Bacterial Blight Resistance Genes Research Articles

Update on Bacterial Blight of Rice: Fourth International Conference on Bacterial Blight

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Genetic diversity of the conserved motifs of six bacterial leaf blight resistance genes in a set of rice landraces

BackgroundBacterial leaf blight (BLB) caused by the vascular pathogen Xanthomonas oryzae pv. oryzae (Xoo) is one of the most serious diseases leading to crop failure in rice growing countries. A total of 37 resistance genes against Xoo has been identified in rice. Of these, ten BLB resistance genes have been mapped on rice chromosomes, while 6 have been cloned, sequenced and characterized. Diversity analysis at the resistance gene level of this disease is scanty, and the landraces from West Bengal and North Eastern states of India have received little attention so far. The objective of this study was to assess the genetic diversity at conserved domains of 6 BLB resistance genes in a set of 22 rice accessions including landraces and check genotypes collected from the states of Assam, Nagaland, Mizoram and West Bengal.ResultsIn this study 34 pairs of primers were designed from conserved domains of 6 BLB resistance genes; Xa1, xa5, Xa21, Xa21(A1), Xa26 and Xa27. The designed primer pairs were used to generate PCR based polymorphic DNA profiles to detect and elucidate the genetic diversity of the six genes in the 22 diverse rice accessions of known disease phenotype. A total of 140 alleles were identified including 41 rare and 26 null alleles. The average polymorphism information content (PIC) value was 0.56/primer pair. The DNA profiles identified each of the rice landraces unequivocally. The amplified polymorphic DNA bands were used to calculate genetic similarity of the rice landraces in all possible pair combinations. The similarity among the rice accessions ranged from 18% to 89% and the dendrogram produced from the similarity values was divided into 2 major clusters. The conserved domains identified within the sequenced rare alleles include Leucine-Rich Repeat, BED-type zinc finger domain, sugar transferase domain and the domain of the carbohydrate esterase 4 superfamily.ConclusionsThis study revealed high genetic diversity at conserved domains of six BLB resistance genes in a set of 22 rice accessions. The inclusion of more genotypes from remote ecological niches and hotspots holds promise for identification of further genetic diversity at the BLB resistance genes.

Quantitative trait Loci mapping for bacterial blight resistance in rice using bulked segregant analysis.

Oryza meyeriana is highly resistant to rice bacterial blight (BB) and this resistance trait has been transferred to cultivated rice (O. sativa) using asymmetric somatic hybridization. However, no resistance genes have yet been cloned. In the present study, a progeny of the somatic hybridization with high BB resistance was crossed with a rice cultivar with high BB susceptibility to develop an F2 population. Using bulked segregant analysis (BSA), 17 polymorphic markers that were linked to rice BB resistance were obtained through scanning a total of 186 simple sequence repeats (SSR) and sequence-tagged site (STS) markers, evenly distributed on 12 chromosomes. A genetic linkage map was then constructed based on the 17 linkage markers and the F2 segregating population, which was followed by mapping for quantitative trait loci (QTLs) for BB resistance. Three QTLs were identified on chromosomes 1, 3 and 5, respectively, and the alleles of the resistant parent at any of the QTLs increased BB resistance. All of the three QTLs had a strong effect on resistance, explaining about 21.5%, 12.3% and 39.2% of the resistance variance, respectively. These QTLs were different from the loci of the BB resistance genes that have been identified in previous studies. The QTLs mapped in this work will facilitate the isolation of novel BB resistance genes and their utilization in rice resistance breeding.

High-resolution genetic mapping of rice bacterial blight resistance gene Xa23

Bacterial blight (BB) caused by Xanthomonas oryzae pv. oryzae (Xoo) is the most devastating bacterial disease of rice (Oryza sativa L.), a staple food crop that feeds half of the world's population. In management of this disease, the most economical and effective approach is cultivating resistant varieties. Due to rapid change of pathogenicity in the pathogen, it is necessary to identify and characterize more host resistance genes for breeding new resistant varieties. We have previously identified the BB resistance (R) gene Xa23 that confers the broadest resistance to Xoo strains isolated from different rice-growing regions and preliminarily mapped the gene within a 1.7cm region on the long arm of rice chromosome 11. Here, we report fine genetic mapping and in silico analysis of putative candidate genes of Xa23. Based on F2 mapping populations derived from crosses between Xa23-containing rice line CBB23 and susceptible varieties JG30 or IR24, six new STS markers Lj36, Lj46, Lj138, Lj74, A83B4, and Lj13 were developed. Linkage analysis revealed that the new markers were co-segregated with or closely linked to the Xa23 locus. Consequently, the Xa23 gene was mapped within a 0.4cm region between markers Lj138 and A83B4, in which the co-segregating marker Lj74 was identified. The corresponding physical distance between Lj138 and A83B4 on Nipponbare genome is 49.8kb. Six Xa23 candidate genes have been annotated, including four candidate genes encoding hypothetical proteins and the other two encoding a putative ADP-ribosylation factor protein and a putative PPR protein. These results will facilitate marker-assisted selection of Xa23 in rice breeding and molecular cloning of this valuable R gene.

