Recent progress on the genetics and molecular breeding of brown planthopper resistance in rice

Identification and candidate analysis of a new brown planthopper resistance locus in an Indian landrace of rice, paedai kalibungga

The brown planthopper (Nilaparvata lugens Stål, BPH) is the most destructive pest of rice (Oryza sativa L.). Utilizing resistant rice cultivars that harbor resistance gene/s is an effective strategy for integrated pest management. Due to the co-evolution of BPH and rice, a single resistance gene may fail because of changes in the virulent BPH population. Thus, it is urgent to explore and map novel BPH resistance genes in rice germplasm. Previously, an indica landrace from India, Paedai kalibungga (PK), demonstrated high resistance to BPH in both in Wuhan and Fuzhou, China. To map BPH resistance genes from PK, a BC1F2:3 population derived from crosses of PK and a susceptible parent, Zhenshan 97 (ZS97), was developed and evaluated for BPH resistance. A novel BPH resistance locus, BPH39, was mapped on the short arm of rice chromosome 6 using next-generation sequencing-based bulked segregant analysis (BSA-seq). BPH39 was validated using flanking markers within the locus. Furthermore, near-isogenic lines carrying BPH39 (NIL-BPH39) were developed in the ZS97 background. NIL-BPH39 exhibited the physiological mechanisms of antibiosis and preference toward BPH. BPH39 was finally delimited to an interval of 84 Kb ranging from 1.07 to 1.15 Mb. Six candidate genes were identified in this region. Two of them (LOC_Os06g02930 and LOC_Os06g03030) encode proteins with a similar short consensus repeat (SCR) domain, which displayed many variations leading to amino acid substitutions and showed higher expression levels in NIL-BPH39. Thus, these two genes are considered reliable candidate genes for BPH39. Additionally, transcriptome sequencing, DEGs analysis, and gene RT-qPCR verification preliminary revealed that BPH39 may be involved in the jasmonic acid (JA) signaling pathway, thus mediating the molecular mechanism of BPH resistance. This work will facilitate map-based cloning and marker-assisted selection for the locus in breeding programs targeting BPH resistance.

Supplementary Information

The online version contains supplementary material available at 10.1007/s11032-024-01485-6.

Identification and Functional Characterization of Two Major Loci Associated with Resistance against Brown Planthoppers (Nilaparvata lugens (Stål)) Derived from Oryza nivara

The brown planthopper (BPH) is a highly destructive pest of rice, causing significant economic losses in various regions of South and Southeast Asia. Researchers have made promising strides in developing resistance against BPH in rice. Introgression line RPBio4918-230S, derived from Oryza nivara, has shown consistent resistance to BPH at both the seedling and adult stages of rice plants. Segregation analysis has revealed that this resistance is governed by two recessive loci, known as bph39(t) and bph40(t), contributing to 21% and 22% of the phenotypic variance, respectively. We later mapped the genes using a backcross population derived from a cross between Swarna and RPBio4918-230S. We identified specific marker loci, namely RM8213, RM5953, and R4M17, on chromosome 4, flanking the bph39(t) and bph40(t) loci. Furthermore, quantitative expression analysis of candidate genes situated between the RM8213 and R4M17 markers was conducted. It was observed that eight genes exhibited up-regulation in RPBio4918-230S and down-regulation in Swarna after BPH infestation. One gene of particular interest, a serine/threonine-protein kinase receptor (STPKR), showed significant up-regulation in RPBio4918-230S. In-depth sequencing of the susceptible and resistant alleles of STPKR from Swarna and RPBio4918-230S, respectively, revealed numerous single nucleotide polymorphisms (SNPs) and insertion-deletion (InDel) mutations, both in the coding and regulatory regions of the gene. Notably, six of these mutations resulted in amino acid substitutions in the coding region of STPKR (R5K, I38L, S120N, T319A, T320S, and F348S) when compared to Swarna and the reference sequence of Nipponbare. Further validation of these mutations in a set of highly resistant and susceptible backcross inbred lines confirmed the candidacy of the STPKR gene with respect to BPH resistance controlled by bph39(t) and bph40(t). Functional markers specific for STPKR have been developed and validated and can be used for accelerated transfer of the resistant locus to elite rice cultivars.

Available cloned genes and markers for genetic improvement of biotic stress resistance in rice

Biotic stress is one of the major threats to stable rice production. Climate change affects the shifting of pest outbreaks in time and space. Genetic improvement of biotic stress resistance in rice is a cost-effective and environment-friendly way to control diseases and pests compared to other methods such as chemical spraying. Fast deployment of the available and suitable genes/alleles in local elite varieties through marker-assisted selection (MAS) is crucial for stable high-yield rice production. In this review, we focused on consolidating all the available cloned genes/alleles conferring resistance against rice pathogens (virus, bacteria, and fungus) and insect pests, the corresponding donor materials, and the DNA markers linked to the identified genes. To date, 48 genes (independent loci) have been cloned for only major biotic stresses: seven genes for brown planthopper (BPH), 23 for blast, 13 for bacterial blight, and five for viruses. Physical locations of the 48 genes were graphically mapped on the 12 rice chromosomes so that breeders can easily find the locations of the target genes and distances among all the biotic stress resistance genes and any other target trait genes. For efficient use of the cloned genes, we collected all the publically available DNA markers (~500 markers) linked to the identified genes. In case of no available cloned genes yet for the other biotic stresses, we provided brief information such as donor germplasm, quantitative trait loci (QTLs), and the related papers. All the information described in this review can contribute to the fast genetic improvement of biotic stress resistance in rice for stable high-yield rice production.

Large scale rice germplasm screening for identification of novel brown planthopper resistance sources

Rice (Oryza sativa L.) is a staple food crop globally. Brown planthopper (Nilaparvata lugens Stål, BPH) is the most destructive insect that threatens rice production annually. More than 40 BPH resistance genes have been identified so far, which provide valuable gene resources for marker-assisted breeding against BPH. However, it is still urgent to evaluate rice germplasms and to explore more new wide-spectrum BPH resistance genes to combat newly occurring virulent BPH populations. To this end, 560 germplasm accessions were collected from the International Rice Research Institute (IRRI), and their resistance to current BPH population of China was examined. A total of 105 highly resistant materials were identified. Molecular screening of BPH resistance genes in these rice germplasms was conducted by developing specific functional molecular markers of eight cloned resistance genes. Twenty-three resistant germplasms were found to contain none of the 8 cloned BPH resistance genes. These accessions also exhibited a variety of resistance mechanisms as indicated by an improved insect weight gain (WG) method, suggesting the existence of new resistance genes. One new BPH resistance gene, Bph44(t), was identified in rice accession IRGC 15344 and preliminarily mapped to a 0-2 Mb region on chromosome 4. This study systematically sorted out the corresponding relationships between BPH resistance genes and germplasm resources using a functional molecular marker system. Newly explored resistant germplasms will provide valualble donors for the identification of new resistance genes and BPH resistance breeding programs.

Supplementary Information

The online version contains supplementary material available at 10.1007/s11032-023-01416-x.

Development and characterization of near-isogenic lines for brown planthopper resistance genes in the genetic background of japonica rice 'Sagabiyori'

The brown planthopper (BPH: Nilaparvata lugens Stål) is one of the most destructive insects in rice production. The use of host plant resistance has potential to reduce damage caused by BPH. The heat tolerance japonica rice 'Sagabiyori', with superior grain quality and high soluble starch in the stem, is highly susceptible to damage by BPH. Here, to enhance its BPH resistance, we developed seven near-isogenic lines (NILs) carrying BPH2, BPH17-ptb, BPH32, BPH3, BPH17, BPH20, and BPH21 through marker-assisted selection and evaluated resistance to two BPH populations. Most lines were more resistant to the Hadano-1966 BPH population than Sagabiyori but were less effective against the highly virulent Koshi-2013 population. Nevertheless, in antixenosis tests, Koshi-2013 settled less on all NILs than on Sagabiyori. In addition, adult mortality and the percentage of fresh weight loss of lines carrying BPH17 and BPH3 indicated that these lines have higher resistance to Koshi-2013 than Sagabiyori. Current study revealed that BPH resistance of Sagabiyori became stronger by transferring BPH3 and BPH17 genes. Thus, BPH3 and BPH17 might be valuable for breeding programs to enhance BPH resistance of high grain quality rice varieties with heat tolerance.

