A whole-genome SNP array (RICE6K) for genomic breeding in rice

Rapid breeding of an early maturing, high-quality, and high-y.ielding rice cultivar using marker‑assisted selection coupled with optimized anther culture

With the global shift towards healthier eating habits, the focus of the rice industry has evolved from quantity to quality. In China, the Yangtze River Basin is the main area consuming long-grain and high-quality indica rice. Hubei Province, a significant rice-producing area, currently cultivates a limited range of rice varieties, risking degradation and diminishing economic returns. Therefore, it is imperative to cultivate elite rice varieties tailored to the local production conditions and can significantly enhance the added value. This study bred the novel rice cultivar "Runxiangyu", characterized by early maturity, high quality, and high yield. It is a hybrid of Ezhong 5, known for its moderate height and excellent quality, albeit with a long growth period and lack of fragrance, and Yuzhenxiang, renowned for its high quality, short growth period, and fragrance but limited by its tall stature and poor tillering ability. The breeding process utilized optimized anther culture coupled with molecular marker-assisted selection (MAS) and phenotype analysis. In the field, the developed cultivar was 120.9 cm tall and had an entire growth period of 117.5 days, demonstrating moderate disease resistance and excellent heat tolerance. Its grains are fragrant, meeting the national standard of grade two high-quality rice set by the Food Quality Supervision and Inspection Center of the Ministry of Agriculture and Rural Areas). Exhibiting superior agronomic traits, such as plant type, height, growth period, and stress resistance, along with and quality attributes, including grain shape, chalkiness, fragrance, and taste, "Runxiangyu" was certified by the Agricultural Crop Variety Certification Commission of Hubei in 2022. These findings suggested that molecular MAS coupled with optimized anther culture and multi-site phenotype analysis is an efficient and rapid method for crop breeding.

Supplementary Information

The online version contains supplementary material available at 10.1007/s11032-024-01495-4.

The GW5-WRKY53-SGW5 module regulates grain size variation in rice

Grain size is a crucial agronomic trait that affects stable yield, appearance, milling quality, and domestication in rice. However, the molecular and genetic relationships among QTL genes (QTGs) underlying natural variation for grain size remain elusive. Here, we identified a novel QTG SGW5 (suppressor of gw5) by map-based cloning using an F2 segregation population by fixing same genotype of the master QTG GW5. SGW5 positively regulates grain width by influencing cell division and cell size in spikelet hulls. Two nearly isogenic lines exhibited a significant differential expression of SGW5 and a 12.2% increase in grain yield. Introducing the higher expression allele into the genetic background containing the lower expression allele resulted in increased grain width, while its knockout resulted in shorter grain hulls and dwarf plants. Moreover, a cis-element variation in the SGW5 promoter influenced its differential binding affinity for the WRKY53 transcription factor, causing the differential SGW5 expression, which ultimately leads to grain size variation. GW5 physically and genetically interacts with WRKY53 to suppress the expression of SGW5. These findings elucidated a new pathway for grain size regulation by the GW5-WRKY53-SGW5 module and provided a novel case for generally uncovering QTG interactions underlying the genetic diversity of an important trait in crops.

Development of Novel KASP Markers for Improved Germination in Deep-Sown Direct Seeded Rice

BACKGROUND

The lack of stable-high yielding and direct-seeded adapted varieties with better germination ability from deeper soil depth and availability of molecular markers are major limitation in achieving the maximum yield potential of rice under water and resource limited conditions. Development of high-throughput and trait-linked markers are of great interest in genomics-assisted breeding. The aim of present study was to develop and validate novel KASP (Kompetitive Allele-Specific PCR) markers associated with traits improving germination and seedling vigor of deep sown direct seeded rice (DSR).

RESULTS

Out of 58 designed KASP assays, four KASP assays did not show any polymorphism in any of the eleven genetic backgrounds considered in the present study. The 54 polymorphic KASP assays were then validated for their robustness and reliability on the F1s plants developed from eight different crosses considered in the present study. The third next validation was carried out on 256 F3:F4 and 713 BC3F2:3 progenies. Finally, the reliability of the KASP assays was accessed on a set of random 50 samples from F3:F4 and 80-100 samples from BC3F2:3 progenies using the 10 random markers. From the 54 polymorphic KASP, based on the false positive rate, false negative rate, KASP utility in different genetic backgrounds and significant differences in the phenotypic values of the positive (desirable) and negative (undesirable) traits, a total of 12 KASP assays have been selected. These 12 KASP include 5 KASP on chromosome 3, 1 on chromosome 4, 3 on chromosome 7 and 3 on chromosome 8. The two SNPs lying in the exon regions of LOC_Os04g34290 and LOC_Os08g32100 led to non-synonymous mutations indicating a possible deleterious effect of the SNP variants on the protein structure.

CONCLUSION

The present research work will provide trait-linked KASP assays, improved breeding material possessing favourable alleles and breeding material in form of expected pre-direct-seeded adapted rice varieties. The marker can be utilized in introgression program during pyramiding of valuable QTLs/genes providing adaptation to rice under DSR. The functional studies of the genes LOC_Os04g34290 and LOC_Os08g32100 possessing two validated SNPs may provide valuable information about these genes.

Development and Assessment of SNP Genotyping Arrays for Citrus and Its Close Relatives

Rapid advancements in technologies provide various tools to analyze fruit crop genomes to better understand genetic diversity and relationships and aid in breeding. Genome-wide single nucleotide polymorphism (SNP) genotyping arrays offer highly multiplexed assays at a relatively low cost per data point. We report the development and validation of 1.4M SNP Axiom® Citrus HD Genotyping Array (Citrus 15AX 1 and Citrus 15AX 2) and 58K SNP Axiom® Citrus Genotyping Arrays for Citrus and close relatives. SNPs represented were chosen from a citrus variant discovery panel consisting of 41 diverse whole-genome re-sequenced accessions of Citrus and close relatives, including eight progenitor citrus species. SNPs chosen mainly target putative genic regions of the genome and are accurately called in both Citrus and its closely related genera while providing good coverage of the nuclear and chloroplast genomes. Reproducibility of the arrays was nearly 100%, with a large majority of the SNPs classified as the most stringent class of markers, "PolyHighResolution" (PHR) polymorphisms. Concordance between SNP calls in sequence data and array data average 98%. Phylogenies generated with array data were similar to those with comparable sequence data and little affected by 3 to 5% genotyping error. Both arrays are publicly available.

Construction of a High-Density Paulownia Genetic Map and QTL Mapping of Important Phenotypic Traits Based on Genome Assembly and Whole-Genome Resequencing

Quantitative trait locus (QTL) mapping based on a genetic map is a very effective method of marker-assisted selection in breeding, and whole-genome resequencing is one of the useful methods to obtain high-density genetic maps. In this study, the hybrid assembly of Illumina, PacBio, and chromatin interaction mapping data was used to construct high-quality chromosomal genome sequences of Paulownia fortunei, with a size of 476.82 Mb, a heterozygosity of 0.52%, and a contig and scaffold N50s of 7.81 Mb and 21.81 Mb, respectively. Twenty scaffolds with a total length of 437.72 Mb were assembled into 20 pseudochromosomes. Repeat sequences with a total length of 243.96 Mb accounted for 51.16% of the entire genome. In all, 26,903 protein-coding gene loci were identified, and 26,008 (96.67%) genes had conserved functional motifs. Further comparative genomics analysis preliminarily showed that the split of P. fortunei with Tectona grandis likely occurred 38.8 (33.3-45.1) million years ago. Whole-genome resequencing was used to construct a merged genetic map of 20 linkage groups, with 2993 bin markers (3,312,780 SNPs), a total length of 1675.14 cm, and an average marker interval of 0.56 cm. In total, 73 QTLs for important phenotypic traits were identified (19 major QTLs with phenotypic variation explained ≥ 10%), including 10 for the diameter at breast height, 7 for the main trunk height, and 56 for branch-related traits. These results not only enrich P. fortunei genomic data but also form a solid foundation for fine QTL mapping and key marker/gene mining of Paulownia, which is of great significance for the directed genetic improvement of these species.

