A novel, major, and validated QTL for the effective tiller number located on chromosome arm 1BL in bread wheat

Quantitative trait loci detection for three tiller-related traits and the effects on wheat (Triticum aestivum L.) yields

KEY MESSAGE

A total of 38 putative additive QTLs and 55 pairwise putative epistatic QTLs for tiller-related traits were reported, and the candidate genes underlying qMtn-KJ-5D, a novel major and stable QTL for maximum tiller number, were characterized. Tiller-related traits play an important role in determining the yield potential of wheat. Therefore, it is important to elucidate the genetic basis for tiller number when attempting to use genetic improvement as a tool for enhancing wheat yields. In this study, a quantitative trait locus (QTL) analysis of three tiller-related traits was performed on the recombinant inbred lines (RILs) of a mapping population, referred to as KJ-RILs, that was derived from a cross between the Kenong 9204 (KN9204) and Jing 411 (J411) lines. A total of 38 putative additive QTLs and 55 pairwise putative epistatic QTLs for spike number per plant (SNPP), maximum tiller number (MTN), and ear-bearing tiller rate (EBTR) were detected in eight different environments. Among these QTLs with additive effects, three major and stable QTLs were first documented herein. Almost all but two pairwise epistatic QTLs showed minor interaction effects accounting for no more than 3.0% of the phenotypic variance. The genetic effects of two colocated major and stable QTLs, i.e., qSnpp-KJ-5D.1 and qMtn-KJ-5D, for yield-related traits were characterized. The breeding selection effect of the beneficial allele for the two QTLs was characterized, and its genetic effects on yield-related traits were evaluated. The candidate genes underlying qMtn-KJ-5D were predicted based on multi-omics data, and TraesKN5D01HG00080 was identified as a likely candidate gene. Overall, our results will help elucidate the genetic architecture of tiller-related traits and can be used to develop novel wheat varieties with high yields.

A major QTL simultaneously increases the number of spikelets per spike and thousand-kernel weight in a wheat line

KEY MESSAGE

A novel and stably expressed QTL QSNS.sicau-SSY-7A for spikelet number per spike in wheat without negative effects on thousand-kernel weight was identified and validated in different genetic backgrounds. Spikelet number per spike (SNS) is an important determinant of yield in wheat. In the present study, we combined bulked segregant analysis (BSA) and the wheat 660 K single-nucleotide polymorphism (SNP) array to rapidly identify genomic regions associated with SNS from a recombinant inbred line (RIL) population derived from a cross between the wheat lines S849-8 and SY95-71. A genetic map was constructed using Kompetitive Allele Specific PCR markers in the SNP-enriched region on the long arm of chromosome 7A. A major and stably expressed QTL, QSNS.sicau-SSY-7A, was detected in multiple environments. It was located in a 1.6 cM interval on chromosome arm 7AL flanked by the markers AX-109983514 and AX-109820548. This QTL explained 6.86-15.72% of the phenotypic variance, with LOD values ranging from 3.66 to 8.66. Several genes associated with plant growth and development were identified in the interval where QSNS.sicau-SSY-7A was located on the 'Chinese Spring' wheat and wild emmer reference genomes. Furthermore, the effects of QSNS.sicau-SSY-7A and WHEAT ORTHOLOG OFAPO1(WAPO1) on SNS were analyzed. Interestingly, QSNS.sicau-SSY-7A significantly increased SNS without negative effects on thousand-kernel weight, anthesis date and plant height, demonstrating its great potential for breeding aimed at improving grain yield. Taken together, these results indicate that QSNS.sicau-SSY-7A is a promising locus for yield improvement, and its linkage markers are helpful for fine mapping and molecular breeding.

