Two new ‘Candidatus Phytoplasma’ species and revision of the Phytoplasma classification
Phytoplasmas are a large group of bacteria lacking cell walls, associated with numerous plant diseases of economic importance. The genus ‘Candidatus Phytoplasma’ was proposed to accommodate species that are molecularly and biochemically incompletely characterized. The classification is based on percent sequence identity of a unique 16S rRNA gene (>1200 bp) with any previously described species. Given the increasing discovery of molecular diversity within the genus ‘Ca. Phytoplasma’, the guidelines for the description of species were revised. The previous guidelines recognized a new ‘Ca. Phytoplasma’ species if the phytoplasma shared <97.5 % 16S rRNA gene sequence identity when compared with a previously published ‘Ca. Phytoplasma’ species. This threshold is now increased to 98.65 %. Strains sharing <98.65 % sequence identity with the reference strain but >98.65 % with other strain(s) within the same ‘Ca. Phytoplasma’ species should be considered related strains to the relevant ‘Ca. Phytoplasma’ species.
There are now 49 officially published ‘Ca. Phytoplasma’ species. Two recently published species are associated with Coconut lethal yellowing disease: ‘Ca. P. cocostanzaniae’ in Africa, and ‘Ca. P. palmae’ in the Americas.
The revised guidelines do not support the ‘Ca. P. stylosanthis’, ‘Ca. P. omanense’, ‘Ca. P. wodyetiae’ and ‘Ca. P. allocasuarinae’ species since they do not have long enough 16S rRNA gene sequences available in GenBank.
Sources
Bertaccini A, Arocha-Rosete Y, Contaldo N, Duduk B, Fiore N, Montano HG, Kube M, Kuo CH, Martini M, Oshima K, Quaglino F (2022) Revision of the ‘Candidatus Phytoplasma’ species description guidelines. International Journal of Systematic and Evolutionary Microbiology 72(4), 005353.