Improvement of Rice Blast Resistance in TGMS Line HD9802S through Optimized Anther Culture and Molecular Marker-Assisted Selection

Rice blast caused by Magnaporthe oryzae is one of the most serious rice diseases worldwide. The early indica rice thermosensitive genic male sterile (TGMS) line HD9802S has the characteristics of stable fertility, reproducibility, a high outcrossing rate, excellent rice quality, and strong combining ability. However, this line exhibits poor blast resistance and is highly susceptible to leaf and neck blasts. In this study, backcross introduction, molecular marker-assisted selection, gene chipping, anther culture, and resistance identification in the field were used to introduce the broad-spectrum blast-resistance gene R6 into HD9802S to improve its rice blast resistance. Six induction media were prepared by varying the content of each component in the culture medium. Murashige and Skoog's medium with 3 mg/L 2,4-dichlorophenoxyacetic acid, 2 mg/L 1-naphthaleneacetic acid, and 1 mg/L kinetin and N6 medium with 800 mg/L casein hydrolysate, 600 mg/L proline, and 500 mg/L glutamine could improve the callus induction rate and have a higher green seedling rate and a lower white seedling rate. Compared to HD9802S, two doubled haploid lines containing R6 with stable fertility showed significantly enhanced resistance to rice blast and no significant difference in spikelet number per panicle, 1000-grain weight, or grain shape. Our findings highlight a rapid and effective method for improving rice blast resistance in TGMS lines.

A Structure Variation in qPH8.2 Detrimentally Affects Plant Architecture and Yield in Rice

Plant height is an important agronomic trait associated with plant architecture and grain yield in rice (Oryza sativa L.). In this study, we report the identification of quantitative trait loci (QTL) for plant height using a chromosomal segment substitution line (CSSL) population with substituted segments from japonica variety Nipponbare (NIP) in the background of the indica variety 9311. Eight stable QTLs for plant height were identified in three environments. Among them, six loci were co-localized with known genes such as semidwarf-1 (sd1) and Grain Number per Panicle1 (GNP1) involved in gibberellin biosynthesis. A minor QTL qPH8.2 on chromosome 8 was verified and fine-mapped to a 74 kb region. Sequence comparison of the genomic region revealed the presence/absence of a 42 kb insertion between NIP and 9311. This insertion occurred predominantly in temperate japonica rice. Comparisons on the near-isogenic lines showed that the qPH8.2 allele from NIP exhibits pleiotropic effects on plant growth, including reduced plant height, leaf length, photosynthetic capacity, delayed heading date, decreased yield, and increased tiller angle. These results indicate that qPH8.2 from temperate japonica triggers adverse effects on plant growth and yield when introduced into the indica rice, highlighting the importance of the inter-subspecies crossing breeding programs.

Construction of High-Density Genetic Map and QTL Mapping for Grain Shape in the Rice RIL Population

Grain shape is an important agronomic trait directly associated with yield in rice. In order to explore new genes related to rice grain shape, a high-density genetic map containing 2193 Bin markers (526957 SNP) was constructed by whole-genome resequencing of 208 recombinant inbred (RILs) derived from a cross between ZP37 and R8605, with a total genetic distance of 1542.27 cM. The average genetic distance between markers was 0.76 cM, and the physical distance was 201.29 kb. Quantitative trait locus (QTL) mapping was performed for six agronomic traits related to rice grain length, grain width, length-to-width ratio, thousand-grain weight, grain cross-sectional area, and grain perimeter under three different environments. A total of 39 QTLs were identified, with mapping intervals ranging from 8.1 kb to 1781.6 kb and an average physical distance of 517.5 kb. Among them, 15 QTLs were repeatedly detected in multiple environments. Analysis of the genetic effects of the identified QTLs revealed 14 stable genetic loci, including three loci that overlapped with previously reported gene positions, and the remaining 11 loci were newly identified loci associated with two or more environments or traits. Locus 1, Locus 3, Locus 10, and Locus 14 were novel loci exhibiting pleiotropic effects on at least three traits and were detected in multiple environments. Locus 14, with a contribution rate greater than 10%, influenced grain width, length-to-width ratio, and grain cross-sectional area. Furthermore, pyramiding effects analysis of three stable genetic loci showed that increasing the number of QTL could effectively improve the phenotypic value of grain shape. Collectively, our findings provided a theoretical basis and genetic resources for the cloning, functional analysis, and molecular breeding of genes related to rice grain shape.

Suppressing a phosphohydrolase of cytokinin nucleotide enhances grain yield in rice

One-step and two-step pathways are proposed to synthesize cytokinin in plants. The one-step pathway is mediated by LONELY GUY (LOG) proteins. However, the enzyme for the two-step pathway remains to be identified. Here, we show that quantitative trait locus GY3 may boost grain yield by more than 20% through manipulating a two-step pathway. Locus GY3 encodes a LOG protein that acts as a 5'-ribonucleotide phosphohydrolase by excessively consuming the cytokinin precursors, which contrasts with the activity of canonical LOG members as phosphoribohydrolases in a one-step pathway. The residue S41 of GY3 is crucial for the dephosphorylation of iPRMP to produce iPR. A solo-LTR insertion within the promoter of GY3 suppressed its expression and resulted in a higher content of active cytokinins in young panicles. Introgression of GY302428 increased grain yield per plot by 7.4% to 16.3% in all investigated indica backgrounds, which demonstrates the great value of GY302428 in indica rice production.

A minimal genome design to maximally guarantee fertile inter-subspecific hybrid rice

Hybrid rice has made considerable contributions to achieve the ambitious goal of food security for the world's population. Hybrid rice from indica/xian and japonica/geng subspecies shows much higher heterosis and is thereby an important innovation in promoting rice production in the next decade. However, such inter-subspecific hybrid rice has long suffered from serious hybrid sterility, which is a major challenge that needs to be addressed. In this study, we performed a genome design strategy to produce fertile inter-subspecific hybrid by creation of wide compatibility varieties that are able to overcome hybrid sterility. Based on combined genetic analyses in two indica-japonica crosses, we determined that four hybrid sterility loci, S5, f5, pf12 and Sc, are the major QTLs controlling inter-subspecific hybrid sterility and thus the minimal targets that can be manipulated for breeding sub-specific hybrid rice. We then cloned the pf12 locus, one of the most effective loci for hybrid male sterility, by map-based cloning, and showed that artificial disruption of pf12A gene at this locus could successfully rescue hybrid fertility. We further dissected the genetic basis of wide compatibility using three pairwise crosses from a wide-compatibility variety Dular and representative indica and japonica varieties. On this basis, we constructed and assembled different combinations of naturally compatible alleles of four loci, S5, Sc, pf12, and f5, and found that the improved lines could fully recover pollen and embryo sac fertility in test-crossed F₁s, thereby completely fulfilling the demands of inter-subspecific hybrid spikelet fertility in agricultural production. This breeding scheme would facilitate redesign of future inter-subspecific hybrid rice with a higher yield potential.

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.

Silveira S, Brommonschenkel SH, Fontes EPB. RLPredictiOme, a Machine Learning-Derived Method for High-Throughput Prediction of Plant Receptor-like Proteins, Reveals Novel Classes of Transmembrane Receptors

Cell surface receptors play essential roles in perceiving and processing external and internal signals at the cell surface of plants and animals. The receptor-like protein kinases (RLK) and receptor-like proteins (RLPs), two major classes of proteins with membrane receptor configuration, play a crucial role in plant development and disease defense. Although RLPs and RLKs share a similar single-pass transmembrane configuration, RLPs harbor short divergent C-terminal regions instead of the conserved kinase domain of RLKs. This RLP receptor structural design precludes sequence comparison algorithms from being used for high-throughput predictions of the RLP family in plant genomes, as has been extensively performed for RLK superfamily predictions. Here, we developed the RLPredictiOme, implemented with machine learning models in combination with Bayesian inference, capable of predicting RLP subfamilies in plant genomes. The ML models were simultaneously trained using six types of features, along with three stages to distinguish RLPs from non-RLPs (NRLPs), RLPs from RLKs, and classify new subfamilies of RLPs in plants. The ML models achieved high accuracy, precision, sensitivity, and specificity for predicting RLPs with relatively high probability ranging from 0.79 to 0.99. The prediction of the method was assessed with three datasets, two of which contained leucine-rich repeats (LRR)-RLPs from Arabidopsis and rice, and the last one consisted of the complete set of previously described Arabidopsis RLPs. In these validation tests, more than 90% of known RLPs were correctly predicted via RLPredictiOme. In addition to predicting previously characterized RLPs, RLPredictiOme uncovered new RLP subfamilies in the Arabidopsis genome. These include probable lipid transfer (PLT)-RLP, plastocyanin-like-RLP, ring finger-RLP, glycosyl-hydrolase-RLP, and glycerophosphoryldiester phosphodiesterase (GDPD, GDPDL)-RLP subfamilies, yet to be characterized. Compared to the only Arabidopsis GDPDL-RLK, molecular evolution studies confirmed that the ectodomain of GDPDL-RLPs might have undergone a purifying selection with a predominance of synonymous substitutions. Expression analyses revealed that predicted GDPGL-RLPs display a basal expression level and respond to developmental and biotic signals. The results of these biological assays indicate that these subfamily members have maintained functional domains during evolution and may play relevant roles in development and plant defense. Therefore, RLPredictiOme provides a framework for genome-wide surveys of the RLP superfamily as a foundation to rationalize functional studies of surface receptors and their relationships with different biological processes.

