Identification of a new resistance gene to a chinese blast fungus isolate in the Japanese rice cultivar aichi asahi

Genetic Variation of Blast (Pyricularia oryzae Cavara) Resistance in the Longistaminata Chromosome Segment Introgression Lines (LCSILs) and Potential for Breeding Use in Kenya

In Kenya's rice-growing areas, Basmati varieties have been produced in monoculture since the late 1980s. This has resulted in the breakdown of the resistance (R) gene-mediated response of the local Basmati varieties to blast disease caused by Pyricularia oryzae. To improve blast resistance in Kenyan Basmati varieties, continuous identification of R genes and suitable breeding materials for Basmati are necessary. Longistaminata chromosome segment introgression lines (LCSILs) with the Kernel Basmati genetic background, developed using a rice line called potential low-input adaptable-1 (pLIA-1) derived from a cross between Taichung 65 (T65) (a rice variety in the Japonica Group) and O. longistaminata, are expected to contain useful blast R genes derived from O. longistaminata or T65. In this study, we investigated the genetic variation of blast R genes in LCSILs and their parents by using a new international differential system for designating blast races based on the gene-for-gene theory and molecular characterization using single nucleotide polymorphism (SNP) markers. LCSILs and their parents were classified into three groups-A, B1, and B2-based on reaction patterns to the standard differential blast isolates (SDBIs). Group A, including pLIA-1, showed the highest resistance in all groups, followed by groups B1 and B2. Kernel Basmati in group B1 was considered to possess Pik-p or Pi7(t), Pi19(t), and other unknown R genes. In addition to these R genes, LCSIL 6, 12, 27, 28, and 40, in group A, were determined to possess one of Pish, Piz-t, or both genes that confer resistance to the Kenyan blast races. These lines can be used for efficiently pyramiding blast R genes in the local Basmati varieties.

Advancement in the Breeding, Biotechnological and Genomic Tools towards Development of Durable Genetic Resistance against the Rice Blast Disease

Rice production needs to be sustained in the coming decades, as the changeable climatic conditions are becoming more conducive to disease outbreaks. The majority of rice diseases cause enormous economic damage and yield instability. Among them, rice blast caused by Magnaportheoryzae is a serious fungal disease and is considered one of the major threats to world rice production. This pathogen can infect the above-ground tissues of rice plants at any growth stage and causes complete crop failure under favorable conditions. Therefore, management of blast disease is essentially required to sustain global food production. When looking at the drawback of chemical management strategy, the development of durable, resistant varieties is one of the most sustainable, economic, and environment-friendly approaches to counter the outbreaks of rice blasts. Interestingly, several blast-resistant rice cultivars have been developed with the help of breeding and biotechnological methods. In addition, 146 R genes have been identified, and 37 among them have been molecularly characterized to date. Further, more than 500 loci have been identified for blast resistance which enhances the resources for developing blast resistance through marker-assisted selection (MAS), marker-assisted backcross breeding (MABB), and genome editing tools. Apart from these, a better understanding of rice blast pathogens, the infection process of the pathogen, and the genetics of the immune response of the host plant are very important for the effective management of the blast disease. Further, high throughput phenotyping and disease screening protocols have played significant roles in easy comprehension of the mechanism of disease spread. The present review critically emphasizes the pathogenesis, pathogenomics, screening techniques, traditional and molecular breeding approaches, and transgenic and genome editing tools to develop a broad spectrum and durable resistance against blast disease in rice. The updated and comprehensive information presented in this review would be definitely helpful for the researchers, breeders, and students in the planning and execution of a resistance breeding program in rice against this pathogen.

Selection of Candidate Genes Conferring Blast Resistance and Heat Tolerance in Rice through Integration of Meta-QTLs and RNA-Seq

