Genome-Wide Analysis of Genetic Variations and the Detection of Rich Variants of NBS-LRR Encoding Genes in Common Wild Rice Lines

The landscape of sequence variations between resistant and susceptible hot peppers to predict functional candidate genes against bacterial wilt disease

BACKGROUND

Bacterial wilt (BW), caused by Ralstonia solanacearum (Ral), results in substantial yield losses in pepper crops. Developing resistant pepper varieties through breeding is the most effective strategy for managing BW. To achieve this, a thorough understanding of the genetic information connected with resistance traits is essential. Despite identifying three major QTLs for bacterial wilt resistance in pepper, Bw1 on chromosome 8, qRRs-10.1 on chromosome 10, and pBWR-1 on chromosome 1, the genetic information of related BW pepper varieties has not been sufficiently studied. Here, we resequenced two pepper inbred lines, C. annuum 'MC4' (resistant) and C. annuum 'Subicho' (susceptible), and analyzed genomic variations through SNPs and Indels to identify candidate genes for BW resistance.

RESULTS

An average of 139.5 Gb was generated among the two cultivars, with coverage ranging from 44.94X to 46.13X. A total of 8,815,889 SNPs was obtained between 'MC4' and 'Subicho'. Among them, 31,190 (0.35%) were non-synonymous SNPs (nsSNPs) corresponding to 10,926 genes, and these genes were assigned to 142 Gene Ontology (GO) terms across the two cultivars. We focused on three known BW QTL regions by identifying genes with sequence variants through gene set enrichment analysis and securing those belonging to high significant GO terms. Additionally, we found 310 NLR genes with nsSNP variants between 'MC4' (R) and 'Subicho' (S) within these regions. Also, we performed an Indel analysis on these genes. By integrating all this data, we identified eight candidate BW resistance genes, including two NLR genes with nsSNPs variations in qRRs-10.1 on chromosome 10.

CONCLUSION

We identified genomic variations in the form of SNPs and Indels by re-sequencing two pepper cultivars with contrasting traits for bacterial wilt. Specifically, the four genes associated with pBWR-1 and Bw1 that exhibit both nsSNP and Indel variations simultaneously in 'Subicho', along with the two NLR genes linked to qRRs-10.1, which are known for their direct involvement in immune responses, are identified as most likely BW resistance genes. These variants in leading candidate genes associated with BW resistance can be used as important markers for breeding pepper varieties.

Identification and application of a candidate gene AhAftr1 for aflatoxin production resistance in peanut seed (Arachis hypogaea L.)

INTRODUCTION

Peanut is susceptible to infection of Aspergillus fungi and conducive to aflatoxin contamination, hence developing aflatoxin-resistant variety is highly meaningful. Identifying functional genes or loci conferring aflatoxin resistance and molecular diagnostic marker are crucial for peanut breeding.

OBJECTIVES

This work aims to (1) identify candidate gene for aflatoxin production resistance, (2) reveal the related resistance mechanism, and (3) develop diagnostic marker for resistance breeding program.

METHODS

Resistance to aflatoxin production in a recombined inbred line (RIL) population derived from a high-yielding variety Xuhua13 crossed with an aflatoxin-resistant genotype Zhonghua 6 was evaluated under artificial inoculation for three consecutive years. Both genetic linkage analysis and QTL-seq were conducted for QTL mapping. The candidate gene was further fine-mapped using a secondary segregation mapping population and validated by transgenic experiments. RNA-Seq analysis among resistant and susceptible RILs was used to reveal the resistance pathway for the candidate genes.

RESULTS

The major effect QTL qAFTRA07.1 for aflatoxin production resistance was mapped to a 1.98 Mbp interval. A gene, AhAftr1 (Arachis hypogaea Aflatoxin resistance 1), was detected structure variation (SV) in leucine rich repeat (LRR) domain of its production, and involved in disease resistance response through the effector-triggered immunity (ETI) pathway. Transgenic plants with overexpression of AhAftr1(ZH6) exhibited 57.3% aflatoxin reduction compared to that of AhAftr1(XH13). A molecular diagnostic marker AFTR.Del.A07 was developed based on the SV. Thirty-six lines, with aflatoxin content decrease by over 77.67% compared to the susceptible control Zhonghua12 (ZH12), were identified from a panel of peanut germplasm accessions and breeding lines through using AFTR.Del.A07.

CONCLUSION

Our findings would provide insights of aflatoxin production resistance mechanisms and laid meaningful foundation for further breeding programs.

