Major Blast Resistance Genes Research Articles

OsLRR-RLP2 Gene Regulates Immunity to Magnaporthe oryzae in Japonica Rice.

Rice is an important cereal crop worldwide, the growth of which is affected by rice blast disease, caused by the fungal pathogen Magnaporthe oryzae. As climate change increases the diversity of pathogens, the disease resistance genes (R genes) in plants must be identified. The major blast-resistance genes have been identified in indica rice varieties; therefore, japonica rice varieties with R genes now need to be identified. Because leucine-rich repeat (LRR) domain proteins possess R-gene properties, we used bioinformatics analysis to identify the rice candidate LRR domain receptor-like proteins (OsLRR-RLPs). OsLRR-RLP2, which contains six LRR domains, showed differences in the DNA sequence, containing 43 single-nucleotide polymorphisms (SNPs) in indica and japonica subpopulations. The results of the M. oryzae inoculation analysis indicated that indica varieties with partial deletion of OsLRR-RLP2 showed susceptibility, whereas japonica varieties with intact OsLRR-RLP2 showed resistance. The oslrr-rlp2 mutant, generated using clustered regularly interspaced palindromic repeats (CRISPR)/CRISPR-associated protein 9 (Cas9), showed increased pathogen susceptibility, whereas plants overexpressing this gene showed pathogen resistance. These results indicate that OsLRR-RLP2 confers resistance to rice, and OsLRR-RLP2 may be useful for breeding resistant cultivars.

Development and Genome-Wide Analysis of a Blast-Resistant japonica Rice Variety.

Rice is one of the most important crops in the world, and its production is severely affected by the rice blast disease caused by the fungus Magnaporthe oryzae. Several major blast resistance genes and QTLs associated with blast resistance have been described and mostly identified in indica rice varieties. In this work, we report the obtention of a blast-resistant rice breeding line derived from crosses between the resistant indica variety CT13432 and the japonica elite cultivar JSendra (highly susceptible to blast). The breeding line, named COPSEMAR9, was found to exhibit resistance to leaf blast and panicle blast, as demonstrated by disease assays under controlled and field conditions. Furthermore, a high-quality genome sequence of the blast-resistant breeding line was obtained using a strategy that combines short-read sequencing (Illumina sequencing) and long-read sequencing (Pacbio sequencing). The use of a whole-genome approach allowed the fine mapping of DNA regions of indica and japonica origin present in the COPSEMAR9 genome and the identification of parental gene regions potentially contributing to blast resistance in the breeding line. Rice blast resistance genes (including Pi33 derived from the resistant parent) and defense-related genes in the genome of COPSEMAR9 were identified. Whole-genome analyses also revealed the presence of microRNAs (miRNAs) with a known function in the rice response to M. oryzae infection in COPSEMAR9, which might also contribute to its phenotype of blast resistance. From this study, the genomic information and analysis methods provide valuable knowledge that will be useful in breeding programs for blast resistance in japonica rice cultivars.

Gene expression of near-isogenic lines (NILs) carrying blast resistance genes Pi9 and Pi54 in the background of rice cultivar Mushk Budji.

Kashmir valley, India is a homeland to rice landraces like Zag, Nunbeoul, Qadirbeigh, Kawkadur, Kamad, Mushk Budji, etc., generally characterized by short grains, aroma, earliness and cold tolerance. Mushk Budji is a commercially important speciality rice known for its taste and aroma, nonetheless, is extremely vulnerable to blast disease. Through the use of the marker-assisted backcrossing (MABC) approach, a set of 24 Near-isogenic lines (NILs) was created, and the lines with the highest background genome recovery were chosen. The expression analysis was carried out for the component genes and other eight pathway genes related to blast resistance. The major blast resistance genes Pi9 (from IRBL-9W) and Pi54 (from DHMAS 70Q 164-1b) were incorporated following simultaneous-but-step-wise MABC. The NILs harbouring genes Pi9 + Pi54, Pi9 and Pi54 expressed resistance to isolate (Mo-nwi-kash-32) under controlled and natural field conditions. The loci controlling ETI (effector triggered immunity) included the gene Pi9 and showed 61.18 and 60.27 fold change in relative gene expression in Pi54 + Pi9 and Pi9 carrying NILs against RP Mushk Budji. Pi54 was up regulated and showed 41 and 21 fold change in relative gene expression for NIL-Pi54 + Pi9 and NIL-Pi54, respectively. Among the pathway genes, LOC_Os01g60600 (WRKY 108) recorded 8 and 7.5 fold up regulation in Pi9 and Pi54 NILs. The NILs showed recurrent parent genome recovery (RPG) per cent of 81.67 to 92.54 and were on par in performance to recurrent parent Mushk Budji. The lines were utilized to study the expression of the loci controlling WRKYs, peroxidases and chitinases that confer overall ETI response.

