Phytoplasma taxonomy: nomenclature, classification and identification

Authors: Wei, Wei; Zhao, Yan

Submitted to: Biology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/25/2022
Publication Date: 7/26/2022
Citation: Wei, W., Zhao, Y. 2022. Phytoplasma taxonomy: nomenclature, classification and identification. Biology. 11/1119. https://doi.org/10.3390/biology11081119.
DOI: https://doi.org/10.3390/biology11081119

Interpretive Summary: Phytoplasmas are phloem-restricted and insect-transmissible bacteria that infect many plant species, including agronomically important crops. So far, phytoplasmas cannot be cultured; thus, as with other unculturable bacteria, phytoplasma taxonomy relies heavily on modern molecular techniques. In the past two decades, molecular technologies have advanced significantly, bringing about tremendous changes in phytoplasma taxonomy. In this article, two ARS scientists, located in Beltsville, Maryland reviewed the latest progress in phytoplasma nomenclature, classification, and identification, the three major aspects of phytoplasma taxonomy. The article also highlights recent hot button issues, such as criteria for naming new phytoplasma species and the recently revised guidelines for phytoplasma species description (published in International Journal of Systematic and Evolutionary, in 2022). At the end, the existing problems and future trends of phytoplasma taxonomy are briefly described. This review will benefit the researchers, professors, and students who are interested in classification, identification, and detection of plant pathogens. In addition, the newly updated information in the article will also be great help to the regulatory agencies in implementing border controls and preventing the spread of quarantine pests. The review will contribute to a better understanding of the up-to-date advances in phytoplasma taxonomy and will help the researchers to diagnose, characterize, and identify phytoplasmas effectively, timely and accurately.

Technical Abstract: Phytoplasmas are pleomorphic, wall-less intracellular bacteria that can cause devastating diseases in a wide variety of plant species. Rapid diagnosis and precise identification of phytoplasmas responsible for emerging plant diseases are crucial to preventing further spread of the diseases and reducing economic losses. Phytoplasma taxonomy (identification, nomenclature, and classification) has lagged in comparison to culturable bacteria, largely due to lack of axenic phytoplasma culture and consequent inaccessibility of phenotypic characteristics. However, the rapid expansion of molecular techniques and the advent of high throughput genome sequencing have tremendously enhanced the nucleotide sequence-based phytoplasma taxonomy. In this article, the key events and milestones that shaped the current phytoplasma taxonomy are highlighted. In addition, the distinctions and relatedness of two parallel systems of 'Candidatus phytoplasma' species/nomenclature system and group/subgroup classification system are clarified. Both systems are indispensable as they serve different purposes. Furthermore, some hot button issues in phytoplasma nomenclature are also discussed, especially those pertinent to the implementation of newly revised guidelines for 'Candidatus Phytoplasma' species description. To conclude, the challenges and future perspectives of phytoplasma taxonomy are briefly outlined.