Major and stably expressed QTL for traits related to the mature wheat embryo independent of kernel size

Two major and stably expressed QTL for traits related to mature wheat embryo independent of kernel size were identified and validated in a natural population that contained 171 Sichuan wheat accessions and 49 Sichuan wheat landraces. As the juvenile of a highly differentiated plant, mature wheat (Triticum aestivum L.) embryos are highly significant to agricultural production. To understand the genetic basis of traits related to wheat embryo size, the embryo of mature kernels in a recombination inbred line that contained 126 lines from four environments was measured. The genetic loci of embryo size, including embryo length (EL), embryo width (EW), embryo area (EA), embryo length/kernel length (EL/KL), embryo width/kernel width (EW/KW), and EL/EW, were identified based on a genetic linkage map constructed based on PCR markers and the Wheat 55 K single nucleotide polymorphism (SNP) array. A total of 50 quantitative trait loci (QTL) for traits related to wheat embryo size were detected. Among them, QEL.sicau-2SY-4A for EL and QEW.sicau-2SY-7B for EW were major and stably expressed and were genetically independent of KL and KW, respectively. Their effects were further verified in a natural population that contained 171 Sichuan wheat accessions and 49 Sichuan wheat landraces. Further analysis showed that TraesCS4A02G343300 and TraesCS7B02G006800 could be candidate genes for QEL.sicau-2SY-4A and QEW.sicau-2SY-7B, respectively. In addition, significant positive correlations between EL and kernel-related traits and the 1,000-grain weight were detected. Collectively, this study broadens our understanding of the genetic basis of wheat embryo size and will be helpful for the further fine-mapping of interesting loci in the future.

Mapping of QTLs and Screening Candidate Genes Associated with the Ability of Sugarcane Tillering and Ratooning

The processes of sugarcane tillering and ratooning, which directly affect the yield of plant cane and ratoon, are of vital importance to the population establishment and the effective stalk number per unit area. In the present study, the phenotypic data of 285 F1 progenies from a cross of sugarcane varieties YT93-159 × ROC22 were collected in eight environments, which consisted of plant cane and ratoon cultivated in three different ecological sites. The broad sense heritability (H2) of the tillering and the ratoon sprouting was 0.64 and 0.63, respectively, indicating that they were middle to middle-high heritable traits, and there is a significantly positive correlation between the two traits. Furthermore, a total of 26 quantitative trait loci (QTLs) related to the tillering ability and 11 QTLs associated with the ratooning ability were mapped on two high-quality genetic maps derived from a 100K SNP chip, and their phenotypic variance explained (PVE) ranged from 4.27-25.70% and 6.20-13.54%, respectively. Among them, four consistent QTLs of qPCTR-R9, qPCTR-Y28, qPCTR-Y60/qRSR-Y60 and PCTR-Y8-1/qRSR-Y8 were mapped in two environments, of which, qPCTR-Y8-1/qRSR-Y8 had the PVEs of 11.90% in the plant cane and 7.88% in the ratoon. Furthermore, a total of 25 candidate genes were identified in the interval of the above four consistent QTLs and four major QTLs of qPCTR-Y8-1, qPCTR-Y8-2, qRSR-R51 and qRSR-Y43-2, with the PVEs from 11.73-25.70%. All these genes were associated with tillering, including eight transcription factors (TFs), while 15 of them were associated with ratooning, of which there were five TFs. These QTLs and genes can provide a scientific reference for genetic improvement of tillering and ratooning traits in sugarcane.

Panicle Ratio Network: streamlining rice panicle measurement by deep learning with ultra-high-definition aerial images in the field

The heading date and effective tiller percentage are important traits in rice, and they directly affect plant architecture and yield. Both traits are related to the ratio of the panicle number to the maximum tiller number, referred to as the panicle ratio (PR). In this study, an automatic PR estimation model (PRNet) based on a deep convolutional neural network was developed. Ultra-high-definition unmanned aerial vehicle (UAV) images were collected from cultivated rice varieties planted in 2384 experimental plots in 2019 and 2020 and in a large field in 2021. The determination coefficient between estimated PR and ground-measured PR reached 0.935, and the root mean square error values for the estimations of the heading date and effective tiller percentage were 0.687 d and 4.84%, respectively. Based on the analysis of the results, various factors affecting PR estimation and strategies for improving PR estimation accuracy were investigated. The satisfactory results obtained in this study demonstrate the feasibility of using UAVs and deep learning techniques to replace ground-based manual methods to accurately extract phenotypic information of crop micro targets (such as grains per panicle, panicle flowering, etc.) for rice and potentially for other cereal crops in future research.

