Publications

4364

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'.

Plants of Convolvulus arvensis exhibiting symptoms of undersized leaves, shoot proliferation and yellowing, collectively defined as bindweed yellows, were sampled in different regions of Europe and assessed for phytoplasma infection by PCR amplification using phytoplasma universal rRNA operon primer pairs. Positive results were obtained for all diseased plants. RFLP analysis of amplicons comprising the16S rRNA gene alone or the16S rRNA gene and 16-23S intergenic spacer region indicated that the detected phytoplasmas were distinguishable from all other previously described rRNA gene sequences. Analysis of 16S rRNA gene sequences derived from seven selected phytoplasma strains (BY-S57/11, BY-S62/11, BY-I1015, BY-I1016, BY-BH1, BY-BH2 and BY-G) showed that they were nearly identical (99.9–100 % gene sequence similarity) but shared less than 97.5 % similarity with comparable sequences of other phytoplasmas. Thus, BY phytoplasmas represent a new taxon whose closest relatives are stolbur phytoplasma strains and ‘ Candidatus Phytoplasma fragariae ’ with which they share 97.2 % and 97.1 % 16S rRNA gene sequence similarity, respectively. Phylogenetic analysis of 16S rRNA gene sequences confirmed that bindweed yellows phytoplasma strains collectively represent a distinct lineage within the phytoplasma clade and share a common ancestor with previously published or proposed ‘Candidatus Phytoplasma’ taxa within a major branch including aster yellows and stolbur phytoplasmas. On the basis of unique 16S rRNA gene sequences and biological properties that include a single host plant species and a geographical distribution limited to parts of Europe, the bindweed yellows (BY) phytoplasmas represent a coherent but discrete taxon, ‘Candidatus Phytoplasma convolvuli’, with strain BY-S57/11 (GenBank accession no. JN833705) as the reference strain.