Application of a simplified marker-assisted backcross technique for hybrid breeding in rice

Hybrid rice has contributed greatly to the self-sufficiency of the food supply in China. However, bacterial blight is a major disease that limits hybrid rice production in China. The study was conducted to develop an efficient breeding technique to improve the bacterial blight resistance in hybrid rice. A marker-assisted backcross breeding technique was adopted to improve HN189, an elite restorer line containing the Pi1 gene. This breeding technique was simplified to foreground selection with only one generation of backcrossing and background selection based on phenotypic selection. A novel bacterial blight resistance gene, Xa23, was introgressed into HN189. Two improved restorer lines, HBH145 (with one generation of backcrossing) and HBH146 (with two generations of backcrossing), were obtained that had a significant bacterial blight resistance advantage over HN189. The corresponding hybrid combination Tianyou H145 (Tianfeng A / HBH145) was certified one year earlier than Qianyou H146 (Qianjiang 1A / HBH146). The use of the marker-assisted backcross breeding technique with one generation of backcrossing and without background selection in rice breeding programs shortened the breeding period of the rice.

Mapping of bacterial blight resistance genexa8in rice (Oryza sativaL.)

Bacterial blight (BB) of rice, caused by Xanthomonas oryzae pv. oryzae (Xoo), is one of the major diseases causing yield losses in rice worldwide. Development and deployment of resistant rice cultivars and pyramiding two or more resistance genes into elite varieties is the only strategy available to combat the disease. The BB resistance gene, xa8 conferred complete resistance to only one pathotype and partial resistance against two pathotypes at seedling stage while complete resistance against five of the seven pathotypes prevalent in Punjab at adult plant stage. The gene was earlier mapped on rice chromosome 7 at 19.9 cM genetic distance from SSR marker, RM214 using a recombinant inbred line (RIL) population generated from the cross between PR106 and IRBB8. However, due to the paucity of markers in that region, the gap could not be narrowed. With the availability of new SSR markers in that region, the gene was placed between two consecutive SSR markers, RM21044 and RM21045 at 7.0 and 9.9 cM respectively. Analysis of the 9.5 kb DNA sequence harboring the xa8 gene revealed the presence of three intact genes, which codes for putative expressed proteins viz., Zinc finger A20 and AN1 domain-containing protein (LOC_Os07g07400), Oxidoreductase, 2OG-Fe oxygenase family protein (LOC_Os07g07410) and Gibberellin 20 oxidase 1-B protein (LOC_Os07g07420). All the three genes are involved in plant response to various stresses and could be the possible candidate genes for xa8.

Construction of SSH Library and Resistance Gene Analyses of Yunnan <italic>Oryza rufipogon</italic> Griff. under <italic>Xanthomonas oryzae</italic> pv. <italic>Oryzae</italic> Stress

A cDNA-SSH library was established by suppression subtractive hybridization(SSH) using leaves of Yunnan Oryza rufipogon Griff. treated by Xanthomonas oryzae pv. oryzae. All the positive clones were sequenced, 494 available expressed sequence tags(ESTs) were obtained by cluster analysis. The results of BlastN showed that 417 ESTs had homologous sequences in GenBank database. The BlastX results indicated that 104 ESTs had higher similarity with the unknown proteins and putative proteins, and 341 ESTs had significant protein homologies while 49 ESTs had no protein homologies. The study suggested that a large group of Xanthomonas oryzae pv. oryzae of stress-induced genes were found in cDNA library, which are involved in many metabolism pathways such as energy metabolism,protein metabolism, nucleic acid metabolism, defense and resistance response, signal transduction, photosynthesis and transmembrane transport. We studied the mRNA expression of several ESTs which may be bacterial blight resistance genes in control and Xanthomonas oryzae pv. oryzae stress of Yunnan Oryza rufipogon Griff. by semi-quantitative RT-PCR. The results showed that the expression of the clone No. OR7, OR68 and OR826 EST were induced by Xanthomonas oryzae pv. oryzae stress, and were up-regulated. EST OR826 that had no protein homology in protein database may belong to a class of new bacterial blight resistance genes. The mRNA expression of OR143 could be detected in the leaf of control and Xanthomonas oryzae pv. oryzae stress. Its expression was gradually strengthened 48 hours after Xanthomonas oryzae pv. oryzae stress. We suggested these genes participated in resistance defense response in Yunnan Oryza rufipogon Griff. directly. This research provided theoretical basis for discovery of new bacterial blight resistance genes, and laid the foundation for further studies of the molecular mechanisms of bacterial blight resistance in Yunnan Oryza rufipogon Griff.