Fine mapping of a stripe rust resistance gene YrZM175 in bread wheat

A stripe rust resistance gene YrZM175 in Chinese wheat cultivar Zhongmai 175 was mapped to a genomic interval of 636.4 kb on chromosome arm 2AL, and a candidate gene was predicted. Stripe rust, caused by Puccinia striiformis f. sp. tritici (PST), is a worldwide wheat disease that causes large losses in production. Fine mapping and cloning of resistance genes are important for accurate marker-assisted breeding. Here, we report the fine mapping and candidate gene analysis of stripe rust resistance gene YrZM175 in a Chinese wheat cultivar Zhongmai 175. Fifteen F₁, 7,325 F₂ plants and 117 F_2:3 lines derived from cross Avocet S/Zhongmai 175 were inoculated with PST race CYR32 at the seedling stage in a greenhouse, and F_2:3 lines were also evaluated for stripe rust reaction in the field using mixed PST races. Bulked segregant RNA-seq (BSR-seq) analyses revealed 13 SNPs in the region 762.50-768.52 Mb on chromosome arm 2AL. By genome mining, we identified SNPs and InDels between the parents and contrasting bulks and mapped YrZM175 to a 0.72-cM, 636.4-kb interval spanned by YrZM175-InD1 and YrZM175-InD2 (763,452,916-764,089,317 bp) including two putative disease resistance genes based on IWGSC RefSeq v1.0. Collinearity analysis indicated similar target genomic intervals in Chinese Spring, Aegilops tauschii (2D: 647.7-650.5 Mb), Triticum urartu (2A: 750.7-752.3 Mb), Triticum dicoccoides (2A: 771.0-774.5 Mb), Triticum turgidum (2B: 784.7-788.2 Mb), and Triticum aestivum cv. Aikang 58 (2A: 776.3-778.9 Mb) and Jagger (2A: 789.3-791.7 Mb). Through collinearity analysis, sequence alignments of resistant and susceptible parents and gene expression level analysis, we predicted TRITD2Bv1G264480 from Triticum turgidum to be a candidate gene for map-based cloning of YrZM175. A gene-specific marker for TRITD2Bv1G264480 co-segregated with the resistance gene. Molecular marker analysis and stripe rust response data revealed that YrZM175 was different from genes Yr1, Yr17, Yr32, and YrJ22 located on chromosome 2A. Fine mapping of YrZM175 lays a solid foundation for functional gene analysis and marker-assisted selection for improved stripe rust resistance in wheat.

Identification and validation of a locus for wheat maximum root length independent of parental reproductive environment

Maximum root length (MRL) plays an important role in the uptake of nutrients and resisting abiotic stresses. Understanding the genetic mechanism of root development is of great significance for genetic improvement of wheat. Previous studies have confirmed that parental reproductive environment (PRE) has a significant impact on growth and development of the next generation in the whole life cycle of a given plant. In this study, a recombinant inbred line population genotyped using the Wheat55K SNP array, was used to map quantitative trait loci (QTL) for wheat seedling MRL based on the harvested seeds from five different PREs. A total of 5 QTL located on chromosomes 3D and 7A were identified. Among them, QMrl.sicau-2SY-3D.2 located in a 4.0 cM interval on chromosome 3D was likely independent of PREs. QMrl.sicau-2SY-7A.2 was detected in two tests and probably influenced by PREs. The effect of QMrl.sicau-2SY-3D.2 was further validated using the tightly linked kompetitive allele specific PCR (KASP) marker, KASP-AX-111589572, in populations with different genetic backgrounds. Lines with a combination of positive alleles from QMrl.sicau-2SY-3D.2 and QMrl.sicau-2SY-7A.2 have significantly longer MRL. Furthermore, four genes (TraesCS3D03G0612000, TraesCS3D03G0608400, TraesCS3D03G0613600, and TraesCS3D03G0602400) mainly expressed in wheat root were predicted to be associated with root growth. Taken together, this study reports on a major QTL independent of PREs and lays a foundation for understanding the regulation mechanism of wheat MRL at the seedling stage.

