Advance in Rice Bacterial Leaf Streak Researches

Rongsheng ZHANG,Zhiyi CHEN,Yongfeng LIU

Institute of Plant Protection,Jiangsu Academy of Agricultural Sciences,Nanjing 210014,China

Bacterial leaf steak(BLS)of rice is caused by Xanthomonas oryzae pv.oryzicola.It is an important quarantine bacterial disease in rice production.Rice bacterial leaf steak is a fulminant and destructive epidemic.It is a major disease in the rice-planting areas in South China and Southeast Asian countries.When weather conditions are suitable,rice bacterial leaf streak can lead to a 40%-60% of yield loss,threatening seriously the high and stable-yield production of rice[1].Although a lot of attention has been paid to the resistance identification and screening of resistance sources,the bacterial leaf streak-resistant rice cultivars are not yet bred.In current rice production,thiazoles are mainly used to control rice bacterial leaf steak[2].With the transporting of rice seeds in production,rice bacterial leaf streak trends to be expanded and aggravated.In order to better guide the scientific control of the disease,the rice bacterial leaf streak was reviewed from the perspectives of occurrence,classification status,mechanism and resistance breeding in this paper.In addition,the areas on which the further researches should be focused were also proposed.

Bacterial Leaf Streak of Rice

Occurrence and classification

Rice bacterial leaf streak first occurred in the Philippines in 1918[3].Currently,it has outbroken in Southeast Asia and Central Africa.Rice bacterial leaf streak first occurred in the Pearl River Delta throughout China in 1953.In China,rice bacterial leaf streak mainly appeared in rice-planting areas in South China.But in recent years,it has also started to occur in late hybrid rice on a large area of the Yangtze River basin.Rice bacterial leaf streak is the largest threat for indica production.In recent 10 years,rice bacterial leaf streak has become the major bacterial disease in rice-planting areas in South China and South Central China.Its damage has exceeded that brought by rice bacterial blight.In severe conditions,the yield loss caused by rice bacterial leaf steak can be up to 40% -60%.According to the pathogenic phenotype,genotype and chemical classification,Xanthomonas oryzae pv.oryzae (Xoo) and Xanthomonas oryzae pv.oryzicola (Xooc)are considered to be the 2 pathogenic variants of X.oryzae.

Symptoms

Rice bacterial leaf streak mainly damages rice leaf blades.The typical bar-type spots can be shown as early as in the seedling stage.Xooc enters the rice leaf tissue through stomas,infecting and damaging parenchyma cells.At the early onset,the spots are dark green,translucent and watersoaked.Then they gradually extend along the veins.Since restricted by veins,the spots finally formed into streaks[4].When rice bacterial leaf steak occurs in field,the streaks on susceptible cultivars will longitudinally extend fastly.Some streaks can be as long as 4-6 cm.At both ends of the steaks,there is much bacterial pus,which is caviar-like.After dried,the pus will be amber-like and attached to the surface of infected leaves.The dried bacterial pus is difficult to fall.There is usually much small bead-like bacterial solution on the surface of streaks.In severe cases,several spots will be connected to each other and form into necrosis lesions.Then the necrosis lesions will extend further,resulting in the generation of wholey redbrown,irregular tan and even withered leaves.The symptoms above are similar to those of bacterial leaf blight.However,the steaks of bacterial leaf steak are translucent.The symptoms of rice bacterial leaf steak are more apparent at the boot stage.On the bacterial leaf steak-resistant cultivars,the length of streaks are smaller (less than 1 cm).Moreover,the steaks are less and the bacterial pus is less.

Detection of pathogenic bacteria of rice bacterial leaf streak

Traditional detection techniques Produced-area quarantineDuring the transporting and before the introducing of seeds,particularly at the boot stage,the produced-area quarantine against rice bacterial leaf streak is effective and necessary to be carried out.

Observation of seedling growthThe rice seeds harvested from the bacterial leaf streak-infected rice plants are nursed in pot experiments and field trials to observe whether the rice bacterial leaf streak will outbreak again.

