Dynamics of Xanthomonas oryzae pv. oryzae Populations in Korea and Their Relationship to Known Bacterial Blight Resistance Genes

ABSTRACT Developing resistant cultivars requires an understanding of the dynamics of the pathogen populations as well as the genetics of host resistance. Bacterial leaf blight (BB), caused by the vascular pathogen Xanthomonas oryzae pv. oryzae, has become one of the most devastating diseases of rice. We demonstrate here the quantitative analyses of responses of near-isogenic lines carrying various BB resistance (R) genes and R-gene combinations against 16 X. oryzae pv. oryzae isolates representing Korean BB pathotypes. The estimated main effects of each R gene against the 16 isolates identified prominent differences in BB pathotypes between Korea and other countries. Three major aspects of our quantitative observations and statistical analysis are (i) strong and broad resistance of xa5; (ii) independent and additive genetic actions of Xa4, xa5, and Xa21 under digenic or trigenic status; and (iii) a strong quantitative complementation effect contributed by the functional alleles of Xa4 and Xa21. We conclude that the pyramid line containing genes Xa4, xa5, and Xa21 would be the most promising and valuable genotype for improving Korean japonica cultivars for BB resistance.     

Identification and molecular mapping of the rice bacterial blight resistance gene allelic to <Emphasis Type="Italic">Xa7</Emphasis> from an elite restorer line Zhenhui 084

Rice bacterial blight (BB), caused by Xanthomonas oryzae pv. Oryzae (Xoo), is a serious disease in rice production worldwide. Rice cv. Zhenhui 084, a newly developed strong indica restorer line, exhibits high resistance to most of the Philippine races of BB and has been widely used in rice hybrids in China; however, the resistance gene has not yet been cloned. Here, we show that the resistance of Zhenhui 084 to Xoo strains is similar to that of IRBB7 containing Xa7, a durable and broad resistance dominant gene for BB. To map the resistance gene in Zhenhui 084, a F₂ population with 331 highly susceptible individuals derived from a cross between Chenghui 448 and Zhenhui 084 was built. We finely mapped the target R gene to a region between two proximal markers RM20576 and MY4 in rice chromosome 6. A marker-based physical map of chromosome six was used to construct the contig covering the genomic region between two markers RM20576 and MY4. The target gene was assumed to be in an interval of approximate 200 kb, in which 16 candidate genes were predicted. Our findings will greatly facilitate the isolation and characterisation of the target R gene allelic to Xa7. Additionally, two PCR-based markers, tightly linked to the target R gene locus, will be a useful tool for the marker-assisted selection of the target R gene allelic to Xa7 in breeding programmes.     

Genetic Diversity of Xanthomonas oryzae pv. oryzae Strains from Sri Lanka

ABSTRACT Sixty strains of Xanthomonas oryzae pv. oryzae, collected from 29 locations in Sri Lanka in 1995, were analyzed by restriction fragment length polymorphism using either polymerase chain reaction-amplified 16S and 23S rDNA or the repetitive DNA element IS1112 from X. oryzae pv. oryzae as hybridization probes. Two different ribogroups were observed in the Sri Lankan strains using rDNA probes, whereas five clusters were identified by the IS1112 probe. Bootstrap analysis revealed that the five clusters defined by IS1112 were relatively robust. Our results suggest that the Sri Lankan strains are phylogenetically composed of five different groups. Each cluster was partially associated with climatic conditions (intermediate zone and wet zone) and was related to groups based on ribotyping. Based on virulence analysis using 12 rice cultivars, each containing a single resistance gene, 14 pathotypes were identified among the Sri Lankan strains. All strains were virulent to resistance genes Xa1, Xa2, Xa4, Xa10, Xa11, and Xa14. Only one strain (pathotype 1) was virulent to all major resistance genes including Xa21, while strains of the other pathotypes were all avirulent to Xa21. A partial relationship was found between the determined phylogenetic groups using the IS1112 probe and pathotypes for all but two clusters. The results of this study will facilitate the further understanding of the population structure of X. oryzae pv. oryzae in Sri Lanka.     