Construction of a novel Wheat 55 K SNP array-derived genetic map and its utilization in QTL mapping for grain yield and quality related traits

Wheat is one of the most important staple crops for supplying nutrition and energy to people world. A new genetic map based on the Wheat 55 K SNP array was constructed using recombinant inbred lines derived from a cross between Zhongkemai138 and Kechengmai2 to explore the genetic foundation for wheat grain features. This new map covered 2,155.72 cM across the 21 wheat chromosomes with 11,455 markers. And 2,846 specific markers for this genetic map and 148 coincident markers among different maps were documented, which was helpful for improving and updating wheat genetic and genomic information. Using this map, a total of 68 additive QTLs and 82 pairs of epistatic QTLs were detected for grain features including yield, nutrient composition, and quality-related traits by QTLNetwork 2.1 and IciMapping 4.1 software. Fourteen additive QTLs and one pair of epistatic QTLs could be detected by both software programs and thus regarded as stable QTLs here, all of which explained higher phenotypic variance and thus could be utilized for wheat grain improvement. Additionally, thirteen additive QTLs were clustered into three genomic intervals (C4D.2, C5D, and C6D2), each of which had at least two stable QTLs. Among them, C4D.2 and C5D have been attributed to the famous dwarfing gene Rht2 and the hardness locus Pina, respectively, while endowed with main effects on eight grain yield/quality related traits and epistatically interacted with each other to control moisture content, indicating that the correlation of involved traits was supported by the pleotropic of individual genes but also regulated by the gene interaction networks. Additionally, the stable additive effect of C6D2 (QMc.cib-6D2 and QTw.cib-6D2) on moisture content was also highlighted, potentially affected by a novel locus, and validated by its flanking Kompetitive Allele-Specific PCR marker, and TraesCS6D02G109500, encoding aleurone layer morphogenesis protein, was deduced to be one of the candidate genes for this locus. This result observed at the QTL level the possible contribution of grain water content to the balances among yield, nutrients, and quality properties and reported a possible new locus controlling grain moisture content as well as its linked molecular marker for further grain feature improvement.

A major quantitative trait locus for wheat total root length associated with precipitation distribution

Optimizing root system architecture (RSA) allows crops to better capture water and nutrients and adapt to harsh environment. Parental reproductive environment (PRE) has been reported to significantly affect growth and development throughout the life cycle of the next generation. In this study, 10 RSA-related traits were evaluated in seedling stage from five independent hydroponic tests using seeds harvested from five different PREs. Based on the Wheat55K SNP array-based genetic map, quantitative trait loci (QTL) for these traits were detected in a recombinant inbred line population. Twenty-eight putative QTL for RSA-related traits were detected, covering thirteen chromosomal regions. A major QTL, QTrl.sicau-2SY-4D for total root length (TRL), which was likely independent of PREs, explained 15.81-38.48% of phenotypic variations and was located at 14.96-19.59 Mb on chromosome arm 4DS. Interestingly, it showed pleiotropic effects on TRL, root area, root volume, root forks, root dry weight, and shoot dry weight. The functional marker KASP-Rht-D1 for Rht-D1 was used to genotype 2SY population and remapping QTL for TRL showed that QTrl.sicau-2SY-4D was not linked to Rht-D1. The kompetitive allele-specific PCR (KASP) marker, KASP-AX-110527441 linked to this major QTL, was developed and used to successfully validate its effect in three different genetic populations. Further analysis suggested that the positive allele at QTrl.sicau-2SY-4D was mainly utilized in wheat breeding of northwest China where precipitation was significantly lower, indicating that wheat requires longer TRL to capture water and nutrients in arid or semi-arid regions due to deficient precipitation. Additionally, four genes (TraesCS4D03G0059800, TraesCS4D03G0057800, TraesCS4D03G0064000, and TraesCS4D03G0064400) possibly related to root development were predicted in physical interval of QTrl.sicau-2SY-4D. Taken together, these results enrich our understanding on the genetic basis of RSA and provide a potentially valuable TRL QTL for wheat breeding.

Quick mapping and characterization of a co-located kernel length and thousand-kernel weight-related QTL in wheat

