Phytoplasma - Taxonomy

Taxonomy

Phytoplasmas are mollicutes and within this group belong to the monophyletic order Acholeplasmatales. In 1992 the Subcommittee on the Taxonomy of Mollicutes proposed the use of the name Phytoplasma in place of the use of the term MLO (Mycoplasma-like organism) "for reference to the phytopathogenic mollicutes". In 2004 the genus name Phytoplasma was adopted and is currently at Candidatus status which is used for bacteria that can not be cultured. It's taxonomy is complicated by the fact that it can not be cultured and thus methods normally used for classification of prokaryotes are not possible. Phytoplasma taxonomic groups are based on differences in the fragment sizes produced by the restriction digest of the 16S rRNA gene sequence (Called RFLP) or by comparison of DNA sequences from the 16s/23s spacer regions. There is some disagreement over how many taxonomic groups the phytoplasmas fall into, recent work involving computer simulated restriction digests of the 16Sr gene suggest there may be up to 28 groups whereas other papers argue for less groups, but more sub-groups. Each group includes at least one Ca. Phytoplasma species, characterised by distinctive biological, phytopathological and genetic properties. The table below summaries some of the major taxonomic groups and the candidatus species that belong in them.