Isolation methodThe Xanthomonas spp.can be isolated from rice seeds with semi-selective culture medium XOS.Then the Xanthomonas oryzae can be detected.

Detection of phagesAdhikari et al.[5]ever reported that the rice bacterial leaf streak can be detected from phages.Although this detection method is simple and rapid,false negative and even missing may occur due to the specialization of phages.

Detection of pathogenicityThe pathogenicity of rice bacterial leaf streak can be detected in vitro[6].

Serological detection techniqueFang et al.[7]investigated the differences in serology among pathogens of rice bacterial blight and leaf streak of rice and Leersia spp.Swartz.They found rice bacterial leaf streak has its own unique serological specificity.Xie et al.[8]developed an IRMA assay for rapidly detecting rice bacterial leaf streak.The sensitivity of the detection method is 100 -1 000 cells/ml.The cross-reaction can not only occur between prepared antibody and rice bacterial leaf streak but also between prepared antibody and rice bacterial blight.However,the cross-reaction between prepared antibody and other pathogens is extremely mild.The serological detection method is sensitive and rapid.However,a false positive may occur due to the poor quality and specialization of antiserum.

PCR detection techniqueWith the development of molecular detection techniques,PCR detection technique has been widely used in plant quarantine due to its specialization,rapidness,sensitivity and simplicity.At current,a variety of PCR detection techniques have been developed for rice bacterial leaf streak,such as TaqMan real-time PCR[9-10],which achieves the real-time detection of rice bacterial leaf streak and rice bacterial blight through 2 specific probes.However,this method has high technical requirements.Moreover,it also requires expensive equipment.So its popularization and promotion are restricted to some extent.The detection methods of rice bacterial leaf streak and rice bacterial blight have also been developed from the perspective of specific primers[10].However,since these 2 kinds of pathogens are pathogenic variants of Xanthomonas oryzae,they are very similar in the molecular genetics.In addition,cross-reaction may occur during the PCR.So the 2 kinds of pathogens are difficult to separate completely with the specific primerbased PCR techniques.Rice bacterial leaf streak and rice bacterial blight can occur in the same plants and even in the same leaves[11].So rice seeds may simultaneously carry pathogens of both rice bacterial leaf streak and rice bacterial blight.These 2 kinds of pathogens need to be detected out separately.Zhang et al.[12]found the genes of electron transfer flavoprotein α subunit and transmembrane proteins of pathogens of rice bacterial leaf streak and rice bacterial blight though comparing genetic sequences registered in GenBank.Then they designed the specific primers so as to realize the specialized detection of pathogen of rice bacterial leaf streak.Hu et al.[13]developed padlock probe-based Macroaary detection methods for rice bacterial leaf streak and rice bacterial blight.This kind of detection methods has higher specificity,sensitivity and stability and has been patented in 2012.

Primary infection sources,transmission routes and occurrence regularity

The overwintering sites and viability of pathogen of rice bacterial leaf streak are similar to those of rice bacterial blight.They all overwinter mainly on the infected rice grains and straw,which will become the primary infection source for the next year.The pathogen of rice bacterial leaf streak infects rice seeds.Then with the transporting of rice seeds,the pathogen is transmitted long-distancely.The pathogen will contact rice seedlings mainly through irrigation water and rainfall.They invade in rice tissue through stomas or wound.After successfully invasion,the pathogen will propagate below the stomas,then expand to the spacing among parenchyma cells and then expand longitudinally,forming streaks.The bacterial pus on the diseased lesions will be transmitted around through wind,rainfall and water,resulting in the re-infection.Farming operations can also lead to the transmission of pathogens.The epidemicity of rice bacterial leaf streak is determined by the disease resistance of rice cultivars,climatic conditions and cultural practices.Although the bacterial leaf streak-resistant rice cultivars have not yet been found,there are still significant differences in resistance against bacterial leaf streak among rice cultivars.Generally,the resistance of conventional rice cultivars is stronger than that of hybrid rice cultivars,and the resistance of japonica and glutinous rice is stronger than that of indica rice[14-15].In addition,the resistance of rice cultivars is also related to stoma density and size.In general conditions,the lower the stoma density is and the smaller the opening degree of stoma is,the stronger the resistance of rice cultivars is.

