Molecular detection of Xanthomonas oryzae pv.oryzae, Xanthomonas oryzae pv. oryzicola, and Burkholderia glumae in infected rice seeds and leaves

The polymerase chain reaction(PCR) is particularly useful for plant pathogen detection. In the present study, multiplex PCR and SYBR Green real-time PCR were developed to facilitate the simultaneous detection of three important rice pathogens, Xanthomonas oryzae pv.oryzae, X. oryzae pv. oryzicola, and Burkholderia glumae. The unique PCR primer sets were designed from portions of a putative glycosyltransferase gene of X. oryzae pv. oryzae, an Avr Rxo gene of X. oryzae pv. oryzicola, and an internal transcribed spacer(ITS) sequence of B. glumae. Using a multiplex PCR assay, X. oryzae pv. oryzae, X. oryzae pv. oryzicola, and B. glumae were detected in one PCR reaction that contained the newly developed primer set mix. Using SYBR Green real-time PCR assays, X. oryzae pv. oryzae, X. oryzae pv. oryzicola, and B. glumae were detected at 1, 1, and 10 fg μL-1, respectively. These newly designed molecular assays are sensitive and could be reliable tools for pathogen detection and disease forecasting.

Assessment of genetic diversity of Xanthomonas oryzae pv.oryzae

Bacterial blight of rice,caused by Xanthomonas oryzaepv.Oryzae(Xoo.),is one of the major rice diseases inChina.Making clear the shift of genetic diversity of the

Exchangeability of Two hrp Gene Fragments from Xanthomonas oryzae pv.oryzae and pv.oryzicola for Hypersensitive Response on Tobacco and Pathogenicity on Rice

hrp mutants were produced from strain JXOIII of Xanthomonas oryzae pv. oryzae (Xoo) and strain RS105 of X.o. pv. oryzicola (Xooc), respectively, by using diethyl sulfate (DES) as a mutagenic che mical. All the hrp mutants lost their pathogenicity on a susceptible host plant, rice (Shanyou63), and elicitation of the hypersensitive response (HR) on a nonhost plant, tobacco (NC89). Extracellular enzyme (amy lase, pectate lyase, proteinase, cellulase and lipase) activities of all the hrp mutants were similar to those of the corresponding wild type strains. The response of tobacco to cell sonicated integrations of the wild type strains and the hrp mutants demonstrated that there existed an HR eliciting substance which was heat stable and sensitive to protease. No HR appeared on tobacco after infiltration of the lipopolysaccharide (LPS) of both the wild strains and hrp mutants into tobacco leaves. The ability of the Xooc hrp mutants to induce HR on tobacco and cause streak disease on rice was restored by complementation with pUHRX245 from JXOIII genomic DNA library and by pUHRS138 from RS105 genomic DNA library, respectively. Subcloning of a 38.6 kb hrp fragment insert in pUHRX245 and a 39.3 kb insert in pUHRS138 revealed that a 3.3 kb Sac Ⅰ fragment from pUHRX245 and a 4.5 kb Bam HⅠ Kpn Ⅰ fragment from pUHRS138 were the minimal functional portions required for restoration of the ability of Xooc hrp mutants to induce HR on tobacco and cause disease on rice. The disease symptom caused by the conjugant (M1005 plus 3.3 kb) on rice was similar to that caused by the wild type of Xooc. It suggests that the two fragments contain the same hrp gene(s) and are responsible reciprocally for HR induction on tobacco and pathogenicity on rice.

Photoinduced palladium-catalyzed 1,3-diene-selective fluoroalkylamination compounds as potential bactericidal agent against Xanthomonas oryzae pv.oryzae

A series of photoinduced palladium-catalyzed 1,3-diene-selective fluoroalkylamination derivatives was rationally synthesized based on diversity-oriented synthesis via cross coupling of 1,3-dienes,amines and fluoroalkyl iodides.The reaction featured good function group tolerance and a broad substrate scope,which could be extended to the late-stage modification of bioactive molecules.Bactericidal activity of all the compounds against Xanthomonas oryzae pv.oryzae(Xoo)was evaluated.Among them,compound E14 showed significant activity against Xanthomonas oryzae pv.oryzae(Xoo)with half maximal effective concentration(EC50)value of 6.61μmol/mL.In pot experiments,the results showed that E14 could control rice bacterial blight with protective and curative efficiencies of 37.5%and 63.2%at 200μg/mL,respectively.Additionally,a plausible mechanism for antibacterial behavior of E14 was proposed by electron microscopy,flow cytometry,reactive oxygen species detection,and biofilm assay.In current work,it can promote the development of photoinduced palladium-catalyzed 1,3-diene-selective fluoroalkyl amination compounds as prospective antibacterial agent bearing an intriguing mode of action.