Genotypic and Pathotypic Diversity in Xanthomonas oryzae pv. oryzae in Nepal

ABSTRACT Among the 171 strains of Xanthomonas oryzae pv. oryzae (the bacterial blight pathogen of rice) collected from eight rice-producing zones in Nepal, 31 molecular haplotypes were distinguished using two polymerase chain reaction-based assays. Six common haplotypes represented nearly 63% of the strains, and some haplotypes were geographically dispersed. Multiple correspondence analysis divided the collection into five putative genetic lineages. Lineages 1, 2, and 4 were the most frequently detected and occurred in diverse geographic populations. Twenty-six pathotypes (virulence phenotypes) of X. oryzae pv. oryzae were identified using 11 near-iso-genic rice lines, each containing a single gene for resistance. The 26 pathotypes grouped into five clusters, and cluster 1 contained wide virulence spectrum strains from all geographic populations. Although molecular variation was greatest between strains of different virulence phenotypes, some variation was observed among strains with identical virulence. There was a weak correlation (r = 0.52) between molecular haplotypes and virulence phenotypes. There are two major groups of X. oryzae pv. oryzae in Nepal. One group consists of strains with high molecular polymorphism and many pathotypes that are either virulent to the 11 major resistance genes or avirulent only to Xa21. Strains in the second group have low molecular polymorphism and are avirulent to Xa4, xa5, Xa7, and Xa21.     

Distribution of Xanthomonas oryzae pv. oryzae Strains Virulent to Xa21 in Korea

ABSTRACT Strains of Xanthomonas oryzae pv. oryzae that are virulent to rice lines carrying the Xa21 resistance gene were widely distributed in Korea. A total of 105 strains collected during 1987 to 1996 in Korea was characterized by pathogenicity tests and restriction fragment length polymorphism analysis of the XorII methyltransferase (xorIIM) and avrXa10 genes. Although the lesion lengths on rice line IRBB21, which carries Xa21, decreased as plant age increased, resistance and susceptibility of the plants to 31 strains were clearly differentiated at the seedling (14, 21, and 28 days old), maximum tillering, and flag leaf stages. The resistance or susceptibility of seedlings was correlated with bacterial populations within an inoculated leaf. There was a significant change in the population structure of X. oryzae pv. oryzae with regard to virulence to Xa21 over the last 10 years; this change in population was confirmed by genome analysis. Lineage I, which is avirulent to Xa21 and does not have a genomic xorIIM homolog, was the predominant lineage found between 1987 and 1989, while lineage II, which is virulent to Xa21 and contains the xorIIM homolog, was predominant in strains collected between 1994 and 1995. Our results demonstrate that introduction of Xa21 into commercial rice should be based on the regional structure of X. oryzae pv. oryzae populations and suggest that Xa21 will not be useful in Korea.     

Inheritance of resistance to bacterial blight in 21 cultivars of rice

ABSTRACT Genetic analysis for resistance to bacterial blight (Xanthomonas oryzae pv. oryzae) of 21 rice (Oryza sativa L.) cultivars was carried out. These cultivars were divided into two groups based on their reactions to Philippine races of bacterial blight. Cultivars of group 1 were resistant to race 1 and those of group 2 were susceptible to race 1 but resistant to race 2. All the cultivars were crossed with TN1, which is susceptible to all the Philippine races of X. oryzae pv. oryzae. F(1) and F(2) populations of hybrids of group 1 cultivars were evaluated using race 1 and F(1) and F(2) populations of hybrids of group 2 cultivars were evaluated using race 2. All the cultivars showed monogenic inheritance of resistance. Allelic relationships of the genes were investigated by crossing these cultivars with different testers having single genes for resistance. Three cultivars have Xa4, another three have xa5, one has xa8, two have Xa3, eight have Xa10, and one has Xa4 as well as Xa10. Three cultivars have new, as yet undescribed, genes. Nep Bha Bong To has a new recessive gene for moderate resistance to races 1, 2, and 3 and resistance to race 5. This gene is designated xa26(t). Arai Raj has a dominant gene for resistance to race 2 which segregates independently of Xa10. This gene is designated as Xa27(t). Lota Sail has a recessive gene for resistance to race 2 which segregates independently of Xa10. This gene is designated as xa28(t).     

