosynthesis pathway were up-regulated in CSSLs under salinity stress. Rice is one of the most consumed staple foods worldwide. Salinity stress is a serious global problem affecting rice productivity. Many attempts have been made to select or produce salinity-tolerant rice varieties. Genetics and biochemical approaches were used to study the salinity-responsive pathway in rice to develop salinity tolerant strains. This study investigated the proteomic profiles of chromosome segment substitution lines (CSSLs) developed from KDML105 (Khao Dawk Mali 105, a Thai jasmine rice cultivar) under salinity stress. The CSSLs showed a clear resistant phenotype in response to 150 mM NaCl treatment compared to the salinity-sensitive line, IR29. Liquid chromatography-tandem mass spectrometry using the Ultimate 3000 Nano/Capillary LC System coupled to a Hybrid Quadrupole Q-Tof Impact II™ equipped with a nano-captive spray ion source was applied for proteomic analysis. Based on our criteria, 178 proteins were identified as differentially expressed proteins under salinity stress. Protein functions in DNA replication and transcription, and stress and defense accounted for the highest proportions in response to salinity stress, followed by protein transport and trafficking, carbohydrate metabolic process, signal transduction, and cell structure. The protein interaction network among the 75 up-regulated proteins showed connections between proteins involved in cell wall synthesis, transcription, translation, and in defense responses.

Genetic Elucidation for Response of Flowering Time to Ambient Temperatures in Asian Rice Cultivars.

Climate resilience of crops is critical for global food security. Understanding the genetic basis of plant responses to ambient environmental changes is key to developing resilient crops. To detect genetic factors that set flowering time according to seasonal temperature conditions, we evaluated differences of flowering time over years by using chromosome segment substitution lines (CSSLs) derived from japonica rice cultivars "Koshihikari" × "Khao Nam Jen", each with different robustness of flowering time to environmental fluctuations. The difference of flowering times in 9 years' field tests was large in "Khao Nam Jen" (36.7 days) but small in "Koshihikari" (9.9 days). Part of this difference was explained by two QTLs. A CSSL with a "Khao Nam Jen" segment on chromosome 11 showed 28.0 days' difference; this QTL would encode a novel flowering-time gene. Another CSSL with a segment from "Khao Nam Jen" in the region around Hd16 on chromosome 3 showed 23.4 days" difference. A near-isogenic line (NIL) for Hd16 showed 21.6 days' difference, suggesting Hd16 as a candidate for this QTL. RNA-seq analysis showed differential expression of several flowering-time genes between early and late flowering seasons. Low-temperature treatment at panicle initiation stage significantly delayed flowering in the CSSL and NIL compared with "Koshihikari". Our results unravel the molecular control of flowering time under ambient temperature fluctuations.

Identification of QTLs and a Candidate Gene for Reducing Pre-Harvest Sprouting in Aegilops tauschii-Triticum aestivum Chromosome Segment Substitution Lines.

Wheat pre-harvest sprouting (PHS) causes serious losses in wheat yield. In this study, precise mapping was carried out in the chromosome segment substitution lines (CSSL) F2 population generated by a direct cross of Zhoumai 18 (PHS-sensitive) and Aegilops tauschii accession T093 (highly PHS-resistant). Three Ae. tauschii-derived quantitative trait loci (QTLs), QDor.3D.1, QDor.3D.2, and QDor.3D.3, were detected on chromosome 3DL using four simple sequence repeats (SSR) markers and 10 developed Kompetitive allele-specific PCR (KASP) markers. Alongside these QTL results, the RNA-Seq and qRT-PCR analysis revealed expression levels of TraesCS3D01G466100 in the QDor.3D.2 region that were significantly higher in CSSLs 495 than in Zhoumai 18 during the seed imbibition treatment. The cDNA sequencing results of TraesCS3D01G466100 showed two single nucleotide polymorphisms (SNPs), resulting in two changed amino acid substitutions between Zhoumai 18 and line 495, and the 148 nt amino acid substitution of TraesCS3D01G466100, derived from Ae. tauschii T093, which may play an important role in the functioning of ubiquitin ligase enzymes 3 (E3) according to the homology protein analysis, which could lead to differential PHS-resistance phenotypes. Taken together, our results may foster a better understanding of the mechanism of PHS resistance and are potentially valuable for marker-assisted selection in practical wheat breeding efforts.