Improving hybrid rice parental lines for blast resistance by introgression of broad‐spectrum resistance genes Pi54 and Pi9 by marker‐assisted selection

AbstractHybrid rice technology offers a great promise to produce 15% to 20% more yield than pure line varieties. The success of hybrid rice hinges on developing superior parental lines. To improve the blast resistance of hybrid rice parental line RP5933‐1‐19‐2R, crosses were made with donors of two major blast resistance genes namely, Pi54 (Tetep) and Pi9 (IR71033–121‐15) and the resulting F1s were confirmed for their hybridity by using Pi54MAS and NMSMPi9‐1 genic markers. The confirmed F1s were intercrossed to obtain ICF1s and selected positive plants by markers were backcrossed to the recurrent parent, as well as selfed for advancing further to BC1F3 and ICF4 generations. The segregating plants were phenotyped for blast resistance at Uniform Blast Nursery. The identified complete restorers namely, RP 6619‐1, RP 6616‐26, RP 6619‐3 and RP 6619‐11 with Pi9 and Pi54 genes would serve as donors for broad spectrum blast resistance. This could ultimately lead to the development of new rice hybrids with improved resistance to blast disease, which is crucial for sustainable rice production and food security.

Phenotypic and Genotypic screening of fifty-two rice (Oryza sativa L.) genotypes for desirable cultivars against blast disease.

Magnaporthe oryzae, the rice blast fungus, is one of the most dangerous rice pathogens, causing considerable crop losses around the world. In order to explore the rice blast-resistant sources, initially performed a large-scale screening of 277 rice accessions. In parallel with field evaluations, fifty-two rice accessions were genotyped for 25 major blast resistance genes utilizing functional/gene-based markers based on their reactivity against rice blast disease. According to the phenotypic examination, 29 (58%) and 22 (42%) entries were found to be highly resistant, 18 (36%) and 29 (57%) showed moderate resistance, and 05 (6%) and 01 (1%), respectively, were highly susceptible to leaf and neck blast. The genetic frequency of 25 major blast resistance genes ranged from 32 to 60%, with two genotypes having a maximum of 16 R-genes each. The 52 rice accessions were divided into two groups based on cluster and population structure analysis. The highly resistant and moderately resistant accessions are divided into different groups using the principal coordinate analysis. According to the analysis of molecular variance, the maximum diversity was found within the population, while the minimum diversity was found between the populations. Two markers (RM5647 and K39512), which correspond to the blast-resistant genes Pi36 and Pik, respectively, showed a significant association to the neck blast disease, whereas three markers (Pi2-i, Pita3, and k2167), which correspond to the blast-resistant genes Pi2, Pita/Pita2, and Pikm, respectively, showed a significant association to the leaf blast disease. The associated R-genes might be utilized in rice breeding programmes through marker-assisted breeding, and the identified resistant rice accessions could be used as prospective donors for the production of new resistant varieties in India and around the world.

Improving blast resistance of maintainer line DRR 9B by transferring broad spectrum resistance gene Pi2 by marker assisted selection in rice.