Mapping quantitative trait Loci for resistance to rice blast

Quantitative trait loci (QTLs) conferring resistance to rice blast, caused by Magnaporthe oryzae, have been under-explored. In the present study, composite interval mapping was used to identify the QTLs that condition resistance to the 6 out of the 12 common races (IB1, IB45, IB49, IB54, IC17, and ID1) of M. oryzae using a recombinant inbred line (RIL) population derived from a cross of the moderately susceptible japonica cultivar Lemont with the moderately resistant indica cultivar Jasmine 85. Disease reactions of 227 F(7) RILs were determined using a category scale of ratings from 0, representing the most resistant, to 5, representing the most susceptible. A total of nine QTLs responsive to different degrees of phenotypic variation ranging from 5.17 to 26.53% were mapped on chromosomes 3, 8, 9, 11, and 12: qBLAST3 at 1.9 centimorgans (cM) to simple sequence repeat (SSR) marker RM282 on chromosome 3 to IB45 accounting for 5.17%; qBLAST8.1 co-segregated with SSR marker RM1148 to IB49 accounting for 6.69%, qBLAST8.2 at 0.1 cM to SSR marker RM72 to IC17 on chromosome 8 accounting for 7.22%; qBLAST9.1 at 0.1 cM to SSR marker RM257 to IB54, qBLAST9.2 at 2.1 cM to SSR marker RM108, and qBLAST9.3 at 0.1 cM to SSR marker RM215 to IC17 on chromosome 9 accounting for 4.64, 7.62, and 4.49%; qBLAST11 at 2.2 cM to SSR marker RM244 to IB45 and IB54 on chromosome 11 accounting for 26.53 and 19.60%; qBLAST12.1 at 0.3 cM to SSR marker OSM89 to IB1 on chromosome 12 accounting for 5.44%; and qBLAST12.2 at 0.3 and 0.1 cM to SSR marker OSM89 to IB49 and ID1 on chromosome 12 accounting for 9.7 and 10.18% of phenotypic variation, respectively. This study demonstrates the usefulness of tagging blast QTLs using physiological races by composite interval mapping.     

Gene-for-gene relationships between rice and diverse avrBs3/pthA avirulence genes in Xanthomonas oryzae pv. oryzae

The interactions between Xanthomonas oryzae pv. oryzae (Xoo) and rice are controlled in a gene-for-gene manner. In this study, a 359 bp DNA fragment of the avrXa3 gene containing three nuclear localization signal (NLS) motifs present in all members of the avrBs3/pthA family was used as a probe to screen a genomic library of the JXOIII strain of Xoo. The results demonstrated that diverse members of the family exist in the pathogen genome. The avrBs3/pthA genes occurred at isolated individual portions or in clusters. The positive avr gene clones were transferred into the virulent recipient PXO99^A. Pathogenicity tests in near isogenic lines of rice confirmed that four resistance (R) genes ( Xa2 , Xa3 , xa5 and xa8 ) matched the four avr genes ( avrXa2 , avrXa3 , avrxa5 and avrxa8 ) in the genome of Xoo strain JXOIII. The avrBs3/PthA -like gene (1·7 kb) present in cosmid p54, may specifically interact with the Xa3 gene present in IRBB3, and is designated avr/pthA3 . Sequencing indicated that there are only 1·5 copies of the 102 bp repeat unit in avr/pthA3 . Alignment of the twelfth and thirteenth amino acids in the repetitive units encoded by this gene with those in other representatives of the AvrBs3 family revealed a unique repeat arrangement which might contribute to variation in the avirulence genes in Xoo. The parental rice line IR24 was found to contain several R genes for resistance to Xoo bacterial blight.     

水稻白叶枯病抗性基因在不同载体品种上及杂种F_1的抗性研究

本研究测试了八个水稻白叶枯病抗性基因Xa3、Xa4、xa5、Xa7、Xa10、Xa11、Xa14和Xa23在相应载体品种上对华南白叶枯病优势致病菌系Ⅳ型菌和强毒菌系Ⅴ型菌的抗性反应;分析了Xa4、xa5、Xa7和Xa23抗病基因与不同感病品种杂交组合F_1的抗性表达模式以及显性抗病基因Xa7和Xa23在杂交水稻上的利用价值。结果表明:大部分抗性基因在不同载体品种上抗性表达一致,但也有少数基因在不同载体品种上抗性表现不一,甚至截然相反,说明不同遗传背景对抗性表达有影响,而这种影响因不同抗性基因而异。隐性抗病基因xa5(IRBB5)和显性抗病基因Xa23(CBB23)与感病亲本组合的F_1代分别表现感病和抗病,符合隐性基因或显性基因的抗性表达模式;而2个显性抗病基因Xa4(IRBB4))和Xa7(IRBB7)与感病亲本组合的F_1代,有部分组合的F_1代表现抗病,符合显性基因的抗性表达模式,有部分组合的F_1代表现感病,不符合显性基因的抗性表达模式。在杂交水稻上的利用价值方面,Xa7与2个不育系组合的F_1代均表现高感,不宜在杂交水稻上利用;Xa23(CBB23)与4个感病亲本,无论是不育系还是常规稻组合的4个F_1代全部表现抗病,抗性在F_1充分表达,在杂交水稻上有重要的利用价值。     