Identification of an avirulence gene,avrxa5,from the rice pathogen Xanthomonas oryzae pv.oryzae

Xanthomonas oryzae pv.oryzae,the causal agent of bacterial blight in rice,interacts with rice plants in a gene-for-gene manner.The specificity of the interaction is dictated by avirulence(avr) genes in the pathogen and resistance(R) genes in the host.To date,no avr genes that correspond to recessive R genes have been isolated.We isolated an avrBs3/pthA family gene,avrxa5,from our previously isolated clone p58,which was originally from strain JXOIII.The avrxa5 gene converted the PXO99A strain from compatible to incompatible in rice cultivars containing the recessive xa5 gene,but not in those containing the dominant Xa5 gene.Sequencing indicated that avrxa5,which is highly similar to members of the avrBs3/pthA family,encodes a protein of 1238 amino acid residues with a conserved carboxy-terminal region containing three nuclear localization signals and a transcription activation domain.It has 19.5 34-amino-acid direct repeats,but the 13th amino acid is missing in the fifth and ninth repetitive units.Domain swapping of the repetitive regions between avrxa5 and avrXa7 changed the avirulence specificity of the genes in xa5 and Xa7 rice lines,respectively.This indicates that avrxa5 is distinct from previously characterized avrBs3/pthA members.The specificity of avrxa5 toward recessive xa5 in rice could help us better understand the molecular mechanisms of plant-pathogen specific interactions.

Genome sequencing of the bacterial blight pathogen DY89031 reveals its diverse virulence and origins of Xanthomonas oryzae pv.oryzae strains

The bacterial pathogen Xanthomonas oryzae pv.oryzae(Xoo),belonging to Xanthomonas sp.,causes one of the most destructive vascular diseases in rice worldwide,particularly in Asia and Africa.To better understand Xoo pathogenesis,we performed genome sequencing of the Korea race 1 strain DY89031(J18)and analyzed the phylogenetic tree of 63 Xoo strains.We found that the rich diversity of evolutionary features is likely associated with the rice cultivation regions.Further,virulence effector proteins secreted by the type III secretion system(T3SS)of Xoo showed pathogenesis divergence.The genome of DY89031 shows a remarkable difference from that of the widely prevailed Philippines race 6 strain PXO99A,which is avirulent to rice Xa21,a well-known disease resistance(R)gene that can be broken down by DY89031.Interestingly,plant inoculation experiments with the PXO99A transformants expressing the DY89031 genes enabled us to identify additional TAL(transcription activator-like)and non-TAL effectors that may support DY89031-specific virulence.Characterization of DY89031 genome and identification of new effectors will facilitate the investigation of the rice-Xoo interaction and new mechanisms involved.

Evaluation of suitable reference genes for normalization of quantitative real-time PCR analysis in rice plants under Xanthomonas oryzae pv.oryzae--infection and melatonin supplementation

Exogenous melatonin(MT)was found to be an interesting tool for enhancing the resistance of rice to Xanthomonasoryzaepv.oryzae(Xoo)-caused bacterial blight(BB).However,the accurate comparison of the expression levels across samples was a challenging task.In this work,the stability of 10 common used housekeeping genes under Xoo-infection and MT supplementation in rice was analyzed using quantitative real-time PCR(qRT-PCR),and algorithms geNorm,NormFinder and BestKeeper.Our results indicated that most reference genes remained stable in Xoo-infected rice plants,while a number of reference genes were affected by MT supplementation.Among all studied genes,the transcript levels of 18S(18S ribosomal RNA)and UBC(Ubiquitin-conjugating enzyme E2)remained unaltered by Xoo infection,while UBC and UBQ5(Ubiquitin 5)were the most stable genes when examining simultaneous Xoo-infection and MT supplementation,demonstrating that UBC is a suitable reference gene for qRT-PCR data normalization in rice under Xoo-infection and MT supplementation.