Due to global warming, high temperature is a significant environmental stress for rice production. Rice (Oryza sativa L.), one of the most crucial cereal crops, is also seriously devastated by Magnaporthe oryzae. Therefore, it is essential to breed new rice cultivars with blast and heat tolerance. Although progress had been made in QTL mapping and RNA-seq analysis in rice in response to blast and heat stresses, there are few reports on simultaneously mining blast-resistant and heat-tolerant genes. In this study, we separately conducted meta-analysis of 839 blast-resistant and 308 heat-tolerant QTLs in rice. Consequently, 7054 genes were identified in 67 blast-resistant meta-QTLs with an average interval of 1.00 Mb. Likewise, 6425 genes were obtained in 40 heat-tolerant meta-QTLs with an average interval of 1.49 Mb. Additionally, using differentially expressed genes (DEGs) in the previous research and GO enrichment analysis, 55 DEGs were co-located on the common regions of 16 blast-resistant and 14 heat-tolerant meta-QTLs. Among, OsChib3H-c, OsJAMyb, Pi-k, OsWAK1, OsMT2b, OsTPS3, OsHI-LOX, OsACLA-2 and OsGS2 were the significant candidate genes to be further investigated. These results could provide the gene resources for rice breeding with excellent resistance to these 2 stresses, and help to understand how plants response to the combination stresses of blast fungus and high temperature.

Pathogenicity of Isolates of the Rice Blast Pathogen (Pyricularia oryzae) From Indonesia

A total of 201 isolates of Pyricularia oryzae (the causal agent of rice blast) were collected from three rice ecosystems (upland, lowland, and swampy) in five regions of Indonesia (West Java, Lampung, South Sumatra, Kalimantan, and Bali). Their pathogenicities were characterized based on the patterns of reaction of 25 differential varieties (DVs) and the susceptible control Lijiangxintuanheigu (LTH), which was susceptible to all blast isolates. A high proportion of isolates (>80.0%) were virulent to DVs for resistance genes Pib, Pit, Pia, Pik-s, and Pi12(t), and a low proportion of isolates (<12.9%) were virulent to DVs for Pik-m, Pi1, Pik-h, Pik, Pik-p, and Pi7(t). Virulence to the other DVs for Pish, Pii, Pi3, Pi5(t), Pi9(t), Piz, Piz-5, Piz-t, Pita-2 (two lines), Pita (two lines), Pi19(t), and Pi20(t) showed intermediate frequencies from 20.0 to 80.0%. These isolates were classified into three cluster groups, Ia, Ib, and II, and the frequencies of cluster groups varied between the three ecosystems and the five regions. The frequencies of cluster groups varied between ecosystems and regions, and races varied according to the ecosystems. A total of 27 standard differential blast isolates (SDBIs) were selected from the 201 isolates collected. The set of 25 DVs and these 27 SDBIs will be used as a new differential system for analysis of the pathogenicity of blast isolates and analysis of resistance genes in rice cultivars, which will contribute to building a durable protection system against blast disease in Indonesia.

Identification of Blast Resistance QTLs Based on Two Advanced Backcross Populations in Rice

BACKGROUND

Rice blast is an economically important and mutable disease of rice. Using host resistance gene to breed resistant varieties has been proven to be the most effective and economical method to control rice blast and new resistance genes or quantitative trait loci (QTLs) are then needed.

RESULTS

In this study, we constructed two advanced backcross population to mapping blast resistance QTLs. CR071 and QingGuAi3 were as the donor parent to establish two BC₃F₁ and derived BC₃F₂ backcross population in the Jin23B background. By challenging the two populations with natural infection in 2011 and 2012, 16 and 13 blast resistance QTLs were identified in Jin23B/CR071 and Jin23B/QingGuAi3 population, respectively. Among Jin23B/CR071 population, 3 major and 13 minor QTLs have explained the phenotypic variation from 3.50% to 34.08% in 2 years. And, among Jin23B/QingGuAi3 population, 2 major and 11 minor QTLs have explained the phenotypic variation from 2.42% to 28.95% in 2 years.

CONCLUSIONS

Sixteen and thirteen blast resistance QTLs were identified in Jin23B/CR071 and Jin23B/QingGuAi3 population, respectively. QTL effect analyses suggested that major and minor QTLs interaction is the genetic basis for durable blast resistance in rice variety CR071 and QingGuAi3.

