Identification of a specific molecular marker for the rice blast-resistant gene <Emphasis Type="Italic">Pigm</Emphasis> and molecular breeding of thermo-sensitive genic male sterile leaf-color marker lines

Rice blast is a damaging disease caused by Magnaportheoryzae. Marker-assisted selection of blast resistance genes could help develop cultivars with blast resistance. Pigm is a broad-spectrum blast-resistant gene. However, few rice resources contain Pigm. In this study, the Pigm gene donor Gumei4 (GM4) was investigated. By analyzing different regions of Pigm sequences, we found that marker G8900 was a specific molecular marker of Pigm gene in GM4. Correlation analysis between molecular marker detection and identification of rice blast disease nursery revealed that G8900 could be used in marker-assisted selection (MAS) of Pigm. Furthermore, we introduced Pigm gene into the KT27S line (a blast-susceptible yellow-green-leaf-color mutant) in G8900-assisted breeding and identified three new yellow-green-leaf-color marker lines that are resistant to blast. The agronomic and economic traits of the three new lines are similar to those of their parental lines. The identification and application of Pigm-specific molecular marker in breeding of yellow-green-leaf-color marker line could play an important role in the production of disease-resistant hybrid rice.     

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.     

Molecular mapping of genes for resistance to rice blast (Pyricularia grisea Sacc.)

Two dominant genes conferring complete resistance to specific isolates of the rice blast fungus, Pyricularia grisea Sacc., were located on the molecular map of rice in this study. Pi-l(t) is a blast resistance gene derived from the cultivar LAC23. Its map location was determined using a pair of nearly isogenic lines (NILs) and a B₆F₃ segregating population from which the isoline was derived. RFLP analysis showed that Pi-l(t) is located near the end of chromosome 11, linked to RZ536 at a distance of 14.0±4.5 centiMorgans (cM). A second gene, derived from the cultivar Apura, was mapped using a rice doubled-haploid (DH) population. This gene was located on chromosome 12, flanked by RG457 and RG869, at a distance of 13.5+-4.3 cM and 17.7+-4.5 cM, respectively. The newly mapped gene on chromosome 12 may be allelic or closely linked toPi-ta. (=Pi-4(t)), a gene derived from Tetep that was previously reported to be linked to RG869 at a distance of 15.4±4.7 cM. The usefulness of markers linked to blast resistance genes will be discussed in the context of breeding for durable blast resistance.     

SSRs for Marker-Assisted Selection for Blast Resistance in Rice (Oryza sativa L.)

Rice blast caused by the fungus Magnaporthe oryzae is one of the most devastating diseases of rice in nearly all rice growing areas of the world including Malaysia. To develop cultivars with resistance against different races of M. oryzae, availability of molecular markers along with marker-assisted selection strategies are essential. In this study, 11 polymorphic simple sequence repeat (SSR) markers with good fit of 1:2:1 ratio for single gene model in F₂ population derived from the cross of Pongsu seribu 2 (Resistant) and Mahsuri (Susceptible) rice cultivars were analysed in 296 F₃ families derived from individual F₂ plants to investigate association with Pi gene conferring resistance to M. oryzae pathotype. Parents and progeny were grouped into two phenotypic classes based on their blast reactions. Chi-square test for the segregation of resistance and susceptibility in F₃ generation fitted a ratio of approximately 3:1. Association of SSR markers with phenotypic trait in F₃ families was identified by statistical analysis. Four SSR markers (RM413, RM5961, RM1233 and RM8225) were significantly associated with blast resistance to pathotype 7.2 of M. oryzae in rice (p ≤ 0.01). These four markers accounted for about 20% of total phenotypic variation. So, these markers were confirmed as suitable markers for use in marker-assisted selection and confirmation of blast resistance genes to develop rice cultivars with durable blast resistance in Malaysian rice breeding programmes.     