Hybrid rice technology offers great promise to further enhance rice production and productivity for global food security. Improving hybrid rice parental lines is the first step in developing heterotic rice hybrids. To improve resistance against blast disease, a maintainer line DRR 9B was fortified with a major broad-spectrum blast resistance gene Pi2 through marker-assisted selection. The rice blast caused by Magnaporthe oryzae is a major disease and can cause severe yield losses upto 100%. The NILs of Samba Mahsuri namely BA-23-11-89-12-168 possessing Pi2 was utilized as a donor parent. The PCR-based molecular marker tightly linked to Pi2 gene was used for the foreground selection at BC1F1 generation. The molecular marker tightly linked to the major fertility restorer gene Rf4 was used for negative selection (i.e., selection of plants possessing non fertility restoring alleles) at BC1F1 generation to identify maintainer lines. The positive plants with Rf4 gene were added to the restorer pool for restorer line development. At each stage, MAS for Pi2 coupled with stringent phenotypic selection for agro-morphological and grain quality traits were exercised. At BC1F3 generation, one hundred families were screened against blast disease at uniform blast nursery (UBN) and selected resistant lines were advanced to next generations. In the BC1F5 generation plants were subjected to agro-morphological evaluation for yield and yield-contributing traits. The selected plants at BC1F5 generation were crossed with DRR 9A to assess the maintainer ability of blast resistance lines and for further CMS line conversion for hybrid rice breeding for developing blast resistance rice hybrids.

Stress responsive OsHyPRP16 promoter driven early expression of resistance gene Pi54 potentiate the resistance against Magnaporthe oryzae in transgenic rice

The rice Hybrid Proline Rich Protein (HyPRP) encoding gene, OsHyPRP16 expression exhibit early upregulation in response to Magnaporthe oryzae inoculation. Here, we functionally characterized the OsHyPRP16 promoter through deletion analysis in transgenic Arabidopsis using GUS (β-glucuronidase) reporter assay. The promoter fragments, sequentially deleted from the 5′ end could induce differential GUS activity in response to stresses induced by different hormones and abiotic stress conditions. In addition, a strong GUS induction was observed in M. oryzae inoculated transgenic Arabidopsis. Based on the insilico and stress-inducibility of D1 promoter fragment against various phytohormones and rice blast fungus, and with no basal activity under control conditions, we rationally selected D1 promoter fragment to drive the expression of a major rice blast resistance gene; Pi54 in the genetic background of blast susceptible TP309 rice line. The D1 promoter fragment was able to induce the expression of Pi54 at immediate-early stages of M. oryzae infection in transgenic rice. The transgenic plants with Pi54 under the control of D1 promoter fragment displayed complete resistance against M. oryzae infection as compared to control plants. The present study suggests that the D1 fragment of OsHyPRP16 promoter is a valuable tool for breeding and development of rice lines with early-inducible and pathogen-responsive enhanced disease resistance.

Integrated Strategies for Durable Rice Blast Resistance in Sub-Saharan Africa.

Rice is a key food security crop in Africa. The importance of rice has led to increasing country-specific, regional, and multinational efforts to develop germplasm and policy initiatives to boost production for a more food-secure continent. Currently, this critically important cereal crop is predominantly cultivated by small-scale farmers under suboptimal conditions in most parts of sub-Saharan Africa (SSA). Rice blast disease, caused by the fungus Magnaporthe oryzae, represents one of the major biotic constraints to rice production under small-scale farming systems of Africa, and developing durable disease resistance is therefore of critical importance. In this review, we provide an overview of the major advances by a multinational collaborative research effort to enhance sustainable rice production across SSA and how it is affected by advances in regional policy. As part of the multinational effort, we highlight the importance of joint international partnerships in tackling multiple crop production constraints through integrated research and outreach programs. More specifically, we highlight recent progress in establishing international networks for rice blast disease surveillance, farmer engagement, monitoring pathogen virulence spectra, and the establishment of regionally based blast resistance breeding programs. To develop blast-resistant, high yielding rice varieties for Africa, we have established a breeding pipeline that utilizes real-time data of pathogen diversity and virulence spectra, to identify major and minor blast resistance genes for introgression into locally adapted rice cultivars. In addition, the project has developed a package to support sustainable rice production through regular stakeholder engagement, training of agricultural extension officers, and establishment of plant clinics.