Fujian cytoplasmic male sterility and the fertility restorer gene OsRf19 provide a promising breeding system for hybrid rice

Although hybrid rice has been widely utilized for nearly half a century, tremendously improving rice productivity worldwide, the breeding of hybrids has been difficult because of genetic complications in male sterility and fertility-restoring systems currently available in rice. Here, we characterized Fujian Abortive cytoplasmic male sterility (CMS-FA) rice, which has shown stable male sterility controlled by the mitochondrial gene FA182; a single nuclear gene, OsRf19, completely restores fertility. This single-gene inheritance has greatly eased the breeding process. By converting CMS-WA hybrids with the CMS-FA system, we developed six hybrids that showed equivalent or better performance relative to their CMS-WA counterparts. CMS-FA/OsRf19 provides a promising system for future hybrid rice breeding.

Cytoplasmic male sterility (CMS) determined by mitochondrial genes and restorer of fertility (Rf) controlled by nuclear-encoded genes provide the breeding systems of many hybrid crops for the utilization of heterosis. Although several CMS/Rf systems have been widely exploited in rice, hybrid breeding using these systems has encountered difficulties due to either fertility instability or complications of two-locus inheritance or both. In this work, we characterized a type of CMS, Fujian Abortive cytoplasmic male sterility (CMS-FA), with stable sporophytic male sterility and a nuclear restorer gene that completely restores hybrid fertility. CMS is caused by the chimeric open reading frame FA182 that specifically occurs in the mitochondrial genome of CMS-FA rice. The restorer gene OsRf19 encodes a pentatricopeptide repeat (PPR) protein targeted to mitochondria, where it mediates the cleavage of FA182 transcripts, thus restoring male fertility. Comparative sequence analysis revealed that OsRf19 originated through a recent duplication in wild rice relatives, sharing a common ancestor with OsRf1a/OsRf5, a fertility restorer gene for Boro II and Hong-Lian CMS. We developed six restorer lines by introgressing OsRf19 into parental lines of elite CMS-WA hybrids; hybrids produced from these lines showed equivalent or better agronomic performance relative to their counterparts based on the CMS-WA system. These results demonstrate that CMS-FA/OsRf19 provides a highly promising system for future hybrid rice breeding.

DRR Dhan 58, a Seedling Stage Salinity Tolerant NIL of Improved Samba Mahsuri Shows Superior Performance in Multi-location Trials

BACKGROUND

Improved Samba Mahsuri (ISM) is an elite, high-yielding, bacterial blight resistant, fine-grained rice variety with low glycaemic index. It is highly sensitive to salt stress, particularly at seedling stage, which significantly reduces its yield potential in coastal areas. A salinity tolerant QTL, Saltol, associated with seedling stage tolerance was previously mapped on chromosome 1 (10.6-11.5 Mb) from the Indian landrace, Pokkali and is effective in different genetic backgrounds. The objective of this study was to enhance salinity tolerance of ISM by incorporating the Saltol QTL through marker-assisted backcross breeding using the breeding line, FL478 (Pokkali/IR29).

RESULTS

Foreground selection was carried out at each generation using five Saltol-specific markers and three bacterial blight resistance genes, Xa21, xa13 and xa5. Background selection was conducted using 66 well distributed polymorphic SSR markers and at the BC₃F₂ generation, a single plant with maximum recurrent parent genome recovery (95.3%) was identified and advanced to the BC₃F₄ generation. Based on bacterial blight resistance, seedling stage salinity tolerance and resemblance to ISM, four advanced breeding lines were selected for testing in replicated experiments near Hyderabad, India. A promising near-isogenic line, DRR Dhan 58, was evaluated in multi-location trials-coastal salinity and it showed significant salinity tolerance, resistance to bacterial blight disease, high yield and excellent grain quality during the 2019 and 2020 trials. DRR Dhan 58 was 95.1% similar to ISM based on genotyping with the 90 K SNP chip. Whole genome resequencing analysis of Pokkali and FL478 which were salinity tolerant checks, ISM and DRR Dhan 58 showed a high degree of relatedness with respect to the candidate gene loci for Saltol and OsSKC1 (Shoot K⁺ Concentration 1).

CONCLUSION

DRR Dhan 58, possessing Saltol and three bacterial blight resistance genes (Xa21, xa13 and xa5) in the genetic background of the Indian mega-variety of rice, Samba Mahsuri, was developed for potential cultivation in areas prone to seedling stage salinity, as well as areas with endemic bacterial blight disease. This entry had a 24% yield advantage over the recurrent parent ISM under coastal saline conditions in multi-location trials and was recently released for commercial cultivation in India.

Genome-Wide Association Mapping Reveals Novel Putative Gene Candidates Governing Reproductive Stage Heat Stress Tolerance in Rice

Temperature rise predicted for the future will severely affect rice productivity because the crop is highly sensitive to heat stress at the reproductive stage. Breeding tolerant varieties is an economically viable option to combat heat stress, for which the knowledge of target genomic regions associated with the reproductive stage heat stress tolerance (RSHT) is essential. A set of 192 rice genotypes of diverse origins were evaluated under natural field conditions through staggered sowings for RSHT using two surrogate traits, spikelet fertility and grain yield, which showed significant reduction under heat stress. These genotypes were genotyped using a 50 k SNP array, and the association analysis identified 10 quantitative trait nucleotides (QTNs) for grain yield, of which one QTN (qHTGY8.1) was consistent across the different models used. Only two out of 10 MTAs coincided with the previously reported QTLs, making the remaing eight novel. A total of 22 QTNs were observed for spikelet fertility, among which qHTSF5.1 was consistently found across three models. Of the QTNs identified, seven coincided with previous reports, while the remaining QTNs were new. The genes near the QTNs were found associated with the protein–protein interaction, protein ubiquitination, stress signal transduction, and so forth, qualifying them to be putative for RSHT. An in silico expression analysis revealed the predominant expression of genes identified for spikelet fertility in reproductive organs. Further validation of the biological relevance of QTNs in conferring heat stress tolerance will enable their utilization in improving the reproductive stage heat stress tolerance in rice.

Verifying the Breeding Value of A Rare Haplotype of Chalk7, GS3, and Chalk5 to Improve Grain Appearance Quality in Rice

Grain quality is a key determinant of commercial value in rice. Efficiently improving grain quality, without compromising grain yield, is a challenge in rice breeding programs. Here we report on the identification and application of a grain quality gene, Chalk7, which causes a slender shape and decreases grain chalkiness in rice. Three allele-specific markers for Chalk7, and two other grain genes (GS3 and Chalk5) were developed, and used to stack the desirable alleles at these loci. The effects of individual or combined alleles at the loci were evaluated using a set of near-isogenic lines, each containing one to three favorable alleles in a common background of an elite variety. We found that the favorable allele combination of the three loci, which rarely occurs in natural rice germplasm, greatly reduces chalky grains without negatively impacting on grain yield. The data for newly developed allele-specific markers and pre-breeding lines will facilitate the improvement of grain appearance quality in rice.

Genetic Analysis of a Collection of Rice Germplasm (Oryza sativa L.) through High-Density SNP Array Provides Useful Information for Further Breeding Practices

Traditional breeding strategies mainly focus on the evaluation of trait performance, but pay less attention to the changing genetic background. A comprehensive understanding of the genetic diversity of germplasms is crucial for the deliberate improvement of specific traits. A collection of 154 highland rice varieties were collected as the initial genetic resource in our breeding program to improve the pathogen resistance and eating and cooking qualities. These varieties were analyzed using a whole-genome SNP array and were clustered into three groups. Further analysis revealed that the favorable alleles of pathogen resistance genes are mostly absent in our collected varieties. However, it showed that most varieties possess favorable alleles of Waxy (Wx) and ALKALI DEGENERATION (ALK), which are able to enhance the eating and cooking qualities. Moreover, only about one fifth of all varieties harbors favorable the allele of fragrance gene Betainealdehyde dehydrogenase (BADH2). Together, these results give an overall view of the genetic constitution of the target traits, which provide useful information for future genetic improvement in breeding practices.