Comparative Field Evaluation and Transcriptome Analysis Reveals that Chromosome Doubling Enhances Sheath Blight Resistance in Rice

Rice sheath blight, caused by Rhizoctonia solani Kihn (R. solani), poses a significant threat to rice production and quality. Autotetraploid rice, developed through chromosome doubling of diploid rice, holds great potential for enhancing biological and yield traits. However, its resistance to sheath blight in the field has remained unclear. In this study, the field resistance of 35 autotetraploid genotypes and corresponding diploids was evaluated across three environments from 2020 to 2021. The booting stage was optimal for inoculating period based on the inoculation and analysis of R. solani at five rice growth stages. We found autotetraploids generally exhibited lower disease scores than diploids, indicating enhanced resistance after chromosome doubling. Among the 35 genotypes, 16 (45.71%) displayed increased resistance, 2 (5.71%) showed decreased resistance, and 17 (48.57%) displayed unstable resistance in different sowing dates. All combinations of the genotype, environment and ploidy, including the genotype-environment-ploidy interaction, contributed significantly to field resistance. Chromosome doubling increased sheath blight resistance in most genotypes, but was also dependent on the genotype-environment interaction. To elucidate the enhanced resistance mechanism, RNA-seq revealed autotetraploid recruited more down-regulated differentially expressed genes (DEGs), additionally, more resistance-related DEGs, were down-regulated at 24 h post inoculation in autotetraploid versus diploid. The ubiquinone/terpenoid quinone and diterpenoid biosynthesis pathways may play key roles in ploidy-specific resistance mechanisms. In summary, our findings shed light on the understanding of sheath blight resistance mechanisms in autotetraploid rice.

Comparative Cytological and Gene Expression Analysis Reveals That a Common Wild Rice Inbred Line Showed Stronger Drought Tolerance Compared with the Cultivar Rice

Common wild rice (Oryza rufipogon Griff.) is an important germplasm resource containing valuable genes. Our previous analysis reported a stable wild rice inbred line, Huaye3, which derives from the common wild rice of Guangdong Province. However, there was no information about its drought tolerance ability. Here, we assessed the germination characteristics and seedling growth between the Dawennuo and Huaye3 under five concentrations of PEG6000 treatment (0, 5%, 10%, 15%, and 20%). Huaye3 showed a stronger drought tolerance ability, and its seed germination rate still reached more than 52.50% compared with Dawennuo, which was only 25.83% under the 20% PEG6000 treatment. Cytological observations between the Dawennuo and Huaye3 indicated the root tip elongation zone and buds of Huaye3 were less affected by the PEG6000 treatment, resulting in a lower percentage of abnormalities of cortical cells, stele, and shrinkage of epidermal cells. Using the re-sequencing analysis, we detected 13,909 genes that existed in the genetic variation compared with Dawennuo. Of these genes, 39 were annotated as drought stress-related genes and their variance existed in the CDS region. Our study proved the strong drought stress tolerance ability of Huaye3, which provides the theoretical basis for the drought resistance germplasm selection in rice.

Genome-wide and pan-genomic analysis reveals rich variants of NBS-LRR genes in a newly developed wild rice line from Oryza alta Swallen

Introduction

Oryza alta Swallen is an allotetraploid perennial wild rice and contains CCDD genome, which may harbor favorable genes for the enrichment of genetic resource.

Methods

A new wild rice line, Huaye 5, was developed from Oryza alta Swallen in our lab. Whole genome re-sequencing and pan-genomic analysis were employed to analyze its genomic variations and novel genes.

Results and Discussion

More than ten million genomic variations were detected when compared with Asian cultivar. Among the variational genes, 724, 197 and 710 genes coded protein kinase, synthetase and transcription factor, respectively. A total of 353, 131 and 135 variational genes were associated with morphological trait, physiological trait, resistance or tolerance, respectively. A total of 62 were NBS-LRR genes were detected, in which 11 NBS-LRR genes expressed in sheath and mature stem, and 26 expressed in young and mature roots expressed. The pan-genome sequences of wild rice species with CCDD genome were constructed by integrating 8 Oryza alta (OA), 2 Oryza grandiglumis (OG) and 18 Oryza latifolia (OL) accessions. A total of 28 non-reference NBS-LRR genes were revealed, and 7 of which were mainly expressed in mature roots. This research demonstrated rich DNA variation in the Oryza alta Swallen that may provide a new germplasm for rice resistance breeding.

