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Title: THE PHYTOPLASMA ASSOCIATED WITH ASH YELLOWS AND LILAC WITCHES'-BROOM: "CANDIDATUS PHYTOPLASMA FRAXINI"

Author
item GRIFFITHS, HELEN - DEPT PLANT PATH, CORNELL
item SINCLAIR, WAYNE - DEPT PLANT PATH, CORNELL
item SMART, CHRISTINE - DEPT PLANT PATH, UC/DAVIS
item Davis, Robert

Submitted to: International Journal of Systematic and Evolutionary Microbiology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/4/1999
Publication Date: N/A
Citation: N/A

Interpretive Summary: Phytoplasmas cause hundreds of plant diseases worldwide. They are minute wall-less bacteria that inhabit the phloem of diseased plants and the bodies of insects, mainly leafhoppers, that transmit them from plant to plant. Ash yellows and lilac witches' broom are highly destructive diseases of ash (Fraxinus spp.) and Syringa spp. (Oleaceae) that are putatively caused by phytoplasmas. Strains of phytoplasma from ash and lilac in different regions of the United States were investigated by methods including analysis of 16S rRNA and 16S-23S rRNA intergenic spacer region sequences, and determination of genome size. The results of the study indicated that the genome size of a representative strain of ash yellows phytoplasma was about 645 kb and that the phytoplasma strains from ash and lilac were heterogeneous but could be grouped in a single taxon. In this paper, we have proposed to name this taxon a new species, "Candidatus Phytoplasma fraxini". This work contributes to the developing taxonomy of phytoplasmas; previously, only three species of phytoplasma had been named.

Technical Abstract: Phytoplasmas associated with the plant diseases ash yellows (AshY, occurring in Fraxinus) and lilac witches'-broom (LWB, occurring in Syringa) represent a putative species-level taxon. We examined phytoplasmal DNA from 19 ash or lilac sources across the known geographic range of AshY (71-113 degrees W) with the objectives of learning if AshY and LWB phytoplasmas are a coherent group, if variability exists in both conserved and anonymous DNA, and if variability in 16S rDNA is related to host or geographic origin. The 16S rRNA gene and 16S-23S spacer were amplified using primer pair P1/P7 and analyzed using 15 restriction enzymes. Sequencing of the amplimers from strains AshY1, AshY3, AshY5 and LWB3, which represent the four 16S rDNA RFLP profile types, revealed only three positions in the 16S rRNA gene and one position in the 16S-23S spacer where differences occurred, and these were single nucleotide substitutions. sequence homology between any two strains was >99.8%. A portion of a ribosomal protein operon, amplified with primer pair rpF1/R1 from each of the four strains noted above, was analyzed with six restriction enzymes, resulting in detection of two RFLP profiles with Mse I. The genome size of strain AshY3 was estimated from pulse-field electrophoresis data to be 645 kbp. Phylogenetic analysis of a 1423-bp 16S rDNA sequence from strains AshY1, AshY3, AshY5 and LWB3, together with sequences of 14 other mollicutes archived in GenBank, produced a tree on which the AshY and LWB strains clustered as a discrete group, consistent with previous analyses utilizing only type strain AshY1. Thus, the AshY phytoplasma group is coherent but heterogeneous. The name "Candidatus Phytoplasma fraxini" is proposed for this group.