QTLs for Tolerance of Drought and Breeding for Tolerance of Abiotic and Biotic Stress: An Integrated Approach

Background

The coupling of biotic and abiotic stresses leads to high yield losses in rainfed rice (Oryza sativa L.) growing areas. While several studies target these stresses independently, breeding strategies to combat multiple stresses seldom exist. This study reports an integrated strategy that combines QTL mapping and phenotypic selection to develop rice lines with high grain yield (GY) under drought stress and non-stress conditions, and tolerance of rice blast.

Methodology

A blast-tolerant BC₂F₃-derived population was developed from the cross of tropical japonica cultivar Moroberekan (blast- and drought-tolerant) and high-yielding indica variety Swarna (blast- and drought-susceptible) through phenotypic selection for blast tolerance at the BC₂F₂ generation. The population was studied for segregation distortion patterns and QTLs for GY under drought were identified along with study of epistatic interactions for the trait.

Results

Segregation distortion, in favour of Moroberekan, was observed at 50 of the 59 loci. Majority of these marker loci co-localized with known QTLs for blast tolerance or NBS-LRR disease resistance genes. Despite the presence of segregation distortion, high variation for DTF, PH and GY was observed and several QTLs were identified under drought stress and non-stress conditions for the three traits. Epistatic interactions were also detected for GY which explained a large proportion of phenotypic variance observed in the population.

Conclusions

This strategy allowed us to identify QTLs for GY along with rapid development of high-yielding purelines tolerant to blast and drought with considerably reduced efforts. Apart from this, it also allowed us to study the effects of the selection cycle for blast tolerance. The developed lines were screened at IRRI and in the target environment, and drought and blast tolerant lines with high yield were identified. With tolerance to two major stresses and high yield potential, these lines may provide yield stability in rainfed rice areas.     

Transfer of bacterial blight and blast resistance from the tetraploid wild rice Oryza minuta to cultivated rice,Oryza sativa

Summary Oryza minuta J. S. Presl ex C. B. Presl is a tetraploid wild rice with resistance to several insects and diseases, including blast (caused by Pyricularia grisea) and bacterial blight (caused by Xanthomonas oryzae pv. oryzae). To transfer resistance from the wild species into the genome of cultivated rice (Oryza sativa L.), backcross progeny (BC₁, BC₂, and BC₃) were produced from interspecific hybrids of O. sativa cv IR31917-45-3-2 (2n=24, AA genome) and O. minuta Acc. 101141 (2n=48, BBCC genomes) by backcrossing to the O. sativa parent followed by embryo rescue. The chromosome numbers ranged from 44 to 47 in the BC₁ progeny and from 24 to 37 in the BC₂ progeny. All F₁ hybrids were resistant to both blast and bacterial blight. One BC₁ plant was moderately susceptible to blast while the rest were resistant. Thirteen of the 16 BC₂ progeny tested were resistant to blast; 1 blast-resistant BC₂, plant 75-1, had 24 chromosomes. A 3 resistant: 1 susceptible segregation ratio, consistent with the action of a major, dominant gene, was observed in the BC₂F₂ and BC₂F₃ generations. Five of the BC₁ plants tested were resistant to bacterial blight. Ten of the 21 BC₂ progeny tested were resistant to Philippine races 2, 3, and 6 of the bacterial blight pathogen. One resistant BC₂, plant 78-1, had 24 chromosomes. The segregation of reactions of the BC₂F₂, BC₂F₃, and BC₂F₄ progenies of plant 78-1 suggested that the same or closely linked gene(s) conferred resistance to races 2, 3, 5, and 6 of the bacterial blight pathogen from the Philippines.     