Pathogenicities of Rice Blast (Pyricularia oryzae Cavara) Isolates From Kenya

A total of 99 isolates of rice blast (Pyricularia oryzae Cavara) were collected from 2010 to 2015 from four regions in Kenya: Kirinyaga County and Embu County, Kisumu County, Tana River County, and Mombasa County. The pathogenicities of these isolates were clarified based on the reaction patterns of Lijiangxintuanheigu and differential varieties (DVs) targeting 23 resistance genes. The frequency of virulent isolates was high for DVs for Pib, Pia, Pii, Pi3, Pi5(t), Pik-s, Pik-m, Pi1, Pik-h, Pik, Pik-p, Pi7(t), Pi19(t), and Pi20(t); low for DVs for Pish, Pi9(t), Piz-5, and Piz-t; and intermediate for the remaining DVs for Pit, Piz, Pita-2, Pita, and Pi12(t). These blast isolates were classified into three cluster groups: Ia, Ib, and II. The frequencies of virulent isolates to DVs for Pit, Pii, Pik-m, Pi1, Pik-h, Pik, Pik-p, Pi7(t), Piz, and Pi12(t) differed markedly between clusters I and II, and those of DVs for Pib, Pit, Pia, Pi3, Pita-2, Pita, and Pi20(t) differed between Ia and Ib. The frequencies of cluster groups in the four geographical regions were different. A total of 62 races were found, with 19 blast isolates categorized into one race (U63-i7-k177-z00-ta003), whereas the other races included only some isolates in each.

The Race Structure of the Rice Blast Pathogen Across Southern and Northeastern China

BACKGROUND

Rice blast, caused by the ascomycete Magnaporthe oryzae (Mo), imposes a major constraint on rice productivity. Managing the disease through the deployment of host resistance requires a close understanding of race structure of the pathogen population.

RESULTS

The host/pathogen interaction between isolates sampled from four Mo populations collected across the rice-producing regions of China was tested using two established panels of differential cultivars. The clearest picture was obtained from the Chinese cultivar panel, for which the frequency of the various races, the race diversity index, the specific race isolate frequency, and the frequency of the three predominant races gave a consistent result, from which it was concluded that the pathogen population present in the southern production region was more diverse than that in the northeastern region. The four blast resistance genes Pi1, Pik, Pik-m, and Piz all still remain effective in the southern China rice production area, as does Pi1 in the northeastern region. The effectiveness of Pita, Pik-p, Piz, and Pib is restricted to single provinces. The distinctive resistance profile shown by the Chinese differential cultivar set implied the presence of at least five as yet unidentified blast resistance genes.

CONCLUSIONS

The Chinese differential cultivar set proved to be more informative than the Japanese one for characterizing the race structure of the rice blast pathogen in China. A number of well characterized host resistance genes, in addition to some as yet uncharacterized ones, remain effective across the major rice production regions in China.

Quantitative trait loci analysis of blast resistance in Oryza sativa L. 'Hokuriku 193'

To investigate the genetic background responsible for blast resistance in Oryza sativa L. 'Hokuriku 193', QTL analysis was conducted using the F₃ lines from the cross [ms-bo] Nekken 2 × Hokuriku 193 that were artificially infected with rice blast fungus (Magnaporthe grisea). QTLs were detected on chromosomes 1, 4, 6 and 12 that correlated with greater blast resistance in the Hokuriku 193-type lines. Notably, the QTL on chromosome 12 had a major effect and localized to the same region where Pi20(t), a broad-spectrum blast resistance gene, is positioned, suggesting strongly that the blast resistance of Hokuriku 193 was controlled by Pi20(t). Also, QTL analysis of the lines found to have no Pi20(t) detected two QTLs on chromosome 4 (qBR4-1 and qBR4-2) and one QTL on chromosome 6 (qBR6), of which qBR4-2 and qBR6 correlated with higher percentages of resistant plants in the Hokuriku 193-type lines. The blast susceptibility of BR_NIL (a NIL of Hokuriku 193 from which Pi20(t) was eliminated) was greater than that of Hokuriku 193, suggesting that elimination of Pi20(t) may markedly increase blast susceptibility. The disease severity of BR_NIL was mild, which might be the effect of qBR4-2 and/or qBR6.

