Submitted to: Plant Disease
Publication Type: Peer reviewed journal
Publication Acceptance Date: 8/24/2010
Publication Date: 3/9/2011
Citation: Liu, R., Zhang, P., Pu, X., Xing, X., Chen, J., Deng, X. 2011. Analysis of a prophage gene frequency revealed population variation of ‘Candidatus Liberibacter asiaticus’ from two geographically distinct citrus growing Provinces in China. Plant Disease. 95:431-435. Interpretive Summary: ‘Candidatus Liberibacter asiaticus’ is a bacterium associated with citrus Huanglongbing (HLB), a highly destructive disease in citrus production world-wide. The bacterium has not been cultured in vitro and little is known about its biology, hampering development of effective HLB control strategies. In this project, a gene of viral origin in the genome of ‘Ca. L. asiaticus’ was used as a marker to evaluate genetic variation in bacterial populations. Bacterial strains were collected from two geographically distinct (different elevations) provinces in China. The gene frequency was significantly lower in the population from Guangdong (elevation < 500 m) than that in Yunnan (elevation > 2,000 m). However, analysis of gene sequences showed that at least some bacterial strains from the two locations shared a common origin. This is the first report on population variation of ‘Ca. L. asiaticus’ in China where HLB was first described. Observations on genetic variation of ‘Ca. L. asiaticus’ populations in different geographical regions could provide useful information for future research on how environmental factors influence the biology of ‘Ca. L. asiaticus’ and subsequent disease development.
Technical Abstract: 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 average elevation of Guangdong Province is <500 m, and that of Yunnan Province is >2,000 m. The frequency of the prophage terminase gene was 15.8% (19/120) in Guangdong and 97.4% (38/39) in Yunnan. The difference was highly significant (P<0.0001) based on Chi-square analysis. This is the first report of population variation of ‘Ca. L. asiaticus’ in China where HLB was first described. 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. The population variation of ‘Ca. L. asiaticus’ in the two geographical regions and the related HLB epidemiology are discussed.