云南疣粒野生稻抗白叶枯病鉴定及叶片组织学观察

 本研究采用水稻白叶枯病菌强致病性代表菌株,评价了云南省自然分布的疣粒野生稻代表生态居群18份材料的抗病性,观察叶片组织学结构,初步分析了疣粒野生稻的抗性基因。结果表明,16份材料表现中抗、高抗甚至免疫;9份材料对4个菌株的抗性有一定差异;同时发现云南地方代表菌株X1和CN9404的致病性比C1和BD8438强。叶片组织学结构表明,高抗材料与感病的栽培稻米泉黑芒都无蜡质层,二者的叶肉组织、薄壁细胞和表皮细胞相同,表皮毛、维管束、厚壁组织等的差异也不明显,不足以引起抗病性差异。疣粒野生稻高抗或免疫白叶枯病并非借助其特殊叶片结构的物理作用,而主要是抗病基因的作用。参试疣粒野生稻中没有与Xa1、Xa21同源的基因,加之独立进化,推断其具有新的优异抗白叶枯病基因,值得深入研究和发掘利用。     

云南水稻白叶枯病菌生理小种初析

从云南省 5个地州14个县市的水稻白叶枯病标本中分离水稻白叶枯病菌菌株 ,选择具有区域代表性的菌株 5 1株 ,利用已知抗病基因的近等基因系 ,在孕穗期应用剪叶法接种明确其生理小种。鉴定结果表明 ,云南省水稻白叶枯病菌小种复杂多样 ,共有 14种类型 ,暂定为YN1～YN14 ,优势生理小种为YN3、YN8、YN11,其中YN3分布在红河、德宏等 4个县市 ;YN8小种分布在大理等地区 ,小种的分布可能受地理区域的影响     

Localizing the Bacterial Blight Resistance Gene, Xa22(t), to a 100-Kilobase Bacterial Artificial Chromosome

ABSTRACT The rice bacterial blight resistance gene, Xa22(t), provides resistance to a broad spectrum of Xanthomonas oryzae pv. oryzae isolates. Here, we localize the gene to a small 100-kb fragment of chromosome 11 by a combination of genetic recombination analysis and physical mapping. Mapping was done with two F(2) populations from the cross between Zhachanglong and Zhenzhuai. The first population consisted of 248 random individuals and 404 highly susceptible individuals selected from an F(2) population of more than 2,000 individuals and was used to construct a linkage map around the Xa22(t) locus. For the second F(2) population, 7,680 plants were examined with simple sequence repeat markers flanking the Xa22(t) locus to identify recombinants useful for fine-genetic mapping. Two large-insert bacterial artificial chromasome (BAC) libraries (from cvs. Teqing and Minghui63) were screened with a marker (R1506) which cosegregated perfectly with Xa22(t) in the first population. Restriction mapping of the resulting BAC clones enabled a physical map of the area to be constructed, and subclones from the BAC clones provided additional restriction fragment length polymorphism probes which could be placed on the fine-structure genetic map using the recombinants from the second mapping population. The Xa22(t) locus was mapped to a approximately 100-kb interval delimited by the R1506 marker and a subclone from the M3H8 BAC clone.     

源于华山新麦草抗条锈病基因 YrH122遗传分析和SSR标记

 H122是1个通过杂交和回交选育的普通小麦-华山新麦草易位系。为明确其抗条锈病基因及遗传特点,建立抗病基因SSR标记,利用我国小麦条锈菌流行小种CYR29、CYR31、CYR32、CYR33和致病类型Su11-4、Su11-11对H122进行苗期抗性鉴定,根据鉴定结果选用CYR32、CYR33和Su11-4对其与铭贤169杂交F1、F2及BC1代进行了遗传分析,同时应用258对SSR引物对将H122/铭贤169 F2代接种Su11-4的185个单株构建的作图群体进行了PCR扩增和电泳分析。结果表明,H122对供试小种均表现免疫或近免疫,对CYR32的抗病性由1对显性基因控制,对CYR33的抗病性由1对隐性基因控制,对Su11-4的抗病性亦由1对显性基因控制,将其暂命名为YrH122。筛选到3个与YrH122连锁的SSR标记Xbarc229、Xwmc339和Xwmc93,遗传距离分别为7.7、4.3和11.0 cM,并将该基因定位于小麦染色体1DL上。SSR标记回检显示,YrH122来源于华山新麦草。通过基因来源、分子检测及染色体位点比较,YrH122可能是1个不同于目前已知抗条锈病基因的新基因。     