If the amount of pathogen in the field is large enough and the rice that is susceptible to rice bacterial leaf streak is planted in a certain area,the epidemicity of rice bacterial leaf steak will be mainly determined by temperature and precipitation.The optimum temperature for outbreak of rice bacterial leaf streak is 30 ℃.During the storm,especially during the typhoon in summer,a large quantity of wounds will appear in the rice leaves,which is conducive to the invasion and transmission of pathogen,resulting in the large-area epidemicity of rice bacterial leaf streak.Considering the occurrence of rice bacterial leaf streak,China can be divided into flowing three main areas:South China (epidemic),Jiang -Huai River basin (accidental)and North China (nonoccurrence).Rice bacterial leaf streak is epidemic in the lower areas of the Yangtze River generally during mid-June to mid-September.The difference in the epidemicity of rice bacterial leaf streak among years mainly depends on the precipitation and humidity.In addition,the occurrence of rice bacterial leaf streak is also closely related to cultivation and management,especially to the irrigation and fertilization.Generally,deep irrigation,constant irrigation,partial fertilization and late fertilization all can lead to the occurrence of rice bacterial leaf streak.

Control strategies

Rice bacterial leaf streak is difficult to control singly depending on chemical agents.Therefore,integrated control measures should be adopted.

Strengthening seed quarantineThe seed producers and sellers should follow the rules in the seed production and transporting processes.The relevant administrative departments should perform good quarantine for rice seeds.The introduction of rice seeds are prohibited from epidemics.The infected seeds are also forbidden from being transmitted long-distancely aiming at avoiding the expansion of epidemics.

Selecting highly disease-resistant cultivarsThe cultivation of highly disease-resistant rice cultivars is an effective way to control rice bacterial leaf streak.The indica rice can be replaced by japonica rice,such as Jinyou 9113,Dingyou 180 and Zhuliangyou[15].

Seed treatmentBefore the sowing,the rice seeds can be soaked in 300-400 times-diluted 8% Strong Chlorine or 1 000 times-diluted 4.2%Jinfeng for disinfection.The rice seeds can be first soaked in water for 12 h and then soaked in drugs for 12 -24 h.After then,the rice seeds should be washed 3-4 times with water.

Strengthening fertilizer and water managementThe irrigation should comply with the concepts of shallowness,thinness,wetness and sunning.The deep irrigation,constant irrigation or excessive irrigation is all forbidden.At the same time,the fertilization should also be reasonable.The excessive N fertilization is forbidden at the middle stage.

Medicine prevention

Chemical controlDuring the 3-leaf stage to the transplanting,the 300 times-diluted Strong Chlorine or 400-500 times-diluted 20% Bismerthiazol can be sprayed once on the leaf surface of rice.Another spraying can be performed 2-3 d before the transplanting.Once rice bacterial leaf streak occurs in field,chemical agents must be sprayed timely,avoiding the spreading of rice bacterial leaf streak caused by crossed irrigation or excessive irrigation.In particularly,chemical agents should also be sprayed timely after rain and waterlogging.

Biological controlCurrently,thiazole fungicides are mainly used to control rice bacterial leaf streak[2].However,the long-time application of biological agents may increase the proportion of drug-resistant variants in the pathogens,leading to the generation of drug resistance.Even worse,the drug resistance can be inherited stably[16].It there are no effective chemical agents or disease-resistant rice cultivars[17],the biocontrol bacteria can be developed to control rice bacterial leaf streak,and this is a worthwhile prevention way.In addition,the biocontrol bacteria have been considered to be a kind of potential biocontrol factors in prevention and control of plant diseases[18].Zhu et al.[19]found the control effects of Helminthosporium gramineum Rabenh.f.sp.echinochloae and its metabolites indoor against rice bacterial leaf streak are significantly better than those of 20% Bismerthiazol WP.The minimum bactericidal concentration of crude toxin of Helminthosporium gramineum Rabenh.f.sp.echinochloae against rice bacterial leaf streak is 1.562 5 mg/ml.Shen et al.[20]considered the mixture of 2 kinds of antagonistic bacterial strains and chemical agents has better control effects against rice bacterial leaf streak compared to those of single chemical agents.The mixture of antagonistic bacterial strains and chemical agents can partially replace pesticides.The starch-degrading Bacillus Lx-11,isolated by Zhang et al[21],has a better growth-promoting effect.In addition,the control effect of Lx-11 against rice bacterial leaf streak is also better than that of 20%Bismerthiazol.