Do transgenesis and marker-assisted backcross breeding produce substantially equivalent plants? - A comparative study of transgenic and backcross rice carrying bacterial blight resistant gene Xa21

BackgroundThe potential impact of genetically modified (GM) plants on human health has attracted much attention worldwide, and the issue remains controversial. This is in sharp contrast to the broad acceptance of plants produced by breeding through Marker Assisted Backcrossing (MAB).ResultsFocusing on transcriptome variation and perturbation to signaling pathways, we assessed the molecular and biological aspects of substantial equivalence, a general principle for food safety endorsed by the Food and Agricultural Organization and the World Health Organization, between a transgenic crop and a plant from MAB breeding. We compared a transgenic rice line (DXT) and a MAB rice line (DXB), both of which contain the gene Xa21 providing resistance to bacterial leaf blight. By using Next-Generation sequencing data of DXT, DXB and their parental line (D62B), we compared the transcriptome variation of DXT and DXB. Remarkably, DXT had 43% fewer differentially expressed genes (DEGs) than DXB. The genes exclusively expressed in DXT and in DXB have pathogen and stress defense functions. Functional categories of DEGs in DXT were comparable to that in DXB, and seven of the eleven pathways significantly affected by transgenesis were also perturbed by MAB breeding.ConclusionsThese results indicated that the transgenic rice and rice from MAB breeding are substantial equivalent at the transcriptome level, and paved a way for further study of transgenic rice, e.g., understanding the chemical and nutritional properties of the DEGs identified in the current study.

A SSR marker linked to the B 12 gene that confers resistance to race 18 of Xanthomonas axonopodis pv. malvacearum in cotton is also associated with other bacterial blight resistance gene complexes

The SSR marker CIR246, which is linked to the B12 gene that confers resistance to race 18 of Xanthomonas axonopodis pv. malvacearum (Xam), was used to evaluate a series of cotton germplasms that were also tested for their response to inoculation with race 18 of Xam (S-295, Delta Opal, 101-102B, Guazuncho-2, Acala 44, Mebane B1, and Stoneville 2B-S9). The allele associated with resistance was amplified in genotypes that carry the B12 gene (S-295 and Delta Opal) as well as in those that carry B2B3 (101-102B) and B9LB10L (Guazuncho-2), indicating that the molecular marker is able to identify genotypes resistant to the Xam races up to race 18, with resistance genes other than B12.

Marker assisted introgression of multiple genes for bacterial blight resistance into aromatic Myanmar rice MK-75

Bacterial blight disease (BB) caused by Xanthomonas oryzae pv. oryzae (Xoo) is a major threat to rice yield in rainfed and irrigated lowland ecosystems in Myanmar. Large-scale and long-term cultivation of released Myanmar rice varieties have continuously caused the outbreak of BB since 1983. In this study, a high yielding Manawthukha rice line ‘Yn 3248-2-128-76-4-3-75’ (MK-75) with fragrance and intermediate amylose content was used as a recipient parent to improve its resistance to BB. RGDU-07097-1-8M-9 (RG-9), an improved line carrying xa5, Xa21 and xa33 was used as the donor parent. Resistant alleles of xa5, Xa21 and xa33 from RG-9 were successfully transferred into MK-75 by using marker-assisted backcrossing (MAB) method while maintaining the cooking quality of MK-75. Twenty eight selected BC3F2 introgression lines (MK-75 ILs) carrying different combinations of these three loci were tested for BB resistance at seedling and maximum tillering stages against ten Thai and five Myanmar Xoo strains. The triple and double resistance gene-introgressed lines (xa5Xa21xa33 or xa5Xa21 or xa5xa33 or Xa21xa33) had higher level of resistance and wider resistance spectrum than MK-75 against both Thai and Myanmar Xoo strains. The single resistance gene-introgressed lines carrying xa5 had higher resistance against Thai Xoo strains, while lines carrying Xa21 showed higher level of resistance to Myanmar Xoo strains. These results clearly indicated that pyramiding of multiple genes is a useful approach for improving BB resistance in MK-75. All ILs had physical grain quality, fragrance and amylose content similar to MK-75. Thus the improvement of BB resistance in MK-75 may have a wide range of benefits in breeding program and should be immediately useful in areas where original Manawthukha, the progenitor or MK-75 is widely grown.