Molecular Screening of Rice Cultivated in Benin for the Identification of Xanthomonas oryzae Pv. oryzae and Bacterial Leaf Blight Resistance Genes

One of the most devastating diseases of rice worldwide is bacterial blight (BLB) caused by Xanthomonas oryzae pv. Oryzae (Xoo). In Benin, Xoo was first described in 2013 on wild rice Oryzae longistaminata. So far, no study has been done on Beninese Xoo strains. We do not know whether the pathogen has already passed into the rice varieties grown, or if they are exposed to other bacteria. Whereas the use of resistant varieties, carrying resistance genes, is the only highly effective and environmentally friendly way to control this disease, no information is available on these Xoo resistance genes in rice varieties grown in Benin apart from the one we recently. This study aims to identify Beninese Xoo strains, causing BLB and screen rice varieties grown in Benin for the main resistance genes. Diseased rice leaves showing typical symptoms of fire blight collected from different rice fields in the three phytogeographic areas of Benin were analyzed by PCR for Xoo-specific sequence identification. Furthermore, seventy-five collected rice accessions were screened to identify xa5, Xa7, xa13, and Xa21 resistance genes to Xoo. The results reveal that Xanthomonas oryzae was identified in two fields in Banikouara and one in Malanville. On the other hand, Sphingomonas sp. has been identified in several other rice fields in Benin. Forty-seven of seventy-five rice accessions examined (62.66%) carried Xoo resistance genes with 3 (4%) and 40 (53.33%) of xa5 and Xa21 respectively. None of the accessions had either Xa7 or xa13 resistance genes. Three accessions possess both xa5 and Xa21 genes. Isogenic lines IRBB60 and IRBB21, supposed to be a positive control, presented a Xoo sensitivity allele. These results indicate that Xoo has moved from the wild rice variety to the cultivated variety in northern Benin and varietal improvement programs must be implemented with varieties having several resistance genes for the efficient response against a possible BLB pandemic in Benin.

湖北省水稻白叶枯病菌致病型分化检测与分析

为了掌握湖北省水稻白叶枯病菌(Xanthomonas oryzae pv. oryzae)的致病型分化和变异动态,采集湖北省水稻种植区白叶枯病样品分离白叶枯病菌,用含不同白叶枯病抗性基因的近等单基因系IRBB2、IRBB3、IRBB5、IRBB13、IRBB14和IR24作为鉴别寄主,采用剪叶接种,根据寄主和分离菌株的互作反应检测病菌的致病型分化。鉴定结果表明,在湖北省水稻白叶枯病的致病型R9(SSSSSS)占鉴定菌株的46.2%,还有Unknown类型的菌株占38.5%。与前人研究结果相比,湖北白叶枯病菌致病型发生了很大的变异,主要致病型由R4变成了R9,且出现了以前没有的R8(SSRSSS)类型和Unknown类型。同时,鉴定了Xa7和Xa23介导的湖北省白叶枯病菌的抗性,Xa7有2个分离菌株可以克服其抗性,而Xa23能抗所有分离菌株,因此在湖北省可以利用这2个基因进行水稻抗白叶枯病育种。

JXOV菌株2个avrBs3/pthA家族基因克隆分析

本研究克隆了水稻白叶枯病病原(Xanthomonas oryzae pv.oryzae,Xoo)菌株JXOV的2个avrBs3/pthA家族成员基因,分别命名为jva1和jva2。通过氨基酸序列分析和系统发育分析、原核表达载体构建及在水稻品种叶片接种试验,探讨其基因功能和毒性。结果表明,Jva1和Jva2的氨基酸序列具有Avr Bs3/pth A家族成员典型序列模式特征;BL21(DE3)原核表达的Jva1和Jva2蛋白在SDS-PAGE胶上呈现出推测的100KD单体蛋白大小,没有出现不依赖于植物蛋白的Avr Bs3类似的二聚体化;jva1和jva2没有增强PXO99A在携带不同抗性基因的15个水稻品种上的毒性。jva1和jva2的基因功能有待于进一步研究。本研究结果为诠释水稻白叶枯病原的致病机理及相关基因家族成员的功能分化提供了重要参考。