Genetic Diversity and Pathogenicity Differentiation of Rice Bacterial Leaf Streak

In the microbial taxonomy,the strains of certain specieses and genera with close generic relationship are often difficult to distinguish due to overlap between specieses with important classification features and effects of environment.Some samely named strains may belong to different specieses,or some strains that come from the same species may be named differently.The genes of many bacteria all contain short repetitive sequences that are of high homology with those of REP,ERIC and BOX.Moreover,the short repetitive sequences are highly conserved among different genus,specieses or strains.Different regions of the genome among REP,ERIC,BOX and other insertion elements are selectively amplified by PCR,and then the amplification products are separated by agarose gel electrophoresis.The techniques above are collectively referred to as rep-PCR DNA fingerprinting[22].At current,the rep-PCR DNA fingerprinting technique has been widely used in the classification,detection and genetic diversity analysis of pathogenic bacterial populations of Acidovorax,Agrobacterium,Clavibacter,Pseudomonas,Rhizobium,Xanthomonas,etc[21-25].The bacteria can be classified and identified at the species,subspecies,pathogenic variant and stain levels through comparing the fingerprints among different strains,revealing the genetic relationships among pathogenic bacteria and the changes in genetic diversity in bacterial populations.In China,there are different types of rice-planting areas,including indica rice-planting areas,japonica rice planting areas and indica-japonica rice planting areas.In the study on pathogenicity differentiation,the pathotype and variant of pathogenic bacteria will be entirely different if the tested rice cultivars are different.Through combining the rep-PCR technology and cultivar identification,the relationship between population structure change and pathogenicity differentiation of pathogens of rice bacterial leaf streak can be further studied.Thus the disease-resistant rice cultivars can be distributed reasonably.

Application of molecular markers in populational genetics of pathogenic bacterial of rice bacterial leaf streak

Ji et al.[26]studied the DNA polymorphism of pathogens of rice bacterial leaf streak that occurs in different rice-planting areas in China using randomly-screened 20 different primers.

They found there was a weak correlation between pathogenic populations and genetic variation of strains.Gonzales et al.[27]carried out a cluster analysis on the pathogenic strains of rice bacterial leaf streak in West Africa and Asia using RFLP and rep-PCR technology.They found the pathogenic populations are related to the origins of strains.Raymundo et al.[28]used repetitive DNAsequences as probes.Then they performed a RFLP analysis on pathogens of rice bacteria leaf streak.In addition,a chromosomal DNA fingerprinting analysis was also carried out though digesting genomic DNA with Pst I.The 124 strains from Philippines were classified as 4 lineages with genetic diversity of 0.92.Ji et al.[23]divided the leaf streak pathogens into 6 and 10 clusters respectively at 80%of similarity rate using ERIC and BOX-PCR.In addition,the resolutions are also inconsistent among different primers.The dendrogram constructed based on the comprehensive analysis on the 2 pairs of primers-amplified DNA fingerprints can reflect more accurately the pathogenic bacterial genetic diversity of rice bacterial leaf streak compared to that constructed based on the analysis on the 1 pair of primers-amplified DNA fingerprint.Zhen et al.[24]could distinguish the pathogenic bacterial of rice bacterial leaf streak from China,Japan and Philippines using specific primers,IS1113 and ERIC.The strains that come from the 3 countries are mainly concentrated in the 2ndand 3rdclusters.In addition,the strains that come from China and Philippines have their own specialized differentiation.Zhang et al.[29]studied the genetic diversity of pathogenic bacteria of rice bacterial leaf streak from Jiangsu,Yunnan,Jiangxi,Hunan and Anhui using ERIC and BOX-PCR primers.They also found the genetic clustering is closely related to geographic position.