Identifying Wild Versus Cultivated Gene-Alleles Conferring Seed Coat Color and Days to Flowering in Soybean.

Annual wild soybean (G. soja) is the ancestor of the cultivated soybean (G. max). To reveal the genetic changes from soja to max, an improved wild soybean chromosome segment substitution line (CSSL) population, SojaCSSLP5, composed of 177 CSSLs with 182 SSR markers (SSR-map), was developed based on SojaCSSLP1 generated from NN1138-2(max)×N24852(soja). The SojaCSSLP5 was genotyped further through whole-genome resequencing, resulting in a physical map with 1366 SNPLDBs (SNP linkage-disequilibrium blocks), which are composed of more markers/segments, shorter marker length and more recombination breakpoints than the SSR-map and caused 721 new wild substituted segments. Using the SNPLDB-map, two loci co-segregating with seed-coat color (SCC) and six loci for days to flowering (DTF) with 88.02% phenotypic contribution were identified. Integrated with parental RNA-seq and DNA-resequencing, two SCC and six DTF candidate genes, including three previously cloned (G, E2 and GmPRR3B) and five newly detected ones, were predicted and verified at nucleotide mutant level, and then demonstrated with the consistency between gene-alleles and their phenotypes in SojaCSSLP5. In total, six of the eight genes were identified with the parental allele-pairs coincided to those in 303 germplasm accessions, then were further demonstrated by the consistency between gene-alleles and germplasm phenotypes. Accordingly, the CSSL population integrated with parental DNA and RNA sequencing data was demonstrated to be an efficient platform in identifying candidate wild vs. cultivated gene-alleles.

Genomic regions responsible for seminal and crown root lengths identified by 2D & 3D root system image analysis.

BACKGROUND: Genetic improvement of root system architecture is a promising approach for improved uptake of water and mineral nutrients distributed unevenly in the soil. To identify genomic regions associated with the length of different root types in rice, we quantified root system architecture in a set of 26 chromosome segment substitution lines derived from a cross between lowland indica rice, IR64, and upland tropical japonica rice, Kinandang Patong, (IK-CSSLs), using 2D & 3D root phenotyping platforms. RESULTS: Lengths of seminal and crown roots in the IK-CSSLs grown under hydroponic conditions were measured by 2D image analysis (RootReader2D). Twelve CSSLs showed significantly longer seminal root length than the recurrent parent IR64. Of these, 8 CSSLs also exhibited longer total length of the three longest crown roots compared to IR64. Three-dimensional image analysis (RootReader3D) for these CSSLs grown in gellan gum revealed that only one CSSL, SL1003, showed significantly longer total root length than IR64. To characterize the root morphology of SL1003 under soil conditions, SL1003 was grown in Turface, a soil-like growth media, and roots were quantified using RootReader3D. SL1003 had larger total root length and increased total crown root length than did IR64, although its seminal root length was similar to that of IR64. The larger TRL in SL1003 may be due to increased crown root length. CONCLUSIONS: SL1003 carries an introgression from Kinandang Patong on the long arm of chromosome 1 in the genetic background of IR64. We conclude that this region harbors a QTL controlling crown root elongation.

Mapping quantitative trait loci for heat tolerance at the booting stage using chromosomal segment substitution lines in rice.

