Oshima, Kenro

The genus ‘Candidatus Phytoplasma’ was proposed to accommodate cell wall-less bacteria that are molecularly and biochemically incompletely characterized, and colonize plant phloem and insect vector tissues. This provisional classification is highly relevant due to its application in epidemiological and ecological studies, mainly aimed at keeping the severe phytoplasma plant diseases under control worldwide. Given the increasing discovery of molecular diversity within the genus ‘Ca. Phytoplasma’, the proposed guidelines were revised and clarified to accommodate those ‘Ca. Phytoplasma’ species strains sharing >98.65 % sequence identity of their full or nearly full 16S rRNA gene sequences, obtained with at least twofold coverage of the sequence, compared with those of the reference strain of such species. 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 that ‘Ca. Phytoplasma’ species. The guidelines herein, keep the original published reference strains. However, to improve ‘Ca. Phytoplasma’ species assignment, complementary strains are suggested as an alternative to the reference strains. This will be implemented when only a partial 16S rRNA gene and/or a few other genes have been sequenced, or the strain is no longer available for further molecular characterization. Lists of ‘Ca. Phytoplasma’ species and alternative reference strains described are reported. For new ‘Ca. Phytoplasma’ species that will be assigned with identity ≥98.65 % of their 16S rRNA gene sequences, a threshold of 95 % genome-wide average nucleotide identity is suggested. When the whole genome sequences are unavailable, two among conserved housekeeping genes could be used. There are 49 officially published ‘Candidatus Phytoplasma’ species, including ‘Ca. P. cocostanzaniae’ and ‘Ca. P. palmae’ described in this manuscript.

SUMMARY Phytoplasmas are plant‐pathogenic bacteria that are associated with numerous plant diseases. We have previously reported the complete genomic sequence of Candidatus Phytoplasma asteris, OY strain, OY‐M line, which causes mild symptoms. The phytoplasma genome lacks several important metabolic genes, implying that the consumption of metabolites by phytoplasmas in plants may cause disease symptoms. Here we show that the approximately 30‐kb region including the glycolytic genes was tandemly duplicated in the genome of OY‐W phytoplasma, which causes severe symptoms. Almost duplicated genes became pseudogenes by frameshift and stop‐codon mutations, probably because of their functional redundancy. However, five kinds of genes, including two glycolytic genes, remained full‐length ORFs, suggesting that it is advantageous for the phytoplasma to retain these genes in its lifestyle. In particular, 6‐phosphofructokinase is known as a rate‐limiting enzyme of glycolysis, implying that the different number of glycolytic genes between OY‐W and OY‐M may influence their respective glycolysis activities. We previously reported that the phytoplasma population of OY‐W was higher than that of OY‐M in their infected plants. Taking this result into account, the higher consumption of the carbon source may affect the growth rate of phytoplasmas and also may directly or indirectly cause more severe symptoms.