Marker-assisted breeding of Thai fragrance rice for semi-dwarf phenotype, submergence tolerance and disease resistance to rice blast and bacterial blight

Rice is a staple food crop for more than half of the world’s population. However, rice production is affected by many types of abiotic and biotic stress. Genetic breeding by utilizing natural resistance or tolerance genes is the most economic and efficient way to combat or adapt to these stresses. Khao Dawk Mali 105 (KDML 105) is an elite cultivar of aromatic rice mainly grown in Thailand. However, the production of KDML 105 is affected by lodging problems due to its tall plant type, regular flash floods or short-term submergence during the monsoon season, and diseases such as blast and bacterial blight. Here we report the pyramiding of semi-dwarf gene sd1, submergence tolerance gene Sub1A, blast resistance gene Pi9 and bacterial blight resistance genes Xa21 and Xa27 in KDML 105 by marker-assisted selection. The improved line, designated T5105, has a semi-dwarf phenotype with improved lodging resistance and a greater harvest index. T5105 survives after 2 weeks of complete submergence without significant loss of viability. T5105 confers high resistance to all five Magnaporthe oryzae isolates tested and provides resistance or moderate resistance to 25 of the 27 Xanthomonas oryzae pv. oryzae strains tested. In addition, T5105 produced higher yield than KDML 105 in two field trials and retains similar good grain quality to KDML 105. The development of T5105 provides a new line to boost the production of high-quality aromatic rice in tropical regions.

Making an Indian traditional rice variety Mahsuri, bacterial blight resistant using marker-assisted selection

Mahsuri a popular traditional variety and the first rain-fed mega variety of the Indian sub- continent. It is highly susceptible to bacterial leaf blight caused by Xanthomonas oryzae pv.oryzae. Nine best performing families of Mahsuri pyramid containing four bacterial blight resistance genes (Xa4, xa5, xa13 and Xa21) were evaluated for agronomic, yield and its related characters viz. days to 50% flowering, plant height, number of tillers, panicle length, filled grains per panicle, grain weight, and yield under natural and disease pressure conditions for three consecutive wet seasons. In addition these pyramids were also evaluated for three different spacings to find out the optimum spacing under disease free and disease pressure conditions.

Analysis of nucleotide diversity among alleles of the major bacterial blight resistance gene Xa27 in cultivars of rice (Oryza sativa) and its wild relatives

Xa27 is one of the important R-genes, effective against bacterial blight disease of rice caused by Xanthomonas oryzae pv. oryzae (Xoo). Using natural population of Oryza, we analyzed the sequence variation in the functionally important domains of Xa27 across the Oryza species. DNA sequences of Xa27 alleles from 27 rice accessions revealed higher nucleotide diversity among the reported R-genes of rice. Sequence polymorphism analysis revealed synonymous and non-synonymous mutations in addition to a number of InDels in non-coding regions of the gene. High sequence variation was observed in the promoter region including the 5'UTR with 'π' value 0.00916 and 'θ w '=0.01785. Comparative analysis of the identified Xa27 alleles with that of IRBB27 and IR24 indicated the operation of both positive selection (Ka/Ks>1) and neutral selection (Ka/Ks≈0). The genetic distances of alleles of the gene from Oryza nivara were nearer to IRBB27 as compared to IR24. We also found the presence of conserved and null UPT (upregulated by transcriptional activator) box in the isolated alleles. Considerable amino acid polymorphism was localized in the trans-membrane domain for which the functional significance is yet to be elucidated. However, the absence of functional UPT box in all the alleles except IRBB27 suggests the maintenance of single resistant allele throughout the natural population.