High temperature stress is a major obstacle in rice productivity. Considerable progress has been made on studying heat tolerance (HT) at different stages. However, the genetic basis of HT at the booting stage is poorly understood. In this study, we analyzed the morphological features of a heat-sensitive japonica cultivar Sasanishiki under natural high temperature stress at the booting stage. The anthers became smaller and the number, and fertility, of pollen grains were decreased significantly. As a result, there was a dramatic reduction in spikelet fertility. In contrast, the indica cultivar Habataki showed high HT and normal spikelet fertility under high temperature stress. Additonally, a set of chromosome segment substitution lines, derived from Sasanishiki and Habataki, were evaluated for HT related quantitative trait loci (QTLs) across two environments in the natural field. A total of 12 QTLs associated with HT were detected, of which, 5 were identified in two environments, and 7 in one environment. Furthermore, one of the major-effect QTLs (qHTB3-3) detected on the long arm of chromosome 3, was confirmed using overlapping substituted lines. qHTB3-3 was finally mapped between the two markers RM3525 and 3-M95, approximately 2.8 Mb apart. These findings and further gene cloning of qHTB3-3 will help us better understand the molecular control of HT in rice, and may contribute to the development of high HT rice varieties.

Chromosome segment detection for seed size and shape traits using an improved population of wild soybean chromosome segment substitution lines.

Size and shape of soybean seeds are closely related to seed yield and market value. Annual wild soybeans have the potential to improve cultivated soybeans, but their inferior seed characteristics should be excluded. To detect quantitative trait loci (QTLs)/segments of seed size and shape traits in annual wild soybean, its chromosome segment substitution lines (CSSLs) derived from NN1138-2 (recurrent parent, Glycine max) and N24852 (donor parent, Glycine soja) and then modified 2 iterations (coded SojaCSSLP3) were improved further to contain more lines (diagonal segments) and less heterozygous and missing portions. The new population (SojaCSSLP4) composed of 195 CSSLs was evaluated under four environments, and 11, 13, 7, 15 and 14 QTLs/segments were detected for seed length (SL), seed width (SW), seed roundness (SR), seed perimeter (SP) and seed cross section area (SA), respectively, with all 60 wild allele effects negative. Among them, 16 QTLs/segments were shared by 2-5 traits, respectively, but 0-3 segments for each of the 5 traits were independent. The non-shared Satt274 and shared Satt305, Satt540 and Satt239 were major segments, along with other segments composed of two different but related sets of genetic systems for SR and the other 4 traits, respectively. Compared with the literature, 7 SL, 5 SW and 2 SR QTLs/segments were also detected in cultivated soybeans; allele distinction took place between cultivated and wild soybeans, and also among cultivated parents. The present mapping is understood as macro-segment mapping, the segments may be further dissected into smaller segments as well as corresponding QTLs/genes.

Comparative quantitative trait loci analysis framework reveals relationships between salt stress responsive phenotypes and pathways.

Soil salinity is a complex abiotic stress that involves several biological pathways. Hence, focusing on a specific or a few salt-tolerant phenotypes is unlikely to provide comprehensive insights into the intricate and interwinding mechanisms that regulate salt responsiveness. In this study, we develop a heuristic framework for systematically integrating and comprehensively evaluating quantitative trait loci (QTL) analyses from multiple stress-related traits obtained by different studies. Making use of a combined set of 46 salinity-related traits from three independent studies that were based on the same chromosome segment substitution line (CSSL) population of rice (Oryza sativa), we demonstrate how our approach can address technical biases and limitations from different QTL studies and calling methods. This allows us to compile a comprehensive list of trait-specific and multi-trait QTLs, as well as salinity-related candidate genes. In doing so, we discover several novel relationships between traits that demonstrate similar trends of phenotype scores across the CSSLs, as well as the similarities between genomic locations that the traits were mapped to. Finally, we experimentally validate our findings by expression analyses and functional validations of several selected candidate genes from multiple pathways in rice and Arabidopsis orthologous genes, including OsKS7 (ENT-KAURENE SYNTHASE 7), OsNUC1 (NUCLEOLIN 1) and OsFRO1 (FERRIC REDUCTASE OXIDASE 1) to name a few. This work not only introduces a novel approach for conducting comparative analyses of multiple QTLs, but also provides a list of candidate genes and testable hypotheses for salinity-related mechanisms across several biological pathways.

