Chen, Jianchi

Abstract Background Huanglongbing (HLB) is a highly destructive disease of citrus production worldwide. 'Candidatus Liberibacter asiaticus', an unculturable alpha proteobacterium, is a putative pathogen of HLB. Information about the biology and strain diversity of 'Ca. L. asiaticus' is currently limited, inhibiting the scope of HLB research and control. Results A genomic region (CLIBASIA_05640 to CLIBASIA_05650) of 'Ca. L. asiaticus' showing hyper-sequence variation or locus mosaicism was identified and investigated using 262 bacterial strains (188 from China and 74 from Florida). Based on the characteristic electrophoretic profiles of PCR amplicons generated by a specific primer set, eight electrophoretic types (E-types) were identified, six E-types (A, B, C, D, E, and F) in China and four E-types (A, C, G, and H) in Florida. The 'Ca. L. asiaticus' strains from China consisted predominately of E-type A (71.3%) and E-type B (19.7%). In contrast, the 'Ca. L. asiaticus' strains from Florida was predominated by E-type G (82.4%). Diversity of 'Ca. L. asiaticus' in China was also evidenced. Strains from the high altitude Yunnan Province consisted of five E-types with E-type B being the majority (62.8%), whereas strains from the low altitude coastal Guangdong Province consisted of only two E-types with E-type A as the majority (97.0%). Sequence analyses revealed that variation of DNA amplicons was due to insertion/deletion events at CLIBASIA_05650 and the downstream intergenic region. Conclusions This study demonstrated the genomic mosaicism of 'Ca. L. asiaticus' resulted from active DNA insertion/deletion activities. Analyses of strain variation depicted the significant inter- and intra-continent diversity of 'Ca. L. asiaticus'.

Prophages are important genetic elements of bacterial genomes and are involved in lateral gene transfer, pathogenicity, environmental adaptations, and interstrain genetic variability. In this study, the sequence of a prophage terminase gene of ‘Candidatus Liberibacter asiaticus’, a bacterium associated with citrus Huanglongbing (HLB), was selected as a molecular marker to assess the genetic variation in two ‘Ca. L. asiaticus’ populations from geographically distinct provinces (Guangdong and Yunnan) in China. The frequency of the prophage terminase gene was 15.8% (19/120) in Guangdong (altitude <500 m) and 97.4% (38/39) in Yunnan (altitude >2,000 m). The difference was highly significant (P < 0.0001) based on χ2 analysis. However, the partial prophage terminase gene sequences obtained from 10 Guangdong strains and 6 Yunnan strains were identical or highly similar, suggesting that at least some bacterial strains in the two locations shared a common recent origin. This is the first report on population variation of ‘Ca. L. asiaticus’ in China, where HLB was first described. The population variation of ‘Ca. L. asiaticus’ in the two geographical regions and the related HLB epidemiology were discussed.

Wampee (Clausena lansium Skeels) is native to southern China and Southeast Asia. Wampee trees are attractive, with grape-like fruits and a muscat taste and are popular in home gardens. Like other members of Rutaceae, wampee has long been suspected to have yellow shoot disease or Huanglongbing (HLB) and Diaphorina citri, the disease vector, was capable of a long-term survival on Wampee. The authors recommend that eradication of wampee trees surrounding citrus orchards should be part of the overall management of citrus HLB. Accepted for publication 20 December 2007. Published 19 April 2007.