Marker-assisted introgression of the major bacterial blight resistance gene, <Emphasis Type="Italic">Xa21</Emphasis> and blast resistance gene, <Emphasis Type="Italic">Pi54</Emphasis> into RPHR-1005, the restorer line of the popular rice hybrid,DRRH3

The elite Indian rice hybrid, DRRH3 is highly susceptible to two major diseases, bacterial blight (BB) and blast, which limit its productivity significantly. In the present study, we have introgressed two major genes, viz., Xa21 and Pi54 conferring resistance against BB and blast, respectively into RPHR-1005, the male parent of DRRH3 through marker-assisted backcross breeding (MABB) and analyzed the backcross derived plants for their resistance against BB and blast. RPBio Patho-2 was used as a donor for both the resistance genes. Gene-specific markers were used for the foreground selection of Xa21 and Pi54 at each stage of backcrossing and markers specific for the major fertility restorer genes, Rf3 and Rf4 were used only at BC₁F₁ generation for foreground selection. Background selection was done using 62 polymorphic SSR markers and marker-assisted backcrossing was continued till BC₃ generation. At BC₃F₄, through intensive phenotype-based selections 15 promising lines (ABLs) possessing high level of resistance against BB and blast, high yield, fine-grain type, complete fertility restoration along with better panicle exsertion and taller plant type as compared to RPHR-1005 were identified and test crossed with APMS 6 A, the female parent of DRRH3. The newly derived hybrids (i.e. improved versions of DRRH3) were observed to possess high level of resistance against BB and blast along with medium-slender grain type and yield level better than or equivalent to that of DRRH3. Our study exemplifies the utility of MABB for targeted improvement of multiple traits in hybrid rice.     

PCR-based DNA markers linked to a gall midge resistance gene, Gm4t, has potential for marker-aided selection in rice

Rice DNAs from a gall midge resistant variety, Abhaya, a susceptible variety, Tulsi and their F₃ progeny were screened using 500 random primers in conjunction with bulked-segregant analysis in a polymerase chain reaction (PCR) with a view to detecting random amplified polymorphic DNAs (RAPDs) linked to the gene, Gm4t, which confers resistance to gall midge, a dipteran insect pest of rice. A total of 454 primers were able to produce a distinct amplification pattern, and 3695 bands/loci were amplified between the phenotypically different parents. Of these, 304 bands were polymorphic between the parents, with 19 being phenotypespecific. One of these primers, E20, amplified 2 bands, E20₅₇₀ and E20₅₈₃, which are tightly linked to resistance and susceptibility, respectively. These specific bands were cloned and sequenced, and a 94% sequence homology was found between the two fragments. Two specific 20-mer oligonucleotides were synthesized, based on the sequence information of E20₅₈₃, for use in PCR amplification directly from genomic DNAs. These PCR primers were able to amplify phenotype-specific bands, a 583-bp fragment in susceptible F₃ lines and a 570-bp fragment in resistant F₃ lines that had been derived from a cross between the parents, indicating their potential and utility for marker-aided selection of the Gm4t gene in rice. Its use would facilitate the early and efficient selection of resistant genes in plant breeding programmes and even in those areas where the insect is not known to occur. These phenotype-specific bands are single-copy sequences and are being mapped to ascertain their chromosomal location in rice.     

Genetic Analysis of Resistance to Rice Blast: A Study on the Inheritance of Resistance to the Blast Disease Pathogen in an F3 Population of Rice

Blast caused by the fungus Magnaporthae grisea (Herbert) Borr. (anamorphe Pyricularia oryza Cav.) is a serious disease of rice (Oryza sativa L.). One method to overcome this disease is to develop disease resistant cultivars. Due to the genetic plasticity in the pathogen genome, there is a continuous threat to the effectiveness of the developed cultivars. Additional studies of the genetics of resistance, virulence stability and functional genomics are required to accelerate research into understanding the molecular basis of blast disease resistance. In this study, individual plants of the F₃ population derived from Pongsu Seribu 2 and Mahsuri were used for pathogenesis assays and inheritance studies of blast resistance. The study was performed with two of the most virulent Malaysian M. grisea pathotypes: P7.2 and P5.0. For blast screening, plants were scored based on the IRRI Standard Evaluation System (SES). F₃ populations showed a segregation ratio of 3R:1S for pathotype P7.2, indicating that resistance to this pathotype is likely controlled by a single nuclear gene. Chi‐square analysis showed that the F₃ families segregated in a 15R:1S ratio for pathotype P5.0. Therefore, locus interactions or epitasis of blast resistance occur against pathotype P5.0 in the F₃ population derived from Pongsu Seribu 2 and Mahsuri. This can be explained by the presence of two independent dominant genes that when present simultaneously, provide resistance to the M. gresia pathotype P5.0. These results indicated that blast resistance in rice is due to the combined effects of multiple loci with major and minor effects. The genetic data generated here will be useful in the breeding of local cultivars for resistance to field blast. The methodology reported here will facilitate the mapping of genes and quantitative trait loci (QTLs) underlying the blast resistance trait.     

