Comparative and functional genomics reveals genetic diversity and determinants of host specificity among reference strains and a large collection of Chinese isolates of the phytopathogen Xanthomonas campestris pv. campestris.
BACKGROUND
Xanthomonas campestris pathovar campestris (Xcc) is the causal agent of black rot disease of crucifers worldwide. The molecular genetic diversity and host specificity of Xcc are poorly understood.


RESULTS
We constructed a microarray based on the complete genome sequence of Xcc strain 8004 and investigated the genetic diversity and host specificity of Xcc by array-based comparative genome hybridization analyses of 18 virulent strains. The results demonstrate that a genetic core comprising 3,405 of the 4,186 coding sequences (CDSs) spotted on the array are conserved and a flexible gene pool with 730 CDSs is absent/highly divergent (AHD). The results also revealed that 258 of the 304 proved/presumed pathogenicity genes are conserved and 46 are AHD. The conserved pathogenicity genes include mainly the genes involved in type I, II and III secretion systems, the quorum sensing system, extracellular enzymes and polysaccharide production, as well as many other proved pathogenicity genes, while the AHD CDSs contain the genes encoding type IV secretion system (T4SS) and type III-effectors. A Xcc T4SS-deletion mutant displayed the same virulence as wild type. Furthermore, three avirulence genes (avrXccC, avrXccE1 and avrBs1) were identified. avrXccC and avrXccE1 conferred avirulence on the hosts mustard cultivar Guangtou and Chinese cabbage cultivar Zhongbai-83, respectively, and avrBs1 conferred hypersensitive response on the nonhost pepper ECW10R.


CONCLUSIONS
About 80% of the Xcc CDSs, including 258 proved/presumed pathogenicity genes, is conserved in different strains. Xcc T4SS is not involved in pathogenicity. An efficient strategy to identify avr genes determining host specificity from the AHD genes was developed.
He
Yong-Qiang
YQ
Guangxi Key Laboratory of Subtropical Bioresources Conservation and Utilization, and College of Life Science and Technology, Guangxi University, Daxue Road, Nanning, Guangxi 530004, People's Republic of China. yqhe@gxu.edu.cn
Zhang
Liang
L
Jiang
Bo-Le
BL
Zhang
Zheng-Chun
ZC
Xu
Rong-Qi
RQ
Tang
Dong-Jie
DJ
Qin
Jing
J
Jiang
Wei
W
Zhang
Xia
X
Liao
Jie
J
Cao
Jin-Ru
JR
Zhang
Sui-Sheng
SS
Wei
Mei-Liang
ML
Liang
Xiao-Xia
XX
Lu
Guang-Tao
GT
Feng
Jia-Xun
JX
Chen
Baoshan
B
Cheng
Jing
J
Tang
Ji-Liang
JL
D019607:Brassicaceae / Q000382:microbiology*; D002681:China; D019295:Computational Biology; D014644:Genetic Variation; D040481:Genome Components; D023281:Genomics / Q000379:methods; D046228:Microarray Analysis; D009693:Nucleic Acid Hybridization; D012680:Sensitivity and Specificity; D013045:Species Specificity; D014774:Virulence / Q000235:genetics; D016959:Xanthomonas campestris / Q000235:genetics* / Q000472:pathogenicity