Diversity and Distribution of Rice Blast (Pyricularia oryzae Cavara) Races in Bangladesh

The pathogenicity of 331 blast isolates (Pyricularia oryzae Cavara) collected from different regions and ecosystems for rice cultivation in Bangladesh was evaluated by compatibility on 23 differential varieties (DV), each harboring a single blast resistance gene, and susceptible 'Lijiangxintuanheigu' (LTH). A wide variation in virulence was found among the isolates, and 267 races were classified using a new designation system. Virulence of blast isolates against DV carrying the resistance genes Pia, Pib, Pit, Pik-s, Piz-t, Pi12(t), Pi19(t), and Pi20(t), as well as avirulence against those carrying Pish, Pi9, Pita-2, and Pita, was distributed widely in Bangladesh. Cluster analysis of the compatibility data on the DV initially classified the isolates into groups I and II. The virulence spectra of the two groups differed mainly according to the reactions of the DV to Pii, Pi3, Pi5(t), Pik-m, Pi1, Pik-h, Pik, Pik-p, and Pi7(t). Group I isolates were distributed mainly in rainfed lowlands, whereas group II isolates were found mainly in irrigated lowlands; however, there were no critical differences in geographic distribution of the blast isolates. In total, 26 isolates, which could be used to identify the 23 resistance genes of the DV on the basis of their reaction patterns, were selected as a set of standard differential blast isolates. To our knowledge, this is the first clear demonstration of the diversity and differentiation of blast races in Bangladesh. This information will be used to develop a durable blast protection system in that country.

Diversity and Distribution of Rice Blast (Pyricularia oryzae Cavara) Races in Japan

In total, 310 rice blast (Pyricularia oryzae Cavara) isolates from Japan showed wide variation in virulence. Virulence on rice (Oryza sativa L.) differential varieties (DV) harboring resistance genes Pish, Pia, Pii, Pi3, Pi5(t), Pik-s, and Pi19(t) ranged from 82.9 to 100.0%. In contrast, virulence on DV possessing Pib, Pit, Pik-m, Pi1, Pik-h, Pik, Pik-p, Pi7(t), Pi9(t), Piz, Piz-5, Piz-t, Pita-2, Pita, Pi12(t), and Pi20(t) ranged from 0 to 21.6%. Cluster analysis using the reaction patterns of the DV classified isolates into three groups: I, virulent to Pik, Pik-h, Pik-p, Pik-m, Pi1, and Pi7(t); IIa, avirulent to the preceding 6 genes and virulent to Pia, Pii, Pi3, and Pi5(t); and IIb, avirulent to all 10 genes. Group I was limited to northern Japan and group IIb to central Japan, while group IIa was distributed throughout Japan. We estimate that group IIa represents the original population and that groups I and IIb arose from it through minor changes in pathogenicity. We classified these isolates into 123 races by a new designation system and conclude that the rice blast races in Japan are less diverse than previously thought.

Molecular progress on the mapping and cloning of functional genes for blast disease in rice (Oryza sativa L.): current status and future considerations

Rice blast disease, which is caused by the fungal pathogen Magnaporthe oryzae, is a recurring problem in all rice-growing regions of the world. The use of resistance (R) genes in rice improvement breeding programmes has been considered to be one of the best options for crop protection and blast management. Alternatively, quantitative resistance conferred by quantitative trait loci (QTLs) is also a valuable resource for the improvement of rice disease resistance. In the past, intensive efforts have been made to identify major R-genes as well as QTLs for blast disease using molecular techniques. A review of bibliographic references shows over 100 blast resistance genes and a larger number of QTLs (∼500) that were mapped to the rice genome. Of the blast resistance genes, identified in different genotypes of rice, ∼22 have been cloned and characterized at the molecular level. In this review, we have summarized the reported rice blast resistance genes and QTLs for utilization in future molecular breeding programmes to introgress high-degree resistance or to pyramid R-genes in commercial cultivars that are susceptible to M. oryzae. The goal of this review is to provide an overview of the significant studies in order to update our understanding of the molecular progress on rice and M. oryzae. This information will assist rice breeders to improve the resistance to rice blast using marker-assisted selection which continues to be a priority for rice-breeding programmes.