Near-isogenic lines for resistance to brown planthopper with the genetic background of Indica Group elite rice (Oryza sativa L.) variety 'IR64'

The brown planthopper (BPH), Nilaparvata lugens Stål, is an insect pest that severely damages rice (Oryza sativa L.) in Asia, causing huge yield loss. Use of resistant variety is a cost-effective and eco-friendly strategy for maintaining BPH populations below the economic injury level. However, current BPH populations have been changed to virulence against resistant varieties. In this study, to estimate effective combinations among eight BPH resistance genes (BPH32, BPH17-ptb, BPH20, BPH17, BPH3, BPH25, BPH26 and qBPH6), eight near-isogenic lines with the genetic background of an Indica Group rice variety 'IR64' (IR64-NIL) were developed using marker-assisted selection. The genome recoveries of these NILs ranged from 89.3% to 98.8% and agronomic traits of them were similar to those of 'IR64'. In modified seed box screening test, resistance level of IR64-NILs was higher than that of 'IR64'. In antibiosis test, high adult mortalities of BPH (from 56.0% to 97.0%) were observed among NILs, in comparison with that of 'IR64'. Among IR64-NILs, the line carrying BPH17 showed the highest resistance level at all tests. Thus, these IR64-NILs with multiple BPH resistance genes could be valuable breeding lines for enhancing resistance levels by gene pyramiding and multiline variety.

Genome-Wide Association Mapping for Yield and Yield-Related Traits in Rice (Oryza Sativa L.) Using SNPs Markers

Rice (Oryza sativa L.) is a staple food for more than 50% of the world's population. Rice cultivar improvement is critical in order to feed the world's growing population. Improving yield is one of the main aims of rice breeders. However, yield is a complex quantitative trait controlled by many genes. The presence of genetic diversity is the key factor to improve the yield hence, the presence of diversity in any germplasm is important for yield improvement. In the current study, the rice germplasm was collected from Pakistan and the United States of America and a panel of 100 diverse genotypes was utilized to identify important yield and yield-related traits. For this, a genome-wide association study (GWAS) was performed to identify the genetic loci related to yield. The GWAS on the diverse germplasm will lead to the identification of new genes which can be utilized in the breeding program for improvement of yield. For this reason, firstly, the germplasm was phenotypically evaluated in two growing seasons for yield and yield-related traits. The analysis of variance results showed significant differences among traits which showed the presence of diversity in the current germplasm. Secondly, the germplasm was also genotypically evaluated using 10K SNP. Genetic structure analysis showed the presence of four groups which showed that enough genetic diversity was present in the rice germplasm to be used for association mapping analysis. The results of GWAS identified 201 significant marker trait associations (MTAs. 16 MTAs were identified for plant height, 49 for days to flowering, three for days to maturity, four for tillers per plant, four for panicle length, eight for grains per panicle, 20 unfilled grains per panicle, 81 for seed setting %, four for thousand-grain weight, five for yield per plot and seven for yield per hectare. Apart from this, some pleiotropic loci were also identified. The results showed that panicle length (PL) and thousand-grain weight (TGW) were controlled by a pleiotropic locus OsGRb23906 on chromosome 1 at 10,116,371 cM. The loci OsGRb25803 and OsGRb15974 on chromosomes 4 and 8 at the position of 14,321,111 cM and 6,205,816 cM respectively, showed pleiotropic effects for seed setting % (SS) and unfilled grain per panicle (UG/P). A locus OsGRb09180 on chromosome 4 at 19,850,601 cM was significantly linked with SS and yield/ha. Furthermore, gene annotation was performed, and results indicated that the 190 candidate genes or QTLs that closely linked with studied traits. These candidate genes and novel significant markers could be useful in marker-assisted gene selection and QTL pyramiding to improve rice yield and the selection of potential parents, recombinants and MTAs which could be used in rice breeding programs to develop high-yielding rice varieties for sustainable food security.

The International Oryza Map Alignment Project (IOMAP): the Americas-past achievements and future directions

The wild relatives of rice hold unexplored genetic diversity that can be employed to feed an estimated population of 10 billion by 2050. The Oryza Map Alignment Project (OMAP) initiated in 2003 has provided comprehensive genomic resources for comparative, evolutionary, and functional characterization of the wild relatives of rice, facilitating the cloning of >600 rice genes, including those for grain width (GW5) and submergence tolerance (SUB1A). Following in the footsteps of the original project, the goal of 'IOMAP: the Americas' is to investigate the present and historic genetic diversity of wild Oryza species endemic to the Americas through the sequencing of herbaria and in situ specimens. The generation of a large diversity panel describing past and current genetic status and potential erosion of genetic variation in the populations will provide useful knowledge for the conservation of the biodiversity in these species. The wild relatives of rice in the Americas present a wide range of resistance traits useful for crop improvement and neodomestication approaches. In the race against time for a sustainable food future, the neodomestication of the first cereal species recently accomplished in O. alta opens the door to the potential neodomestication of the other wild Oryza species in Americas.

Functional Bph14 from Rathu Heenati promotes resistance to BPH at the early seedling stage of rice (Oryza sativa L.) as revealed by QTL-seq

A QTL associated with BPH resistance at the early seedling stage was identified on chromosome 3. Functional Bph14 in Rathu Heenati was associated with BPH resistance at the early seedling stage. Brown planthopper (BPH; Nilaparvata lugens Stål) is considered the most important rice pest in many Asian countries. Several BPH resistance genes have previously been identified. However, there are few reports of genes specific for BPH resistance at the early seedling stage, a crucial stage for direct-seeding cultivation. In this study, we performed a QTL-seq analysis using two bulks (20 F₂ lines in each bulk) of the F₂ population (n = 300) derived from a cross of Rathu Heenati (RH) × HCS-1 to identify QTL/genes associated with BPH resistance at the early seedling stage. An important QTL was identified on chromosome 3 and Bph14 was identified as a potential candidate gene based on the differences in gene expression and sequence variation when compared with the two parents. All plants in the resistant bulks possessed the functional Bph14 from RH and all plants in the susceptible bulk and HCS-1 contained a large deletion (2703 bp) in Bph14. The functional Bph14 gene of RH appears to be important for BPH resistance at the early seedling stage of rice and could be used in conjunction with other BPH resistance genes in rice breeding programs that confer resistance to BPH at the early and later growth stages.

The role of polyplexes in developing a green sustainable approach in agriculture

Rise in global population has increased the food demands and thus the competition among farmers to produce more and more. In the race to obtain higher productivity, farmers have resorted to injudicious farming practices that include the reckless use of nitrogenous fertilizers and intensive cropping on farmlands. Such practices have paved the path for large scale infestations of crops and plants by pests thus affecting the plant productivity and crop vigour. There are several traditional techniques to control pest infestations in plants such as the use of chemical or bio-pesticides, and integrated pest management practices which face several drawbacks. Delivery of gene/nucleic acid in plants through genetic engineering approaches is a more sustainable and effective method of protection against pests. The technology of RNA interference (RNAi) provides a sustainable solution to counter pest control problems faced by other traditional techniques. The RNAi technique involves delivery of dsDNA/dsRNA or other forms of nucleic acids into target organisms thereby bringing about gene silencing. However, RNAi is also limited to its use because of their susceptibility to degradation wherein the use of cationic polymers can provide a tangible solution. Cationic polymers form stable complexes with the nucleic acids known as "polyplexes", which may be attributed to their high positive charge densities thus protecting the exogenous nucleic acids from extracellular degradation. The current paper focuses on the utility of nucleic acids as a sustainable tool for pest control in crops and the use of cationic polymers for the efficient delivery of nucleic acids in pests thus protecting the plant from infestations.