Development and Application of a Target Capture Sequencing SNP-Genotyping Platform in Rice

The development of efficient, robust, and high-throughput SNP genotyping platforms is pivotal for crop genetics and breeding. Recently, SNP genotyping platforms based on target capture sequencing, which is very flexible in terms of the number of SNP markers, have been developed for maize, cassava, and fava bean. We aimed to develop a target capture sequencing SNP genotyping platform for rice. A target capture sequencing panel containing 2565 SNPs, including 1225 SNPs informative for japonica and 1339 SNPs informative for indica, was developed. This platform was used in diversity analysis of 50 rice varieties. Of the 2565 SNP markers, 2341 (91.3%) produced useful polymorphic genotype data, enabling the production of a phylogenetic tree of the 50 varieties. The mean number of markers polymorphic between any two varieties was 854. The platform was used for QTL mapping of preharvest sprouting (PHS) resistance in an F₈ recombinant inbred line population derived from the cross Odae × Joun. A genetic map comprising 475 markers was constructed, and two QTLs for PHS resistance were identified on chromosomes 4 and 11. This system is a powerful tool for rice genetics and breeding and will facilitate QTL studies and gene mapping, germplasm diversity analysis, and marker-assisted selection.

Dissection of the practical soybean breeding pipeline by developing ZDX1, a high-throughput functional array

KEY MESSAGE

We developed the ZDX1 high-throughput functional soybean array for high accuracy evaluation and selection of both parents and progeny, which can greatly accelerate soybean breeding. Microarray technology facilitates rapid, accurate, and economical genotyping. Here, using resequencing data from 2214 representative soybean accessions, we developed the high-throughput functional array ZDX1, containing 158,959 SNPs, covering 90.92% of soybean genes and sites related to important traits. By application of the array, a total of 817 accessions were genotyped, including three subpopulations of candidate parental lines, parental lines and their progeny from practical breeding. The fixed SNPs were identified in progeny, indicating artificial selection during the breeding process. By identifying functional sites of target traits, novel soybean cyst nematode-resistant progeny and maturity-related novel sources were identified by allele combinations, demonstrating that functional sites provide an efficient method for the rapid screening of desirable traits or gene sources. Notably, we found that the breeding index (BI) was a good indicator for progeny selection. Superior progeny were derived from the combination of distantly related parents, with at least one parent having a higher BI. Furthermore, new combinations based on good performance were proposed for further breeding after excluding redundant and closely related parents. Genomic best linear unbiased prediction (GBLUP) analysis was the best analysis method and achieved the highest accuracy in predicting four traits when comparing SNPs in genic regions rather than whole genomic or intergenic SNPs. The prediction accuracy was improved by 32.1% by using progeny to expand the training population. Collectively, a versatile assay demonstrated that the functional ZDX1 array provided efficient information for the design and optimization of a breeding pipeline for accelerated soybean breeding.

Applications of Genomic Tools in Plant Breeding: Crop Biofortification

Crop breeding has mainly been focused on increasing productivity, either directly or by decreasing the losses caused by biotic and abiotic stresses (that is, incorporating resistance to diseases and enhancing tolerance to adverse conditions, respectively). Quite the opposite, little attention has been paid to improve the nutritional value of crops. It has not been until recently that crop biofortification has become an objective within breeding programs, through either conventional methods or genetic engineering. There are many steps along this long path, from the initial evaluation of germplasm for the content of nutrients and health-promoting compounds to the development of biofortified varieties, with the available and future genomic tools assisting scientists and breeders in reaching their objectives as well as speeding up the process. This review offers a compendium of the genomic technologies used to explore and create biodiversity, to associate the traits of interest to the genome, and to transfer the genomic regions responsible for the desirable characteristics into potential new varieties. Finally, a glimpse of future perspectives and challenges in this emerging area is offered by taking the present scenario and the slow progress of the regulatory framework as the starting point.

From Green Super Rice to green agriculture: Reaping the promise of functional genomics research

Producing sufficient food with finite resources to feed the growing global population while having a smaller impact on the environment has always been a great challenge. Here, we review the concept and practices of Green Super Rice (GSR) that have led to a paradigm shift in goals for crop genetic improvement and models of food production for promoting sustainable agriculture. The momentous achievements and global deliveries of GSR have been fueled by the integration of abundant genetic resources, functional gene discoveries, and innovative breeding techniques with precise gene and whole-genome selection and efficient agronomic management to promote resource-saving, environmentally friendly crop production systems. We also provide perspectives on new horizons in genomic breeding technologies geared toward delivering green and nutritious crop varieties to further enhance the development of green agriculture and better nourish the world population.

Construction of transgenic detection system of Brassica napus L. based on single nucleotide polymorphism chip

Brassica napus L. is a vital oil crop in China. As auxiliary tools for rapeseed breeding, transgenic technologies play a considerable role in heterosis, variety improvement, and pest resistance. Research on transgenic detection technologies is of great significance for the introduction, supervision, and development of transgenic rapeseed in China. However, the transgenic detection methods currently in use are complex and time-consuming, with low output. A single nucleotide polymorphism (SNP) chip can effectively overcome such limitations. In the present study, we collected 40 transgenic elements and designed 291 probes. The probe sequences were submitted to Illumina Company, and the Infinium chip technology was used to prepare SNP chips. In the present Brassica napus transgenic detection experiment, 84 high-quality probes of 17 transgenic elements were preliminarily screened, and genotyping effect was optimised for the probe signal value. Ultimately, a transgenic detection system for B. napus was developed. The developed system has the advantages of simple operation, minimal technical errors, and stable detection outcomes. A transgenic detection sensitivity test revealed that the probe designed could accurately detect 1% of transgenic samples and had high detection sensitivity. In addition, in repeatability tests, the CaMV35S promoter coefficient of variation was approximately 3.58%. Therefore, the SNP chip had suitable repeatability in transgene detection. The SNP chip developed could be used to construct transgenic detection systems for B. napus.

SUPPLEMENTARY INFORMATION

The online version contains supplementary material available at 10.1007/s13205-021-03062-6.

Identification and verification of grain shape QTLs by SNP array in rice

Grain shape strongly influences the economic value and grain yield of rice. Thus, identifying quantitative trait loci (QTLs) for grain shape has been a longstanding goal in rice genetic research and breeding programs. Single nucleotide polymorphism (SNP) markers are ubiquitous in the rice genome and are more abundant and evenly distributed on the 12 rice chromosomes than traditional markers. An F₂ population was genotyped using the RICE6K SNP array to elucidate the mechanisms governing grain shape. Thirty-five QTLs for grain shape were detected on 11 of 12 chromosomes over 2 years. The major QTL cluster qGS7 was detected in both years and displayed strong genetic effects on grain length and width, showing consistency with GL7/GW7. Some minor QTLs were also detected, and the effects of four QTLs on seed size were then validated using BC₁F₆ populations with residual heterozygous lines in each QTL region. Our findings provide insights into the molecular basis of grain shape as well as additional resources and approaches for producing hybrid high-yield rice varieties.

Genomic Variation in Korean japonica Rice Varieties

Next-generation sequencing technologies have enabled the discovery of numerous sequence variations among closely related crop varieties. We analyzed genome resequencing data from 24 Korean temperate japonica rice varieties and discovered 954,233 sequence variations, including 791,121 single nucleotide polymorphisms (SNPs) and 163,112 insertions/deletions (InDels). On average, there was one variant per 391 base-pairs (bp), a variant density of 2.6 per 1 kbp. Of the InDels, 10,860 were longer than 20 bp, which enabled conversion to markers resolvable on an agarose gel. The effect of each variant on gene function was predicted using the SnpEff program. The variants were categorized into four groups according to their impact: high, moderate, low, and modifier. These groups contained 3524 (0.4%), 27,656 (2.9%), 24,875 (2.6%), and 898,178 (94.1%) variants, respectively. To test the accuracy of these data, eight InDels from a pre-harvest sprouting resistance QTL (qPHS11) target region, four highly polymorphic InDels, and four functional sequence variations in known agronomically important genes were selected and successfully developed into markers. These results will be useful to develop markers for marker-assisted selection, to select candidate genes in map-based cloning, and to produce efficient high-throughput genome-wide genotyping systems for Korean temperate japonica rice varieties.

Balancing selection and wild gene pool contribute to resistance in global rice germplasm against planthopper

Interactions and co-evolution between plants and herbivorous insects are critically important in agriculture. Brown planthopper (BPH) is the most severe insect of rice, and the biotypes adapt to feed on different rice genotypes. Here, we present genomics analyses on 1,520 global rice germplasms for resistance to three BPH biotypes. Genome-wide association studies identified 3,502 single nucleotide polymorphisms (SNPs) and 59 loci associated with BPH resistance in rice. We cloned a previously unidentified gene Bph37 that confers resistance to BPH. The associated loci showed high nucleotide diversity. Genome-wide scans for trans-species polymorphisms revealed ancient balancing selection at the loci. The secondarily evolved insect biotypes II and III exhibited significantly higher virulence and overcame more rice varieties than the primary biotype I. In response, more SNPs and loci evolved in rice for resistance to biotypes II and III. Notably, three exceptional large regions with high SNP density and resistance-associated loci on chromosomes 4 and 6 appear distinct between the resistant and susceptible rice varieties. Surprisingly, these regions in resistant rice might have been retained from wild species Oryza nivara. Our findings expand the understanding of long-term interactions between rice and BPH and provide resistance genes and germplasm resources for breeding durable BPH-resistant rice varieties.

