Genetic Diversity Of Xanthomonas Citri Subsp.citri Isolates From Gannan And Comparson Of Resistance To Its Host

Citrus canker disease is one of the most dangerous quarantine disease worldwide,it is widely distributed in China,Japan,the United States,India and other citrus-growing regions and caused great econmic losses.When citrus plants infected with the disease,the leaf,branch and fruit of plants shows some typical symptoms,such as disease spot,more seriously,citrus canker induces potential decline,falling flowers and fruit trees,finally cause the loss of citrus yield and quality.Citrus canker disease mainly caused by Xanthomonas citri subsp.citri(Xac)in china.In this study,the following two aspects have been focused: 1)the population structure of citrus canker pathogen from Gannan’s 18 counties through microsatellite molecular markers,to systematically evaluate genetic diversity and population differentiation of citrus canker pathogen in Gannan,and providing useful informations to the epidemiology,prevention and control the pathogenic bacteria,2)The risistance differences to the citrus canker disease of three local citrus cultivarieties,including ‘Newhall’,‘Gannanzao’ and ‘Satsuma mandarin’.The main results obtained are as follows.The first,According to 14 reported SSR loci form 306 strains of Xcc,the polymorphism of Xanthomonas citri subsp.citri(Xcc)from 18 counties of Gannan were analyzed.The results showed that 7 SSR loci(XL1,XL2,XL3,XL4,XL5,XL6 and XL7)were identified,it exist significant difference in Gannan’ Xcc.Through Nei’s gene diversity index of 7 SSR loci ranging from 0.66 to 0.83,it suggesting high genetic diversity in Gannan’s citrus canker pathogen.Xcc isolates from Dayu country had high diversity,Effective number of alleles,Nei’s gene diversity,Shannon ’ s information index was highest,which are respectively 3.33,0.70 and 1.29;Xcc isolates from Dingnan country’s had low diversity,the three values are respectively 1.26,0.19 and0.32.Marking clustering analysis for all of Xcc isolates from Gannan by Powermarker 3.25 and structure 2.3.4,All isolates from Gannan were clustered into two groups.Xcc isolates from Xinfeng,Dingnan,Zhanggongqu,Xingguo and Huichang were constituted nearly in a single group.Among them,samples collected from Xinfeng,Dingnan and Zhanggongqu were clustered in one group,samples collected from Xingguo and Huichang belonged to the other group,The remaining samples were distributed in the both of the two groups.The second,The needling inoculation study found that,inoculated point of three citrus varieties is appear with particular disease spot at 2 days.At 3 days,back of leaf is uplift,disease spot appears white spongy material,morbidity of ‘Newhall’,‘Gannanzao’ and ‘Satsuma mandarin’was respectively 63.15%,42.1% and 5.12%.form 4 day to 6 day,disease symptoms of three citrus varieties with aggravation,incidence rate also gradually increased;inoculation after 7 days,‘Newhall’ and ‘Gannanzao’ vaccination point is disease,disease spot size was respectively 9.5mm2 and 7.2 mm2,‘Satsuma mandarin’ attack lighter,disease incidence rate is only 49.5%,disease spot size was 4.1 mm2.Among three varieties of citrus,The lowest infected concentration of‘Newhall ’ is lowest(1.0 x 106 CFU/mL),which ‘Gannanzao’ and ‘Satsuma mandarin’ is close(1.0 x 107 CFU/mL),it suggesting Newhall than ‘Gannanzao’ and ‘Satsuma mandarin’ is more susceptible to citrus canker.In smearing inoculation experiment,inoculation after 10 days,‘Newhall’ have 26 disease spot,‘Gannanzao’ have 3.5 disease spot,they are appear with disease spot,but ‘Satsuma mandarin’ was no disease.Therefore,with the results of the two inoculation site morbidity,disease spot size and the lowest infected concentration,resistance size of three citrus varieties followed by ‘Satsuma mandarin’ > ‘Gannanzao’ > ‘Newhall’.The third,Through scanning electron microscope,it found significant difference in stomatal density and morphology of three varieties.In terms of stomatal density,stomatal density of‘Gannanzao’and ‘Satsuma mandarin’ higher than that of ‘Newhall’,but around the blade rough and pores smooth degree,‘Newhall’ is more easily adhered by pathogenic bacteria.The stomatal opening size of ‘Newhall’ is large,the room space is large,making the pathogenic bacteria is the most easily into citrus;the stomatal opening size of ‘Gannanzao’ is small,the room space is small,the small stomatal opening ruduce the chances the pathogenic bacteria enter cells,and reduce the possibility to forming disease;stoma size of ‘Satsuma mandarin’ is small,smooth around outside the guard cells,closely related to the guard cells,forming the room of a slit type invagination state,least favorable to enter of pathogenic bacteria.By observation of leaf anatomical structure,it found the differences in blade thickness,palisade tissue thickness,spongy tissue thickness,up-down cortex layer and the degree among three varieties,but the differences isn’t relation to the resistance of citrus canker.After the leaves of ‘Newhall’,‘Gannanzao’ and ‘Satsuma mandarin’ varieties was inoculated by Xanthomonas citri subsp.citri,the phenylalanine ammonia lyase(PAL),peroxidase(POD),catalase(CAT),and superoxide dismutase(SOD)activities were happen a series of changes,but there were no significantly differences in the susceptible response of external stimulus and resistance to pathogen invasion between ‘Newhall’,‘Gannanzao’ and ‘Satsuma mandarin’ varieties.