ARS | Publication request: Diversity of the citrus HLB bacterium, ‘Candidatus Liberibacter asiaticus’, in psyllids (Diaphorina citri) collected from Murraya paniculata and citrus spp. in Thailand

Submitted to: American Phytopathological Society Annual Meeting
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: June 22, 2012
Publication Date: N/A

Interpretive Summary: Citrus huanglongbing (HLB), also known as citrus greening, is associated with three phloem inhabiting a-Proteobacteria bacteria. The most common of these bacteria is ‘Candidatus Liberibacter asiaticus’ (Las). The disease is transmitted by the Asian citrus psyllid (ACP). The Las genome was sequenced and contained two prophage regions. A prophage is a viral genome inserted and integrated into the circular bacterial DNA chromosome and they are associated with gene gain and loss, lateral gene transfer, pathogenicity, environmental adaptations, and interstrain genetic differences. The hypervariable gene, hyvI, was identified within the prophage region of the Las genome and in this study was used as a molecular marker to investigate the genetic differences in two ACP populations in Thailand. The one population was collected from HLB-affected Citrus sp. and the other from the ornamental citrus relative Murraya paniculata. The hyvI gene occurred in 81.25% (65/80) of HLB-infected psyllids from Citrus and 90.91% (20/22) of HLB-infected psyllids from M. paniculata. The hyvI genes obtained from psyllids fed on Citrus and M. paniculata were identical or highly similar, suggesting that at least some Las strains in the two hosts shared a common recent origin. This is the first report on diversity of Las isolated from psyllids in Thailand.

Technical Abstract: ‘Candidatus Liberibacter asiaticus’ (Las) is a phloem inhabiting bacterium that causes huanglongbing disease (HLB), also known as citrus greening associated with three species of a-Proteobacteria in the genus ‘Candidatus Liberibacter sp’. Prophage is an important genetic element of bacterial genomes and is involved in lateral gene transfer, pathogenicity, environmental adaptations, and interstrain genetic variability. Hypervariable gene hyvI was identified in the prophage region of the Las Psy 62 genome. In this study, the sequence of a hyvI gene from ‘Candidatus Liberibacter asiaticus’ was selected as a molecular marker for the assessment of the genetic variation in two ‘Ca. Liberibacter asiaticus’ populations infected by psyllid vector collected from Citrus sp. and Murraya paniculata in Thailand. The frequency of the hyvI gene was 81.25% (65/80) in HLB-infected psyllid fed on citrus and 90.91% (20/22) in HLB-infected psyllid fed on M. paniculata. Frequent deletions and insertions of these repeats did not disrupt the open reading frames. However, the hyvI genes obtained from psyllids fed on Citrus and M. paniculata were identical or highly similar, suggesting that at least some bacterial strains in the two hosts shared a common recent origin. This is the first report on diversity variation of ‘Ca. Liberibacter asiaticus’ isolated from psyllids in Thailand. This understanding could be applied to studies on ecology and epidemiology of ‘Ca. Liberibacter asiaticus’ for further effective disease management.