Development and application of the Faba_bean_130K targeted next-generation sequencing SNP genotyping platform based on transcriptome sequencing

KEY MESSAGE

Large-scale faba bean transcriptome data are available, and the first genotyping platform based on liquid-phase probe targeted capture technology was developed for genetic and molecular breeding studies. Faba bean (Vicia faba L., 2n = 12) is an important food legume crop that is widely grown for multiple uses worldwide. However, no reference genome is currently available due to its very large genome size (approximately 13 Gb) and limited single nucleotide polymorphism (SNP) markers as well as highly efficient genotyping tools have been reported for faba bean. In this study, 16.7 billion clean reads were obtained from transcriptome libraries of flowers and leaves of 102 global faba bean accessions. A total of 243,120 unigenes were de novo assembled and functionally annotated. Moreover, a total of 1,579,411 SNPs were identified and further filtered according to a selection pipeline to develop a high-throughput, flexible, low-cost Faba_bean_130K targeted next-generation sequencing (TNGS) genotyping platform. A set of 69 Chinese faba bean accessions were genotyped with the TNGS genotyping platform, and the average mapping rate of captured reads to reference transcripts was 93.14%, of which 53.23% were located in the targeted regions. The TNGS genotyping results were validated by Sanger sequencing and the average consistency rate reached 93.6%. Comprehensive population genetic analysis was performed on the 69 Chinese faba bean accessions and identified four genetic subgroups correlated with the geographic distribution. This study provides valuable genomic resources and a reliable genotyping tool that could be implemented in genetic and molecular breeding studies to accelerate new cultivar development and improvement in faba bean.

The identification of grain size genes by RapMap reveals directional selection during rice domestication

Cloning quantitative trait locus (QTL) is time consuming and laborious, which hinders the understanding of natural variation and genetic diversity. Here, we introduce RapMap, a method for rapid multi-QTL mapping by employing F2 gradient populations (F2GPs) constructed by minor-phenotypic-difference accessions. The co-segregation standard of the single-locus genetic models ensures simultaneous integration of a three-in-one framework in RapMap i.e. detecting a real QTL, confirming its effect, and obtaining its near-isogenic line-like line (NIL-LL). We demonstrate the feasibility of RapMap by cloning eight rice grain-size genes using 15 F2GPs in three years. These genes explain a total of 75% of grain shape variation. Allele frequency analysis of these genes using a large germplasm collection reveals directional selection of the slender and long grains in indica rice domestication. In addition, major grain-size genes have been strongly selected during rice domestication. We think application of RapMap in crops will accelerate gene discovery and genomic breeding.

Genomic Selection in Sugarcane: Current Status and Future Prospects

Sugarcane is a C4 and agro-industry-based crop with a high potential for biomass production. It serves as raw material for the production of sugar, ethanol, and electricity. Modern sugarcane varieties are derived from the interspecific and intergeneric hybridization between Saccharum officinarum, Saccharum spontaneum, and other wild relatives. Sugarcane breeding programmes are broadly categorized into germplasm collection and characterization, pre-breeding and genetic base-broadening, and varietal development programmes. The varietal identification through the classic breeding programme requires a minimum of 12-14 years. The precise phenotyping in sugarcane is extremely tedious due to the high propensity of lodging and suckering owing to the influence of environmental factors and crop management practices. This kind of phenotyping requires data from both plant crop and ratoon experiments conducted over locations and seasons. In this review, we explored the feasibility of genomic selection schemes for various breeding programmes in sugarcane. The genetic diversity analysis using genome-wide markers helps in the formation of core set germplasm representing the total genomic diversity present in the Saccharum gene bank. The genome-wide association studies and genomic prediction in the Saccharum gene bank are helpful to identify the complete genomic resources for cane yield, commercial cane sugar, tolerances to biotic and abiotic stresses, and other agronomic traits. The implementation of genomic selection in pre-breeding, genetic base-broadening programmes assist in precise introgression of specific genes and recurrent selection schemes enhance the higher frequency of favorable alleles in the population with a considerable reduction in breeding cycles and population size. The integration of environmental covariates and genomic prediction in multi-environment trials assists in the prediction of varietal performance for different agro-climatic zones. This review also directed its focus on enhancing the genetic gain over time, cost, and resource allocation at various stages of breeding programmes.

The Bnapus50K array: a quick and versatile genotyping tool for Brassica napus genomic breeding and research

Rapeseed is a globally cultivated commercial crop, primarily grown for its oil. High-density single nucleotide polymorphism (SNP) arrays are widely used as a standard genotyping tool for rapeseed research, including for gene mapping, genome-wide association studies, germplasm resource analysis, and cluster analysis. Although considerable rapeseed genome sequencing data have been released, DNA arrays are still an attractive choice for providing additional genetic data in an era of high-throughput whole-genome sequencing. Here, we integrated re-sequencing DNA array data (32,216, 304 SNPs) from 505 inbred rapeseed lines, allowing us to develop a sensitive and efficient genotyping DNA array, Bnapus50K, with a more consistent genetic and physical distribution of probes. A total of 42,090 high-quality probes were filtered and synthesized, with an average distance between adjacent SNPs of 8 kb. To improve the practical application potential of this array in rapeseed breeding, we also added 1,618 functional probes related to important agronomic traits such as oil content, disease resistance, male sterility, and flowering time. The additional probes also included those specifically for detecting genetically modified material. These probes show a good detection efficiency and are therefore useful for gene mapping, along with crop variety improvement and identification. The novel Bnapus50K DNA array developed in this study could prove to be a quick and versatile genotyping tool for B. napus genomic breeding and research.

Development and validation of a 1 K sika deer (Cervus nippon) SNP Chip

Background

China is the birthplace of the deer family and the country with the most abundant deer resources. However, at present, China’s deer industry faces the problem that pure sika deer and hybrid deer cannot be easily distinguished. Therefore, the development of a SNP identification chip is urgently required.

Results

In this study, 250 sika deer, 206 red deer, 23 first-generation hybrid deer (F1), 20 s-generation hybrid deer (F2), and 20 third-generation hybrid deer (F3) were resequenced. Using the chromosome-level sika deer genome as the reference sequence, mutation detection was performed on all individuals, and a total of 130,306,923 SNP loci were generated. After quality control filtering was performed, the remaining 31,140,900 loci were confirmed. From molecular-level and morphological analyses, the sika deer reference population and the red deer reference population were established. The Fst values of all SNPs in the two reference populations were calculated. According to customized algorithms and strict screening principles, 1000 red deer-specific SNP sites were finally selected for chip design, and 63 hybrid individuals were determined to contain red deer-specific SNP loci. The results showed that the gene content of red deer gradually decreased in subsequent hybrid generations, and this decrease roughly conformed to the law of statistical genetics. Reaction probes were designed according to the screening sites. All candidate sites met the requirements of the Illumina chip scoring system. The average score was 0.99, and the MAF was in the range of 0.3277 to 0.3621. Furthermore, 266 deer (125 sika deer, 39 red deer, 56 F1, 29 F2,17 F3) were randomly selected for 1 K SNP chip verification. The results showed that among the 1000 SNP sites, 995 probes were synthesized, 4 of which could not be typed, while 973 loci were polymorphic. PCA, random forest and ADMIXTURE results showed that the 1 K sika deer SNP chip was able to clearly distinguish sika deer, red deer, and hybrid deer and that this 1 K SNP chip technology may provide technical support for the protection and utilization of pure sika deer species resources.

Conclusion

We successfully developed a low-density identification chip that can quickly and accurately distinguish sika deer from their hybrid offspring, thereby providing technical support for the protection and utilization of pure sika deer germplasm resources.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12863-021-00994-z.

Genetic relationships and identification of core germplasm among rice photoperiod- and thermo-sensitive genic male sterile lines

BACKGROUND

Harnessing heterosis is one of the major approaches to increase rice yield and has made a great contribution to food security. The identification and selection of outstanding parental genotypes especially among male sterile lines is a key step for exploiting heterosis. Two-line hybrid system is based on the discovery and application of photoperiod- and thermo-sensitive genic sensitive male sterile (PTGMS) materials. The development of wide-range of male sterile lines from a common gene pool leads to a narrower genetic diversity, which is vulnerable to biotic and abiotic stress. Hence, it is valuable to ascertain the genetic background of PTGMS lines and to understand their relationships in order to select and design a future breeding strategy.