HrcQ is necessary for Xanthomonas oryzae pv. oryzae HR-induction in non-host tobacco and pathogenicity in host rice

Bacterial blight, caused by Xanthomonas oryzae pv. oryzae(Xoo), is one of the most destructive diseases of rice(Oryza sativa L.) worldwide. The type III secretion system(T3SS) of Xoo, encoded by the hrp(hypersensitive response and pathogenicity) genes, plays critical roles in conferring pathogenicity in host rice and triggering a hypersensitive response(HR) in non-host plants. To investigate the major genes conferring the pathogenicity and avirulence of Xoo, we previously constructed a random Tn5-insertion mutant library of Xoo strain PXO99A. We report here the isolation and characterization of a Tn5-insertion mutant PXM69. Tn5-insertion mutants were screened on indica rice JG30, which is highly susceptible to PXO99A, by leaf-cutting inoculation.Four mutants with reduced virulence were obtained after two rounds of screening. Among them, the mutant PXM69 had completely lost virulence to the rice host and ability to elicit HR in non-host tobacco. Southern blotting analysis showed a single copy of a Tn5-insertion in the genome of PXM69. PCR walking and sequencing analysis revealed that the Tn5 transposon was inserted at nucleotide position 70,192–70,201 in the genome of PXO99A, disrupting the type III hrc(hrp-conserved) gene hrcQ, the first gene in the D operon of the hrp cluster in Xoo. To confirm the relationship between the Tn5-insertion and the avirulence phenotype of PXM69, we used the marker exchange mutagenesis to create a PXO99Amutant, ΔhrcQ::KAN, in which the hrcQ was disrupted by a kanamycin-encoding gene cassette at the same site as that of the Tn5-insertion. ΔhrcQ::KAN showed the same phenotype as mutant PXM69. Reintroduction of the wild-type hrcQ gene partially complemented the pathogenic function of PXM69. RT-PCR and cellulase secretion assays showed that the Tn5-disruption of hrcQ did not affect transcription of downstream genes in the D operon and function of the type II secretion system. Our results provide new insights into the pathogenic functions of clustered hrp genes in Xoo.

An improved protein expression system for T3SS genes regulation analysis in Xanthomonas oryzae pv. oryzae

Xanthomonas oryzea pv.oryzae(Xoo)is the causal agent of bacterial blight of rice,which is a significant threat to many of rice-growing regions.The type Ⅲ secretion system(T3SS)is an essential virulence factor in Xoo.Expression of the T3SS is often induced in the host environment or in hrp-inducing medium but is repressed in nutrient-rich medium.The elucidation of molecular mechanism underlying induction of T3SS genes expression is a very important step to lift the veil on global virulence regulation network in Xoo.Thus,an efficient and reliable genetic tool system is required for detection of the T3SS proteins.In this study,we constructed a protein expression vector pH3-flag based on the backbone of pHM1,a most widely used vector in Xoo strains,especially a model strain PXO99A.This vector contains a synthesized MCS-FLAG cassette that consists of a multiple cloning site(MCS),containing a modified pUC18 polylinker,and Flag as a C-terminal tag.The cassette is flanked by transcriptional terminators to eliminate interference of external transcription enabling detection of accurate protein expression.We evaluated the potential of this expression vector as T3SS proteins detection system and demonstrated it is applicable in the study of T3SS genes expression regulation in Xoo.This improved expression system could be very effectively used as a molecular tool in understanding some virulence genes expression and regulation in Xoo and other Xanthomonas spp.

