Phytoplasma detection and identification: from 16S ribosomal gene to multiple gene identification.

Phytoplasmas are bacteria lacking cell wall that are located in the phloem of plants and in the hemolymph of insect vectors. The phytoplasma classification scheme is based on PCR/RFLP analyses of 16S rDNA: a reliable tool for the differentiation that has become the most comprehensive and widely accepted classification system. On the other hand, the ‘Candidatus Phytoplasma’ species description, recently under adoption, refers to 16S rRNA gene sequence with a threshold <97.5% similarity to that of any previously described ‘Ca. Phytoplasma’ species. However because of the highly conserved nature of the 16S rDNA, many biologically or ecologically distinct phytoplasma strains, which may warrant designation of a new taxon may fail to meet this requirement. Additional unique biological properties such as antibody specificity, host range and vector transmission specificity as well as other molecular criteria need to be included for speciation. Some additional tools for phylogenetic analyses and finer strain differentiation of phytoplasmas such as rp, secY, tuf, groEL genes, and the 16S-23S rRNA intergenic spacer region sequences are now used as supplementary tools and multilocus analyses is providing useful and reliable information in combination with 16SrDNA for phytoplasma strains differentiation.