Conjunctive Analysis of BSA-Seq and SSR Markers Unveil the Candidate Genes for Resistance to Rice False Smut

Rice false smut (RFS) caused by the fungus Ustilaginoidea virens (Cook) leads to serious yield losses in rice. Identification of the gene or quantitative trait loci (QTLs) is crucial to resistance breeding and mitigation of RFS damage. In this study, we crossed a resistant variety, IR77298-14-1-2::IRGC117374-1, with a susceptible indica cultivar, 9311, and evaluated recombinant inbred lines in a greenhouse. The genetic analysis showed that the RFS resistance of IR77298-14-1-2::IRGC117374-1 was controlled by multiple recessive loci. We identified a novel QTL, qRFS12.01, for RFS resistance in IR77298-14-1-2::IRGC117374-1 by combining bulked segregant analysis with whole genome resequencing (BSA-seq) and simple sequence repeat (SSR) marker mapping approaches. The phenotypic effect of qRFS12.01 on RFS resistance reached 28.74%, suggesting that SSR markers linked to qRFS12.01 are valuable for marker-assisted breeding of RFS resistance in rice. The prediction of putative candidate genes within qRFS12.01 revealed five disease resistance proteins containing NB-ARC domains. In conclusion, our findings provide a new rice chromosome region carrying genes/QTLs for resistance to RFS.

Natural allelic variation in GRAIN SIZE AND WEIGHT 3 of wild rice regulates the grain size and weight

Grain size is important for yield in rice (Oryza sativa L.). Although many genes involved in grain size have been isolated, few can be used in breeding due to their interactions and phenotypic effects. Here, we describe natural variation in the granule-type quantitative trait locus GRAIN SIZE AND WEIGHT 3 (GSW3) located on chromosome 3 in wild rice (Oryza rufipogon Griff.) that encodes a GTPase-regulated protein and negatively regulates grain length, grain width, and 1,000-grain weight. The insertion of a 232-bp fragment of the genomic sequence in the wild rice, a natural allelic variant gene (GSW3), increased the expression levels and reduced the grain length and width and 1,000-grain weight. Knockout of GSW3 in the wild rice inbred line Huaye 3 increased the grain length and width and 1,000-grain weight. Introducing GSW3Huaye3 into cultivated rice line KJ01 and overexpressing GSW3Huaye3 in Huaye 3 resulted in reduced grain length and width and 1,000-grain weight, and grain size and 1,000-grain weight changes were closely related to GSW3 expression levels. GSW3 regulated the grain length and width simultaneously by promoting grain glume cell division and longitudinal and transverse cell growth. GSW3 was also involved in regulating the gibberellic acid signaling pathway and negatively regulated plant growth. Furthermore, a critical SNP in the GSW3 coding region was obviously correlated with grain size variation in a core collection of cultivated rice. This SNP resulted in an amino acid substitution from Gln to Arg at position 161 in GSW3, which reduced the grain size. Our study shows that GSW3 negatively regulates the grain shape, which could explain different grain shapes in modern cultivars and wild rice. GSW3 may also be used for breeding rice varieties with improved grain shapes and higher yield.

Cytological observation and transcriptome analysis reveal dynamic changes of Rhizoctonia solani colonization on leaf sheath and different genes recruited between the resistant and susceptible genotypes in rice

Sheath blight, caused by Rhizoctonia solani, is a big threat to the global rice production. To characterize the early development of R. solani on rice leaf and leaf sheath, two genotypes, GD66 (a resistant genotype) and Lemont (a susceptible genotype), were observed using four cytological techniques: the whole-mount eosin B-staining confocal laser scanning microscopy (WE-CLSM), stereoscopy, fluorescence microscopy, and plastic semi-thin sectioning after in vitro inoculation. WE-CLSM observation showed that, at 12 h post-inoculation (hpi), the amount of hyphae increased dramatically on leaf and sheath surface, the infection cushions occurred and maintained at a huge number from about 18 to 36 hpi, and then the infection cushions disappeared gradually from about 42 to 72 hpi. Interestingly, R. solani could not only colonize on the abaxial surfaces of leaf sheath but also invade the paraxial side of the leaf sheath, which shows a different behavior from that of leaf. RNA sequencing detected 6,234 differentially expressed genes (DEGs) for Lemont and 7,784 DEGs for GD66 at 24 hpi, and 2,523 DEGs for Lemont and 2,719 DEGs for GD66 at 48 hpi, suggesting that GD66 is recruiting more genes in fighting against the pathogen. Among DEGs, resistant genes, such as OsRLCK5, Xa21, and Pid2, displayed higher expression in the resistant genotype than the susceptible genotype at both 24 and 48 hpi, which were validated by quantitative reverse transcription-PCR. Our results indicated that the resistance phenotype of GD66 was the consequence of recruiting a series of resistance genes involved in different regulatory pathways. WE-CLSM is a powerful technique for uncovering the mechanism of R. solani invading rice and for detecting rice sheath blight-resistant germplasm.