Identification of the rice genes and metabolites involved in dual resistance against brown planthopper and rice blast fungus

Brown planthopper (BPH) and blast disease jointly or individually cause big yield losses every year. To identify genes and metabolites with potential contributions to the dual resistance against both biotic-stress factors, we carried out a transcriptome and metabolome analysis for susceptible and resistant rice varieties after BPH and rice blast infestations. Coexpression network analysis identified a modular pattern that had the highest correlation coefficients (0.81) after the BPH and rice blast (-0.81) treatments. In total, 134 phenylpropanoid biosynthesis pathway-related genes were detected in this group. We found that the flavanone 3-hydroxylase gene (OsF3H) had opposite expression trends in response to BPH and rice blast infestations whereas the OsF3'H had similar expression patterns. Genetics analysis confirmed that the OsF3H gene knockdown lines demonstrated the opposite resistance phenotypes against BPH and rice blast, whereas the OsF3'H knockout lines enhanced rice resistance against both pests. Consistently, our metabolomics analysis identified the metabolite eriodictyol, one putative essential product of these two genes, that was more highly accumulated in the resistant rice variety of RHT than in the susceptible variety MDJ. This study highlights a useful strategy for identifying more genes and metabolites that have potential synergistic effects on rice against to multiple biotic stresses.

Introgression Lines: Valuable Resources for Functional Genomics Research and Breeding in Rice (Oryza sativa L.)

The narrow base of genetic diversity of modern rice varieties is mainly attributed to the overuse of the common backbone parents that leads to the lack of varied favorable alleles in the process of breeding new varieties. Introgression lines (ILs) developed by a backcross strategy combined with marker-assisted selection (MAS) are powerful prebreeding tools for broadening the genetic base of existing cultivars. They have high power for mapping quantitative trait loci (QTLs) either with major or minor effects, and are used for precisely evaluating the genetic effects of QTLs and detecting the gene-by-gene or gene-by-environment interactions due to their low genetic background noise. ILs developed from multiple donors in a fixed background can be used as an IL platform to identify the best alleles or allele combinations for breeding by design. In the present paper, we reviewed the recent achievements from ILs in rice functional genomics research and breeding, including the genetic dissection of complex traits, identification of elite alleles and background-independent and epistatic QTLs, analysis of genetic interaction, and genetic improvement of single and multiple target traits. We also discussed how to develop ILs for further identification of new elite alleles, and how to utilize IL platforms for rice genetic improvement.

Inter-Alpha-Trypsin Inhibitor Heavy Chain 4 Plays an Important Role in the Development and Reproduction of Nilaparvata lugens

Simple Summary

The brown planthopper, Nilaparvata lugens (Hemiptera: Delphacidae), is a destructive insect pest of rice. It causes reductions in rice yield and great economic losses. In this study, we used RNAi to explore the function of the inter-alpha-trypsin inhibitor heavy chain 4 (ITIH4) gene in the development and reproduction of the brown planthopper. Our results revealed that ITIH4 influences the survival, ovarian development, egg production, and egg hatching of this insect, indicating that ITIH4 plays important roles in development and reproduction. Considering the importance of ITIH4 in the brown planthopper, it may be a potential target for pest management.

Abstract

The brown planthopper, Nilaparvata lugens, is a difficult-to-control insect pest affecting rice yields in Asia. As a structural component of the inter-alpha-trypsin inhibitor (ITI), the inter-alpha-trypsin inhibitor heavy chain (ITIH) has been reported to be involved in various inflammatory or malignant disorders, ovarian development, and ovulation. To reveal the function of ITIH4 in N. lugens, the gene encoding N. lugens ITIH4 (NlITIH4) was cloned and characterized. NlITIH4 contains a signal peptide, a vault protein inter-alpha-trypsin domain, and a von Willebrand factor type A domain. qPCR analysis showed that NlITIH4 was expressed at all developmental stages and in all tissues (fat body, ovary, and gut), with the highest expression in the fat body. Double stranded NlITIH4 (dsNlITIH4) injection clearly led to an RNAi-mediated inhibition of the expression of NlITIH4 and resulted in reduced survival, delayed ovarian development, and reduced egg production and egg hatching. These results indicate that NlITIH4 plays an important role in the development and reproduction of N. lugens.

Heat Shock 70 kDa Protein Cognate 3 of Brown Planthopper Is Required for Survival and Suppresses Immune Response in Plants

The brown planthopper (Nilaparvata lugens) is a monophagous pest of rice (Oryza sativa), which threatens food security around the world. Insect Heat shock proteins 70 kDa (Hsp70s) play a key role in insect growth and development, however, if they also modulate the plant physiological processes is still unclear. In this study, we identified the Heat shock 70 kDa protein cognate 3 (NlHSC70-3) of BPH from compared protein profiles of Nipponbare tissues after BPH infestation via LC/MS. NlHSC70-3 has a predicted signal peptide and displays high transcription levels in the salivary glands, which further supported that it is secreted into plants by BPH during the feeding process. Using RNA interference (RNAi), we showed that NlHSC70-3 is indispensable for the survival of BPH on rice. Most importantly, NlHSC70-3 mediates the plant immune responses including cell death, flg22-induced ROS burst and defense-related gene expression in N. benthamiana. These results demonstrate that NlHSC70-3 may function as an effector manipulating plant physiological processes to facilitate pest survival on rice, which provides a new potential target for future pest control.

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.

RNA-Sequencing Reveals Differentially Expressed Rice Genes Functionally Associated with Defense against BPH and WBPH in RILs Derived from a Cross between RP2068 and TN1

BACKGROUND

Rice is staple food for over two billion people. Planthoppers like BPH and WBPH occur together in most of rice growing regions across Asia and cause extensive yield loss by feeding and transmission of disease-causing viruses. Chemical control of the pest is expensive and ecologically disastrous; breeding resistant varieties is an acceptable option. But most of such efforts are focused on BPH with an assumption that these varieties will also be effective against WBPH. No critical studies are available to understand rice resistance, common or otherwise, against these two planthoppers.

RESULTS

Our studies aimed to understand the defense mechanisms in rice line RP2068 against BPH and WBPH through RNA sequencing analysis of a RIL line TR3RR derived from the cross TN1 (susceptible) and RP2068 (resistant) after infestation with BPH or WBPH. Results revealed higher number of differentially expressed genes (DEGs) in BPH infested plants than in WBPH infested plants when compared with the uninfested plants. These DEGs could be grouped into UPUP, DNDN, UPDN and DNUP groups based on whether the DEGs were up (UP) or down (DN) regulated against BPH and WBPH, respectively. Gene ontology analysis, specially of members of the last two groups, revealed differences in plant response to the two planthoppers. Abundance of miRNAs and detection of their target genes also indicated that separate sets of genes were suppressed or induced against BPH and WBPH. These results were validated through the analysis of expression of 27 genes through semi-quantitative and quantitative real-time RT-PCR using a set of five RILs that were genetically identical but with different reaction against the two planthoppers. Coupled with data obtained through pathway analysis involving these 27 genes, expression studies revealed common and differential response of rice RP2068 against BPH and WBPH. Trehalose biosynthesis, proline transport, methylation were key pathways commonly upregulated; glucosinolate biosynthesis, response to oxidative stress, proteolysis, cytokinesis pathways were commonly down regulated; photosynthesis, regulation of transcription, expression and transport of peptides and defense related pathways were exclusively upregulated against WBPH; MYB transcription factor mediated defense induction was exclusive to BPH.

CONCLUSION

Rice defense against the two sympatric planthoppers: BPH and WBPH has distinct features in RP2068. Hence, a conscious combination of resistance to these two pests is essential for effective field management.