Close linkage of a blast resistance gene, Pias(t), with a bacterial leaf blight resistance gene, Xa1-as(t), in a rice cultivar 'Asominori'

It has long been known that a bacterial leaf blight-resistant line in rice obtained from a crossing using 'Asominori' as a resistant parent also has resistance to blast, but a blast resistance gene in 'Asominori' has not been investigated in detail. In the present study, a blast resistance gene in 'Asominori', tentatively named Pias(t), was revealed to be located within 162-kb region between DNA markers YX4-3 and NX4-1 on chromosome 4 and to be linked with an 'Asominori' allele of the bacterial leaf blight resistance gene Xa1, tentatively named Xa1-as(t). An 'Asominori' allele of Pias(t) was found to be dominant and difference of disease severity between lines having the 'Asominori' allele of Pias(t) and those without it was 1.2 in disease index from 0 to 10. Pias(t) was also closely linked with the Ph gene controlling phenol reaction, suggesting the possibility of successful selection of blast resistance using the phenol reaction. Since blast-resistant commercial cultivars have been developed using 'Asominori' as a parent, Pias(t) is considered to be a useful gene in rice breeding for blast resistance.

A rice calcium-dependent protein kinase OsCPK12 oppositely modulates salt-stress tolerance and blast disease resistance

Calcium-dependent protein kinases (CDPKs) regulate the downstream components in calcium signaling pathways. We investigated the effects of overexpression and disruption of an Oryza sativa (rice) CDPK (OsCPK12) on the plant's response to abiotic and biotic stresses. OsCPK12-overexpressing (OsCPK12-OX) plants exhibited increased tolerance to salt stress. The accumulation of hydrogen peroxide (H(2) O(2) ) in the leaves was less in OsCPK12-OX plants than in wild-type (WT) plants. Genes encoding reactive oxygen species (ROS) scavenging enzymes (OsAPx2 and OsAPx8) were more highly expressed in OsCPK12-OX plants than in WT plants, whereas the expression of the NADPH oxidase gene, OsrbohI, was decreased in OsCPK12-OX plants compared with WT plants. Conversely, a retrotransposon (Tos17) insertion mutant, oscpk12, and plants transformed with an OsCPK12 RNA interference (RNAi) construct were more sensitive to high salinity than were WT plants. The level of H(2) O(2) accumulation was greater in oscpk12 and OsCPK12 RNAi plants than in the WT. These results suggest that OsCPK12 promotes tolerance to salt stress by reducing the accumulation of ROS. We also observed that OsCPK12-OX seedlings had increased sensitivity to abscisic acid (ABA) and increased susceptibility to blast fungus, probably resulting from the repression of ROS production and/or the involvement of OsCPK12 in the ABA signaling pathway. Collectively, our results suggest that OsCPK12 functions in multiple signaling pathways, positively regulating salt tolerance and negatively modulating blast resistance.

Abscisic acid interacts antagonistically with salicylic acid signaling pathway in rice-Magnaporthe grisea interaction

Plant hormones play pivotal signaling roles in plant-pathogen interactions. Here, we report characterization of an antagonistic interaction of abscisic acid (ABA) with salicylic acid (SA) signaling pathways in the rice-Magnaporthe grisea interaction. Exogenous application of ABA drastically compromised the rice resistance to both compatible and incompatible M. grisea strains, indicating that ABA negatively regulates both basal and resistance gene-mediated blast resistance. ABA markedly suppressed the transcriptional upregulation of WRKY45 and OsNPR1, the two key components of the SA signaling pathway in rice, induced by SA or benzothiadiazole or by blast infection. Overexpression of OsNPR1 or WRKY45 largely negated the enhancement of blast susceptibility by ABA, suggesting that ABA acts upstream of WRKY45 and OsNPR1 in the rice SA pathway. ABA-responsive genes were induced during blast infection in a pattern reciprocal to those of WRKY45 and OsPR1b in the compatible rice-blast interaction but only marginally in the incompatible one. These results suggest that the balance of SA and ABA signaling is an important determinant for the outcome of the rice-M. grisea interaction. ABA was detected in hyphae and conidia of M. grisea as well as in culture media, implying that blast-fungus-derived ABA could play a role in triggering ABA signaling at host infection sites.