Antagonistic Interactions Between Strains of Xanthomonas oryzae pv. oryzae

ABSTRACT The ability of some phytopathogenic bacterial strains to inhibit the growth of others in mixed infections has been well documented. Here we report that such antagonistic interactions occur between several wild-type strains of the rice bacterial blight pathogen Xanthomonas oryzae pv. oryzae. In mixed inoculations, a wild-type Philippine strain was found to inhibit the growth of a wild-type Korean strain. Furthermore, a nonpathogenic mutant of the Philippine strain maintained these antagonistic properties. Growth curve analysis indicated that both the wild-type Philippine strain and its nonpathogenic mutant inhibited the growth of the Korean strain 2 days after infection and prior to the onset of disease symptoms. When mixed with the nonpathogenic mutant, 10 out of 18 diverse wild-type X. oryzae pv. oryzae strains did not cause disease. Conversely, three of the strains that were not affected by the nonpathogenic mutant were found to inhibit the growth of both the wild-type and mutant Philippine strains, indicating that antagonism is widespread and strain specific. The observed growth inhibition occurred only in planta and did not correlate with bacteriocin activity in vitro. Antagonistic interactions also were found to affect resistance (R) gene-mediated resistance. The R gene Xa21 was capable of protecting rice plants coinoculated with nonantagonistic virulent and avirulent strains; however, when avirulent strains were coinoculated with virulent antagonistic strains, disease ensued. Taken together, these results indicate that X. oryzae pv. oryzae has evolved strategies to compete with rival strains in a fashion that allows virulent strains to evade R gene-mediated protection even when avirulent strains are present in the inoculum.     

Identification of the rice blast resistance gene Pib in the National Small Grains Collection

The Pib gene in rice confers resistance to a wide range of races of the rice blast pathogen, Magnaporthe oryzae, including race IE1k that overcomes Pita, another broad-spectrum resistance gene. In this study, the presence of Pib was determined in 164 rice germplasm accessions from a core subset of the National Small Grains Collection utilizing DNA markers and pathogenicity assays. The presence of Pib was evaluated with two simple sequence repeat (SSR) markers and a dominant marker (Pib-dom) derived from the Pib gene sequence. Pathogenicity assays using two avirulent races (IE1k and IB1) and a virulent race (IB54) were performed to verify the resistance responses of accessions. Of the 164 accessions evaluated, 109 contained the Pib gene as determined using both SSR markers and pathogenicity assays, albeit different haplotypes were detected. The remaining 52 germplasm accessions were different in their responses to the blast races IB54, IE1k, and IB1, thus indicating the presence of R gene(s) other than Pib. The accessions characterized in this study could be used for marker-assisted breeding to improve blast resistance in indica and japonica cultivars worldwide.     

Xa21转基因水稻对白叶枯病的抗性及其遗传

 采用包括与Xa21基因相对应的白叶枯病鉴别小种P6在内的12个国际鉴别小种和我国7个病原型,对通过农杆菌介导的8个不同遗传背景的Xa21转基因品系,及其不同世代的植株,进行接种和抗性分析。结果表明:Xa21在不同的遗传背景下呈现显性,并且在转基因后代中可以稳定遗传,且保留其广谱抗性。在不同的遗传背景下,Xa21的抗性表达略有差异。     

Identification and fine mapping of the new bacterial blight resistance gene, <Emphasis Type="Italic">Xa31(t)</Emphasis>, in rice