Pathogenicity differentiation of rice bacterial leaf streak

Pathogenicity differentiation at the species levelThe study on pathogenicity differentiation of pathogenic bacteria of rice bacterial leaf streak will contribute to the breeding of disease-resistant cultivars and forecasting of epidemics.It is also a

fundamental work for controlling diseases.Guo et al.[30]determined the pathogenicity differentiation of 62 pathogenic strains of bacterial leaf streak in rice-planting areas in South China.The tested pathogenic strains of bacterial leaf streak can be divided into 6 small populations.They considered there is a weak interaction between pathogenic strains and cultivars,and there is even a strong interaction between partial pathogenic strains and cultivars.In the study on pathogenicity differentiation of pathogenic bacteria of plants,the selection of tested cultivars is extremely important.The classification of pathotype and physiological variant of pathogenic bacteria will be entirely different for different test cultivars.Liu et al.[31]used Jingang 30,Zhaiyeqing,XM5,XM6 and M41 as the tested rice cultivars.They divided the 75 pathogenic strains of bacterial leaf streak in South China as 7 pathotypes.Wang et al.[32]determined the pathogenicity of 75 pathogenic strains of bacterial leaf streak in IRBB4,IRBB5,IRBB14,IRBB18,IRBB21 and IR24 in Southwest China,and the 75 pathogenic strains are divided into 13 small populations.Chen et al.[14]investigated the virulence of 82 pathogenic strains of bacterial leaf streak from IRBB4,IRBB5,IRBB14,IRBB21,IR24 and Jingang 30 in Xuhuai area and divided the 82 pathogenic strains into 8 pathotypes.The classification standards for variants of pathogenic bacteria of rice bacterial leaf streak are still not yet unified.In different ecological rice planting areas,the tested rice cultivars are different.So the classification of pathotype and pathogenic variant of bacterial strains of rice bacterial leaf streak are also entirely different.However,IRBB4,IRBB5,IRBB14,IRBB21,IR24 and Jingang 30 are commonly used[14,29].At current,most of the studies on pathogenicity differentiation of bacterial strains of rice bacterial leaf streak are focused on adult rice plants.There are rare reports on the pathogenicity differentiation of bacterial strains of bacterial leaf streak in rice seedlings.

Difference in pathogenic variants of rice bacterial leaf streak at the gene levelThe results of genomic sequencing show the quantity of avrBs3/pthA genes determines the virulence differentiation of bacterial strains of bacterial leaf streak in rice,which is predicted from the functions of avrBs3/pthA genes in different Xoos within the same genus[33].There are 28 and 33 avrBs3/pthA genes respectively in the genomic sequences of BLS256 and BLS303 from Philippines[34],and at least 20 avrBs3/pthA genes exist in the genomic sequence of RS105 from China[35-37].However,it is still needed further study to classify pathogenic strains of rice bacterial leaf streak based on the quantity of avrBs3/pthA genes in the genomic sequences.

Pathogenic Genes of Rice Bacterial Leaf Streak

Characteristics of hrp gene cluster of Xanthomonas oryzae

The gram-negative plant-derived bacteria can cause the withering,wiltng,ulcers and leaf spots of plants.They have pathogenicity for host plants and can lead to allergic reaction in antagonistic hosts and non-host plants.The pathogenicity for host plants is mainly determined by hrp gene cluster.The hrp gene cluster consists of hrp,hrc and hpa,which encode combinedly the type III secretion system.The pathogenic bacteria can inject effector proteins directly into plant cells through the type III secretion system.Zou et al.[38]successfully cloned the entire hrp gene cluster by referring to the genomic sequence of Xooc.In addition,the complete sequences of hrp gene cluster are obtained.The hrp gene cluster of Xooc consists of core gene cluster and regulatory gene cluster.The core gene cluster is composed by 27 hrp genes,including 8 hpa (hrp-associated)genes,9 hrc (hrp-conserved) genes and 10 hrp genes.Based on the structural conservatism of T3SS components in pathogenic bacteria in animals and plants,it can be speculated that HrcV,HrcT,HrcR and HrcS form a complex that is located on the inner membrane of bacteria.HrcC is located on the outer membrane of bacteria[39-41].The effector molecules are secreted through T3SS to the top of Hrp pilus that are connected to HrcC and extend into host cell wall[42-43].Then the effector molecules are transferred into host cells by HrpF that is located on the cell membrane[44-45].The study also found that hrcQ and hrcJ are key factors for the pathogenicity of bacterial strains of rice bacterial leaf streak and the stimulation of allergic reactions in non-host plants[46-47].