Deficiency of mitochondrial outer membrane protein 64 confers rice resistance to both piercing-sucking and chewing insects in rice

The brown planthopper (BPH) and striped stem borer (SSB) are the most devastating insect pests in rice (Oryza sativa) producing areas. Screening for endogenous resistant genes is the most practical strategy for rice insect-resistance breeding. Forty-five mutants showing high resistance against BPH were identified in a rice T-DNA insertion population (11,000 putative homozygous lines) after 4 years of large-scale field BPH-resistance phenotype screening. Detailed analysis showed that deficiency of rice mitochondrial outer membrane protein 64 (OM64) gene resulted in increased resistance to BPH. Mitochondrial outer membrane protein 64 protein is located in the outer mitochondrial membrane by subcellular localization and its deficiency constitutively activated hydrogen peroxide (H₂ O₂ ) signaling, which stimulated antibiosis and tolerance to BPH. The om64 mutant also showed enhanced resistance to SSB, a chewing insect, which was due to promotion of Jasmonic acid biosynthesis and related responses. Importantly, om64 plants presented no significant changes in rice yield-related characters. This study confirmed OM64 as a negative regulator of rice herbivore resistance through regulating H₂ O₂ production. Mitochondrial outer membrane protein 64 is a potentially efficient candidate to improve BPH and SSB resistance through gene deletion. Why the om64 mutant was resistant to both piercing-sucking and chewing insects via a gene deficiency in mitochondria is discussed.

A, Chakraborty A, Thirunavukkarasu N. Multiple Genome Wide Association Mapping Models Identify Quantitative Trait Nucleotides for Brown Planthopper (Nilaparvata lugens) Resistance in MAGIC Indica Population of Rice

Brown planthopper (BPH), one of the most important pests of the rice (Oryza sativa) crop, becomes catastrophic under severe infestations and causes up to 60% yield loss. The highly disastrous BPH biotype in the Indian sub-continent is Biotype 4, which also known as the South Asian Biotype. Though many resistance genes were mapped until now, the utility of the resistance genes in the breeding programs is limited due to the breakdown of resistance and emergence of new biotypes. Hence, to identify the resistance genes for this economically important pest, we have used a multi-parent advanced generation intercross (MAGIC) panel consisting of 391 lines developed from eight indica founder parents. The panel was phenotyped at the controlled conditions for two consecutive years. A set of 27,041 cured polymorphic single nucleotide polymorphism (SNPs) and across-year phenotypic data were used for the identification of marker–trait associations. Genome-wide association analysis was performed to find out consistent associations by employing four single and two multi-locus models. Sixty-one SNPs were consistently detected by all six models. A set of 190 significant marker-associations identified by fixed and random model circulating probability unification (FarmCPU) were considered for searching resistance candidate genes. The highest number of annotated genes were found in chromosome 6 followed by 5 and 1. Ninety-two annotated genes identified across chromosomes of which 13 genes are associated BPH resistance including NB-ARC (nucleotide binding in APAF-1, R gene products, and CED-4) domain-containing protein, NHL repeat-containing protein, LRR containing protein, and WRKY70. The significant SNPs and resistant lines identified from our study could be used for an accelerated breeding program to develop new BPH resistant cultivars.

Recent Strategies for Detection and Improvement of Brown Planthopper Resistance Genes in Rice: A Review

Brown planthopper (BPH; Nilaparvata lugens Stal) is considered the main rice insect pest in Asia. Several BPH-resistant varieties of rice have been bred previously and released for large-scale production in various rice-growing regions. However, the frequent surfacing of new BPH biotypes necessitates the evolution of new rice varieties that have a wide genetic base to overcome BPH attacks. Nowadays, with the introduction of molecular approaches in varietal development, it is possible to combine multiple genes from diverse sources into a single genetic background for durable resistance. At present, above 37 BPH-resistant genes/polygenes have been detected from wild species and indica varieties, which are situated on chromosomes 1, 3, 4, 6, 7, 8, 9, 10, 11 and 12. Five BPH gene clusters have been identified from chromosomes 3, 4, 6, and 12. In addition, eight BPH-resistant genes have been successfully cloned. It is hoped that many more resistance genes will be explored through screening of additional domesticated and undomesticated species in due course.

Gene Pyramiding for Sustainable Crop Improvement against Biotic and Abiotic Stresses

Sustainable agricultural production is endangered by several ecological factors, such as drought, extreme temperatures, excessive salts, parasitic ailments, and insect pest infestation. These challenging environmental factors may have adverse effects on future agriculture production in many countries. In modern agriculture, conventional crop-breeding techniques alone are inadequate for achieving the increasing population’s food demand on a sustainable basis. The advancement of molecular genetics and related technologies are promising tools for the selection of new crop species. Gene pyramiding through marker-assisted selection (MAS) and other techniques have accelerated the development of durable resistant/tolerant lines with high accuracy in the shortest period of time for agricultural sustainability. Gene stacking has not been fully utilized for biotic stress resistance development and quality improvement in most of the major cultivated crops. This review emphasizes on gene pyramiding techniques that are being successfully deployed in modern agriculture for improving crop tolerance to biotic and abiotic stresses for sustainable crop improvement.

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.

Accelerated development of rice stripe virus-resistant, near-isogenic rice lines through marker-assisted backcrossing

The development of new improved varieties is one of the major goals of plant breeding. Concomitantly, the demand for stable, eco-friendly, and high-quality rice production is constantly increasing. However, most farmers prefer to cultivate familiar rice varieties developed more than 10 years ago to minimize economic risk. A strategy is needed to develop rice varieties without the limitations of the preferred old varieties. Here, we tested the rapid development of near isogenic lines (NILs) using a rapid generation advance system together with marker-assisted backcrossing to overcome the shortcomings of parental materials. For this purpose, we chose rice stripe virus (RSV) susceptible variety Unkwang and RSV resistant variety Haedamssal as experimental materials. First, we backcrossed and screened BC₁F₁ and BC₂F₁ plants having similar agronomic traits as Unkwang and the heterozygous genotype for RSV resistant specific marker InDel7 from Haedamssal. Secondly, the genetic background of 11 BC₂F₁ plants was identified with 73 KASP markers; plants of line YR32548-8 showed 84.5% of recovery of the recurrent parent genome. Among 28 BC₂F₂ plants, YR32548-8-16 was the line that showed maximum recovery of the recurrent parent genome (96.2%) while effectively introgressed with RSV-resistance loci on chromosome 11. Finally, we selected line YR32548-8-16 as an NIL showing an RSV resistant phenotype and similar agronomic traits to Unkwang. This fast breeding approach will be useful in rice breeding programs for the improvement of varieties preferred by farmers for their stress tolerance, yield, or quality.

Genetics of novel brown planthopper Nilaparvata lugens (Stål) resistance genes in derived introgression lines from the interspecific cross O. sativa var. Swarna × O. nivara

The brown planthopper (BPH) Nilaparvata lugens (Stål) (Homoptera: Delphacidae) is considered a threat to rice (Oryza sativa ssp.) crop in many parts of the world including India. Among the BPH-resistance (R) genes so far reported in rice, most of them are ineffective against BPH biotype 4 predominant in the Indian sub-continent. In this study, we show the introgression line RPBio4918-230S was identified as BPH resistant after five years of rigorous screening at seedling stage and two years at tillering and reproductive stages. The inheritance of resistance indicated that two recessive genes are involved at seedling and reproductive stages. The allelic relation with known genes using linked reported markers suggested that the genes present in RPBio4918-230S are different. We report here the genetics of the two newly introgressed BPH resistance genes from O. nivara in the background of Swarna which are effective at all the important growth stages. The genes have been tentatively named as bph39(t) and bph40(t). The honeydew area (feeding rate) and days to wilt parameters observed at 30 days after sowing in BC₁F₃ indicated that newly introgressed genes have both antibiosis and tolerance mechanisms for resistance. The BPH resistance genes identified in this study would facilitate the breeding of broad spectrum and durable resistance in rice against BPH biotype 4.

Global metabolite profiles of rice brown planthopper-resistant traits reveal potential secondary metabolites for both constitutive and inducible defenses

INTRODUCTION

Brown planthopper (BPH) is a phloem feeding insect that causes annual disease outbreaks, called hopper burn in many countries throughout Asia, resulting in severe damage to rice production. Currently, mechanistic understanding of BPH resistance in rice plant is limited, which has caused slow progression on developing effective rice varieties as well as effective farming practices against BPH infestation.

OBJECTIVE

To reveal rice metabolic responses during 8 days of BPH attack, this study examined polar metabolome extracts of BPH-susceptible (KD) and its BPH-resistant isogenic line (IL308) rice leaves.

METHODS

Ultra high performance liquid chromatography-quadrupole time-of-flight mass spectrometry (UPLC-QToF-MS) was combined with multi-block PCA to analyze potential metabolites in response to BPH attack.