Chlorophyll fluorescence and lipid peroxidation changes in rice somaclonal lines subjected to salt stress

The aim of the present work was to explore physiological changes provoked by somaclonal variation in response to salinity. Two parental cultivars (La Candelaria and Yerua) and their derived somaclones were used as a source for breeding new rice lines with improved salt tolerance. We studied the effect of NaCl salt stress on chlorophyll fluorescence-related parameters, such as the maximum quantum yield of primary PSII photochemistry (F _v/F _m) and the performance index for energy conservation from photon absorbed by PSII antenna (PI_ABS). In addition malondialdehyde (MDA) content and leaf temperature (LT) responses were also measured. In somaclonal lines, F _v/F _m, PI_ABS, MDA and LT showed coefficients of variation of 13.7, 39.3, 25.5, and 3 %, respectively, for La Candelaria and 1.4, 17.6, 34.4 and 3 % for Yerua. However, the fragrant character did not differ in the aromatic somaclonal lines with respect to their parentals. Our results suggest that the F _v/F _m ratio would not be as good marker of PSII vitality as PI_ABS for salinized rice somaclones, unless they are highly susceptible to salinity. On other hand, the MDA content showed a strong negative correlation with the PI_ABS content in somaclones of both rice cultivars, suggesting that MDA levels could also be used as an oxidative damage index in rice somaclones.     

Inheritance of resistance to the cowpea aphid in cowpea

Summary Inheritance of resistance to cowpea aphid, Aphis craccivora Koch, in three resistant cultivars of cowpea, Vigna unguiculata (L.) Walp, was studied. The parents, F₁ and F₂ population were grown in an insect-proof screenhouse. Each 3-day-old seedling was infested with 10 apterous adult aphids. Seedling reaction was recorded when the susceptible check was killed. The segregation data revealed that the resistance of ICV11 and TVU310 is governed by single dominant genes. All the F₂ seedlings of the cross ICV10xTVU310 were resistant, indicating that they have the same gene for resistance. However, the F₂ populations from the crosses ICV10xICV11 and ICV11xTVU310 segregated in a ratio of 151, indicating that the dominant genes in ICV11 and TVU310 are non-allelic and independent of each other. The resistance gene of ICV10 and TVU310 is designated as Ac₁ and that of ICV11 as Ac₂.     