Zhachanglong (ZCL), a regional rice variety from Yunnan province in southwest China, has a high level of resistance to a broad spectrum of Xanthomones oryzae pv. oryzae (Xoo) isolates. In a previous study, a bacterial blight (BB) resistance (R) gene, Xa22(t), with resistance against Xoo strain Px061 on chromosome 11 was identified in ZCL. Here, we report another BB R-gene, tentatively named Xa31(t), with resistance against Xoo strain OS105 and susceptible to Px061 identified in ZCL. To determine the location of Xa31(t), 102 polymorphic RFLP markers on 12 rice chromosomes were selected for bulked segregation analysis (BSA). Twelve RFLP markers on chromosome 4 detected DNA polymorphisms between ZCL and Zhengzhu Ai (ZZA), as well as in the OS105-resistant and -susceptible bulks from F₂ populations derived from ZCL × ZZA. Genetic linkage analysis and fine mapping localised Xa31(t) within a genetic distance of 0.2 cM between two RFLP markers, G235 and C600, on the end of the long arm of chromosome 4, using two F₂ populations from the cross ZCL × ZZA and two F₃ populations, consisting of 3,311 plants with 301 F₃ random families and 3,333 plants with 303 F₃ Pxo61-susceptible families, derived from the same F₂ populations from the cross ZCL × ZZA. Using two flanking markers, G235 and C600, to screen the MH63 BAC library, the Xa31(t) locus was limited to one BAC clone with a length of about 100 kb.     

Molecular mapping of the new blast resistance genes Pi47 and Pi48 in the durably resistant local rice cultivar Xiangzi 3150

The indica rice cultivar Xiangzi 3150 (XZ3150) confers a high level of resistance to 95% of the isolates of Magnaporthe oryzae (the agent of rice blast disease) collected in Hunan Province, China. To identify the resistance (R) gene(s) controlling the high level of resistance in this cultivar, we developed 286 F(9) recombinant inbred lines (RILs) from a cross between XZ3150 and the highly susceptible cultivar CO39. Inoculation of the RILs and an F(2) population from a cross between the two cultivars with the avirulent isolate 193-1-1 in the growth chamber indicated the presence of two dominant R genes in XZ3150. A linkage map with 134 polymorphic simple sequence repeat and single feature polymorphism markers was constructed with the genotype data of the 286 RILs. Composite interval mapping (CIM) using the results of 193-1-1 inoculation showed that two major R genes, designated Pi47 and Pi48, were located between RM206 and RM224 on chromosome 11, and between RM5364 and RM7102 on chromosome 12, respectively. Interestingly, the CIM analysis of the four resistant components of the RILs to the field blast population revealed that Pi47 and Pi48 were also the major genetic factors responsible for the field resistance in XZ3150. The DNA markers linked to the new R genes identified in this study should be useful for further fine mapping, gene cloning, and marker-aided breeding of blast-resistant rice cultivars.     

Pathogenesis-related gene expression in rice is correlated with developmentally controlled Xa21-mediated resistance against Xanthomonas oryzae pv. oryzae

Disease resistance mediated by the resistance gene Xa21 is developmentally controlled in rice. We examined the relationship between Pathogenesis Related (PR) defense gene expression and Xa21-mediated developmental disease resistance induced by Xanthomonas oryzae pv. oryzae (Xoo). OsPR1a, OsPR1b, and OsPR1c genes were cloned and their induction was analyzed, in addition to the OsPR10a gene, at the juvenile and adult stages in response to a wildtype Xoo strain that induces a resistance response (incompatible interaction) and an isogenic mutant Xoo strain that does not (compatible interaction). We found that the adult stage leaves are more competent to express these OsPR1 genes and that the Xa21 locus is required for the highest levels of induction.     

Genetic Analysis of the Relationship between the Browning Reaction and Bacterial Blight Resistance Gene Xa3 in Rice

The relationship between bacterial blight resistance gene Xa3 and browning reaction was genetically analyzed using F₂ plants from the cross of rice cultivar Kuntulan with Kinmaze. Kuntulan harbors resistance Xa3 and developes a browning reaction to avirulent races of Xanthomonas oryzae pv. oryzae, whereas Kinmaze has a typical susceptible reaction to all known Japanese races of X. o. pv. oryzae. The F₂ plants were tested for their resistance to avirulent race II strain of X. o. pv. oryzae and the development of browning. Of 337 F₂ plants tested, 251 had resistance to the strain. In all the resistant plants, a browning reaction developed around the point of inoculation. The remaining 86 had a susceptible reaction to the strain without a browning reaction. The F₂ population of Kuntulan × Kinmaze had a clear-cut segregation ratio of 3R : 1S. These facts led to the conclusion that the browning reaction is a pleiotropic effect of Xa3. Received 29 January 2001/ Accepted in revised form 24 April 2001