RESULTS

This multivariate statistical model revealed different metabolic response patterns between the BPH-susceptible and BPH-resistant varieties during BPH infestation. The metabolite responses of the resistant IL308 variety occurred on Day 1, which was significantly earlier than those of the susceptible KD variety which showed an induced response by Days 4 and 8. BPH infestation caused metabolic perturbations in purine, phenylpropanoid, flavonoid, and terpenoid pathways. While found in both susceptible and resistant rice varieties, schaftoside (1.8 fold), iso-schaftoside (1.7 fold), rhoifolin (3.4 fold) and apigenin 6-C-α-L-arabinoside-8-C-β-L-arabinoside levels (1.6 fold) were significantly increased in the resistant variety by Day 1 post-infestation. 20-hydroxyecdysone acetate (2.5 fold) and dicaffeoylquinic acid (4.7 fold) levels were considerably higher in the resistant rice variety than those in the susceptible variety, both before and after infestation, suggesting that these secondary metabolites play important roles in inducible and constitutive defenses against the BPH infestation.

CONCLUSIONS

These potential secondary metabolites will be useful as metabolite markers and/or bioactive compounds for effective and durable approaches to address the BPH problem.

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.

The rice genome revolution: from an ancient grain to Green Super Rice

Rice is a staple crop for half the world's population, which is expected to grow by 3 billion over the next 30 years. It is also a key model for studying the genomics of agroecosystems. This dual role places rice at the centre of an enormous challenge facing agriculture: how to leverage genomics to produce enough food to feed an expanding global population. Scientists worldwide are investigating the genetic variation among domesticated rice species and their wild relatives with the aim of identifying loci that can be exploited to breed a new generation of sustainable crops known as Green Super Rice.

High-resolution mapping and breeding application of a novel brown planthopper resistance gene derived from wild rice (Oryza. rufipogon Griff)

BACKGROUND

The brown planthopper (Nilaparvata lugens Stål; BPH), one of the most destructive pests of rice, has proven to be a substantial threat, conferring enormous production losses in Asia and becoming a difficult challenge to manipulate and control under field conditions. The continuous use of insecticides promotes the resurgence of BPH, which results in resistant varieties adapting through the upgrading of new BPH biotypes. To overcome resistance acquired by BPH against resistance varieties, different forms of novel resistant gene fusions act as functional domains for breeding to enhance insect resistance.

RESULTS

The current study reports on the novel BPH resistance gene Bph36 derived from two introgression lines (RBPH16 and RBPH17) developed from wild rice GX2183 which was previously reported to be resistant to BPH. Using two F₂ crossing populations (Kangwenqizhan × RBPH16 and Huanghuazhan × RBPH17) in a bulked segregant analysis (BSA) for identification of resistant genes and QTL analysis, two QTLs for BPH resistance were generated on the long and short arms of chromosome 4, which was further confirmed by developing BC₁F_2:3 populations by backcrossing via marker assisted selection (MAS) approach. One BPH resistance locus on the short arm of chromosome 4 was mapped to a 38-kb interval flanked by InDel markers S13 and X48, and then was named Bph36, whereas another locus on the long arm of chromosome 4 was also detected in an interval flanked by RM16766 and RM17033, which was the same as that of Bph27. An evaluation analysis based on four parameters (BPH host selection, honeydew weight, BPH survival rate and BPH population growth rate) shows that Bph36 conferred high levels antibiosis and antixenosis to BPH. Moreover, Bph36 pyramided with Bph3, Bph27, and Bph29 through MAS into elite cultivars 9311 and MH511 (harbored Xa23), creating different background breeding lines that also exhibited strong resistance to BPH in the seedling or tillering stage.

CONCLUSION

Bph36 can be utilized in BPH resistance breeding programs to develop high resistant rice lines and the high-resolution fine mapping will facilitate further map-based cloning and marker-assisted gene pyramiding of resistant gene. MAS exploited to pyramid with Bph3, Bph27, Bph29, and Xa23 was confirmed the effectiveness for BPH resistance breeding in rice and provided insights into the molecular mechanism of defense to control this devastating insect.

Development of a drought stress-resistant rice restorer line through Oryza sativa-rufipogon hybridization

Restorer line F6 (Oryza sativa L. ssp. indica) has been widely used in hybrid rice breeding systems in southern China. However, line F6 is susceptible to drought stress, which restricts its utilization in many areas. Dongxiang wild rice (DXWR, Oryza rufipogon Griff.) has strong drought stress resistance, but the molecular factors responsible for drought resistance in DXWR remain unknown. In this study, we used the combination of phenotypic selection and molecular marker-assisted selection (MAS) to improve the drought stress resistance of line F6 by introgression of qSDT12-2, a large effect drought stress-related quantitative trait locus identified in DXWR. Molecular MAS was carried out using linked marker RM1226, which is associated with qSDT12-2. Genomic background assessmentwas performed using 112 polymorphic markers. Finally, a stable drought stress-resistant backcross inbred line (BIL) was developed from a BC5F5 population, termed BIL627. Genetic constitution analysis revealed that the genome of BIL627 is almost identical (99.1%) to that of the restorer line F6. Further, BIL627 showed no yield penalty and no decrease in restorationability under normal conditions. Taken together, our findings reveal the intrinsic value of using genetic resources present in wild species of Oryza to improve undesirable traits found in cultivated rice.

Comparative metabolomics analysis of different resistant rice varieties in response to the brown planthopper Nilaparvata lugens Hemiptera: Delphacidae

INTRODUCTION

The brown planthopper (BPH, Nilaparvata lugens Stål, Hemiptera: Delphacidae) is one of the most devastating insect pests of the crucially important cereal crop, rice (Oryza sativa L.). Currently, multiple BPH-resistant rice varieties have been cultivated and generalized to control BPH. However, the defence metabolic responses and their modes of action against BPH in different rice cultivars remain uncharacterized.

OBJECTIVE

We used a non-biased metabolomics approach to explore the differences in metabolite profiles in response to BPH infestation in the susceptible TN1 rice cultivar and two resistant cultivars (IR36 and IR56).

METHODS

The metabolomic detection based on gas chromatography-mass spectrometry (GC-MS) and liquid chromatography-mass spectrometry (LC-MS) was performed to investigate the content changes of identified metabolites in TN1, IR36 and IR56 rice varieties at various time points (0 h, 24 h, 48 h and 96 h) post BPH feeding. The differentially expressed metabolites were screened and the corresponding metabolic pathways were further enriched.

RESULTS

The results showed that compared to that in TN1, the content changes of most primary metabolites were more stable, but the concentration alterations of some defence-related metabolites were more acute and persistent in IR36 and IR56. Furthermore, the differentially expressed pathways analysis revealed that cyanoamino acids and lipids metabolism was persistently induced in IR36, but changes in thiamine, taurine and hypotaurine metabolism were more significant in IR56 during BPH infestation. Besides, the contents of quercetin and spermidine which were harmful to BPH fitness, were significantly elevated by BPH in TN1 and IR36, and the quercetin level was significantly decreased during BPH feeding in IR56.

CONCLUSION

The results of the differences in metabolite profiles in response to BPH infestation in different rice cultivars were useful to clarify the metabolic mechanism of rice plants during BPH infestation and to provide new resources to control this insect pest.

Development of molecular marker and introgression of Bph3 into elite rice cultivars by marker-assisted selection

The brown planthopper (BPH) is a serious insect pest of rice and a substantial threat to rice production. Identification of new BPH resistance genes and their transfer into modern rice cultivars are effective breeding approaches to reduce the damage caused by BPH. In this study, we mapped a BPH resistance gene to a 50-kb genomic interval between two InDel markers 4M03980 and 4M04041 on the short arm of chromosome 4 in indica rice cultivar BP60, where the BPH resistance gene was mapped in Rathu Heenati by Liu et al. (2015) and named "Bph3". This region contains two annotated genes Os04g0201900 and Os04g0202300, which encode lectin receptor kinases responsible for BPH resistance. We also developed a molecular marker "MM28T" for Bph3, and introgression Bph3 into susceptible rice restorer lines Guihui582 and Gui7571 by the marker-assisted selection (MAS) approach. The BPH resistance level is significantly enhanced in the Bph3-introgression lines, the resistance scores decrease from 8.2 to 3.6 for Guihui582 and decrease from 8.7 to around 3.8 for Gui7571. Therefore, developing molecular markers for the BPH resistance gene Bph3 and using them for molecular breeding will facilitate the creation of BPH-resistance rice cultivars to reduce damage caused by BPH.

