The groEL gene as an additional marker for finer differentiation of ‘Candidatus Phytoplasma asteris'-related strains

Abstract

Phytoplasma classification established using 16S ribosomal groups and ‘Candidatus Phytoplasma' taxon are mainly based on the 16S rDNA properties and do not always provide molecular distinction of the closely related strains such as those in the aster yellows group (16SrI or ‘Candidatus Phytoplasma asteris'-related strains). Moreover, because of the highly conserved nature of the 16S rRNA gene, and of the not uncommon presence of 16S rDNA interoperon sequence heterogeneity, more variable single copy genes, such as ribosomal protein (rp), secY and tuf, were shown to be suitable for differentiation of closely related phytoplasma strains. Specific amplification of fragments containing phytoplasma groEL allowed studying its variability in 27 ‘Candidatus Phytoplasma asteris'-related strains belonging to different 16SrI subgroups, of which 11 strains were not studied before and 8 more were not studied on other genes than 16S rDNA. The restriction fragment length polymorphism (RFLP) analyses of the amplified fragments confirmed differentiation among 16SrI-A, I-B, I-C, I-F and I-P subgroups, and showed further differentiation in strains assigned to 16SrI-A, 16SrI-B and 16SrI-C subgroups. However, analyses of groEL gene failed to discriminate strains in subgroups 16SrI-L and 16SrI-M (described on the basis of 16S rDNA interoperon sequence heterogeneity) from strains in subgroup 16SrI-B. On the contrary, the 16SrI unclassified strain ca2006/5 from carrot (showing interoperon sequence heterogeneity) was differentiable on both rp and groEL genes from the strains in subgroup 16SrI-B. These results indicate that interoperon sequence heterogeneity of strains AY2192, PRIVA (16SrI-L), AVUT (16SrI-M) and ca2006/5 resulted in multigenic changes - one evolutionary step further - only in the latter case. Phylogenetic analyses carried out on groEL are in agreement with 16Sr, rp and secY based phylogenies, and confirmed the differentiation obtained by RFLP analyses on groEL amplicons.