Mapping and validation of QTLs for rice sheath blight resistance.

Sheath blight, caused by Rhizoctonia solani, is one of the most serious diseases of rice. Among 33 rice accessions, mainly from National Institute of Agrobiological Sciences (NIAS) Core Collection, we found three landraces from the Himalayas-Jarjan, Nepal 555 and Nepal 8-with resistance to sheath blight in 3 years' field testing. Backcrossed inbred lines (BILs) derived from a cross between Jarjan and the leading Japanese cultivar Koshihikari were used in QTL analyses. Since later-heading lines show fewer lesions, we used only earlier-heading BILs to avoid association with heading date. We detected eight QTLs; the Jarjan allele of three of these increased resistance. Only one QTL, on chromosome 9 (between markers Nag08KK18184 and Nag08KK18871), was detected in all 3 years. Chromosome segment substitution lines (CSSLs) carrying it showed resistance in field tests. Thirty F2 lines derived from a cross between Koshihikari and one CSSL supported the QTL.

Identification and fine mapping of a new bacterial blight resistance gene, Xa43(t), in Zhangpu wild rice (Oryza rufipogon).

Bacterial blight (BB) is currently considered one of the most serious rice diseases and is caused by Xanthomonas oryzae pv. oryzae (Xoo). Numerous studies have shown that breeding resistant rice varieties is one of the most effective methods to prevent BB, and it is important to identify and isolate more BB resistance (R) genes from different rice resources. Using a map-based approach, we identified a new QTL/gene, Xa43(t), from ZhangPu wild rice, which was highly resistant to the BB isolate PX099. We performed bulked segregant analysis combined with candidate gene prediction to identify the candidate gene. The Xa43(t) gene was narrowed down to a 29-kb region containing four putative genes. More importantly, the candidate gene Xa43(t) did not affect the main agronomic traits of rice. We also identified a widely applicable molecular marker, namely Inde1-18, which co-segregates with the Xa43(t) gene. The Xa43(t) gene is a new broad-spectrum BB resistance gene without identified alleles and has good application prospects for rice disease resistance breeding.

Genetic Variation of Blast (Pyricularia oryzae Cavara) Resistance in the Longistaminata Chromosome Segment Introgression Lines (LCSILs) and Potential for Breeding Use in Kenya.

In Kenya's rice-growing areas, Basmati varieties have been produced in monoculture since the late 1980s. This has resulted in the breakdown of the resistance (R) gene-mediated response of the local Basmati varieties to blast disease caused by Pyricularia oryzae. To improve blast resistance in Kenyan Basmati varieties, continuous identification of R genes and suitable breeding materials for Basmati are necessary. Longistaminata chromosome segment introgression lines (LCSILs) with the Kernel Basmati genetic background, developed using a rice line called potential low-input adaptable-1 (pLIA-1) derived from a cross between Taichung 65 (T65) (a rice variety in the Japonica Group) and O. longistaminata, are expected to contain useful blast R genes derived from O. longistaminata or T65. In this study, we investigated the genetic variation of blast R genes in LCSILs and their parents by using a new international differential system for designating blast races based on the gene-for-gene theory and molecular characterization using single nucleotide polymorphism (SNP) markers. LCSILs and their parents were classified into three groups-A, B1, and B2-based on reaction patterns to the standard differential blast isolates (SDBIs). Group A, including pLIA-1, showed the highest resistance in all groups, followed by groups B1 and B2. Kernel Basmati in group B1 was considered to possess Pik-p or Pi7(t), Pi19(t), and other unknown R genes. In addition to these R genes, LCSIL 6, 12, 27, 28, and 40, in group A, were determined to possess one of Pish, Piz-t, or both genes that confer resistance to the Kenyan blast races. These lines can be used for efficiently pyramiding blast R genes in the local Basmati varieties.

