| Xanthomonas oryzae pv.oryzae(Xoo) is one of the most important bacterial pathogens,which caused bacterial blight in rice.This pathogen possesses hrp genes and avirulence gene(avr) or virulence gene(vir) like other gram-negative plant pathogenic bacteria.The hrp genes confer the pathogenic bacteria the ability to trigger hypersensitive response(HR) on non-host plants and to cause diseade on host plants.The avirulence(avr) or virulence(vir) genes determine host specificity via gene-for-gene interactions.Research on avr genes in Xoo can help to identify and clone R genes from rice and to understand the molecule mechanisms between pathogenic bacteria and host plant.There are many avrBs3/pthA genes in Xoo,in order to understand the existance of avirulence genes in Xoo strain PXO99,its genomic library was screened by using a probe of avrXa3 which covers a 2.67kb- avrXa3 gene from Xoo JXOâ…¢.87 positive clones were screened out of 1100 library clones.It indicates that there are multiple avrBs3/pthA genesin PXO99 genome.The positive cosmids DNA was digested with BamHI and Southern blot showed that there were 13 different clones in the pathogenic genome.The avrBs3/pthA genes occurred at isolated individual portions or in clusters.This study definitude the quantity,types and arrange manner of avirulence gene in the genome of PXO99.And it may provide some clues for further study.According to the previouse work done in our lab,an avirulence gene mutant PXOâ–³avr was constructed by homologous recombination of avrXa3 from Jxoâ…¢in PXO99.In this study,by comparing genomic DNA Southern blot result of PXO99 and PXOâ–³avr,for mutant PXOâ–³avr,three bands disapeared in size 4.3Kb,3.3Kb,2.9Kb and two bands weakened in size 4.0Kb and 3.6Kb as compared to wild-type.By analyzing the above 87 positive cosmid clones screened from PXO99 genomic library,three cosmid clones P5,P8 and P67 were found to contain five,three and two of the same size bands respectively as that changed in PXO99â–³avr.P5,P8 and P67 were introduced into PXOâ–³avr, pathogenicity tests in near isogenic lines of rice confirmed that the cosmid P5 could restored the pathogenicity of PXOâ–³avr;cosmid P8 made no change;cosmid P67 partly restored the pathogenicity of PXOâ–³avr.The results showd bands in cosmid P5 were relevant with the changed bands in PXOâ–³avr,but as to what role of each band play in the pathogenicity is still needed to be revealed.By screening the genomic library of PXO99,two avrBs3/pthA genes P3.1 and P3.4 were obtained.The two putative proteins of these genes were similar to other avrBs3/pthA genes,possessing the same 5'-and 3'-terminals,one leucine zipper(LZ),three nuclear localization signals(NLSs) and one acidic transcriptional activation domain(AAD).The repeat numbers of 102bp unit contained in P3.1 and P3.4 were 12.5 and 15.5,which were same as S11 from JXOV and avrXa10 from PXO86 respectively.Alignments of 12-13th residues in 34-amino-acid unit showed that they are different.In order to compare whether these genes have the same function or not,constructed vectors pZW::P3.4,pZW:: avrXa10;pZW::P3.1,pZW::S11 were introduced into Chinese strains OS-198 and KS6-6 which were mutanted with 5-anacytidine.pathogenicity tests in near isogenic lines of rice showed that:although these genes have the same number of repeats,but their function are not complely identical. |