Genetics and Improvement of Bacterial Blight Resistance of Hybrid Rice in China

Since 1980s, rice breeding for resistance to bacterial blight has been rapidly progressing in China. The gene Xa4 was mainly used in three-line indica hybrid and two-line hybrid rice. The disease has been ＇quiet＇ for 20 years in China, yet in recent years it has gradually emerged and been prevalent in fields planted with newly released rice varieties in the Changjiang River valley. Under the circumstances, scientists inevitably raised several questions： what causes the resurgence and what should we do next？ And/or is resistance breeding still one of the main objectives in rice improvement？ Which approach do we take on resistance breeding so that the resistance will be more durable, and the resistance gene will be used more efficiently？ A combined strategy involving traditional method, molecular marker-assisted selection, and transgenic technology should bring a new era to the bacterial blight resistance hybrid rice breeding program. This review also briefly discusses and deliberates on issues related to the broadening of bacterial blight resistance, and suitable utilization of resistance genes, alternate planting of available resistance genes; and understands the virulent populations of the bacterial pathogen in China even in Asia.

Virulence of Xanthomonas oryzae pv. oryzae on Rice Near-Isogenic Lines with Single Resistance Gene and Pyramiding Lines in China

Ninety one isolates of Xanthomonas oryzae pv. oryzae were collected from different rice- growing regions in China and determined for their virulence on 24 rice near-isogenic lines containing single resistance gene and 2-4 genes: IRBB1 (Xa1), IRBB2 (Xa2), IRBB3 (Xa3), IRBB4 (Xa4), IRBB5 (xa5), IRBB7 (Xa7), IRBB8 (xa8), IRBB10 (Xa10), IRBB11 (Xa11), IRBB13 (xa13), IRBB14 (Xa14), IRBB21 (Xa21), IR24 (Xa18), IRBB50 (Xa4 + xa5), IRBB51 (Xa4 + xa13), IRBB52 (Xa4 + Xa21), IRBB53 (xa5 + xa13), IRBB54 (xa5 + Xa21), IRBB55 (xa13 + Xa21), IRBB56 (Xa4 + xa5 + xa13), IRBB57 (Xa4 + xa5 + Xa21), IRBB58 (Xa4 + xa13 + Xa21), IRBB59 (xa5 + xa13 + Xa21) and IRBB60 (Xa4 + xa5 + xa13 + Xa21). The results showed that most isolates were less virulent on lines with more than one genes pyramided than those with single resistance gene. The isolates tested were more virulent on IR24 and IRBB10, less virulent on IRBB5, IRBB7 and IRBB21. Based on interactions between isolates and rice near-isogenic lines, 7 cultivars with single gene (IRBB5, IRBB4, IRBB3, IRBB14, IRBB2, IRBB1 and IR24) were chosen as the differentials, and the tested isolates were classified into 7 virulence groups. The reaction patterns of the 7 groups in order were: RRRRRRR, RRRRRRS, RRRRRSS, RR/SRRSSS, RRRSSSS, RRSSSSS, RSSSSSS. The virulence frequencies were 7.69, 6.59, 14.29, 12.09, 14.29, 28.57 and 16.48% respectively. The elementary system for races identification has been established in China based on the results. It will be possible to compare with races in other countries, and the results will facilitate the development of rice resistance breeding to bacterial blight in China.

Functional identification of phenazine biosynthesis genes in plant pathogenic bacteria Pseudomonas syringae pv. tomato and Xanthomonas oryzae pv. oryzae

Phenazines are secondary metabolites with broad spectrum antibiotic activity and thus show high potential in biological control of pathogens. In this study, we identified phenazine biosynthesis （phz） genes in two genome-completed plant pathogenic bacteria Pseudomonas syringae pv. tomato （Pst） DC3000 and Xanthomonas oryzae pv. oryzae （Xoo） PXO99A. Unlike the phz genes in typical phenazine-producing pseudomonads, phz homologs in Pst DC3000 and Xoo PXO99A consisted of phzC/D/E/F/G and phzC/E1/E2/F/G, respectively, and the both were not organized into an operon. Detection experiments demonstrated that phenazine-l-carboxylic acid （PCA） of Pst DC3000 accumulated to 13.4 IJg L-1, while that of Xoo PXO99A was almost undetectable. Moreover, Pst DC3000 was resistant to 1 mg mL-1 PCA, while Xoo PXO99A was sensitive to 50 IJg mL ~ PCA. Furthermore, mutation of phzF blocked the PCA production and significantly reduced the pathogenicity of Pst DC3000 in tomato, while the complementary strains restored these phenotypes. These results revealed that Pst DC3000 produces low level of and is resistant to phenazines and thus is unable to be biologically controlled by phenazines. Additionally, phz-mediated PCA production is required for full pathogenicity of Pst DC3000. To our knowledge, this is the first report of PCA production and its function in pathogenicity of a plant pathogenic P. syringae strain.