Proteomic analysis of the rice (Oryza officinalis) provides clues on molecular tagging of proteins for brown planthopper resistance

BACKGROUND

Among various pests, the brown planthopper (BPH) that damages rice is the major destructive pests. Understanding resistance mechanisms is a critical step toward effective control of BPH. This study investigates the proteomics of BPH interactions with three rice cultivars: the first resistant (PR) to BPH, the second susceptible (PS), and the third hybrid (HR) between the two, in order to understand mechanisms of BPH resistance in rice.

RESULTS

Over 4900 proteins were identified from these three rice cultivars using iTRAQ proteomics study. A total of 414, 425 and 470 differentially expressed proteins (DEPs) were detected from PR, PS and HR, respectively, after BPH infestation. Identified DEPs are mainly enriched in categories related with biosynthesis of secondary metabolites, carbon metabolism, and glyoxylate and dicarboxylate metabolism. A two-component response regulator protein (ORR22) may participate in the early signal transduction after BPH infestation. In the case of the resistant rice cultivar (PR), 6 DEPs, i.e. two lipoxygenases (LOXs), a lipase, two dirigent proteins (DIRs) and an Ent-cassa-12,15-diene synthase (OsDTC1) are related to inheritable BPH resistance. A heat shock protein (HSP20) may take part in the physiological response to BPH infestation, making it a potential target for marker-assisted selection (MAS) of rice. Quantitative real-time polymerase chain reaction (qRT-PCR) revealed eight genes encoding various metabolic proteins involved in BPH resistance. During grain development the expressions of these genes varied at the transcriptional and translational levels.

CONCLUSIONS

This study provides comprehensive details of key proteins under compatible and incompatible interactions during BPH infestation, which will be useful for further investigation of the molecular basis of rice resistance to BPH and for breeding BPH-resistant rice cultivars.

Unanticipated benefits and potential ecological costs associated with pyramiding leafhopper resistance loci in rice

We tested the hypotheses that increasing the number of anti-herbivore resistance loci in crop plants will increase resistance strength, increase the spectrum of resistance (the number of species affected), and increase resistance stability. We further examined the potential ecological costs of pyramiding resistance under benign environments. In our experiments, we used 14 near-isogenic rice lines with zero (T65: recurrent parent), one, two or three resistance loci introgressed through marker-assisted selection. Lines with two or more loci that were originally bred for resistance to the green rice leafhopper, Nephotettix cincticeps, significantly reduced egg-laying by the green leafhopper, N. virescens. Declines in egg-number and in nymph weight were correlated with the numbers of resistance loci in the rice lines. To test the spectrum of resistance, we challenged the lines with a range of phloem feeders including the zig-zag leafhopper, Recilia dorsalis, brown planthopper, Nilaparvata lugens, and whitebacked planthopper, Sogatella furcifera. There was an increase in the number of tested species showing significant declines in egg-laying and nymph survival on lines with increasing numbers of loci. In a screen house trial that varied rates of nitrogenous fertilizer, a line with three loci had stable resistance against the green leafhopper and the grain yields of infested plants were maintained or increased (overcompensation). Under benign conditions, plant growth and grain yields declined with increasing numbers of resistance loci. However, under field conditions with natural exposure to herbivores, there were no significant differences in final yields. Our results clearly indicate the benefits, including unanticipated benefits such as providing resistance against multiple herbivore species, of pyramiding anti-herbivore resistance genes/loci in crop plants. We discuss our results as part of a review of existing research on pyramided resistance against leafhoppers and planthoppers in rice. We suggest that potential ecological costs may be overcome by the careful selection of gene combinations for pyramiding, avoidance of high (potentially redundant) loci numbers, and introgression of loci into robust plant types such as hybrid rice varieties.

Bacillus velezensis YC7010 Enhances Plant Defenses Against Brown Planthopper Through Transcriptomic and Metabolic Changes in Rice

Brown planthopper (BPH; Nilaparvata lugens Stål) is one of the most serious insect pests, which reduce rice yield remarkably in many rice-growing areas. A few plant growth-promoting rhizobacteria induce systemic resistance against herbivorous insects. Here we show that root drenching of rice seedlings with an endophytic strain Bacillus velezensis YC7010 enhanced defenses against BPH. Based on high-throughput transcriptome analysis, systemic resistance against BPH was induced by B. velezensis YC7010 via salicylic acid (SA)- and jasmonic acid (JA)-dependent pathways. Increased leaf contents of secondary metabolites, tricin and C-glycosyl flavone and cell-wall contents of lignin and cellulose were the key defense mechanisms inducing resistance against BPH during the three-way interaction. This study shows for the first time that chemical changes and strengthening of physical barriers play important roles simultaneously in plant defense against BPH in rice by the endophytic bacteria. This defense was induced by lipopeptides including a novel bacillopeptin X.

Systems model-guided rice yield improvements based on genes controlling source, sink, and flow

A large number of genes related to source, sink, and flow have been identified after decades of research in plant genetics. Unfortunately, these genes have not been effectively utilized in modern crop breeding. This perspective paper aims to examine the reasons behind such a phenomenon and propose a strategy to resolve this situation. Specifically, we first systematically survey the currently cloned genes related to source, sink, and flow; then we discuss three factors hindering effective application of these identified genes, which include the lack of effective methods to identify limiting or critical steps in a signaling network, the misplacement of emphasis on properties, at the leaf, instead of the whole canopy level, and the non-linear complex interaction between source, sink, and flow. Finally, we propose the development of systems models of source, sink and flow, together with a detailed simulation of interactions between them and their surrounding environments, to guide effective use of the identified elements in modern rice breeding. These systems models will contribute directly to the definition of crop ideotype and also identification of critical features and parameters that limit the yield potential in current cultivars.

Virulence adaptation in a rice leafhopper: Exposure to ineffective genes compromises pyramided resistance

Pyramiding resistance genes is predicted to increase the durability of resistant rice varieties against phloem-feeding herbivores. We examined responses by the green leafhopper, Nephotettix virescens (Hemiptera: Cicadellidae), to near-isogenic rice lines with zero, one and two resistance genes. The recurrent parent (T65) and monogenic lines (GRH2-NIL and GRH4-NIL) with genes for resistance to the green rice leafhopper, Nephotettix cincticeps (Hemiptera: Cicadellidae), were susceptible to the green leafhopper, but the pyramided line (GRH2/GRH4-PYL) was highly resistant to the green leafhopper. We selected green leafhoppers, N. virescens, from five sites in the Philippines for over 20 generations on each of the four lines. Populations selected on GRH2/GRH4-PYL gained partial virulence (feeding and development equal to that on T65) to the pyramided line within 10 generations and complete virulence (egg-laying equal to that on T65) within 20 generations. After 20 generations of rearing on the susceptible monogenic lines, green leafhoppers were also capable of developing and laying eggs on GRH2/GRH4-PYL. Furthermore, green leafhoppers reared on the susceptible GRH4-NIL for 20 generations showed equal preferences for T65 and GRH2/GRH4-PYL in choice bioassays. Our results indicate that previous long-term exposure to ineffective genes (including unperceived resistance genes) could dramatically reduce the durability of pyramided resistance. We suggest that informed crop management and deployment strategies should be developed to accompany rice lines with pyramided resistance and avoid the build-up of virulent herbivore populations.

Identification and fine mapping of Bph33, a new brown planthopper resistance gene in rice (Oryza sativa L.)

BACKGROUND

Host-plant resistance is the most desirable and economic way to overcome BPH damage to rice. As single-gene resistance is easily lost due to the evolution of new BPH biotypes, it is urgent to explore and identify new BPH resistance genes.