Identification of qGL3.5, a Novel Locus Controlling Grain Length in Rice Through Bulked Segregant Analysis and Fine Mapping

Grain length (GL) directly affects the yield and quality of rice. Very few cloned GL-related genes are applied in production because their yield-increasing effects are not obvious, and the overall regulatory networks underlying the associated processes remain poorly understood. DNA samples from two bulk DNA pools (L-pool and S-pool) and their parents (KJ01 and Huaye 3) were subjected to high-throughput sequencing. Using bulked segregant analysis (BSA), qGL3.5 was mapped to a 0.34-Mb "hotspot" region on chromosome 3 that contains 37 genes related to various traits. Then, qGL3.5 was mapped to the genomic interval between the flanking markers M2 and M3 using 2786 BC₄F₂ individuals. Because the region from B5 to B6 was not the associated region under BSA-seq analysis, qGL3.5 was narrowed down to the interval between B6 and M3, which spanned 24.0-kb. Of all 37 genes with non-synonymous single-nucleotide polymorphisms (SNPs) between KJ01 and Huaye 3 based on BSA-seq analysis, only one complete annotated gene, ORF18 (Gene ID: LOC_Os03g42790.1) was found. ORF18 encodes an IBR-RING zinc-finger-related protein, with one really interesting new gene (RING) and two in between ring finger (IBR) domains. The knockout of ORF18 derived from Huaye 3 using clustered, regularly interspaced, short palindromic repeat (CRISPR)/CRISPR-associated 9 (Cas9) editing technology increased the GL of the mutant by approximately 2.2 mm. The novel locus qGL3.5 negatively regulated GL by promoting elongation of the longitudinal cell of the grain outer glume. These results provide a new genetic resource for rice grain shape breeding and a starting point for the functional characterization of the wild rice GL gene.

Omics: a tool for resilient rice genetic improvement strategies

Rice is pivotal pyramid of about half of the world population. Bearing small genome size and worldwide utmost food crop rice has been known as ideal cereal crop for genome research. Currently, decreasing water table and soil fatigue are big challenges and intense consequences in changing climate. Whole sequenced genome of rice sized 389 Mb of which 95% is covered with excellent mapping order. Sequenced rice genome helps in molecular biology and transcriptomics of cereals as it provides whole genome sequence of indica and japonica sub species. Through rice genome sequencing and functional genomics, QTLs or genes, genetic variability and halophyte blocks for agronomic characters were identified which have proved much more useful in molecular breeding and direct selection. There are different numbers of genes or QTLs identified for yield related traits i.e., 6 QTLs/genes for plant architecture, 6 for panicle characteristics, 4 for grain number, 1 gene/QTL for tiller, HGW, grain filling and shattering. QTLS/genes for grain quality, biotic stresses and for abiotic stresses are 7, 23 and 13 respectively. Low yield, inferior quality and susceptibility to biotic and abiotic stresses of a crop is due to narrow genetic background of new evolving rice verities. Wild rice provides genetic resources for improvement of these characters, molecular and genomics tool at different stages can overcome these stresses and improve yield and quality of rice crop.

Genomics Analyses Reveal Unique Classification, Population Structure and Novel Allele of Neo-Tetraploid Rice

BACKGROUND

Neo-tetraploid rice (NTR) is a useful new germplasm that developed from the descendants of the autotetraploid rice (ATR) hybrids. NTR showed improved fertility and yield potential, and produced high yield heterosis when crossed with indica ATR for commercial utilization. However, their classification, population structure and genomic feature remain elusive.

RESULTS

Here, high-depth genome resequencing data of 15 NTRs and 18 ATRs, together with 38 publicly available data of diploid rice accessions, were analyzed to conduct classification, population structure and haplotype analyses. Five subpopulations were detected and NTRs were clustered into one independent group that was adjacent to japonica subspecies, which maybe the reason for high heterosis when NTRs crossed with indica ATRs. Haplotype patterns of 717 key genes that associated with yield and other agronomic traits were revealed in these NTRs. Moreover, a novel specific SNP variation was detected in the first exon of HSP101, a known heat-inducible gene, which was conserved in all NTRs but absent in ATRs, 3KRG and RiceVarMap2 databases. The novel allele was named as HSP101-1, which was confirmed to be a heat response factor by qRT-PCR, and knockout of HSP101-1 significantly decreased the thermotolerance capacity of NTR. Interestingly, HSP101-1 was also specifically expressed in the anthers of NTR at pre-meiotic and meiosis stages under optimal environment without heat stress, and its loss-of-function mutant showed significant decrease in fertility of NTR.