Exopolysaccharide(EPS)

The extracellular polysaccharide(EPS) of a variety of plant-derived pathogenic bacteria,such as Pseudomonas,Erwinia amylovora and pathogenic bacteria of corn wilting and cabbage black rot,have been proved to be one of the major virulence factors[48].The generation of EPS in Xanthomonas is mainly controlled by gum,xan and wxoc gene clusters.However,the genetic composition of the 3 gene clusters differs greatly among different Xanthomonas spp[49].The genetic composition of wxoc gene cluster differs greatly between the 2 variants of Xan thomonas oryzae,and wxocA and wxocB only exist in Xooc but not in Xoo[50].Zhou et al.[51]found the xopQ,pilY and fimO,of which the gene functions are known,are involved in the formation of EPS in Xanthomonas,and the virulence of variants of which the EPS is increased is not significantly increased.

Diffusible signal factor(DSF)

Intercellular signal transduction exists widespread in monera.This intercellular signal transduction is often called as quorum sensing (QS).The DSF-dependent QS system found in Xanthomonas is a new regulatory mechanism[52-53].Studies have reported rpf gene cluster,composed of rpfF,rpfC and rpfG,is the core genes of DSF-dependent QS system.rpfF is a key enzyme that catalyzes the synthesis of DSF signaling molecules.rpfG(response regulator protein) and rpfC(sensing phosphokinase) form a twocomponent system to sense and transmit DSF signals among cells[54,56].The genomic and genetic analysis shows that DSF-dependent QS signaling pathway regulates a variety of biological functions,such as pathogenicity,mobility and biofilm dissipation[57].Zhao et al.[58]compared the rpfF wild mutant strains at the proteomics level.They found the expression of 48 proteins is up-regulated.Among them,18 proteins are mainly involved in nitrogen transportation,protein folding,scavenging of superoxide radicals and flagella formation.So they considered the DSF signaling molecules play an important role in the virulence of pathogenic strains of rice bacterial leaf streak.Yin et al.[59]found FlgD and FlgE are the necessary factors for the flagella formation in pathogenic strains of rice bacterial leaf streak; DSF affects the virulence of pathogenic strains of rice bacterial leaf streak through regulating the expression of flgDxoc and flgExoc.

Resistance Breeding of Rice Bacterial Leaf Streak

Cultivar resistance is the result of interaction between host and pathogen.The utilization of diseaseresistant cultivars can effectively and economically control diseases in plants.In addition,this mean will produce no environmental pollution.Although a lot of attention has been paid to the resistance identification and screening of resistance sources,the bacterial leaf streak-resistant rice cultivars are not yet bred.Therefore,the exploration and utilization of new resistance sources,the positioning and cloning of resistance genes and the indepth understanding of the mechanism of interaction between pathogenic bacteria and rice are all of great significance for controlling rice bacterial leaf streak.