RESULTS

In this study, using F_2:3 populations and near-isogenic lines (NILs) derived from crosses between two BPH-resistant Sri Lankan rice cultivars (KOLAYAL and POLIYAL) and a BPH-susceptible cultivar 9311, a new resistance gene Bph33 was fine mapped to a 60-kb region ranging 0.91-0.97 Mb on the short arm of chromosome 4 (4S), which was at least 4 Mb distant from those genes/QTLs (Bph12, Bph15, Bph3, Bph20, QBph4 and QBph4.2) reported before. Seven genes were predicted in this region. Based on sequence and expression analyses, a Leucine Rich Repeat (LRR) family gene (LOC_Os04g02520) was identified as the most possible candidate of Bph33. The gene exhibited continuous and stable resistance from seedling stage to tillering stage, showing both antixenosis and antibiosis effects on BPH.

CONCLUSION

The results of this study will facilitate map-based cloning and marker-assisted selection of the gene.

Lipid profiles reveal different responses to brown planthopper infestation for pest susceptible and resistant rice plants

INTRODUCTION

Brown planthopper (BPH) is the most destructive insect pest for rice, causing major reductions in rice yield and large economic losses. More than 31 BPH-resistance genes have been located, and several of them have been isolated. Nevertheless, the metabolic mechanism related to BPH-resistance genes remain uncharacterized.

OBJECTIVES

To elucidate the resistance mechanism of the BPH-resistance gene Bph6 at the metabolic level, a Bph6-transgenic line R6 (BPH-resistant) and the wild-type Nipponbare (BPH-susceptible) were used to investigate their lipid profiles under control and BPH treatments.

METHODS

In conjunction with multivariate statistical analysis and quantitative real-time PCR, BPH-induced lipid changes in leaf blade and leaf sheath were investigated by GC-MS-based lipidomics.

RESULTS

Forty-five lipids were identified in leaf sheath extracts. Leaf sheath lipidomics analysis results show that BPH infestation induces significant differences in the lipid profiles of Nipponbare and R6. The levels of hexadecanoic acid, methyl ester, linoleic acid, methyl ester, linolenic acid, methyl ester, glycidyl palmitate, eicosanoic acid, methyl ester, docosanoic acid, methyl ester, beta-monolinolein, campesterol, beta-sitosterol, cycloartenol, phytol and phytyl acetate had undergone enormous changes after BPH feeding. These results illustrate that BPH feeding enhances sterol biosynthetic pathway in Nipponbare plants, and strengthens wax biosynthesis and phytol metabolism in R6 plants. The results of quantitative real-time PCR of 5 relevant genes were consistent with the changes in metabolic level. Forty-five lipids were identified in the leaf blade extracts. BPH infestation induces distinct changes in the lipid profiles of the leaf blade samples of Nipponbare and R6. Although the lipid changes in Nipponbare are more drastic, the changes within the two varieties are similar. Lipid profiles in leaf sheath brought out significant differences than in leaf blade within Nipponbare and R6. We propose that Bph6 mainly affects the levels of lipids in leaf sheath, and mediates resistance by deploying metabolic re-programming during BPH feeding.

CONCLUSION

The results indicate that wax biosynthesis, sterol biosynthetic pathway and phytol metabolism play vital roles in rice response to BPH infestation. This finding demonstrated that the combination of lipidomics and quantitative real-time PCR is an effective approach to elucidating the interactions between brown planthopper and rice mediated by resistance genes.

OsEXPA10 mediates the balance between growth and resistance to biotic stress in rice

OsEXPA10 gene coordinates the balance between rice development and biotic resistance. Expansins are proteins that can loosen the cell wall. Previous studies have indicated that expansin-encoding genes were involved in defense against abiotic stress, but little is known about the involvement of expansins in biotic stress. Brown planthopper (BPH) is one of the worst insect pests of rice in the Asia-Pacific planting area, and many efforts have been made to identify and clone BPH-resistance genes for use in breeding resistant cultivars. At the same time, rice blast caused by Magnaporthe grisea is one of the three major diseases that severely affect rice production worldwide. Here, we demonstrated that one rice expansin-encoding gene, OsEXPA10, functions in both rice growth and biotic resistance. Over expression of OsEXPA10 improved rice growth but also increased susceptibility to BPH infestation and blast attack, while knock-down OsEXPA10 gene expression resulted in reduced plant height and grain size, but also increased resistance to BPH and the blast pathogen. These results imply that OsEXPA10 mediates the balance between rice development and biotic resistance.

Wild Relatives of Maize, Rice, Cotton, and Soybean: Treasure Troves for Tolerance to Biotic and Abiotic Stresses

Global food demand is expected to nearly double by 2050 due to an increase in the world's population. The Green Revolution has played a key role in the past century by increasing agricultural productivity worldwide, however, limited availability and continued depletion of natural resources such as arable land and water will continue to pose a serious challenge for global food security in the coming decades. High yielding varieties with proven tolerance to biotic and abiotic stresses, superior nutritional profiles, and the ability to adapt to the changing environment are needed for continued agricultural sustainability. The narrow genetic base of modern cultivars is becoming a major bottleneck for crop improvement efforts and, therefore, the use of crop wild relatives (CWRs) is a promising approach to enhance genetic diversity of cultivated crops. This article provides a review of the efforts to date on the exploration of CWRs as a source of tolerance to multiple biotic and abiotic stresses in four global crops of importance; maize, rice, cotton, and soybean. In addition to the overview of the repertoire and geographical spread of CWRs in each of the respective crops, we have provided a comprehensive discussion on the morphological and/or genetic basis of the traits along with some examples, when available, of the research in the transfer of traits from CWRs to cultivated varieties. The emergence of modern molecular and genomic technologies has not only accelerated the pace of dissecting the genetics underlying the traits found in CWRs, but also enabled rapid and efficient trait transfer and genome manipulation. The potential and promise of these technologies has also been highlighted in this review.

High-resolution genetic mapping of a novel brown planthopper resistance locus, Bph34 in Oryza sativa L. X Oryza nivara (Sharma & Shastry) derived interspecific F2 population

A BPH-resistant locus designated as Bph34 identified in Oryza nivara acc. IRGC104646 on long arm of chromosome 4 using high-resolution mapping with 50 K SNP chip. BPH resistance contributed by locus showed dominant inheritance in F₂ and F₃. The Bph34 locus is 91 kb in size and contains 11 candidate genes. In addition to SNP markers, SSR markers, RM16994 and RM17007 co-segregated with the BPH resistance. These two SSR markers can facilitate marker-assisted transfer of the Bph34 locus into elite rice cultivars in all labs. Brown planthopper (BPH, Nilaparvata lugen Stål) is one of the most destructive insects of rice (Oryza sativa L.) causing significant yield losses annually. Exploiting host plant resistance to BPH and incorporating resistant genes in susceptible commercial cultivars is economical and environmentally friendly approach to manage this pest. Here, we report high-resolution mapping of a novel genetic locus for resistance to BPH, designated as Bph34 on long arm of rice chromosome 4. The locus was mapped using an interspecific F₂ population derived from a cross between susceptible indica cultivar PR122 and BPH-resistant wild species, O. nivara acc. IRGC104646. Inheritance studies performed using F₂ and F_2:3 populations revealed the presence of single dominant gene. Construction of high-density linkage map using 50 K SNP chip (OsSNPnks) followed by QTL mapping identified single major locus at 28.8 LOD score between SNP markers, AX-95952039 and AX-95921548. The major locus contributing resistance to BPH designated as Bph34 and explained 68.3% of total phenotypic variance. The Bph34 locus is 91 Kb in size on Nipponbare reference genome-IRGSP-1.0 and contains 11 candidate genes. In addition to associated SNP markers, two SSR markers, RM16994 and RM17007, also co-segregated with the Bph34 which can be used efficiently for markers assisted transfer into elite rice cultivars across the labs.

Evaluation and breeding application of six brown planthopper resistance genes in rice maintainer line Jin 23B

BACKGROUND

The brown planthopper (BPH), an insect species that feeds on rice plants (Oryza sativa L.), is one of the most destructive insect pests of rice. BPH can be controlled with chemical pesticides, but they are expensive in terms of the cost and environmental hazards. Hence, utilization of resistance genes in resistant varieties is considered as a more economical and eco-friendly effective method for control.