Differential Detection and Analysis of Pathotypes and Differentiation against Xanthomonas oryzae pv. oryzae in Southern China

Pathogenic isolates were collected from different rice-cropping regions in southern China to dissect the pathogenic disintegration and variation of Xanthornonas oryzae pv. oryzae （Xoo）. Two sets of rice-Xoo differential hosts, including Chinese system with such five euhivars as IR26, Java14, Nangeng15, Tetep and Jingang 30 and international system with a series of nearisogenic lines （NILs） including IRBB5, IRBB13, IRBB3, IRBB14, IRBB2 and IR24 carrying different known resistance genes,were used to detect pathogenic disintegration for the reaction between host and pathogen with leaf-clippingmethod at the rice booting stage. The results showed the type of the pathogen were divided intosix pathotypes, i.e. , Ⅰ, Ⅱ, Ⅲ, Ⅳ, Ⅴ and Ⅸ based on the Chinese differential system, and seven pathogenicraces including RI, R2, R3, R4, R5, R8 and R10 based on the international differential system. The pathogenicity frequency of Xoo pathotypes V and IV and pathogenic races R8 and R5 were 27.40% , 19.30% and 44.67% , 15.34%, which were considered to be the prevailing races in southern China. Pathogenic rates of pathotypes Ⅳ, Ⅴ, Ⅸ and races R8, R5 against 500 varieties derived from southernChina were 96.40% , 95.00% , 50.40% , 62.00% , and 42.60%, respectively. Among which pathotype Ⅸ was the most virulent pathotype. The pathotype Ⅴ became preponderant pathotype and the new pathotype Ⅸ grew up quickly.

Evaluation of the Pathotypes of Xanthomonas oryzae pv. oryzae and Preliminary Analysis of the Resistant Reactions of Main Japonica Rice in the Yunnan Plateau, China

The pathogenicity of 36 isolates of Xanthomonas oryzae pv. oryzae （Xoo）, which were collected from japonica rice varieties in the Yunnan Plateau, China, was evaluated. It was evaluated on 29 rice varieties including a set of seven varieties to identify pathogenicity, i.e., Haonuoyang, TN1, Kogyoku, Zhenzhu＇ai, IR26, Nanjing 33, and Kinmaze, which may be considered as a set of differential varieties for Xoo races from Yunnan japonica rice. The efficiency of the seven varieties was further confirmed. The results showed reversible and specific interactions between isolates and varieties. The isolates were classified into nine pathotypes from pathotyp Ⅰ to Ⅸ according to their pathogenic reactions on the seven rice varieties. The pathotype V was the epidemic, whereas pathogen Ⅶ was the most pathogenic. Most japonica varieties grown in the Yunnan Plateau were susceptible to Xoo. The rice lines IRBB21 （Xa-21）, Zhachanglong （Xa-22,, Xa- 24,）, and IR1545-339 （xa-5）, which were resistant to all the isolates tested, can be used as donors of resistant genes for bacterial blight in japonica rice breeding in the Yunnan Plateau.

Tal6b/AvrXa27A,a hidden TALE targeting the susceptibility gene OsSWEET11a and the resistance gene Xa27 in rice

