Comparison of the 16S/23S ribosomal intergenic regions of "Candidatus Liberobacter asiaticum" and "Candidatus Liberobacter africanum," the two species associated with citrus huanglongbing (greening) disease

This study compared the 16S/23S ribosomal intergenic spacer regions of two Liberobacter species associated with citrus huanglongbing (greening) disease: "Candidatus Liberobacter asiaticum" (found in Asia) and "Candidatus Liberobacter africanum" (found in Africa). Researchers extracted DNA from infected citrus plants and amplified the intergenic spacer regions using PCR. The intergenic spacer of C. L. asiaticum strains from India and China was identical at 595 bp in length and contained two tRNA genes: isoleucine tRNA and alanine tRNA separated by 11 nucleotides. The intergenic spacer of C. L. africanum was shorter at 498 bp and contained only the alanine tRNA gene. DNA sequence analysis revealed 79.46% overall homology between the two species' intergenic regions, with 87.8% homology in the alanine tRNA regions. The two C. L. asiaticum strains from geographically distant regions showed 100% sequence identity despite belonging to different serotypes, suggesting limited genetic variation within species at this locus. These findings provide molecular differentiation between the two Liberobacter species causing citrus greening disease.

Key findings

• The C. L. asiaticum intergenic spacer (595 bp) contains both isoleucine and alanine tRNA genes, while C. L. africanum (498 bp) contains only alanine tRNA
• Overall sequence homology between the two Liberobacter species is 79.46% in their intergenic regions
• The two C. L. asiaticum strains from geographically distant regions (India and China) have identical intergenic spacer sequences despite different serotypes
• The 16S/23S intergenic spacer region shows minimal variation within a species, suggesting monoclonal antibodies are needed for strain differentiation