Improvement of Upland Rice Variety by Pyramiding Drought Tolerance QTL with Two Major Blast Resistance Genes for Sustainable Rice Production

Varalu is an early maturing rice variety widely grown in the rainfed ecosystem preferred for its grain type and cooking quality. However, the yield of Varalu is substantially low since it is being affected by reproductive drought stress along with the blast disease. The genetic improvement of Varalu was done by introgressing a major yield QTL, qDTY12.1, along with two major blast resistance genes i.e. Pi54 and Pi1 through marker-assisted backcross breeding. Both traits were transferred till BC2 generation and intercrossing was followed to pyramid the two traits. Stringent foreground selection was carried out using linked markers as well as peak markers (RM28099, RM28130, RM511 and RM28163) for the targeted QTL (qDTY12.1), RM206 for Pi54 and RM224 for Pi1. Extensive background selection was done using genome-wide SSR markers. Six best lines (MSM-36, MSM-49, MSM-53, MSM-57, MSM-60 and MSM-63) having qDTY12.1 and two blast resistance genes in homozygous condition with recurrent parent genome of 95.0%-96.5% having minimal linkage drag of about 0.1 to 0.7 Mb were identified. These lines showed yield advantage under drought stress as well as irrigated conditions. MSM-36 showed better performance in the national coordinated trials conducted across India, which indicated that improved lines of Varalu expected to replace Varalu and may have an important role in sustaining rice production. The present study demonstrated the successful marker-assisted pyramiding strategy for introgression of genes/QTLs conferring biotic stress resistance and yield under abiotic stress in rice.

SSR Marker based Molecular Screening of Blast Resistant Genes in selected Rice genotypes

Rice blast caused by Magnaporthe oryzae is one of the most devastating disease causing major yield losses worldwide in every year. Use of resistant rice varieties is one of the most effective ways to control the disease and reduce yield loss. Molecular screening and allelic diversity of major rice blast resistant genes were determined in forty-eight (48) genotypes of rice germplasms of Bangladesh with ten (10) previously synthesized gene based SSR (Single Sequence Repeats) markers [RM 541, RM 224, RM21, RM527, RM 208, RM 247,RM 72, RM 259, RM 246 & RM 206]. The genetic frequencies of ten (10) major blast resistance genes (Pi-9, Pi-1, Pi-5(t), Piz-5, Pi-b, Pi-b, Pi-ta, Pi-33, Pi27(t), Pitp(t) and Pi-kh) were ranged from 2% to 93%. The blast resistance gene Pi-kh was widely (93%) and Pi5 (2%) was sparsely distributed among the studied genotypes. Nine (9) genotypes sonaanjana, jabedshail, karaja, pajon, IR-09, BAU 39, binni, katiabagdad and kataribhug, had maximum eight (8) blast resistance genes and only one BRRIdhan 46 had minimum two (2) blast resistance genes. Twelve genotypes possessed seven, twelve had six, seven had five and five had four resistant genes. Out of forty-eight (48) genotypes, forty-five (45) genotypes occupied at least five (5) positive fragments of expected product size. Allelic diversity was also found among the rice genotypes for different blast resistance genes.

Epistasis and Quantitative Resistance to Pyricularia oryzae Revealed by GWAS in Advanced Rice Breeding Populations

Rice blast caused by Pyricularia oryzae is a major rice disease worldwide. Despite the detailed knowledge on major resistance genes available to date, little is known about how these genes interact with quantitative blast resistance loci and with the genetic background. Knowledge on these interactions is crucial for assessing the usefulness of introgressed resistance loci in breeding germplasm. Our goal was to identify quantitative trait loci (QTL) for blast resistance in rice breeding populations and to describe how they interact among each other and with the genetic background. To that end, resistance to blast was mapped by genome-wide association study (GWAS) in two advanced rice breeding subpopulations, one made of 305 indica type inbred lines, and the other of 245 tropical japonica inbred lines. The interactions and main effects of blast resistance loci were assessed in a multilocus model. Well known, major effect blast resistance gene clusters were detected in both tropical japonica (Pii/Pi3/Pi5) and indica (Piz/Pi2/Pi9) subpopulations with the GWAS scan 1. When these major effect loci were included as fixed cofactors in subsequent GWAS scans 2 and 3, additional QTL and more complex genetic architectures were revealed. The multilocus model for the tropical japonica subpopulation showed that Pii/Pi3/Pi5 had significant interaction with two QTL in chromosome 1 and one QTL in chromosome 8, together explaining 64% of the phenotypic variance. In the indica subpopulation a significant interaction among the QTL in chromosomes 6 and 4 and the genetic background, together with Piz/Pi2/Pi9 and QTL in chromosomes 1, 4 and 7, explained 35% of the phenotypic variance. Our results suggest that epistatic interactions can play a major role modulating the response mediated by major effect blast resistance loci such as Pii/Pi3/Pi5. Furthermore, the additive and epistatic effects of multiple QTL bring additional layers of quantitative resistance with a magnitude comparable to that of major effect loci. These findings highlight the need of genetic background-specific validation of markers for molecular assisted blast resistance breeding and provide insights for developing quantitative resistance to blast disease in rice.