Identification and mapping of Pi41, a major gene conferring resistance to rice blast in the Oryza sativa subsp. indica reference cultivar, 93-11

The Oryza sativa subsp. indica reference cultivar (cv.), 93-11 is completely resistant to many Chinese isolates of the rice blast fungus. Resistance segregated in a 3:1 (resistance/susceptible) ratio in an F(2) population from the cross between 93-11 and the japonica reference cv. Nipponbare, when challenged with two independent blast isolates. The chromosomal location of this monogenic resistance was mapped to a region of the long arm of chromosome 12 by bulk segregant analysis, using 180 evenly distributed SSR markers. Five additional SSR loci and nine newly developed PCR-based markers allowed the target region to be reduced to ca. 1.8 cM, equivalent in Nipponbare to about 800 kb. In the reference sequence of Nipponbare, this region includes an NBS-LRR cluster of four genes. The known blast resistance gene Pi-GD-3 also maps in this region, but the 93-11 resistance was distinguishable from Pi-GD-3 on the basis of race specificity. We have therefore named the 93-11 resistance Pi41. Seven markers completely linked to Pi41 will facilitate both marker-assisted breeding and gene isolation cloning.

Rice Pti1a negatively regulates RAR1-dependent defense responses

Tomato (Solanum lycopersicum) Pto encodes a protein kinase that confers resistance to bacterial speck disease. A second protein kinase, Pti1, physically interacts with Pto and is involved in Pto-mediated defense signaling. Pti1-related sequences are highly conserved among diverse plant species, including rice (Oryza sativa), but their functions are largely unknown. Here, we report the identification of a null mutant for the Pti1 homolog in rice and the functional characterization of Os Pti1a. The rice pti1a mutant was characterized by spontaneous necrotic lesions on leaves, which was accompanied by a series of defense responses and resistance against a compatible race of Magnaporthe grisea. Overexpression of Pti1a in rice reduced resistance against an incompatible race of the fungus recognized by a resistance (R) protein, Pish. Plants overexpressing Pti1a were also more susceptible to a compatible race of the bacterial pathogen Xanthomonas oryzae pv oryzae. These results suggest that Os Pti1a negatively regulates defense signaling for both R gene-mediated and basal resistance. We also demonstrated that repression of the rice RAR1 gene suppressed defense responses induced in the pti1a mutant, indicating that Pti1a negatively regulates RAR1-dependent defense responses. Expression of a tomato Pti1 cDNA in the rice pti1a mutant suppressed the mutant phenotypes. This contrasts strikingly with the previous finding that Sl Pti1 enhances Pto-mediated hypersensitive response (HR) induction when expressed in tobacco (Nicotiana tabacum), suggesting that the molecular switch controlling HR downstream of pathogen recognition has evolved differently in rice and tomato.

Identification and fine mapping of Pi39(t), a major gene conferring the broad-spectrum resistance to Magnaporthe oryzae

Blast, caused by the ascomycete fungus Magnaporthe oryzae, is one of the most devastating diseases of rice worldwide. The Chinese native cultivar (cv.) Q15 expresses the broad-spectrum resistance to most of the isolates collected from China. To effectively utilize the resistance, three rounds of linkage analysis were performed in an F(2) population derived from a cross of Q15 and a susceptible cv. Tsuyuake, which segregated into 3:1 (resistant/susceptible) ratio. The first round of linkage analysis employing simple sequence repeat (SSR) markers was carried out in the F(2) population through bulked-segregant assay. A total of 180 SSR markers selected from each chromosome equally were surveyed. The results revealed that only two polymorphic markers, RM247 and RM463, located on chromosome 12, were linked to the resistance (R) gene. To further define the chromosomal location of the R gene locus, the second round of linkage analysis was performed using additional five SSR markers, which located in the region anchored by markers RM247 and RM463. The locus was further mapped to a 0.27 cM region bounded by markers RM27933 and RM27940 in the pericentromeric region towards the short arm. For fine mapping of the R locus, seven new markers were developed in the smaller region for the third round of linkage analysis, based on the reference sequences. The R locus was further mapped to a 0.18 cM region flanked by marker clusters 39M11 and 39M22, which is closest to, but away from the Pita/Pita(2) locus by 0.09 cM. To physically map the locus, all the linked markers were landed on the respective bacterial artificial chromosome clones of the reference cv. Nipponbare. Sequence information of these clones was used to construct a physical map of the locus, in silico, by bioinformatics analysis. The locus was physically defined to an interval of approximately 37 kb. To further characterize the R gene, five R genes mapped near the locus, as well as 10 main R genes those might be exploited in the resistance breeding programs, were selected for differential tests with 475 Chinese isolates. The R gene carrier Q15 conveys resistances distinct from those conditioned by the carriers of the 15 R genes. Together, this valuable R gene was, therefore, designated as Pi39(t). The sequence information of the R gene locus could be used for further marker-based selection and cloning.