Resistance identification of rice bacterial leaf streak

The resistance sources are extremely rice in cultivated rice against bacterial leaf streak.Wang et al.[60]determined the resistance of 3 343 rice cultivars (lines).They found the resistant cultivars account for 5.77% and the moderately resistant cultivars account for 15.55%,indicating the presence of abundant resistance sources in rice germplasms.Xia et al.[61]identified the resistance of 969 rice cultivars at the seedling stage.The results showed the resistant cultivars account for 14.55%; the moderately resistant cultivars account for 55.83%; a few rice cultivars have strong anti-expansion capability.Xiao et al.[15]identified the resistance of 54 rice cultivars in Hunan Province.There are 8 rice cultivars of which the resistance is higher than the average,accounting for 14.81% of all the tested rice cultivars.Chen et al.[14]studied the resistance of different rice cultivars in Jiangsu Province.They found the resistance cultivars account for 0% and 40.00%of all the tested middle indica rice cultivars and middle hybrid japonica rice cultivars,respectively.In the future,the breeding of bacterial leaf streakresistant cultivars should be strengthened;the resistance of rice cultivars in large-scale cultivation and regional trial against bacterial leaf streak should be monitored timely;the susceptible cultivars should be eliminated timely.Thus the resistance of rice cultivars against bacterial leaf streak will be improved.

Genetic analysis of resistance genes against rice bacterial leaf streak

The results of genetic studies on resistance against bacterial leaf streak differ among different researchers.Zhang et al.[62]divided the rice cultivars according to the distribution characteristics of lesion length.They considered the resistance of Dular and IR36 is controlled by a pair of major genes.However,Zhou et al.[63]considered the resistance of Dular and IR36 against bacterial leaf steak is controlled by 2 pairs of recessive genes.He et al.[64]thought the resistance of hybrid rice depends on the restorer; the resistance of disease-resistant restorer is controlled by 1-2 pairs of major genes;the resistance is a dominant inheritance.Huang et al.[65]found the resistance of 8 conventional wild rice cultivars is all a recessive inheritance.Due to the presence of gene-to-gene inhibitor[66],the pathogenicity differentiation occurs during the interaction between rice and pathogenic bacteria of rice bacterial leaf streak.So the resistance of rice cultivars against bacterial leaf streak is inherited as a quantitative trait.Tang et al.[67]considered the resistance of Acc8518 and Acc8558 against bacterial leaf streak is a quantitative trait that is controlled by multiple genes.The differences in inoculation strains and genetic composition of tested material all affect the judgment of designed resistance.At current,the studies on the genetic laws of resistance of rice cultivars against bacterial leaf streak are only at the initial stage.The genetic composition and laws of resistance of the same disease-resistant cultivar and different tested cultivars in different hybrid combinations against different strains are not consistent,and this still needs further study.

Exogenetic rice bacterial leaf streak-resistant genes

The cloning of resistance genes against bacterial leaf streak has made no significant progress.So far,no resistance genes against bacterial leaf streak in rice have been successfully cloned.However,non-rice resistance gene singly against bacterial leaf streak has been identified in the domestic and foreign related researches.Rxol is the first non-host resistance gene that is cloned from cereal crop(maize)[68].It is an excellent resistance source for modified rice,especially hybrid rice,against bacterial leaf streak.Therefore,with the rapid development of genetic cloning and plant genetic transformation techniques,it will become possible to improve rice resistance and breed bacterial leaf streakresistant cultivars with Rxol.

Issues and Prospect

Currently,rice bacterial leaf streak may still possibly outbreak in local riceplanting areas in China in different years,threatening potentially the safe production of rice.Rice bacterial leaf streak has an incubation period.The pathogenic bacteria may have invaded rice plants before the onset,but without apparent symptoms.When weather conditions are suitable,the disease will be spread rapidly.Therefore,the early diagnosis,forecasting,epidemic laws and effective control measures of rice bacterial leaf steak still need systematic studies.A great genetic differentiation has occurred in the pathogenic strains of bacterial leaf streak.The emergence and spread of new pathogenic strains seriously threaten the effectiveness of existing resistance genes.However,the studies on resistance genes against bacterial leaf streak still need to be further strengthened.The resistance of different rice cultivars against bacterial leaf streak is extremely significantly different,indicating the presence of pathogenicity differentiation.It is still unclear whether there are genetic differences in pathogenic factors among different pathogenic strains.The mechanism of interaction between rice and pathogenic strains of bacterial leaf streak also remains unclear.The key pathogenic factor,type III secretion system,has been studied deeply.However,the roles and action mechanisms of other pathogenic factors,such as EPS,pectinase,cellulase,protease,toxins and effector proteins secreted through the type III secretion system still remain unknown.

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