Marker Assisted Gene Pyramiding (MAGP) for bacterial blight and blast resistance into mega rice variety "Tellahamsa".

Bacterial blight (BB) and fungal blast diseases are the major biotic constraints that limit rice productivity. To sustain yield improvement in rice, it is necessary to developed yield potential of the rice varieties by incorporation of biotic stress resistance genes. Tellahamsa is a well-adapted popular high yielding rice variety in Telangana state, India. However, the variety is highly susceptible to BB and blast. In this study, simultaneous stepwise transfer of genes through marker-assisted backcross breeding (MABB) strategy was used to introgress two major BB (Xa21 and xa13) and two major blast resistance genes (Pi54 and Pi1) into Tellahamsa. In each generation (from F1 to ICF3) foreground selection was done using gene-specific markers viz., pTA248 (Xa21), xa13prom (xa13), Pi54MAS (Pi54) and RM224 (Pi1). Two independent BC2F1 lines of Tellahamsa/ISM (Cross-I) and Tellahamsa/NLR145 (Cross-II) possessing 92% and 94% recurrent parent genome (RPG) respectively were intercrossed to develop ICF1—ICF3 generations. These gene pyramided lines were evaluated for key agro-morphological traits, quality, and resistance against blast at three different hotspot locations as well as BB at two locations. Two ICF3 gene pyramided lines viz., TH-625-159 and TH-625-491 possessing four genes exhibited a high level of resistance to BB and blast. In the future, these improved Tellahamsa lines could be developed as mega varieties for different agro-climatic zones and also as potential donors for different pre-breeding rice research.

Molecular profiling of blast resistance genes and evaluation of leaf and neck blast disease reaction in rice

In the present investigation, phenotypic evaluation of blast disease reaction was conducted at Ponnampet and Mandya districts of Karnataka, India, which indicated that the rice varieties such as IR64, Jaya, KMP153, IR30864, Mandya Sona-1, Mandya Sona-2, KCP-1, Dodda Byra, and Malgudi Sanna were susceptible to both leaf and neck blasts. Further, the rice varieties that were resistant to leaf blast such as KMP200, DHMAS70Q164-1b, Karibatta, Coimbatore Sanna and others showed susceptible reaction to neck blast only. In contrast, the varieties such as JyothixBR2655, Punkutt Kodi, Sirsi, 222 and Gangadale which were resistant to neck blast were found to be susceptible to leaf blast also. Only one variety, BR2655 showed resistance to both leaf and neck blast diseases. The genotypic studies using simple sequence repeat markers showed that the analysis of the distribution of resistance genes and genotyping of the selected rice varieties, and traditional rice varieties from different ecological regions with allele specific markers helped to identify 20 major blast resistance genes. The individual gene frequencies of the 20 major rice blast resistance genes varied from 10.34 to 100%. Less and more frequency of resistance gene distribution occurred in Pi9 and Pizt gene, respectively. The result of this study would help to create strategies for improving rice blast resistance through genetic studies and plant-pathogen interaction.

Identification of the blast resistance gene Picl(t) from Chaling common wild rice (Oryza rufipogon Griff.)

