Submitted to: International Journal of Systematic and Evolutionary Microbiology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/15/2014
Publication Date: 1/8/2015
Citation: Zhao, Y., Davis, R.E., Wei, W., Lee, I. 2015. Should ‘Candidatus Phytoplasma’ be retained within the order Acholeplasmatales?. International Journal of Systematic and Evolutionary Microbiology. 65:1075-1082.
Interpretive Summary: Phytoplasmas are a diverse group of bacteria that lack a cell wall. These bacteria infect plants and are spread by insects, responsible for diseases in numerous agriculturally and environmentally important plant species worldwide. Although almost a half of a century has passed since their discovery, phytoplasmas are still among the least characterized pathogens, and their taxonomy remained unsettled. In previous studies, scientists in the Molecular Plant Pathology Laboratory, ARS-USDA discovered the distinct genome architecture of phytoplasmas and identified the key genomic events that likely triggered the evolutionary emergence of phytoplasma species. In the present study, we contrasted the habitat and life style of phytoplasmas with those of other known cell wall-less bacteria, and reasoned that there are striking differences in nutritional, biochemical, and physiological properties that distinguish phytoplasmas from all other cell wall-less bacteria. Through analyses of available genome data, we identified such distinguishing properties and concluded that phytoplasmas should be recognized as representing a new family within a new order in the bacterial taxonomic system. Our conclusion is also supported by evolutionary relationships based on the most reliable molecular taxonomic marker, the ribosomal small subunit RNA gene. The findings and insights from our work will be of interest to scientists studying evolution of pathogens, scientists studying genome features dictating bacterium-host interactions, and be useful to universities, government agencies, and diagnostics companies that are involved in work aimed at disease control.
Technical Abstract: Phytoplasmas are a diverse but phylogenetically coherent group of cell wall-less bacteria affiliated with the class Mollicutes. Due to difficulties in establishing axenic culture crucial for polyphasic characterization, phytoplasmas were assigned to a provisional genus, ‘Candidatus Phytoplasma’, and the genus was embraced within the order Acholeplasmatales. However, phytoplasmas differ significantly from acholeplasmas and other mollicutes in their habitat specificities, life modes, metabolic capabilities, genomic architectures, and phylogenetic positions. This communication describes unique ecological, nutritional, biochemical, genomic, and phylogenetic properties that distinguish phytoplasmas from other taxa in the class Mollicutes. Since such distinguishing properties of the phytoplasmas are not referable to the descriptions of any of the four existing orders, namely Acholeplasmatales, Mycoplasmatales, Entomoplasmatales, and Anaeroplasmatales, we propose to remove ‘Candidatus Phytoplasma’ from the order Acholeplasmatales, and to erect a new provisional order ‘Candidatus Phytoplasmatales’ and a new provisional family ‘Candidatus Phytoplasmataceae’, accommodating the genus ‘Candidatus Phytoplasma’.