RESULTS

A collection of 118 male sterile rice lines and 13 conventional breeding lines from the major rice growing regions of China was evaluated and screened against the photosensitive (pms3) and temperature sensitive male sterility (tms5) genes. The total gene pool was divided into four major populations as P1 possessing the pms3, P2 possessing tms5, P3 possessing both pms3 and tms5 genes, and P4 containing conventional breeding lines without any male sterility allele. The high genetic purity was revealed by homozygous alleles in all populations. The population admixture, principle components and the phylogenetic analysis revealed the close relations of P2 and P3 with P4. The population differentiation analysis showed that P1 has the highest differentiation coefficient. The lines from P1 were observed as the ancestors of other three populations in a phylogenetic tree, while the lines in P2 and P3 showed a close genetic relation with conventional lines. A core collection of top 10% lines with maximum within and among populations genetic diversity was constructed for future research and breeding efforts.

CONCLUSION

The low genetic diversity and close genetic relationship among PTGMS lines in P2, P3 and P4 populations suggest a selection sweep and they might result from a backcrossing with common ancestors including the pure lines of P1. The core collection from PTGMS panel updated with new diverse germplasm will serve best for further two-line hybrid breeding.

Designing Future Crops: Genomics-Assisted Breeding Comes of Age

Over the past decade, genomics-assisted breeding (GAB) has been instrumental in harnessing the potential of modern genome resources and characterizing and exploiting allelic variation for germplasm enhancement and cultivar development. Sustaining GAB in the future (GAB 2.0) will rely upon a suite of new approaches that fast-track targeted manipulation of allelic variation for creating novel diversity and facilitate their rapid and efficient incorporation in crop improvement programs. Genomic breeding strategies that optimize crop genomes with accumulation of beneficial alleles and purging of deleterious alleles will be indispensable for designing future crops. In coming decades, GAB 2.0 is expected to play a crucial role in breeding more climate-smart crop cultivars with higher nutritional value in a cost-effective and timely manner.

OsGRETCHENHAGEN3-2 modulates rice seed storability via accumulation of abscisic acid and protective substances

Seed storability largely determines the vigor of seeds during storage and is significant in agriculture and ecology. However, the underlying genetic basis remains unclear. In the present study, we report the cloning and characterization of the rice (Oryza sativa) indole-3-acetic acid (IAA)-amido synthetase gene GRETCHEN HAGEN3-2 (OsGH3-2) associated with seed storability. OsGH3-2 was identified by performing a genome-wide association study in rice germplasms with linkage mapping in chromosome substitution segment lines, contributing to the wide variation of seed viability in the populations after long periods of storage and artificial ageing. OsGH3-2 was dominantly expressed in the developing seeds and catalyzed IAA conjugation to amino acids, forming inactive auxin. Transgenic overexpression, knockout, and knockdown experiments demonstrated that OsGH3-2 affected seed storability by regulating the accumulation level of abscisic acid (ABA). Overexpression of OsGH3-2 significantly decreased seed storability, while knockout or knockdown of the gene enhanced seed storability compared with the wild-type. OsGH3-2 acted as a negative regulator of seed storability by modulating many genes related to the ABA pathway and probably subsequently late embryogenesis-abundant proteins at the transcription level. These findings shed light on the molecular mechanisms underlying seed storability and will facilitate the improvement of seed vigor by genomic breeding and gene-editing approaches in rice.

Genomic resources in plant breeding for sustainable agriculture

Climate change during the last 40 years has had a serious impact on agriculture and threatens global food and nutritional security. From over half a million plant species, cereals and legumes are the most important for food and nutritional security. Although systematic plant breeding has a relatively short history, conventional breeding coupled with advances in technology and crop management strategies has increased crop yields by 56 % globally between 1965-85, referred to as the Green Revolution. Nevertheless, increased demand for food, feed, fiber, and fuel necessitates the need to break existing yield barriers in many crop plants. In the first decade of the 21st century we witnessed rapid discovery, transformative technological development and declining costs of genomics technologies. In the second decade, the field turned towards making sense of the vast amount of genomic information and subsequently moved towards accurately predicting gene-to-phenotype associations and tailoring plants for climate resilience and global food security. In this review we focus on genomic resources, genome and germplasm sequencing, sequencing-based trait mapping, and genomics-assisted breeding approaches aimed at developing biotic stress resistant, abiotic stress tolerant and high nutrition varieties in six major cereals (rice, maize, wheat, barley, sorghum and pearl millet), and six major legumes (soybean, groundnut, cowpea, common bean, chickpea and pigeonpea). We further provide a perspective and way forward to use genomic breeding approaches including marker-assisted selection, marker-assisted backcrossing, haplotype based breeding and genomic prediction approaches coupled with machine learning and artificial intelligence, to speed breeding approaches. The overall goal is to accelerate genetic gains and deliver climate resilient and high nutrition crop varieties for sustainable agriculture.

Identification and Characterization of Base-Substitution Mutations in the Macronuclear Genome of the Ciliate Tetrahymena thermophila

Polyploidy can provide adaptive advantages and drive evolution. Amitotic division of the polyploid macronucleus (MAC) in ciliates acts as a nonsexual genetic mechanism to enhance adaptation to stress conditions and thus provides a unique model to investigate the evolutionary role of polyploidy. Mutation is the primary source of the variation responsible for evolution and adaptation; however, to date, de novo mutations that occur in ciliate MAC genomes during these processes have not been characterized and their biological impacts are undefined. Here, we carried out long-term evolution experiments to directly explore de novo MAC mutations and their molecular features in the model ciliate, Tetrahymena thermophila. A simple but effective method was established to detect base-substitution mutations in evolving populations whereas filtering out most of the false positive base-substitutions caused by repetitive sequences and the programmed genome rearrangements. The detected mutations were rigorously validated using the MassARRAY system. Validated mutations showed a strong G/C→A/T bias, consistent with observations in other species. Moreover, a progressive increase in growth rate of the evolving populations suggested that some of these mutations might be responsible for cell fitness. The established mutation identification and validation methods will be an invaluable resource to make ciliates an important model system to study the role of polyploidy in evolution.

Dynamic genome-wide association analysis and identification of candidate genes involved in anaerobic germination tolerance in rice

BACKGROUND

In Asian rice production, an increasing number of countries now choose the direct seeding mode because of rising costs, labour shortages and water shortages. The ability of rice seeds to undergo anaerobic germination (AG) plays an important role in the success of direct seeding.

RESULTS

In this study, we used 2,123,725 single nucleotide polymorphism (SNP) markers based on resequencing to conduct a dynamic genome-wide association study (GWAS) of coleoptile length (CL) and coleoptile diameter (CD) in 209 natural rice populations. A total of 26 SNP loci were detected in these two phenotypes, of which 5 overlapped with previously reported loci (S1_ 39674301, S6_ 20797781, S7_ 18722403, S8_ 9946213, S11_ 19165397), and two sites were detected repeatedly at different time points (S3_ 24689629 and S5_ 27918754). We suggest that these 7 loci (-log₁₀ (P) value > 7.3271) are the key sites that affect AG tolerance. To screen the candidate genes more effectively, we sequenced the transcriptome of the flooding-tolerant variety R151 in six key stages, including anaerobic (AN) and the oxygen conversion point (AN-A), and obtained high-quality differential expression profiles. Four reliable candidate genes were identified: Os01g0911700 (OsVP1), Os05g0560900 (OsGA2ox8), Os05g0562200 (OsDi19-1) and Os06g0548200. Then qRT-PCR and LC-MS/ MS targeting metabolite detection technology were used to further verify that the up-regulated expression of these four candidate genes was closely related to AG.

CONCLUSION

The four novel candidate genes were associated with gibberellin (GA) and abscisic acid (ABA) regulation and cell wall metabolism under oxygen-deficiency conditions and promoted coleoptile elongation while avoiding adverse effects, allowing the coleoptile to obtain oxygen, escape the low-oxygen environment and germinate rapidly. The results of this study improve our understanding of the genetic basis of AG in rice seeds, which is conducive to the selection of flooding-tolerant varieties suitable for direct seeding.