Fine Mapping of a Novel Major Quantitative Trait Locus, qPAA7, That Controls Panicle Apical Abortion in Rice.

The panicle apical abortion (PAA) causes severe yield losses in rice production, but details about its development and molecular basis remain elusive. Here, we detected PAA quantitative trait loci (QTLs) in three environments using a set of chromosome segment substitution lines (CSSLs) that was constructed with indica Changhui121 as the recurrent parent and japonica Koshihikari as the donor parent. First, we identified a novel major effector quantitative trait locus, qPAA7, and selected a severe PAA line, CSSL176, which had the highest PAA rate among CSSLs having Koshihikari segments at this locus. Next, an F2 population was constructed from a cross between CSS176 and CH121. Using F2 to make recombinantion analysis, qPAA7 was mapped to an 73.8-kb interval in chromosome 7. Among nine candidate genes within this interval, there isn't any known genes affecting PAA. According to the gene annotation, gene expression profile and alignment of genomic DNA, LOC_Os07g41220 and LOC_Os07g41280 were predicted as putative candidate genes of qPAA7. Our study provides a foundation for cloning and functional characterization of the target gene from this locus.

Investigation of clomazone-tolerance mechanism in a long-grain cultivar of rice.

BACKGROUND: Clomazone is a potent herbicide for controlling weeds that have evolved resistance to other herbicides due to its unique mode of action. Clomazone is used in rice cultivation, but is limited to long-grain cultivars because other cultivars are highly sensitive to it. In this study, we investigated the mechanism of clomazone tolerance in a long-grain cultivar. RESULTS: The long-grain cultivar Kasalath tolerated approximately five-fold higher doses of clomazone compared to two short-grain cultivars, Nipponbare and Koshihikari. While Arabidopsis thaliana transformed with a rice cytochrome P450, CYP81A6, showed resistance to clomazone, the cyp81a6 knockout Kasalath was unchanged in its clomazone sensitivity. The inheritance of clomazone sensitivity in the F1 and F2 of Kasalath and Nipponbare indicated the involvement of multiple loci for clomazone tolerance. Four chromosome segment substitution lines of Nipponbare/Kasalath and Koshihikari/Kasalath exhibited partial tolerance to clomazone. The overlapping DNA region among the four lines is on chromosome 5 within 11.5 Mb. CONCLUSION: Multiple loci are involved in clomazone tolerance in Kasalath, one of which is located on chromosome 5. This information will help develop short-grain cultivars tolerant to clomazone. © 2021 Society of Chemical Industry.

Analysis of Soybean Somatic Embryogenesis Using Chromosome Segment Substitution Lines and Transcriptome Sequencing.

Soybean is an important cash crop that is widely used as a source of vegetable protein and edible oil. The regeneration ability of soybean directly affects the application of biotechnology. In this study, we used the exogenous hormone 2,4-D to treat immature embryos. Different levels of somatic incidence were selected from the chromosome segment substitution lines (CSSLs) constructed by SN14 and ZYD00006. Transcriptome sequencing of extreme materials was performed, and 2666 differentially expressed genes were obtained. At the same time, a difference table was generated by combining the data on CSSL rearrangement. In the extreme materials, a total of 93 differentially expressed genes were predicted and were then analyzed by cluster analysis and Gene Ontology (GO) annotation. After screening and annotating the target genes, three differentially expressed genes with hormone pathways were identified. The expression patterns of the target genes were verified by real-time quantitative PCR (qRT-PCR). Haplotype polymorphism detection and linkage disequilibrium analysis were performed on the candidate gene Glyma.09g248200. This study provided more information on the regulation network of soybean somatic embryogenesis and regeneration processes, and further identified important genes in the soybean regeneration process and provided a theoretical basis for accelerating the application of biotechnology to soybean for improving its breeding efficiency.

