Description of a putative novel phytoplasma taxon isolated from Periwinklein Spain

Phytoplasmas are cell-wall less bacteria of class Mollicutes that inhabite phloem sieve cells in plants and colonize different tissues in insect vectors. Their evolutionary genome reduction resulted in loss of genes encoding various metabolic pathways making difficult the isolation in axenic culture and the classification based on phenotypic character observation. A phytoplasma classification has been established on the basis of 16S rRNA gene sequences, and a provisional 'Candidatus Phytoplasma' species taxonomy has been adopted. In this study, we assessed the taxonomic position and group classification of a phytoplasma responsible for virescence and phyllody symptoms in naturally diseased Madagascar periwinkle plants in southern Spain. Unique regions in the 16S rRNA gene of this phytoplasma distinguished the isolate from all previously described 'Candidatus Phytoplasma' species. In the alignment with full-length 16S rRNA gene sequences the Spanish isolate shared less than 97.5% sequence similarity with that of previously described 'Ca. Phytoplasma' species, justifying the recognition of a putative novel taxon. The 16S rRNA gene F2nR2 fragment from the Spanish isolate exhibited a distinct restriction fragment length polymorphism (RFLP) profile, different from representative phytoplasmas classified in any of the 31 previously delineated 16Sr groups. Phylogenetic analyses of full length sequences of three other genes, encoding the Elongation Factor Tu, ribosomal proteins L22 and S3 and protein translocase subunit SecY, shared less than 92% sequence similarity with other 'Ca. Phytoplasma' species sequences, supporting the identification of a novel taxon.