Genetic basis of vascular bundle variations in rice revealed by genome-wide association study

The vascular bundles play important roles in transportation of photoassimilate, and the number, size, and capacity of vascular bundles influence the transportation efficiency. Dissecting the genetic basis may help to make better use of naturally occurring vascular bundle variations. Here, we conducted a genome-wide association study (GWAS) of the vascular bundle variations in a worldwide collection of 529 Oryza sativa accessions. A total of 42 and 93 significant association loci were identified in the neck panicle and flag leaf, respectively. The introgression lines showing extreme values of the target traits harbored at least one GWAS signal, indicating the reliability of the GWAS loci. Based on the data of near-isogenic lines and transgenic plants, Grain number, plant height, and heading date7 (Ghd7) was identified as a major locus for the natural variation of vascular bundles in the neck panicle at the heading stage. In addition, Narrow leaf1 (NAL1) was found to influence the vascular bundles in both the neck panicle and flag leaf, and the effects of the major haplotypes of NAL1 were characterized. The loci or candidate genes identified would help to improve vascular bundle system in rice breeding.

Identification of Novel Alleles of the Rice Blast-Resistance Gene Pi9 through Sequence-Based Allele Mining

BACKGROUND

As rice (Oryza sativa) is the staple food of more than half the world's population, rice production contributes greatly to global food security. Rice blast caused by the fungus Magnaporthe oryzae (M. oryzae) is a devastating disease that affects rice yields and grain quality, resulting in substantial economic losses annually. Because the fungus evolves rapidly, the resistance conferred by most the single blast-resistance genes is broken after a few years of intensive agricultural use. Therefore, effective resistance breeding in rice requires continual enrichment of the reservoir of resistance genes, alleles, or QTLs. Seed banks represent a rich source of genetic diversity; however, they have not been extensively used to identify novel genes and alleles.

RESULTS

We carried out a large-scale screen for novel blast-resistance alleles in 1883 rice varieties from major rice-producing areas across China. Of these, 361 varieties showed at least moderate resistance to natural infection by rice blast at rice blast nurseries in Enshi and Yichang, Hubei Province. We used sequence-based allele mining to amplify and sequence the allelic variants of the major rice blast-resistance genes at the Pi2/Pi9 locus of chromosome 6 from the 361 blast-resistant varieties, and the full-length coding region of this gene could be amplified from 107 varieties. Thirteen novel Pi9 alleles (named Pi9-Type1 to Pi9-Type13) were identified in these 107 varieties based on comparison to the Pi9 referenced sequence. Based on the sequencing results, the Pi2/Pi9 locus of the 107 varieties was divided into 15 genotypes (including three different genotypes of Pi9-Type5). Fifteen varieties, each representing one genotype, were evaluated for resistance to 34 M. oryzae isolates. The alleles from seven varieties with the highest resistance and widest resistance spectra were selected for transformation into the susceptible variety J23B to construct near-isogenic lines (NILs). These NILs showed resistance in a field test in Enshi and Yichang, indicating that the seven novel rice blast-resistance tandem-repeat regions at the Pi2/Pi9 locus of chromosome 6 could potentially serve as a genetic resource for molecular breeding of resistance to rice blast.

CONCLUSIONS

The thirteen novel Pi9 alleles identified in this study expand the list of available of blast-resistance alleles. Seven tandem-repeat regions of the Pi2/Pi9 locus from different donors were characterized as broad-spectrum rice blast-resistance fragments; these donors enrich the genetic resources available for rice blast-resistance breeding programs.

Genetic Dissection of Seed Dormancy in Rice (Oryza sativa L.) by Using Two Mapping Populations Derived from Common Parents

BACKGROUND

Seed dormancy, a quality characteristic that plays a role in seed germination, seedling establishment and grain yield, is affected by multiple genes and environmental factors. The genetic and molecular mechanisms underlying seed dormancy in rice remain largely unknown.

RESULTS

Quantitative trait loci (QTLs) for seed dormancy were identified in two different mapping populations, a chromosome segment substitution line (CSSL) and backcross inbred line (BIL) population, both derived from the same parents Nipponbare, a japonica cultivar with seed dormancy, and 9311, an indica cultivar lacking seed dormancy. A total of 12 and 27 QTL regions for seed dormancy were detected in the CSSLs and BILs, respectively. Among these regions, four major loci (qSD3.1, qSD3.2, qSD5.2 and qSD11.2) were commonly identified for multiple germination parameters associated with seed dormancy in both populations, with Nipponbare alleles delaying the seed germination percentage and decreasing germination uniformity. Two loci (qSD3.1 and qSD3.2) were individually validated in the near-isogenic lines containing the QTL of interest. The effect of qSD3.2 was further confirmed in a CSSL-derived F₂ population. Furthermore, both qSD3.1 and qSD3.2 were sensitive to abscisic acid and exhibited a significant epistatic interaction to increase seed dormancy.

CONCLUSIONS

Our results indicate that the integration of the developed CSSLs and BILs with high-density markers can provide a powerful tool for dissecting the genetic basis of seed dormancy in rice. Our findings regarding the major loci and their interactions with several promising candidate genes that are induced by abscisic acid and specifically expressed in the seeds will facilitate further gene discovery and a better understanding of the genetic and molecular mechanisms of seed dormancy for improving seed quality in rice breeding programs.

Role of NGS and SNP genotyping methods in sugarcane improvement programs

Sugarcane (Saccharum spp.) is one of the most economically significant crops because of its high sucrose content and it is a promising biomass feedstock for biofuel production. Sugarcane genome sequencing and analysis is a difficult task due to its heterozygosity and polyploidy. Long sequence read technologies, PacBio Single-Molecule Real-Time (SMRT) sequencing, the Illumina TruSeq, and the Oxford Nanopore sequencing could solve the problem of genome assembly. On the applications side, next generation sequencing (NGS) technologies played a major role in the discovery of single nucleotide polymorphism (SNP) and the development of low to high throughput genotyping platforms. The two mainstream high throughput genotyping platforms are the SNP microarray and genotyping by sequencing (GBS). This paper reviews the NGS in sugarcane genomics, genotyping methodologies, and the choice of these methods. Array-based SNP genotyping is robust, provides consistent SNPs, and relatively easier downstream data analysis. The GBS method identifies large scale SNPs across the germplasm. A combination of targeted GBS and array-based genotyping methods should be used to increase the accuracy of genomic selection and marker-assisted breeding.

Recommendations for Choosing the Genotyping Method and Best Practices for Quality Control in Crop Genome-Wide Association Studies

High-throughput genotyping boosts genome-wide association studies (GWAS) in crop species, leading to the identification of single-nucleotide polymorphisms (SNPs) associated with economically important traits. Choosing a cost-effective genotyping method for crop GWAS requires careful examination of several aspects, namely, the purpose and the scale of the study, crop-specific genomic features, and technical and economic matters associated with each genotyping option. Once genotypic data have been obtained, quality control (QC) procedures must be applied to avoid bias and false signals in genotype–phenotype association tests. QC for human GWAS has been extensively reviewed; however, QC for crop GWAS may require different actions, depending on the GWAS population type. Here, we review most popular genotyping methods based on next-generation sequencing (NGS) and array hybridization, and report observations that should guide the investigator in the choice of the genotyping method for crop GWAS. We provide recommendations to perform QC in crop species, and deliver an overview of bioinformatics tools that can be used to accomplish all needed tasks. Overall, this work aims to provide guidelines to harmonize those procedures leading to SNP datasets ready for crop GWAS.

An improved 7K SNP array, the C7AIR, provides a wealth of validated SNP markers for rice breeding and genetics studies

Single nucleotide polymorphisms (SNPs) are highly abundant, amendable to high-throughput genotyping, and useful for a number of breeding and genetics applications in crops. SNP frequencies vary depending on the species and populations under study, and therefore target SNPs need to be carefully selected to be informative for each application. While multiple SNP genotyping systems are available for rice (Oryza sativa L. and its relatives), they vary in their informativeness, cost, marker density, speed, flexibility, and data quality. In this study, we report the development and performance of the Cornell-IR LD Rice Array (C7AIR), a second-generation SNP array containing 7,098 markers that improves upon the previously released C6AIR. The C7AIR is designed to detect genome-wide polymorphisms within and between subpopulations of O. sativa, as well as O. glaberrima, O. rufipogon and O. nivara. The C7AIR combines top-performing SNPs from several previous rice arrays, including 4,007 SNPs from the C6AIR, 2,056 SNPs from the High Density Rice Array (HDRA), 910 SNPs from the 384-SNP GoldenGate sets, 189 SNPs from the 44K array selected to add information content for elite U.S. tropical japonica rice varieties, and 8 trait-specific SNPs. To demonstrate its utility, we carried out a genome-wide association analysis for plant height, employing the C7AIR across a diversity panel of 189 rice accessions and identified 20 QTLs contributing to plant height. The C7AIR SNP chip has so far been used for genotyping >10,000 rice samples. It successfully differentiates the five subpopulations of Oryza sativa, identifies introgressions from wild and exotic relatives, and is useful for quantitative trait loci (QTL) and association mapping in diverse materials. Moreover, data from the C7AIR provides valuable information that can be used to select informative and reliable SNP markers for conversion to lower-cost genotyping platforms for genomic selection and other downstream applications in breeding.