A co-located KL and TKW-related QTL with no negative effect on PH and AD was rapidly identified using BSA and wheat 660 K SNP array. Its effect was validated in a panel of 218 wheat accessions. Kernel length (KL) and thousand-kernel weight (TKW) of wheat (Triticum aestivum L.) contribute significantly to kernel yield. In the present study, a recombinant inbred line (RIL) population derived from the cross between the wheat line S849-8 with larger kernels and more spikelets per spike and the line SY95-71 was developed. Further, of both the bulked segregant analysis (BSA) and the wheat 660 K single nucleotide polymorphism (SNP) array were used to rapidly identify genomic regions for kernel-related traits from this RIL population. Kompetitive Allele Specific PCR markers were further developed in the SNP-enriched region on the 2D chromosome to construct a genetic map. Both QKL.sicau-SSY-2D for KL and QTKW.sicau-SSY-2D for TKW were identified at multiple environments on chromosome arm 2DL. These two QTLs explained 9.68-23.02% and 6.73-18.32% of the phenotypic variation, respectively. The effects of this co-located QTL were successfully verified in a natural population consisting of 218 Sichuan wheat accessions. Interestingly, the major QTL was significantly and positively correlated with spike length, but did not negatively affect spikelet number per spike (SNS), plant height, or anthesis date. These results indicated that it is possible to synchronously improve kernel weight and SNS by using this QTL. Additionally, several genes associated with kernel development and filling rate were predicted and sequenced in the QTL-containing physical intervals of reference genomes of 'Chinese spring' and Aegilops tauschii. Collectively, these results provide a QTL with great breeding potential and its linked markers which should be helpful for fine mapping and molecular breeding.

Four-Year and Five-Developing-Stage Dynamic QTL Mapping for Tiller Number in the Hybrid Population of Agropyron Gaertn

Tiller number (TN) is an important agronomic trait affecting gramineous crop yield. To understand the static and dynamic information of quantitative trait locus (QTLs) controlling TN of Agropyron Gaertn., both the unconditional and conditional quantitative trait loci (QTL) mapping of TN were conducted using a cross-pollinated (CP) hybrid population with a total of 113 plant lines from the cross between Agropyron cristatum (L.) Gaertn. Z1842 and Allium mongolicum Keng Z2098, based on the phenotypic data of TN at five developmental stages [i.e., recovering stage (RS), jointing stage (JS), heading stage (HS), flowering stage (FS), and maturity stage (MS)] in 4 years (i.e., 2017, 2018, 2020, and 2021) and the genetic map constructed of 1,023 single-nucleotide polymorphism (SNP) markers. Thirty-seven QTLs controlling TN were detected using two analysis methods in 4 years, which were distributed in six linkage groups. Each QTL explained 2.96-31.11% of the phenotypic variation, with a logarithum of odds (LOD) value of 2.51-13.95. Nine of these loci detected both unconditional and conditional QTLs. Twelve unconditional major QTLs and sixteen conditional major QTLs were detected. Three relatively major stable conditional QTLs, namely, cQTN1-3, cQTN1-5, and cQTN4-1, were expressed in 2020 and 2021. Meantime, two pairs of major QTLs cQTN1-5 and qTN1-4 and also cQTN2-4 and qTN2-3 were located at the same interval but in different years. Except for qTN2-2 and qTN3-5/cQTN3-5, other thirty-four QTLs were first detected in this study. This study provides a better interpretation of genetic factors that selectively control tiller at different developmental stages and a reference for molecular marker-assisted selection in the related plant improvement.

Improvement and Re-Evolution of Tetraploid Wheat for Global Environmental Challenge and Diversity Consumption Demand

Allotetraploid durum wheat is the second most widely cultivated wheat, following hexaploid bread wheat, and is one of the major protein and calorie sources of the human diet. However, durum wheat is encountered with a severe grain yield bottleneck due to the erosion of genetic diversity stemming from long-term domestication and especially modern breeding programs. The improvement of yield and grain quality of durum wheat is crucial when confronted with the increasing global population, changing climate environments, and the non-ignorable increasing incidence of wheat-related disorders. This review summarized the domestication and evolution process and discussed the durum wheat re-evolution attempts performed by global researchers using diploid einkorn, tetraploid emmer wheat, hexaploid wheat (particularly the D-subgenome), etc. In addition, the re-evolution of durum wheat would be promoted by the genetic enrichment process, which could diversify allelic combinations through enhancing chromosome recombination (pentaploid hybridization or pairing of homologous chromosomes gene Ph mutant line induced homoeologous recombination) and environmental adaptability via alien introgressive genes (wide cross or distant hybridization followed by embryo rescue), and modifying target genes or traits by molecular approaches, such as CRISPR/Cas9 or RNA interference (RNAi). A brief discussion of the future perspectives for exploring germplasm for the modern improvement and re-evolution of durum wheat is included.

Characterization and fine mapping of a lesion mimic mutant (Lm5) with enhanced stripe rust and powdery mildew resistance in bread wheat (Triticum aestivum L.)