Large-scale deployment of a rice 6 K SNP array for genetics and breeding applications.

BACKGROUND: Fixed arrays of single nucleotide polymorphism (SNP) markers have advantages over reduced representation sequencing in their ease of data analysis, consistently higher call rates, and rapid turnaround times. A 6 K SNP array represents a cost-benefit "sweet spot" for routine genetics and breeding applications in rice. Selection of informative SNPs across species and subpopulations during chip design is essential to obtain useful polymorphism rates for target germplasm groups. This paper summarizes results from large-scale deployment of an Illumina 6 K SNP array for rice. RESULTS: Design of the Illumina Infinium 6 K SNP chip for rice, referred to as the Cornell_6K_Array_Infinium_Rice (C6AIR), includes 4429 SNPs from re-sequencing data and 1571 SNP markers from previous BeadXpress 384-SNP sets, selected based on polymorphism rate and allele frequency within and between target germplasm groups. Of the 6000 attempted bead types, 5274 passed Illumina's production quality control. The C6AIR was widely deployed at the International Rice Research Institute (IRRI) for genetic diversity analysis, QTL mapping, and tracking introgressions and was intensively used at Cornell University for QTL analysis and developing libraries of interspecific chromosome segment substitution lines (CSSLs) between O. sativa and diverse accessions of O. rufipogon or O. meridionalis. Collectively, the array was used to genotype over 40,000 rice samples. A set of 4606 SNP markers was used to provide high quality data for O. sativa germplasm, while a slightly expanded set of 4940 SNPs was used for O. sativa X O. rufipogon populations. Biparental polymorphism rates were generally between 1900 and 2500 well-distributed SNP markers for indica x japonica or interspecific populations and between 1300 and 1500 markers for crosses within indica, while polymorphism rates were lower for pairwise crosses within U.S. tropical japonica germplasm. Recently, a second-generation array containing ~7000 SNP markers, referred to as the C7AIR, was designed by removing poor-performing SNPs from the C6AIR and adding markers selected to increase the utility of the array for elite tropical japonica material. CONCLUSIONS: The C6AIR has been successfully used to generate rapid and high-quality genotype data for diverse genetics and breeding applications in rice, and provides the basis for an optimized design in the C7AIR.

Mapping and characterization of quantitative trait loci for mesocotyl elongation in rice (Oryza sativa L.).

Mesocotyl elongation is an important trait for seedling emergence in direct-seeding cultivation in rice. In this study, a backcross inbred line (BIL) population from a cross between Kasalath and Nipponbare was employed to map quantitative trait loci (QTLs) for mesocotyl elongation. A total of 5 QTLs for mesocotyl length were identified on chromosomes 1, 3, 7, 9, and 12 in 2 independent experiments. At all QTL, the Kasalath alleles contributed to an increase in mesocotyl length. Two QTLs (qMel-1 and qMel-3) on chromosomes 1 and 3 were consistently detected in both experiments. To fine map the QTLs, a cross was made between 2 chromosome segment substitution lines (CSSL-6 and CSSL-15), each harboring the Kasalath allele across the qMel-1 and qMel-3 regions, and an F2:3 population was developed. A two-way ANOVA indicated that no epistatic interaction was detected between the 2 QTLs in the F2 population (P = 0.31). Moreover, analysis of two F3 near-isogenic lines (NILs) derived from the same cross, indicated that the 2 QTLs act additively in distinct or complementary pathways in controlling mesocotyl elongation. Substitution mapping indicated that the qMel-1 QTL was located between the 2 SSR markers RM5448 and RM5310, which are 3,799-kb apart, and that the qMel-3 QTL was located between the 2 SSR markers RM3513 and RM1238, which are 6,964-kb apart. To our knowledge, this is the first report to fine-map QTLs for mesocotyl elongation and to analyze their interaction.