AbstractRice blast, caused by the fungal pathogen Magnaporthe oryzae (M. oryzae), is one of the most destructive and widespread plant diseases in the world. Utilization of resistance genes in rice breeding is considered to be an effective and economical method to control this disease. To identify new sources of blast resistance, a set of 1160 introgression lines (ILs) containing chromosome segments of Chaling common wild rice (Oryza rufipogon Griff.) in the genetic background of an elite indica rice variety 93‐11 were developed and phenotyped in the blast nursery. Thirty‐three ILs displaying stable blast resistance in three consecutive years were obtained. Among them, one line, IL1043, was subsequently found to be resistant to all of the 28 M. oryzae isolates from different regions through artificial inoculation in greenhouse. By combining bulk segregant analysis coupled with next‐generation sequencing (BSA‐seq) and recessive class analysis (RCA), a major blast resistance gene in IL1043, designated Picl(t), was mapped on rice chromosome 6 flanked by the markers RM527 and Indel6 with an interval of approximately 925 kb, which covers the Pi2/9 locus. These results will facilitate fine mapping and cloning of Picl(t), and the linked markers will further provide a useful tool for rice blast resistance breeding.

Analysis of the Relationship between Blast Resistance Genes and Disease Resistance of Rice Germplasm via Functional Molecular Markers

Rice blast disease is one of the most devastating diseases of rice (Oryza sativa L.) caused by the fungus Magnaporthe oryzae (M. oryzae), and neck blast is the most destructive phase of this illness. The underlying molecular mechanisms of rice blast resistance are not well known. Thus, we collected 150 rice varieties from different ecotypes in China and assessed the rice blast resistances under the natural conditions that favoured disease development in Jining, Shandong Province, China in 2017. Results showed that 92 (61.3%) and 58 (38.7%) rice varieties were resistant and susceptible to M. oryzae, respectively. Among the 150 rice varieties screened for the presence of 13 major blast resistance (R) genes against M. oryzae by using functional markers, 147 contained one to eight R genes. The relationship between R genes and disease response was discussed by analysing the phenotype and genotype of functional markers. The results showed that the rice blast resistance gene Pita was significantly correlated with rice blast resistance. Our results provided a basis for the further understanding of the distribution of 13 major R genes of rice blast in the germplasm resources of the tested rice varieties, and were meaningful for rice disease resistance breeding.

Marker-assisted pyramiding of major blast resistance genes in Swarna-Sub1, an elite rice variety (Oryza sativa L.)

Swarna-Sub1, a ruling submergence tolerant variety of rice, showed a significant decrease in the yield in major blast disease prone areas, which is caused by fungus Magnaporthe oryzae (Pyricularia oryzae). To overcome this problem, three major blast resistance genes viz., Pi1, Pi2, and Pi54 were pyramided through marker-assisted backcross breeding using donors, Swarna-LT (having Pi1 + Pi54) and Swarna-A51 (having Pi2). Foreground selection was carried out using molecular markers tightly linked to three blast resistance genes and also submergence tolerance for retaining the Sub1 gene in the recurrent parent. Based on disease resistance and agro-morphological performance of backcross-derived lines, we selected the best 25 plants of two- and three-gene pyramided BC3F2 homozygous lines with a maximum of ~ 93.5% recurrent parent genome and were further advanced to the BC3F5 generation. The stringent recurrent parent genome recovery analysis using SSR markers limited the linkage drag from a minimum of 0.2 Mb to a maximum of 2 Mb in all three-gene pyramided lines. The two and three blast resistance genes pyramided lines, i.e., SS30-24-82, SS30-24-73, and SS30-24-46, displayed a high level of blast resistance and submergence tolerance. The successful use of marker-assisted backcrossing strategy coupled with phenotypic selection helped in the development of Swarna-Sub1 lines having multiple blast resistance genes with superior agro-morphological and grain quality traits, which could serve as valuable genetic stocks in the rice breeding program.

Development of InDel marker for rice blast resistance gene Pi9

Pi9 is one of the major blast resistance genes which encodes a nucleotide-binding site-leucine-rich repeat (NBS-LRR) domain-containing protein. This gene was observed to show resistance against many pathotypes of the blast pathogen in Malaysia. Resistance allele Pi9 from rice variety 75-1-127 had previously been cloned using map-based cloning strategy. The gene sequence was used to design specific primers to amplify susceptible Pi9 allele from MR219 rice variety prior to cloning. The resistance and susceptible allele of Pi9 were 8.587kb and 8.785kb in length respectively. Allele mining was carried out by comparing between the susceptible and resistance allele of Pi9. One potential InDels polymorphism at position 590bp and 920bp was identified. Primer named as Pi9_InDel was designed targeting this region in such a way that the resistance and susceptible genotypes yielded 327 bp and 438 bp amplicon respectively.