A novel light intensity-dependent lesion mimic mutant with enhanced disease resistance was physiologically, biochemically, and genetically characterized, and the causative gene was fine mapped to a 1.28 Mbp interval containing 17 high-confidence genes. Lesion mimic mutants are ideal for studying disease resistance and programmed cell death photosynthesis in plants to improve crop yield. In this study, a novel light intensity-dependent lesion mimic mutant (MC21) was obtained from the wheat variety Chuannong16 (CN16) by ethyl methane sulfonate treatment. The mutant initially developed tiny lesion spots on the basal part of the leaves, which then gradually proceeded down to leaf sheaths, stems, shells, and awns at the flowering stage. The major agronomic traits were significantly altered in the mutant compared to that in the wild-type CN16. Furthermore, the mutant exhibited a lesion phenotype with degenerated chloroplast structure, decreased chlorophyll content, increased level of reactive oxygen species, and increased resistance to stripe rust and powdery mildew. Genetic analysis indicated that the lesion phenotype was controlled by a novel single semi-dominant nuclear gene. The target gene was mapped on chromosome arm 2AL located between Kompetitive Allele Specific PCR (KASP) markers, KASP-4211 and KASP-5353, and tentatively termed as lesion mimic 5 (Lm5). The fine mapping suggested that Lm5 was located in a 1.28 Mbp interval between markers KASP-5825 and KASP-9366; 17 high-confidence candidate genes were included in this genomic region. This study provides an important foundational step for further cloning of Lm5 using a map-based approach.

Fine mapping of the tiller inhibition gene TIN4 contributing to ideal plant architecture in common wheat

A tiller inhibition gene, TIN4, was mapped to an approximately 311 kb genomic interval on chromosome arm 2DL of wheat. The tiller is one of the key components of plant morphological architecture and a central agronomic trait affecting spike number in wheat. Low tiller number has been proposed as a major component of crop ideotypes for high yield potential. In this study, we characterized the development of tillering in near-isogenic lines (NIL7A and NIL7B), indicating that the TIN4 gene inhibited the growth of tillering buds and negatively regulated tiller number. Low-tillering was controlled by a single gene (TIN4) located on chromosome 2DL by genetic analysis and bulked segregant RNA-seq analysis. A total of 17 new polymorphic markers were developed in this study, and 61 recombinants were identified in the secondary F2 population containing 4,266 individuals. TIN4 was finally mapped on a 0.35 cM interval, co-segregated with molecular marker M380, within a 311 kb genomic interval of the wheat cultivar Chinese Spring reference genome sequence that contained twelve predicted genes. Yield experiments showed that the yield of low-tillering lines was higher than that of high-tillering lines at a higher density. Overall, this study provides a foundation for the construction of a low-tillering ideotype for improving wheat yield and further cloning TIN4 by map-based cloning approach.

Spike Density Quantitative Trait Loci Detection and Analysis in Tetraploid and Hexaploid Wheat Recombinant Inbred Line Populations

Spike density (SD) is an agronomically important character in wheat. In addition, an optimized spike structure is a key basis for high yields. Identification of quantitative trait loci (QTL) for SD has provided a genetic basis for constructing ideal spike morphologies in wheat. In this study, two recombinant inbred line (RIL) populations (tetraploid RIL AM and hexaploid RIL 20828/SY95-71 (2SY)) previously genotyped using the wheat55K SNP array were used to identify SD QTL. A total of 18 QTL were detected, and three were major and one was stably expressed (QSd.sau-2SY-7A.2, QSd.sau-AM-5A.2, QSd.sau-AM-7B, and QSd.sau-2SY-2D). They can explain up to 23.14, 19.97, 12.00, and 9.44% of phenotypic variation, respectively. QTL × environment and epistatic interactions for SD were further analyzed. In addition, pyramiding analysis further revealed that there were additive effects between QSd.sau-2SY-2D and QSd.sau-2SY-7A.2 in 2SY, and QSd.sau-AM-5A.2 and QSd.sau-AM-7B in AM. Pearson's correlation between SD and other agronomic traits, and effects of major or stable QTL on yield related traits indicated SD significantly impacted spike length (SL), spikelet number per spike (SNS) and kernel length (KL). Several genes related to spike development within the physical intervals of major or stable QTL were predicted and discussed. Collectively, our research identified QTL with potential applications for modern wheat breeding and broadening the genetic basis of SD.