Functional markers for bacterial blight resistance gene Xa3 in rice

Bacterial blight (BB) of rice (Oryza sativa L.) caused by Xanthomonas oryzae pv. oryzae (Xoo) is a destructive disease in rice worldwide. Xa3, a gene conferring resistance to BB at the booting stage of the rice plant, has been characterized previously using map-based cloning. We cloned and sequenced the Xa3/xa3 gene in the Korean cultivars Hwayeong, Ilmi, and Goun and conferred resistance or susceptibility to BB. We detected polymorphisms, and polymerase chain reaction-based functional markers were developed based on the single nucleotide polymorphism from the Xa3 and xa3 nucleotide sequence. Susceptible or resistant individuals from an F2 population developed from a cross between Milyang 244 and Ilmi, near-isogenic lines carrying BB resistance genes, were screened with functional markers. The BB3-RF and BB3-RR primers consistently amplified a resistance-specific fragment of 255 bp only in resistant plants, whereas the BB3-SF and BB3-SR primers were specific to susceptible plants. Genotyping results were co-segregated with phenotype by conducting the BB resistance test with the K3 race. These markers could be effective for marker-assisted selection of the Xa3 gene in rice breeding programs.

Identification of Indian rice germplasm lines with bacterial leaf blight (BLB) resistance genes

Bacterial leaf blight of rice, caused by Xanthomonas oryzae pv. oryzae is one of the major constraints limiting rice production and productivity in South East Asia, particularly in Japan, Philippines, Indonesia, China and India. No effective chemical control for the disease is available. It is therefore, important to identify lines resistant to blight, which can then be used as parental lines in breeding programmes. Gene-linked and gene-based molecular markers and well defined donors are available for most of these genes. In the present study, a total of 386 Indian rice germplasm lines including commercially cultivated Indian rice varieties were screened for the presence of four bacterial blight resistance genes i.e. Xa4, xa5, xa13 and Xa21. The rationale behind the study was to identify new hitherto unreported, rice lines carrying BLB resistance genes and studying the allelic distribution of these genes in the germplasm.

Evaluation on Effect of Resistance Improvement of Bacterial Leaf Blight and Brown Planthopper for Three Dominant Indica Restorer Lines

A novel bacterial blight (BB) resistance gene, Xa23, and brown planthopper (BPH) resistance gene, Bph18(t) were introgressed into three popular restorer lines (Minghui 86, Shuhui 527, and Zhehui 7954) for wild abortive cytoplasmic male sterility by marker-assisted backcross and pyramiding breeding approaches in combination with artificial inoculation and stringent phenotypic selections. The four derived BB resistant restorer lines [Minghui 86-Xa23, Shuhui 527-Xa23, Zhehui 7954-Xa23, and Zheshu-Xa23-Bph18(t)] and their derived hybrid combinations with Ⅱ-32A and Huhan11A demonstrated similar BB resistance spectrum as the donor parent, IRBB23. The three derived BPH resistant restorer lines [Shu 527-Bph18(t), Zhehui 7954-Bph18(t), and Zheshu-Xa23-Bph18(t)] showed resistant or moderate resistant to BPH. The newly developed BB and BPH resistant restorers and their derived hybrids were identical to their respective original versions for agronomic traits especially under disease free condition. However, under severe disease condition, the four BB resistant restorer lines exhibited significantly higher grain weight and spikelet fertility as compared to the respective original restorer lines thus further resulting in BB resistant hybrids with significantly higher grain yields than their respective original hybrids. The results indicated that the Xa23 gene could completely express its dominant resistance in different backgrounds of both restorer and male sterile lines across different growth stages,suggesting its immense breeding value in BB resistance improvement for hybrid rice. However, effect of resistant improvement of Bph18(t) for BPH depends on genetic background. Improvement of resistance of BB and BPH for restorer lines by molecular marker-assisted backcross and pyramiding was deeply discussed.

Developing Restorer Lines Pyramiding Different Resistant Genes to Blast and Bacterial Leaf Blight by Marker-assisted Selection in Rice

