Submitted to: Phytopathology
Publication Type: Peer reviewed journal
Publication Acceptance Date: 8/4/2013
Publication Date: 2/1/2014
Citation: Ma, W., Wen, X., Gu, Y., Deng, X., Chen, J. 2014. Population structures of “Candidatus Liberibacter asiaticus” in Southern China. Phytopathology. 104:158-162. Interpretive Summary: Citrus Huanglongbing (HLB, yellow shoot disease), also called citrus greening disease, is highly destructive to worldwide citrus production. The disease is associated with a non-culturable bacterium, “Candidatus Liberibacter asiaticus” (CLas). Currently, little is known about this bacterium. Knowledge about CLas population structure and spread is critical for HLB epidemiological research and development of control strategies. In this study, CLas population structure was examined for southern China, where HLB was first described, using sequence variation in a hyper-variable region of the CLas genome. Results were summarized by three parameters: 1) Nei’s diversity index (H), 2) ratio of tandem repeat genotypes, and 3) percentage of multiple DNA band samples. Comparison of all three parameters observed significant differences between CLas populations in Guangdong Province and provinces outside Guangdong. It is hypothesized that the bacterial population structure in Guangdong is due to the endemic occurrence of CLas in Guangdong (> 100 years) and the population structure outside Guangdong reflects later CLas spread (since 1970s).
Technical Abstract: Huanglongbing (HLB, yellow shoot disease) is highly detrimental to worldwide citrus production. The disease is associated with “Candidatus Liberibacter asiaticus”, a non-culturable alpha-proteobacterium. Characterization of “Ca. L. asiaticus” population is important for HLB epidemiological research. Thus, the population structure of “Ca. L. asiaticus” in southern China was analyzed using variation of tandem repeat numbers (TRNs) in a genomic locus. PCR results separated the 224 HLB samples collected from eight provinces into two categories: single amplicon group (SAG) with 175 samples (78.3%), and multiple amplicon group (MAG) with 49 samples (21.7%). Three parameters: Nei’s diversity index (H), ratio of TRN genotypes, and percentage of MAG samples were used to describe bacterial populations. Regional population difference of “Ca. L. asiaticus” was first noted in H values within SAG samples from three individual orchards. The Guangdong orchard (n=24) had an H value of 0.50, which was lower than those of the Yunnan (0.83) and the Hainan (0.88) orchards. Calculations based on all Guangdong samples (n=78) also yielded a H value (0.77), which was significantly lower than the H value in non-Guangdong orchards (0.91). The ratio of TRN=10 / TRN>10 genotype in Guangdong was 25 (75/3), significantly greater than that of 1.6 (50/30) for the non-Guangdong population. Percentage of MAG samples in the Guangdong population was low (1.3%). Based on these results, the “Ca. L. asiaticus” population in southern China was identified into two sub-structures: the Guangdong sub-structure was different from that of outside regions. Whereas the bacterial population sub-structure in Guangdong was likely related to a long history of HLB (since 1890s), “Ca. L. asiaticus” population sub-structure outside Guangdong reflects later bacterial spread (since 1970s).