RESULTS

In this study, six dominant BPH-resistance genes (Bph3, Bph14, Bph15, Bph18, Bph20 and Bph21) were introduced into an elite indica rice cultivar Jin 23B using the marker-assisted selection (MAS) method of breeding. One line combining Bph14 and Bph15 and six single gene introgression lines were used to evaluate the gene effects based on three parameters: seedling and tillering resistance of the rice genotypes, honeydew weight, and BPH survival rate. Among all improved lines, combination of Bph14 and Bph15 had the largest effect in conferring resistance to BPH. Bioassays showed that the order of the gene effects was Bph14/Bph15 > Bph15 ≥ Bph14 ≥ Bph20 ≥ Bph21 ≥ Bph3 > Bph18 > none at the rice seedling stage. The pyramided or single-gene introgression lines showed enhanced resistance relative to the control. Furthermore, field trial data demonstrated that yields of improved Jin 23B lines were similar to the control under BPH-free field conditions.

CONCLUSIONS

Each of the BPH resistance genes reduced BPH growth and development, and was effective at both the seedling and tillering growth stages. These lines can be used in rice hybrid and production in China.

Marker assisted pyramiding of Bph6 and Bph9 into elite restorer line 93-11 and development of functional marker for Bph9

BACKGROUND

The brown planthopper (BPH) has become the most destructive and a serious threat to the rice production in Asia. Breeding the resistant varieties with improved host resistance is the most effective and ecosystem-friendly strategy of BPH biological management. As host resistance was always broken down by the presence of the upgrading BPH biotype, the more resistant varieties with novel resistance genes or pyramiding known identified BPH resistance genes would be needed urgently for higher resistant level and more durability of resistance.

RESULTS

Here, we developed near isogenic lines of Bph9 (NIL-Bph9) by backcrossing elite cultivar 93-11 with Pokkali (harboring Bph9) using marker-assisted selection (MAS). Subsequently, we pyramided Bph6 and Bph9 in 93-11 genetic background through MAS. The resulting Bph6 and Bph9 pyramided line LuoYang69 had stronger antixenotic and antibiosis effects on BPH and exhibited significantly enhanced resistance to BPH than near isogenic lines NIL-Bph6 and NIL-Bph9. LuoYang69 derived hybrids, harboring heterozygous Bph6 and Bph9 genes, also conferred high level of resistance to BPH. Furthermore, LuoYang69 did not affect the elite agronomic traits and rice grain quality of 93-11. The current study also developed functional markers for Bph9. Using functional dominant marker, we screened and evaluated worldwide accessions of rice germplasm. Of the 673 varieties tested, 8 cultivars were identified to harbor functional Bph9 gene.

CONCLUSION

The development of Bph6 and Bph9 pyramided line LuoYang69 provides valuable resource to develop hybrid rice with highly and durable BPH resistance. The development of functional markers will promote MAS of Bph9. The identified Bph9 containing cultivars can be used as new sources for BPH resistance breeding programs.

Development of 25 near-isogenic lines (NILs) with ten BPH resistance genes in rice (Oryza sativa L.): production, resistance spectrum, and molecular analysis

A first set of 25 NILs carrying ten BPH resistance genes and their pyramids was developed in the background of indica variety IR24 for insect resistance breeding in rice. Brown planthopper (Nilaparvata lugens Stal.) is one of the most destructive insect pests in rice. Development of near-isogenic lines (NILs) is an important strategy for genetic analysis of brown planthopper (BPH) resistance (R) genes and their deployment against diverse BPH populations. A set of 25 NILs with 9 single R genes and 16 multiple R gene combinations consisting of 11 two-gene pyramids and 5 three-gene pyramids in the genetic background of the susceptible indica rice cultivar IR24 was developed through marker-assisted selection. The linked DNA markers for each of the R genes were used for foreground selection and confirming the introgressed regions of the BPH R genes. Modified seed box screening and feeding rate of BPH were used to evaluate the spectrum of resistance. BPH reaction of each of the NILs carrying different single genes was variable at the antibiosis level with the four BPH populations of the Philippines. The NILs with two- to three-pyramided genes showed a stronger level of antibiosis (49.3-99.0%) against BPH populations compared with NILs with a single R gene NILs (42.0-83.5%) and IR24 (10.0%). Background genotyping by high-density SNPs markers revealed that most of the chromosome regions of the NILs (BC₃F₅) had IR24 genome recovery of 82.0-94.2%. Six major agronomic data of the NILs showed a phenotypically comparable agronomic performance with IR24. These newly developed NILs will be useful as new genetic resources for BPH resistance breeding and are valuable sources of genes in monitoring against the emerging BPH biotypes in different rice-growing countries.

Geographic and Research Center Origins of Rice Resistance to Asian Planthoppers and Leafhoppers: Implications for Rice Breeding and Gene Deployment

This study examines aspects of virulence to resistant rice varieties among planthoppers and leafhoppers. Using a series of resistant varieties, brown planthopper, Nilaparvata lugens, virulence was assessed in seedlings and early-tillering plants at seven research centers in South and East Asia. Virulence of the whitebacked planthopper, Sogatella furcifera, in Taiwan and the Philippines was also assessed. Phylogenetic analysis of the varieties using single-nucleotide polymorphisms (SNPs) indicated a clade of highly resistant varieties from South Asia with two further South Asian clades of moderate resistance. Greenhouse bioassays indicated that planthoppers can develop virulence against multiple resistance genes including genes introgressed from wild rice species. Nilaparvata lugens populations from Punjab (India) and the Mekong Delta (Vietnam) were highly virulent to a range of key resistance donors irrespective of variety origin. Sogatella furcifera populations were less virulent to donors than N. lugens; however, several genes for resistance to S. furcifera are now ineffective in East Asia. A clade of International Rice Research Institute (IRRI)-bred varieties and breeding lines, without identified leafhopper-resistance genes, were highly resistant to the green leafhopper, Nephotettix virescens. Routine phenotyping during breeding programs likely maintains high levels of quantitative resistance to leafhoppers. We discuss these results in the light of breeding and deploying resistant rice in Asia.

Genomics of interaction between the brown planthopper and rice

Rice (Oryza sativa L.) and the brown planthopper (Nilaparvata lugens (Stål)) form a model system for dissection of the mechanism of interaction between insect pest and crop. In this review, we focus on the genomics of BPH-rice interaction. On the side of rice, a number of BPH-resistance genes have been identified genetically. Thirteen of these genes have been cloned which shed a light on the molecular basis of the interaction. On the aspect of BPH, a lot of salivary proteins have been identified using transcriptome and proteome techniques. The genetic loci of virulence were mapped in BPH genome based on the linkage map. The understanding of interaction between BPH and rice will provide novel insights into efficient control of this pest.

Development and evaluation of near-isogenic lines for brown planthopper resistance in rice cv. 9311

Brown planthopper (BPH) is the most destructive pest of rice in Asia. To date 29 BPH resistance genes have been identified, but only a few genes are being used in breeding due to inefficient markers for marker-assisted selection (MAS) and little knowledge of the real effects of the genes. In this study we individually transferred 13 genes or QTLs (Bph14, QBph3, QBph4, Bph17, Bph15, Bph20, Bph24, Bph6, Bph3, Bph9, Bph10, Bph18 and Bph21) into cultivar 9311 by marker assisted backcross breeding (MABB). Through positive and negative selection we narrowed the segments from donors containing Bph14, Bph15, Bph6 and Bph9 to 100-400 kb. Whole-genome background selection based on a high resolution SNP array was performed to maximize reconstitution of the recurrent parent genome (RPG 99.2-99.9%). All genes reduced BPH growth and development and showed antibiotic responses in seedlings. Based on genetic effects and amino acid sequences of genes in three clusters we inferred that Bph10 and Bph21 might be identical to Bph26, whereas Bph9 and Bph18 were different. Bph15 might be same with Bph17, but QBph4, Bph20 and Bph24 might be different. We believe that these NILs will be useful in rice BPH resistance research and breeding.