‘Candidatus Phytoplasma’, a taxon for the wall-less, non-helical prokaryotes that colonize plant phloem and insects

Abstract

The trivial name ‘phytoplasma’ has been adopted to collectively name wall-less, non-helical prokaryotes that colonize plant phloem and insects, which were formerly known as mycoplasma-like organisms. Although phytoplasmas have not yet been cultivated in vitro, phylogenetic analyses based on various conserved genes have shown that they represent a distinct, monophyletic clade within the class Mollicutes. It is proposed here to accommodate phytoplasmas within the novel genus ‘Candidatus (Ca.) Phytoplasma’. Given the diversity within ‘Ca. Phytoplasma’, several subtaxa are needed to accommodate organisms that share <97·5 % similarity among their 16S rRNA gene sequences. This report describes the properties of ‘Ca. Phytoplasma’, a taxon that includes the species ‘Ca. Phytoplasma aurantifolia’ (the prokaryote associated with witches'-broom disease of small-fruited acid lime), ‘Ca. Phytoplasma australiense’ (associated with Australian grapevine yellows), ‘Ca. Phytoplasma fraxini’ (associated with ash yellows), ‘Ca. Phytoplasma japonicum’ (associated with Japanese hydrangea phyllody), ‘Ca. Phytoplasma brasiliense’ (associated with hibiscus witches'-broom in Brazil), ‘Ca. Phytoplasma castaneae’ (associated with chestnut witches'-broom in Korea), ‘Ca. Phytoplasma asteris' (associated with aster yellows), ‘Ca. Phytoplasma mali’ (associated with apple proliferation), ‘Ca. Phytoplasma phoenicium’ (associated with almond lethal disease), ‘Ca. Phytoplasma trifolii’ (associated with clover proliferation), ‘Ca. Phytoplasma cynodontis' (associated with Bermuda grass white leaf), ‘Ca. Phytoplasma ziziphi’ (associated with jujube witches'-broom), ‘Ca. Phytoplasma oryzae’ (associated with rice yellow dwarf) and six species-level taxa for which the Candidatus species designation has not yet been formally proposed (for the phytoplasmas associated with X-disease of peach, grapevine flavescence dorée, Central American coconut lethal yellows, Tanzanian lethal decline of coconut, Nigerian lethal decline of coconut and loofah witches'-broom, respectively). Additional species are needed to accommodate organisms that, despite their 16S rRNA gene sequence being >97·5 % similar to those of other ‘Ca. Phytoplasma’ species, are characterized by distinctive biological, phytopathological and genetic properties. These include ‘Ca. Phytoplasma pyri’ (associated with pear decline), ‘Ca. Phytoplasma prunorum’ (associated with European stone fruit yellows), ‘Ca. Phytoplasma spartii’ (associated with spartium witches'-broom), ‘Ca. Phytoplasma rhamni’ (associated with buckthorn witches'-broom), ‘Ca. Phytoplasma allocasuarinae’ (associated with allocasuarina yellows), ‘Ca. Phytoplasma ulmi’ (associated with elm yellows) and an additional taxon for the stolbur phytoplasma. Conversely, some organisms, despite their 16S rRNA gene sequence being <97·5 % similar to that of any other ‘Ca. Phytoplasma’ species, are not presently described as Candidatus species, due to their poor overall characterization.

Phytoplasmas and Phytoplasma Diseases: A Severe Threat to Agriculture

“…The 'Candidatus Phytoplasma' genus was described together with 19 'Candidatus species' [92] and formal descriptions for 35 'Ca. Phytoplasma species' total have since been published.…”

Phytoplasma classification and phylogeny based on in silico and in vitro RFLP analysis of cpn60 universal target sequences

“…In 2004, the International Committee of Systematic Bacteriology Subcommittee for the Taxonomy of Mollicutes , the International Research Program for Comparative Mycoplasmology (IRPCM), proposed the provisional genus ‘ Candidatus Phytoplasma’ (IRPCM, 2004). This classification is based on the similarity of 16S rRNA gene sequences supported by phylogenetic analysis, and using this strategy, 38 ‘ Candidatus Phytoplasma’ species have been formally described to date (Davis et al , 2013; Harrison et al , 2014; IRPCM, 2004; Nejat et al , 2013). Classification of phytoplasmas is further supported by the 16S rRNA gene through the use of restriction fragment length polymorphism (RFLP) of the 16S rRNA F2nR2 fragment with a set of seventeen endonucleases (Lee et al , 1993, 1998).…”

Expanding and exploring the diversity of phytoplasmas from lucerne (Medicago sativa)

“…The ‘ Candidatus Phytoplasma’ taxon (trivial name: phytoplasma) comprises a group of wall-less, non-helical prokaryotes that are unusual in being able to invade and reproduce within both living plants and insects1. They are associated with a broad variety of plant pathologies and are generally vectored among host plants by sap feeding insects in the order Hemiptera.…”

“…Formal recognition of bacteria species in the class Mollicutes requires description of cultured growth activity which is not observable with phytoplasmas. Accordingly, a new ‘species’ is assigned if it has <97.5% sequence similarity to existing described taxa1. Based on this system, 37 formally described candidate species of ‘ Ca .…”

“…Based on this system, 37 formally described candidate species of ‘ Ca . Phytoplasma’ and more than 10 putatively novel species have been reported13 and classified to 33 RFLP based 16 S rRNA subgroups (and others undetermined) associated with various phytoplasma-induced plant diseases4. The list of formally described ‘ Ca .…”