Improving chickpea yield by incorporating resistance to ascochyta blight

Ascochyta blight [Ascochyta rabiei (Pass.) Lab.] is the most destructive disease of chickpea (Cicer arietinum L.), but it can be managed effectively by the use of resistant cultivars. Therefore, a breeding programme was initiated during 1977–78 at ICARDA, Syria, to breed blight-resistant, high-yielding chickpeas with other desirable agronomic traits. Crosses were made in main season at Tel Hadya, Syria, and the F₁s were grown in the off season at Terbol, Lebanon. The F₂, F₄ and F₅ generations were grown in a blight nursery in the main season where blight epidemic was artificially created. The plants and progenies were scored for blight resistance and other traits. The F₃ and F₆ generations were grown in the off season under normal day length to eliminate late-maturing plants. The pedigree method of breeding was followed initially, but was later replaced by the F₄-derived family method. The yield assessment began with F₇ lines, first at ICARDA sites and later internationally. A total of 1584 ascochyta blight-resistant chickpea lines were developed with a range of maturity, plant height, and seed size not previously available to growers in the blight-endemic areas in the Mediterranean region. These included 92 lines resistant to six races of the ascochyta pathogen, and 15 large-seeded and 28 early maturity lines. New cultivars produced 33% more seed yield than the original resistant sources. The yield of chickpea declined by 340 kg ha^-1, with an increase in blight severity by one class on a 1–9 scale, reaching zero yield with the 8 and 9 classes. Development of blight-resistant lines made the introduction of winter sowing possible in the Mediterranean region with the prospect of doubling chickpea production. Twenty three cultivars have been released so far in 11 countries.Joint contribution from ICARDA and ICRISAT. ICRISAT Journal Article no. JA 1886.     

The use of bulk segregant analysis to identify a RAPD marker linked to leaf rust resistance in barley

An F₂ population from a cross between barley accession Q21861 and the Australian barley variety Galleon was used to develop RAPD markers for resistance to barley leaf rust (Puccinia hordei). Resistant and susceptible DNA bulks were constructed following the classification of F₂ plants by leaf rust infection type. Bulked segregant analysis was then used to identify a 2.7-kb marker, designated OU02₂₇₀₀ and located approximately 12cM from RphQ, a leaf rust resistance gene in Q21861. The marker was generated by PCR with the oligonucleotide primer OPU-02 (Operon). Infection types of F₃ progeny were used to confirm assignment of F₂ genotypes. OU02₂₇₀₀ was shown, retrospectively, to be useful in the identification of individual F₂ plants that had been originally misclassified as having susceptible infection types. Both the RAPD marker and RphQ will be potentially useful in the development of new barley cultivars.     

Quantitative trait locus mapping reveals conservation of major and minor loci for powdery mildew resistance in four sources of resistance in mungbean [Vigna radiata (L.) Wilczek]

Powdery mildew (PM) is a common and serious disease of mungbean [Vigna radiata (L.) Wilczek]. A few quantitative trait loci (QTL) for PM resistance in mungbean have been reported. The objective of this study was to locate QTL for PM resistance in two resistant accessions V4718 and RUM5. Simple sequence repeat markers were analyzed in an F₂ population from a cross between Kamphaeng Saen 1 (KPS1; susceptible to PM) and V4718 (resistant to PM), and in F₂ and BC₁F₁ populations from a cross between Chai Nat 60 (CN60; susceptible to PM) and RUM5 (resistant to PM). Progenies of 134 F_2:3 and F_2:4 lines derived from KPS1 × V4718, and 190 F_2:3 and 74 BC₁F_1:2 lines derived from CN60 × RUM5 and CN60 × (CN60 × RUM5), respectively, were evaluated for response to PM under field conditions. Multiple interval mapping identified a major QTL on linkage group (LG) 9 and two minor QTL on LG4 for the resistance in V4718, and detected two major QTL on LG6 and LG9 and one minor QTL on LG4 for the resistance in RUM5. Comparative linkage analysis of the QTL for PM resistance in this study and in previous reports suggests that the resistance QTL on LG9 in V4718, RUM5, ATF3640 and VC6468-11-1A are the same locus or linked. One QTL on LG4 is the same in three sources (V4718, RUM5 and VC1210A). Another QTL on LG6 is the same in two sources (RUM5 and VC6468-11-1A). In addition, one QTL in V4718 on LG4 appears to be a new resistance locus. These different resistance loci will be useful for breeding durably PM-resistant mungbean cultivars.     