Genomic Breeding of Green Super Rice Varieties and Their Deployment in Asia and Africa

KEY MESSAGE

The "Green Super Rice" (GSR) project aims to fundamentally transform crop production techniques and promote the development of green agriculture based on functional genomics and breeding of GSR varieties by whole-genome breeding platforms. Rice (Oryza sativa L.) is one of the leading food crops of the world, and the safe production of rice plays a central role in ensuring food security. However, the conflicts between rice production and environmental resources are becoming increasingly acute. For this reason, scientists in China have proposed the concept of Green Super Rice for promoting resource-saving and environment-friendly rice production, while still achieving a yield increase and quality improvement. GSR is becoming one of the major goals for agricultural research and crop improvement worldwide, which aims to mine and use vital genes associated with superior agronomic traits such as high yield, good quality, nutrient efficiency, and resistance against insects and stresses; establish genomic breeding platforms to breed and apply GSR; and set up resource-saving and environment-friendly cultivation management systems. GSR has been introduced into eight African and eight Asian countries and has contributed significantly to rice cultivation and food security in these countries. This article mainly describes the GSR concept and recent research progress, as well as the significant achievements in GSR breeding and its application.

Improving nutritional quality of rice for human health

This review surveys rice nutritional value, mainly focusing on breeding achievements via adoption of both genetic engineering and non-transgenic strategies to improve key nutrients associated with human health. Rice (Oryza sativa) is an essential component of the diets and livelihoods of over 3.5 billion people. Polished rice is mostly consumed as staple food, fulfilling daily energy demands and part of the protein requirement. Brown rice is comparatively more nutritious, containing more lipids, minerals, vitamins, dietary fiber, micronutrients, and bioactive compounds. In this article, we review the nutritional facts about rice including the level of γ-aminobutyric acid, resistant starch, lysine, iron, zinc, β-carotene, folate, anthocyanin, various carotenoids, and flavonoids, focusing on their synthesis and metabolism and the advances in their biofortification via adoption of both conventional and genetic engineering strategies. We conclude that besides representing a staple food, rice has the potential to become a source of various essential nutrients or bioactive compounds through appropriate genetic improvements to benefit human health and prevent certain chronic diseases. Finally, we discuss the available, non-genetically engineering strategies for the nutritional improvement of rice, including their main strengths and constraints.

A xylan glucuronosyltransferase gene exhibits pleiotropic effects on cellular composition and leaf development in rice

Leaf chlorophyll content is an important physiological indicator of plant growth, metabolism and nutritional status, and it is highly correlated with leaf nitrogen content and photosynthesis. In this study, we report the cloning and identification of a xylan glucuronosyltransferase gene (OsGUX1) that affects relative chlorophyll content in rice leaf. Using a set of chromosomal segment substitution lines derived from a cross of wild rice accession ACC10 and indica variety Zhenshan 97 (ZS97), we identified numerous quantitative trait loci for relative chlorophyll content. One major locus of them for relative chlorophyll content was mapped to a 10.3-kb region that contains OsGUX1. The allele OsGUX1^AC from ACC10 significantly decreases nitrogen content and chlorophyll content of leaf compared with OsGUX1^ZS from ZS97. The overexpression of OsGUX1 reduced chlorophyll content, and the suppression of this gene increased chlorophyll content of rice leaf. OsGUX1 is located in Golgi apparatus, and highly expressed in seedling leaf and the tissues in which primary cell wall synthesis occurring. Our experimental data indicate that OsGUX1 is responsible for addition of glucuronic acid residues onto xylan and participates in accumulation of cellulose and hemicellulose in the cell wall deposition, thus thickening the primary cell wall of mesophyll cells, which might lead to reduced chlorophyll content in rice leaf. These findings provide insights into the association of cell wall components with leaf nitrogen content in rice.

Directional upgrading of brown planthopper resistance in an elite rice cultivar by precise introgression of two resistance genes using genomics-based breeding

Brown planthopper (BPH) is a devastating pest that threatens the food security of rice-producing countries. At present, most cultivars planted in farmers' paddies lack effective BPH resistance, which constitutes a potential threat to rice yield. Moreover, developing BPH-resistant rice varieties using traditional breeding approaches is time-consuming, labor-intensive, and unpredictable. In this study, we successfully enhanced BPH resistance of the elite rice cultivar Wushansimiao by introgressing the resistance genes BPH14 and BPH15 through positive selection, negative selection, and whole genome background selection. Through backcrossing, the introgression fragments were reduced to 428.3 kb for BPH14 and 413.1 kb for BPH15. Except for these two fragments, the residual genetic background of the selected near-isogenic lines (NILs) was nearly identical to that of the recurrent parent, with a genetic background recovery rate of 99.78%. As a result, the selected NILs exhibited much stronger BPH resistance at the seedling and adult stages compared to the recurrent parent. Moreover, field tests showed that grain yield, major agronomic traits, and grain quality of the five selected NILs were statistically indistinguishable from those of the recurrent parent. Our results provide an effective approach for directionally upgrading the target traits and will inform and facilitate rice breeding.

Genotyping-by-sequencing based QTL mapping for rice grain yield under reproductive stage drought stress tolerance

QTLs for rice grain yield under reproductive stage drought stress (qDTY) identified earlier with low density markers have shown linkage drag and need to be fine mapped before their utilization in breeding programs. In this study, genotyping-by-sequencing (GBS) based high-density linkage map of rice was developed using two BC₁F₃ mapping populations namely Swarna*2/Dular (3929 SNPs covering 1454.68 cM) and IR11N121*2/Aus196 (1191 SNPs covering 1399.68 cM) with average marker density of 0.37 cM to 1.18 cM respectively. In total, six qDTY QTLs including three consistent effect QTLs were identified in Swarna*2/Dular while eight qDTY QTLs including two consistent effect QTLs were identified in IR11N121*2/Aus 196 mapping population. Comparative analysis revealed four stable and novel QTLs (qDTY_2.4, qDTY_3.3, qDTY_6.3, and qDTY_11.2) which explained 8.62 to 14.92% PVE. However, one of the identified stable grain yield QTL qDTY_1.1 in both the populations was located nearly at the same physical position of an earlier mapped major qDTY QTL. Further, the effect of the identified qDTY_1.1 was validated in a subset of lines derived from five mapping populations confirming robustness of qDTY_1.1 across various genetic backgrounds/seasons. The study successfully identified stable grain yield QTLs free from undesirable linkages of tall plant height/early maturity utilizing high density linkage maps.

Improving rice blast resistance of Feng39S through molecular marker-assisted backcrossing

BACKGROUND

Rice blast caused by Magnaporthe oryzae is one of the most widespread biotic constraints that threaten rice production. Using major resistance genes for rice blast resistance improvement is considered to be an efficient and technically feasible approach to achieve optimal grain yield.

RESULTS

We report here the introgression of the broad-spectrum blast resistance gene Pi2 into the genetic background of an elite PTGMS line, Feng39S, for enhancing it and its derived hybrid blast resistance through marker-assisted backcrossing (MABC) coupled with genomics-based background selection. Two PTGMS lines, designated as DB16206-34 and DB16206-38, stacking homozygous Pi2 were selected, and their genetic background had recurrent parent genome recovery of 99.67% detected by the SNP array RICE6K. DB16206-34 and DB16206-38 had high resistance frequency, with an average of 94.7%, when infected with 57 blast isolates over 2 years, and the resistance frequency of their derived hybrids ranged from 68.2% to 95.5% under inoculation of 22 blast isolates. The evaluation of results under natural blast epidemic field conditions showed that the selected PTGMS lines and their derived hybrids were resistant against leaf and neck blast. The characterizations of the critical temperature point of fertility-sterility alternation of the selected PTGMS lines, yield, main agronomic traits, and rice quality of the selected PTGMS lines and their hybrids were identical to those of the recurrent parent and its hybrids. DB16206-34/9311 or DB16206-38/9311 can be used as a blast-resistant version to replace the popular hybrid Fengliangyou 4. Likewise, DB16206-34/FXH No.1 or DB16206-38/FXH No.1 can also be used as a blast-resistant version to replace another popular hybrid Fengliangyou Xiang 1.

CONCLUSIONS

Our evaluation is the first successful case to apply MABC with genomics-based background selection to improve the blast resistance of PTGMS lines for two-line hybrid rice breeding.