Molecular mapping of two cultivar-specific avirulence genes in the rice blast fungus Magnaporthe grisea

Rice blast, caused by the fungus Magnaporthe grisea, is a globally important disease of rice that causes annual yield losses. The segregation of genes controlling the virulence of M. grisea on rice was studied to establish the genetic basis of cultivar specificity in the interaction of rice and M. grisea. The segregation of avirulence and virulence was studied in 87 M. grisea F(1) progeny isolates from a cross of two isolates, Guy11 and JS153, using resistance-gene-differential rice cultivars. The segregation ratio indicated that avirulence and virulence in the rice cultivars Aichi-asahi and K59, respectively, are controlled by single major genes. Genetic analyses of backcrosses and full-sib crosses in these populations were also performed. The chi(2 )test of goodness-of-fitness for a 1:1 ratio indicated that one dominant gene controls avirulence in Aichi-asahi and K59 in this population. Based on the resistance reactions of rice differential lines harboring known resistance genes to the parental isolates, two genetically independent avirulence genes, AVR-Pit and AVR-Pia, were identified. Genetic linkage analysis showed that the SSR marker m355-356 is closely linked to AVR-Pit, on the telomere of chromosome 1 at a distance of approximately 2.3 cM. The RAPD marker S487, which was converted to a sequence-characterized amplified region (SCAR) marker, was found to be closely linked to AVR-Pia, on the chromosome 7 telomere at a distance of 3.5 cM. These molecular markers will facilitate the positional cloning of the two AVR genes, and can be applied to molecular-marker-assisted studies of M. grisea populations.

Identification of an Avirulence Gene in the Fungus Magnaporthe grisea Corresponding to a Resistance Gene at the Pik Locus

ABSTRACT A rice isolate of Magnaporthe grisea collected from China was avirulent on rice cvs. Hattan 3 and 13 other Japanese rice cultivars. The rice cv. Hattan 3 is susceptible to almost all Japanese blast fungus isolates from rice. The genetic basis of avirulence in the Chinese isolate on Japanese rice cultivars was studied using a cross between the Chinese isolate and a laboratory isolate. The segregation of avirulence or virulence was studied in 185 progeny from the cross, and monogenic control was demonstrated for avirulence to the 14 rice cultivars. The resistance gene that corresponds to the avirulence gene (Avr-Hattan 3) is thought to be located at the Pik locus. Resistance and susceptibility in response to the Chinese isolate in F(3) lines of a cross of resistant and susceptible rice cultivars were very similar to the Pik tester isolate, Ken54-20. Random amplified polymorphic DNA markers and restriction fragment length polymorphism markers from genetic maps of the fungus were used to construct a partial genetic map of Avr-Hattan 3. We obtained several flanking markers and one co-segregated marker of Avr-Hattan 3 in the 144 mapping population.

Identification of Magnaporthe oryzae Avirulence Genes to Three Rice Blast Resistance Genes

The segregation of avirulence/virulence was studied in 115 F₁ progeny isolates of Magnaporthe oryzae from a cross of two field isolates on three Japanese race-differential rice cultivars Kanto 51, Fukunishiki, and Toride 1. The χ² tests of goodness-of-fit for a 1:1 ratio indicated that avirulence on cvs. Kanto 51, Fukunishiki, and Toride 1 was under monogenic control. The relationship between the avirulence (Avr) gene in the parental isolate and the Avr gene in the standard isolate was investigated by using 100 lines each of three F₃ families from the crosses of the rice cultivars Norin 3/Kanto 51, AK61/Fukunishiki, and Norin 3/Toride 1, respectively. Based on the resistant reactions of the F₃ rice lines to the parental isolates and the standard isolates harboring three known Avr genes, three genetically independent Avr genes, AvrPik, AvrPiz, and AvrPiz-t, were identified. The three identified Avr genes were mapped using random amplified polymorphic DNA (RAPD) analysis, and a partial linkage map was constructed with 17 RAPD markers closely linked to the Avr genes. Twelve markers and AvrPik, three markers and AvrPiz, and two markers and AvrPiz-t, as well as mating locus MAT1, constructed linkage groups A, B, and C, respectively.