Genetic dissection of rice blast resistance by QTL mapping approach using an F3 population

Rice blast is one of the major fungal diseases that badly reduce rice production in Asia including Malaysia. There is not much information on identification of QTLs as well as linked markers and their association with blast resistance within local rice cultivars. In order to understanding of the genetic control of blast in the F₃ families from indica rice cross Pongsu seribu2/Mahsuri, an analysis of quantitative trait loci against one of the highly virulent Malaysian rice blast isolate Magnaporthe oryzae, P5.0 was carried out. Result indicated that partial resistance to this pathotype observed in the present study was controlled by multiple loci or different QTLs. In QTL analysis in F₃ progeny fifteen QTLs on chromosomes 1, 2, 3, 5, 6, 11 and 12 for resistance to blast nursery tests was identified. Three of detected QTLs (qRBr-6.1, qRBr-11.4, and qRBr-12.1) had significant threshold (LOD >3) and approved by both IM and CIM methods. Twelve suggestive QTLs, qRBr-1.2, qRBr-2.1, qRBr-4.1, qRBr-5.1, qRBr-6.2, qRBr-6.3, qRBr-8.1, qRBr-10.1, qRBr-10.2, qRBr-11.1, qRBr-11.2 and qRBr-11.3) with Logarithmic of Odds (LOD) <3.0 or LRS <15) were distributed on chromosomes 1, 2, 4, 5, 6, 8, 10, and 11. Most of the QTLs detected using single isolate had the resistant alleles from Pongsu seribu 2 which involved in the resistance in the greenhouse. We found that QTLs detected for deferent traits for the using isolate were frequently located in similar genomic regions. Inheritance study showed among F₃ lines resistance segregated in the expected ratio of 15: 1 for resistant to susceptible. The average score for blast resistance measured in the green house was 3.15, 1.98 and 29.95 % for three traits, BLD, BLT and % DLA, respectively.     

Fine-mapping and molecular marker development for <Emphasis Type="Italic">Pi56(t),</Emphasis> a NBS-LRR gene conferring broad-spectrum resistance to <Emphasis Type="Italic">Magnaporthe oryzae</Emphasis> in rice

The major quantitative trait locus qBR9.1 confers broad-spectrum resistance to rice blast, and was mapped to a ~69.1 kb region on chromosome 9 that was inherited from resistant variety Sanhuangzhan No 2 (SHZ-2). Within this region, only one predicted disease resistance gene with nucleotide binding site and leucine-rich repeat (NBS-LRR) domains was found. Specific markers corresponding to this gene cosegregated with blast resistance in F₂ and F₃ populations derived from crosses of susceptible variety Texianzhan 13 (TXZ-13) to SHZ-2 and the resistant backcross line BC-10. We tentatively designate the gene as Pi56(t). Sequence analysis revealed that Pi56(t) encodes an NBS-LRR protein composed of 743 amino acids. Pi56(t) was highly induced by blast infection in resistant lines SHZ-2 and BC-10. The corresponding allele of Pi56(t) in the susceptible line TXZ-13 encodes a protein with an NBS domain but without LRR domain, and it was not induced by Magnaporthe oryzae infection. Three new cosegregating gene-specific markers, CRG4-1, CRG4-2 and CRG4-3, were developed. In addition, we evaluated polymorphism of the gene-based markers among popular varieties from national breeding programs in Asia and Africa. The presence of the CRG4-2 SHZ-2 allele cosegregated with a blast-resistant phenotype in two BC₂F₁ families of SHZ-2 crossed to recurrent parents IR64-Sub1 and Swarna-Sub1. CRG4-1 and CRG4-3 showed clear polymorphism among 19 varieties, suggesting that they can be used in marker-assisted breeding to combine Pi56(t) with other target genes in breeding lines.     

Gametic selection in anther culture of rice (Oryza sauva L.)