CONCLUSION

The construction of first genomic variation repository and the revelation of population structure provide invaluable information for optimizing the designs of tetraploid rice breeding. The detection of specific genomic variations offered useful genomic markers and new directions to resolve high fertility mechanism of NTR.

Cytological Observations and Bulked-Segregant Analysis Coupled Global Genome Sequencing Reveal Two Genes Associated with Pollen Fertility in Tetraploid Rice

Neo-tetraploid rice with high fertility is a useful germplasm for polyploid rice breeding, which was developed from the crossing of different autotetraploid rice lines. However, little information is available on the molecular mechanism underlying the fertility of neo-tetraploid rice. Here, two contrasting populations of tetraploid rice, including one with high fertility (hereafter referred to as JG) and another with low fertility (hereafter referred to as JD), were generated by crossing Huaduo 3 (H3), a high fertility neo-tetraploid rice that was developed by crossing Jackson-4x with 96025-4x, and Huajingxian74-4x (T452), a low fertility autotetraploid rice parent. Cytological, global genome sequencing-based bulked-segregant (BSA-seq) and CRISPR/Cas9 technology were employed to study the genes associated with pollen fertility in neo-tetraploid rice. The embryo sacs of JG and JD lines were normal; however, pollen fertility was low in JD, which led to scarce fertilization and low seed setting. Cytological observations displayed low pollen fertility (25.1%) and approximately 31.3 and 27.2% chromosome lagging at metaphase I and II, and 28.8 and 24.8% chromosome straggling at anaphase I and II in JD, respectively. BSA-seq of F_2–3 generations and RNA-seq of F₄ generation detected a common fragment, i.e., 18,915,234–19,500,000, at chromosome 7, which was comprised of 78 genes associated with fertility. Among 78 genes, 9 genes had been known to be involved in meiosis and pollen development. Two mutants ny1 (LOC_Os07g32406) and ny2 (LOC_Os07g32040) were generated by CRISPR/Cas9 knockout in neo-tetraploid rice, and which exhibited low pollen fertility and abnormal chromosome behavior. Our study revealed that two unknown genes, LOC_Os07g32406 (NY1) and LOC_Os07g32040 (NY2) play an important role in pollen development of neo-tetraploid rice and provides a new perspective about the genetic mechanisms of fertility in polyploid rice.

Production Assessment and Genome Comparison Revealed High Yield Potential and Novel Specific Alleles Associated with Fertility and Yield in Neo-Tetraploid Rice

BACKGROUND

Neo-tetraploid rice (NTR) is a new tetraploid rice germplasm that developed from the crossing and directional selection of different autotetraploid rice lines, which showed high fertility and promising yield potential. However, systematic yield assessment, genome composition and functional variations associated with fertility and yield remain elusive.

RESULTS

Two season's field trials of 15 NTRs and 27 autotetraploid rice (ATR) lines revealed that the improvement of YPP (yield per plant, 4.45 g increase) were significantly associated with the increase of SS (seed setting, 29.44% increase), and yield and seed setting of NTRs improved significantly compared to parental lines. Whole genome resequencing of 13 NTR sister lines and their parents at about 48.63 depth were conducted and genome compositions were illustrated using inherited chromosomal blocks. Interestingly, 222 non-parental genes were detected between NTRs and their low fertility parental lines, which were conserved in 13 NTRs. These genes were overlapped with yield and fertility QTLs, and RNA-Seq analysis revealed that 81 of them were enriched in reproductive tissues. CRISPR/Cas9 gene knockout was conducted for 9 non-parental genes to validate their function. Knockout mutants showed on an average 25.63% and 4.88 g decrease in SS and YPP, respectively. Notably, some mutants showed interesting phenotypes, e.g., kin7l (kinesin motor gene) and kin14m (kinesin motor gene), bzr3 (BES1/BZR1 homolog) and nrfg4 (neo-tetraploid rice fertility related gene) exhibited 44.65%, 24.30%, 24.42% and 28.33% decrease in SS and 8.81 g, 4.71 g, 5.90 g, 6.22 g reduction in YPP, respectively.

CONCLUSION

Comparative genomics provides insights into genome composition of neo-tetraploid rice and the genes associated with fertility and yield will play important role to reveal molecular mechanisms for the improvement of tetraploid rice.