QTL Analysis of Five Morpho-Physiological Traits in Bread Wheat Using Two Mapping Populations Derived from Common Parents

Traits such as plant height (PH), juvenile growth habit (GH), heading date (HD), and tiller number are important for both increasing yield potential and improving crop adaptation to climate change. In the present study, these traits were investigated by using the same bi-parental population at early (F₂ and F₂-derived F₃ families) and late (F₆ and F₇, recombinant inbred lines, RILs) generations to detect quantitative trait loci (QTLs) and search for candidate genes. A total of 176 and 178 lines were genotyped by the wheat Illumina 25K Infinium SNP array. The two genetic maps spanned 2486.97 cM and 3732.84 cM in length, for the F₂ and RILs, respectively. QTLs explaining the highest phenotypic variation were found on chromosomes 2B, 2D, 5A, and 7D for HD and GH, whereas those for PH were found on chromosomes 4B and 4D. Several QTL detected in the early generations (i.e., PH and tiller number) were not detected in the late generations as they were due to dominance effects. Some of the identified QTLs co-mapped to well-known adaptive genes (i.e., Ppd-1, Vrn-1, and Rht-1). Other putative candidate genes were identified for each trait, of which PINE1 and PIF4 may be considered new for GH and TTN in wheat. The use of a large F₂ mapping population combined with NGS-based genotyping techniques could improve map resolution and allow closer QTL tagging.

The 55K SNP-Based Exploration of QTLs for Spikelet Number Per Spike in a Tetraploid Wheat (Triticum turgidum L.) Population: Chinese Landrace "Ailanmai" × Wild Emmer

Spikelet number per spike (SNS) is the primary factor that determines wheat yield. Common wheat breeding reduces the genetic diversity among elite germplasm resources, leading to a detrimental effect on future wheat production. It is, therefore, necessary to explore new genetic resources for SNS to increase wheat yield. A tetraploid landrace "Ailanmai" × wild emmer wheat recombinant inbred line (RIL) population was used to construct a genetic map using a wheat 55K single- nucleotide polymorphism (SNP) array. The linkage map containing 1,150 bin markers with a total genetic distance of 2,411.8 cm was obtained. Based on the phenotypic data from the eight environments and best linear unbiased prediction (BLUP) values, five quantitative trait loci (QTLs) for SNS were identified, explaining 6.71-29.40% of the phenotypic variation. Two of them, QSns.sau-AM-2B.2 and QSns.sau-AM-3B.2, were detected as a major and novel QTL. Their effects were further validated in two additional F₂ populations using tightly linked kompetitive allele-specific PCR (KASP) markers. Potential candidate genes within the physical intervals of the corresponding QTLs were predicted to participate in inflorescence development and spikelet formation. Genetic associations between SNS and other agronomic traits were also detected and analyzed. This study demonstrates the feasibility of the wheat 55K SNP array developed for common wheat in the genetic mapping of tetraploid population and shows the potential application of wheat-related species in wheat improvement programs.

QTL mapping and validation of bread wheat flag leaf morphology across multiple environments in different genetic backgrounds

Eight major and stably expressed QTL for flag leaf morphology across eleven environments were identified and validated using newly developed KASP markers in seven biparental populations with different genetic backgrounds. Flag leaf morphology is a determinant trait influencing plant architecture and yield potential in wheat (Triticum aestivum L.). A recombinant inbred line (RIL) population with a 55 K SNP-based constructed genetic map was used to map quantitative trait loci (QTL) for flag leaf length (FLL), width (FLW), area (FLA), angle (FLANG), opening angle (FLOA), and bend angle (FLBA) in eleven environments. Eight major QTL were detected in 11 environments with 5.73-54.38% of explained phenotypic variation. These QTL were successfully verified using the newly developed Kompetitive Allele Specific PCR (KASP) markers in six biparental populations with different genetic backgrounds. Among these 8 major QTL, two co-located intervals were identified. Significant interactions for both FLL- and FLW-related QTL were detected. Comparison analysis showed that QFll.sau-SY-2B and QFla.sau-SY-2B are likely new loci. Significant relationships between flag leaf- and yield-related traits were observed and discussed. Several genes associated with leaf development including the ortholog of maize ZmRAVL1, a B3-domain transcription factor involved in regulation of leaf angle, were predicted in physical intervals harboring these major QTL on reference genomes of bread wheat 'Chinese spring', T. turgidum, and Aegilops tauschii. Taken together, these results broaden our understanding on genetic basis of flag leaf morphology and provide clues for fine mapping and marker-assisted breeding wheat with optimized plant architecture for promising loci.