The novel blast resistance genes, Pi-GD-1(t), Pi-GD-2(t), and GLP8-6(t) from rice variety Sanhuangzhan 2 (here abbreviated them as G1, G2, and G8, respectively) and bacterial blight (BB) resistance gene, Xa23, were introgressed into three elite restorer lines (Minghui 86, Shuhui 527, and Zhehui 7954) by marker-assisted pyramiding breeding approaches in combination with artificial inoculation and stringent phenotypic selections. The five derived blast and bacterial blight resistance restorer lines, Minghui 86-G1-G2-Xa23, Shuhui 527-G2-Xa23, Mingzhe-G2-G8-Xa23-1, Mingzhe-G2-G8-Xa23-2, and Mingzhe-G1-G2-G8Xa23, demonstrated similar or wider blast resistance spectrum as compared with the donor parent, Sanhuangzhan 2. The five derived BB resistant restorer lines and their derived hybrid combinations with II-32A demonstrated similar BB resistance spectrum to the donor parent, IRBB23. The newly developed BB resistant restorers and their derived hybrids were identical to their respective original versions for agronomic traits especially under disease free condition. However, under severe disease condition, the five BB resistant restorer lines exhibited significantly higher grain weight, spikelet fertility, and grain yield as compared with the respective original restorer lines, thus further resulting in significantly higher grain yields in BB resistant hybrids than in their 第 9期 潘晓飚等: 分子标记辅助选育水稻抗白叶枯病和稻瘟病多基因聚合恢复系 1583 respective original hybrids.

In vitro Screening and Molecular Characterization of a Bacterial BlightResistance Gene in Rice

Bacterial blight (BB) is one of the most serious bacterial diseases of rice in the world. Since 1986, our laboratory has conducted in vitro screening and application of BB resistance somaclonal mutants for rice breeding. In this paper, we review our nearly 30-year research on BB resistance somaclonal variation, including the development of an in vitro selection system, discovery of BB resistance mutants, evaluation of BB resistance in a somaclonal mutant named HX-3, genetic analysis and molecular mapping of the new BB resistance gene Xa-25. In addition, we also used the new resistance gene to develop BB resistance cultivar and hybrid rice. Our long-term research contributed a novel resistance gene in rice, it also provided an example for new gene creation and discovery using somaclonal variation.

Molecular breeding for the development of multiple disease resistance in Basmati rice.

Basmati rice grown in the Indian subcontinent is highly valued for its unique culinary qualities. Production is, however, often constrained by diseases such as bacterial blight (BB), blast and sheath blight (ShB). The present study developed Basmati rice with inbuilt resistance to BB, blast and ShB using molecular marker-assisted selection. The rice cultivar 'Improved Pusa Basmati 1' (carrying the BB resistance genes xa13 and Xa21) was used as the recurrent parent and cultivar 'Tetep' (carrying the blast resistance gene Pi54 and ShB resistance quality trait loci (QTL), qSBR11-1) was the donor. Marker-assisted foreground selection was employed to identify plants possessing resistance alleles in the segregating generations along with stringent phenotypic selection for faster recovery of the recurrent parent genome (RPG) and phenome (RPP). Background analysis with molecular markers was used to estimate the recovery of RPG in improved lines. Foreground selection coupled with stringent phenotypic selection identified plants homozygous for xa13, Xa21 and Pi54, which were advanced to BC(2)F(5) through pedigree selection. Marker-assisted selection for qSBR11-1 in BC(2)F(5) using flanking markers identified seven homozygous families. Background analysis revealed that RPG recovery was up to 89.5%. Screening with highly virulent isolates of BB, blast and ShB showed that the improved lines were resistant to all three diseases and were on a par with 'Improved Pusa Basmati 1' for yield, duration and Basmati grain quality. This is the first report of marker-assisted transfer of genes conferring resistance to three different diseases in rice wherein genes xa13 and Xa21 for BB resistance, Pi54 for blast resistance, and a major QTL qSBR11-1 have been combined through marker-assisted backcross breeding. In addition to offering the potential for release as cultivars, the pyramided lines will serve as useful donors of gene(s) for BB, blast and ShB in future Basmati rice breeding programmes.

Marker Based Estimation of Gene Flow in Tropical Rice Fields and Its Ecological Consequences

Pollen mediated gene flow is the most widely debated biosafety issue in case of genetically modified crop plants and is a primary determining factor for permitting their field release, particularly major food crops like rice. The alleged consequences of gene escape into wild/weedy relatives that coexist together with cultivated forms in rice fields in several countries is perceived to be a major concern. In an effort to estimate the gene flow in rice in a tropical environment, rice varieties stacked with three bacterial blight (BB) resistance alleles and a cytoplasmic male sterile (CMS) line were used as donors and receiver of pollen respectively and the pollen flow was tracked with molecular markers that are closely linked to BB resistance genes. The study could detect gene flow up to 20 m distance with environmental factors like temperature and humidity influencing it. The gene flow observed at longer distances than the 10 m distance suggested earlier advocate caution and suggests that all the relevant biosafety issues need to be addressed prior to release of GM rice in tropical countries where sympatric association of wild relatives with cultivated rice with frequent crop-weedy gene flow both ways is quite common.