Molecular identification of blast resistance genes in rice landraces from northeastern India

Rice blast disease caused by the fungus Magnaporthe oryzae is one of the most devastating diseases causing huge losses worldwide. In the present study, major blast resistance genes were investigated in landraces originating from northeastern India. Based on phenotypic evaluation, 288 landraces were classified into three distinct groups: resistant (75), moderately resistant (127) and susceptible (86). The genetic frequencies of the 18 major blast resistance genes were between 6.2% and 27.4%, with only two genotypes possessing a maximum of nine blast resistance genes. The cluster and population structure analysis grouped the landraces into two groups. Through principal coordinate analysis, the scatter plots partitioned the resistant and moderately resistant landraces into different groups. Analysis of molecular variance showed maximum (96%) diversity within populations and least (4%) diversity between populations. Association analysis identified six markers, CRG4_2, RM72, tk59‐2, pi21_79‐3, RM1233 and RM6648, that are significantly associated with blast disease and explained a phenotypic variance of 1.1–6.5%. The associated genes could be used in marker‐assisted rice breeding programmes for gene pyramiding to develop rice varietal resistance against blast disease. The present study represents a valuable blast resistance genetic resource that could be used for identification of new R genes, donors for blast resistance, and genomic studies.

Functional Interactions Between Major Rice Blast Resistance Genes, Pi-ta and Pi-b, and Minor Blast Resistance Quantitative Trait Loci.

Major blast resistance (R) genes confer resistance in a gene-for-gene manner. However, little information is available on interactions between R genes. In this study, interactions between two rice blast R genes, Pi-ta and Pi-b, and other minor blast resistance quantitative trait loci (QTLs) were investigated in a recombinant inbred line (RIL) population comprising 243 RILs from a Cybonnet (CYBT) × Saber (SB) cross. CYBT has the R gene Pi-ta and SB has Pi-b. Ten differential isolates of four Magnaporthe oryzae races (IB-1, IB-17, IB-49, and IE-1K) were used to evaluate disease reactions of the 243 RILs under greenhouse conditions. Five resistance QTLs were mapped on chromosomes 2, 3, 8, 9, and 12 with a linkage map of 179 single nucleotide polymorphism markers. Among them, qBR12 (Q1), was mapped at the Pi-ta locus and accounted for 45.41% of phenotypic variation while qBR2 (Q2) was located at the Pi-b locus and accounted for 24.81% of disease reactions. The additive-by-additive epistatic interaction between Q1 (Pi-ta) and Q2 (Pi-b) was detected; they can enhance the disease resistance by an additive 0.93 using the 0 to 9 standard phenotyping method. These results suggest that Pi-ta interacts synergistically with Pi-b.

Improvement of blast resistance of the popular high-yielding, medium slender-grain type, bacterial blight resistant rice variety, Improved Samba Mahsuri by marker-assisted breeding

Improved Samba Mahsuri (ISM) is a popular, high-yielding, bacterial blight resistant rice variety possessing medium-slender grain type. As ISM is highly susceptible to blast disease of rice, through the present study we have transferred two major blast resistance genes, Pi2 and Pi54 into the elite variety by marker-assisted backcross breeding. The two blast resistance genes were transferred to ISM through sets of backcrosses. In every backcross generation, PCR-based markers, specific for the blast resistance genes (Pi2 and Pi54) and bacterial blight resistance genes (Xa21, xa13 and xa5) were utilized for foreground selection, while a set of 144 parental polymorphic SSR markers were used for background selection and backcrossing was carried out until BC2 generation. A solitary BC2F1 plant possessing Pi2 or Pi54 along with Xa21, xa13 and xa5 and > 90% recovery of ISM genome was selected from the two sets of backcrosses were crossed and the intercross F1s (ICF1s) thus obtained were selfed to generate ICF2s. Homozygous ICF2 plants carrying all the five resistance genes were identified through markers and advanced through selfing till ICF5 generation by adopting pedigree method of selection. Three best lines at ICF5, possessing excellent resistance against bacterial blight and blast and closely resembling or superior to ISM in terms of grain quality: yield and agro-morphological traits have been identified and advanced for multi-location trials.