Genetic Mapping and Validation of Loci for Kernel-Related Traits in Wheat (Triticum aestivum L.)

Kernel size (KS) and kernel weight play a key role in wheat yield. Phenotypic data from six environments and a Wheat55K single-nucleotide polymorphism array-based constructed genetic linkage map from a recombinant inbred line population derived from the cross between the wheat line 20828 and the line SY95-71 were used to identify quantitative trait locus (QTL) for kernel length (KL), kernel width (KW), kernel thickness (KT), thousand-kernel weight (TKW), kernel length-width ratio (LWR), KS, and factor form density (FFD). The results showed that 65 QTLs associated with kernel traits were detected, of which the major QTLs QKL.sicau-2SY-1B, QKW.sicau-2SY-6D, QKT.sicau-2SY-2D, and QTKW.sicau-2SY-2D, QLWR.sicau-2SY-6D, QKS.sicau-2SY-1B/2D/6D, and QFFD.sicau-2SY-2D controlling KL, KW, KT, TKW, LWR, KS, and FFD, and identified in multiple environments, respectively. They were located on chromosomes 1BL, 2DL, and 6DS and formed three QTL clusters. Comparison of genetic and physical interval suggested that only QKL.sicau-2SY-1B located on chromosome 1BL was likely a novel QTL. A Kompetitive Allele Specific Polymerase chain reaction (KASP) marker, KASP-AX-109379070, closely linked to this novel QTL was developed and used to successfully confirm its effect in two different genetic populations and three variety panels consisting of 272 Chinese wheat landraces, 300 Chinese wheat cultivars most from the Yellow and Huai River Valley wheat region, and 165 Sichuan wheat cultivars. The relationships between kernel traits and other agronomic traits were detected and discussed. A few predicted genes involved in regulation of kernel growth and development were identified in the intervals of these identified major QTL. Taken together, these stable and major QTLs provide valuable information for understanding the genetic composition of kernel yield and provide the basis for molecular marker-assisted breeding.

A novel, validated, and plant height-independent QTL for spike extension length is associated with yield-related traits in wheat

A novel, stably expressed, and plant height-independent QTL for spike extension length on 5AS was identified and validated in different populations using a newly developed and tightly linked KASP marker. As an important component of plant height (PH), spike extension length (SEL) plays a significant role in formation of an ideotype in wheat. Despite the fact that numerous loci for SEL in wheat have been reported, our knowledge on PH-independent loci remains to be limited. In this study, two recombinant inbred line (RIL) populations genotyped using the Wheat55K SNP were used to detect quantitative trait loci (QTL) controlling SEL across six environments. A total of 30 QTL for SEL were detected in these two RIL populations, and four of them, i.e., QSEL.sicau-2CN-4D, QSEL.sicau-2SY-4B.2, QSEL.sicau-2SY-4D.1, and QSEL.sicau-2CN-5A, were stably expressed. Genetic and conditional QTL analysis showed that the first three were significantly associated with PH, while the last one, QSEL.sicau-2CN-5A, is independent of PH. Comparison of genetic and physical maps suggested that only QSEL.sicau-2CN-5A located on chromosome arm 5AS is likely a novel QTL. A Kompetitive Allele-Specific PCR (KASP) marker, KASP-AX-110413733, tightly linked to this novel QTL was developed to successfully confirm its effect in three different genetic populations. Further, in the interval where QSEL.sicau-2CN-5A was located on 'Chinese Spring' wheat reference genome, three promising genes mainly expressed in wheat stem were predicated and they all encode the cytochrome P450 that was demonstrated to be closely associated with SEL elongation in rice. In addition, significant correlations between SEL and PH, spikelet number per spike, and thousand-grain weight were also detected. Altogether, our results broaden our understanding on genetic basis of SEL and will be useful for marker-based selection of lines with different SELs and fine mapping the novel and PH-independent QTL QSEL.sicau-2CN-5A.