Xanthomonas oryzae pv.oryzae(Xoo)secretes transcription activator-like effectors(TALEs)to activate rice susceptibility(S)genes,causing bacterial blight(BB),as well as resistance(R)genes,leading to de-fense against BB.This activation follows a gene-for-gene paradigm that results in an arms race between the TALE of the pathogen and effector-binding elements(EBEs)in the promoters of host genes.In this study,we characterized a novel TALE,designated Tal6b/AvrXa27A,that activates the rice S gene OsSWEET11a and the rice R gene Xa27.Tal6b/AvrXa27A is a member of the AvrXa27/TalAO class and contains 16 repeat variable diresidues(RVDs);one RVD is altered and one is deleted in Tal6b/AvrXa27A compared with AvrXa27,a known avirulence(avr)effector of Xa27.Tal6b/AvrXa27A can transcriptionally activate the expression of Xa27 and OsSWEET11a via EBEs in their corresponding promoters,leading to effector-triggered immunity and susceptibility,respectively.The 16 RVDs in Tal6b/AvrXa27A have no obvious similarity to the 24 RVDs in the effector PthXo1,but EBETal6b and EBEPthXo1 are overlapped in the OsSWEET11a promoter.Tal6b/AvrXa27A is prevalent among Asian Xoo isolates,but PthXo1 has only been reported in the Philippine strain PXO99A.Genome editing of EBETal6b in the OsSWEET11a pro-moter further confirmed the requirement for OsSWEET11a expression in Tal6b/AvrXa27A-dependent susceptibility to Xoo.Moreover,Tal6b/AvrXa27A resulted in higher transcription of Xa27 than of OsSWEET11a,which led to a strong,rapid resistance response that blocked disease development.Thesefindings suggest that Tal6b/AvrXa27A has a dual function:triggering resistance by activating Xa27 gene expression as an avirulence factor and inducing transcription of the S gene OsSWEET11a,resulting in virulence.Intriguingly,Tal6b/AvrXa27A,but not AvrXa27,can bind to the promoter of OsSWEET11a.The underlying recognition mechanism for this binding remains unclear but appears to deviate from the currently accepted TALE code.

The MRE11–ATM–SOG1 DNA damage signaling pathway confers rice immunity to Xanthomonas oryzae

Plants are constantly exposed to microbial pathogens in the environment.One branch of innate plant immunity is mediated by cell-membrane-localized receptors,but less is known about associations between DNA damage and plant immune responses.Here,we show that rice(Oryza sativa)mesophyll cells are prone to DNA double-stranded breaks(DSBs)in response to ZJ173,a strain of Xanthomonas oryzae pv.oryzae(Xoo).The DSB signal transducer ataxia telangiectasia mutated(ATM),but not the ATM and Rad3-related branch,confers resistance against Xoo.Mechanistically,the MRE11–ATM module phosphorylates suppressor of gamma response 1(SOG1),which activates several phenylpropanoid pathway genes and prompts downstream phytoalexin biosynthesis during Xoo infection.Intriguingly,overexpression of the topoisomerase gene TOP6A3 causes a switch from the classic non-homologous end joining(NHEJ)pathway to the alternative NHEJ and homologous recombination pathways atXoo-induced DSBs.The enhanced ATM signaling of the alternative NHEJ pathway strengthens the SOG1-regulated phenylpropanoid pathway and thereby boosts Xoo-induced phytoalexin biosynthesis in TOP6A3-OE1 overexpression lines.Overall,the MRE11–ATM–SOG1 pathway serves as a prime example of plant–pathogen interactions that occur via host non-specific recognition.The function of TOP6-facilitated ATM signaling in the defense response makes it a promising target for breeding of rice germplasm that exhibits resistance to bacterial blight disease without a growth penalty.

水稻白叶枯病菌在寄主体内外不同生长条件下致病基因差异表达的分析

To demonstrate the expression profiling of Xanthomonas oryzae pv.oryzae(Xoo) in vitro and in planta,DNA microarrays of 371 genes potentially associated with pathogenicity and virulence were used to compare the transcriptional level alteration of the wild-type strain PXO99A and gene deletion mutants ΔgacAxoo and ΔfleQxoo of Xoo grown in the rich medium NBY vs.hrp-inducing minimal medium XOM2 or leaf tissues of rice.Results indicated that 17 and 38 genes of PXO99A were differentially expressed in XOM2 and the leaf tissues of rice relative to NBY,respectively.Twenty-eight genes of ΔgacAxoo grown in XOM2 and 12 genes of ΔfleQxoo in NBY were differentially expressed relative to PXO99A.The identification of differentially-expressed genes,GacAxoo-and FleQxoo-regulons and novel candidate genes of Xoo strains would provide us the target genes for further functional analysis in pathogenesis of Xoo.