Summary The segregation and recombination of heterozygous isozyme markers have been monitored in anther culture derivatives (i.e., six nonmorphogenic microspore-derived callus [NMC] populations and two anther culture plant [ACP] populations) and F₂ plants generated from six F₁ hybrids of rice, including five japonica upland/improved indica tropical hybrids. The alleles in excess at some loci displaying skewed segregations in the F₂s were consistently overrepresented in the NMC populations. These alleles were also generally found to be overabundant in the two ACP populations except for certain loci that contrastingly segregated in a 11 ratio. Additional distortions were found to be specific to AC derivatives indicating the existence of in vitro gametic selection. Overall, however, the gametic selection in the ACP materials was neutral with regard to the indica and japonica differentiation. Estimates of linkages between markers borne by chromosome 6 using AC-derivative data were consistent with those noted in the F₂s and with current knowledge of the isozyme locus linkage map. Given the average neutrality of gametic selection and the consistency of linkage relationships in the ACPs, their further use as rice molecular mapping and gene tagging populations can be investigated with confidence.Joint contribution with Research Institute for Tropical Food Crops, Department of the International Centre of Agronomical Research for Development (IRAT-CIRAD), 45bis avenue de la Belle Gabrielle, F-94736 Nogent s/Marne Cédex, France and International Rice Research Institute (IRRI), P.O.Box 933, Manila, Philippines     

Genetic difference in somatic embryogenesis from seeds in melon (Cucumis melo L.)

Significant differences in somatic embryogenesis from melon seeds were observed among 18 cultivars; especially, cultivars Earl's Favorite and Barnett which produced a large number of somatic embryos. F₁ seeds were obtained by reciprocal crosses between cultivars. Some lines produced a large number of somatic embryos whereas others showed no or poor embryogenic response. Most of the F₁ seeds formed somatic embryos. The frequency of somatic embryogenesis decreased as compared to the parents with the highest potential. Transfer of the frequency of somatic embryogenesis from superior responding cultivars to inferior cultivars was proved. It was difficult to determine the mode of inheritance of somatic embryogenesis because there was a large variation in the range of somatic embryogenesis from F₂ seeds, and cytoplasmic effect was recognized in certain combinations.Abbreviations 2,4-D 2,4-dichlorophenoxyacetic acid - BAP 6-benzylaminopurine     

Molecular mapping of two cultivar-specific avirulence genes in the rice blast fungus <Emphasis Type="Italic">Magnaporthe grisea</Emphasis>

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 F₁ 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 χ² 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.     

Improvement of seedling and panicle blast resistance in <Emphasis Type="Italic">Xian</Emphasis> rice varieties following <Emphasis Type="Italic">Pish</Emphasis> introgression

Rice blast is a serious disease caused by the filamentous ascomycetous fungus Magnaporthe oryzae. Incorporating disease resistance genes in rice varieties and characterizing the distribution of M. oryzae isolates form the foundation for enhancing rice blast resistance. In this study, the blast resistance gene Pish was observed to be differentially distributed in the genomes of rice sub-species. Specifically, Pish was present in 80.5% of Geng varieties, but in only 2.3% of Xian varieties. Moreover, Pish conferred resistance against only 23.5% of the M. oryzae isolates from the Geng-planting regions, but against up to 63.2% of the isolates from the Xian-planting regions. Thus, Pish may be an elite resistance gene for improving rice blast resistance in Xian varieties. Therefore, near-isogenic lines (NILs) with Pish and the polygene pyramid lines (PPLs) PPL^Pish/Pi1, PPL^Pish/Pi54, and PPL^Pish/Pi33 in the Xian background Yangdao 6 were generated using a molecular marker-assisted selection method. The results suggested that (1) Pish significantly improved rice blast resistance in Xian varieties, which exhibited considerably improved seedling and panicle blast resistance after Pish was introduced; (2) PPLs with Pish were more effective than the NILs with Pish regarding seedling and panicle blast resistance; (3) the PPL seedling and panicle blast resistance was improved by the complementary and overlapping effects of different resistance genes; and (4) the stability of NIL and PPL resistance varied under different environmental conditions, with only PPL^Pish/Pi54 exhibiting highly stable resistance in three natural disease nurseries (Jianyang, Jinggangshan, and Huangshan). This study provides new blast resistance germplasm resources and describes a novel molecular strategy for enhancing rice blast resistance.