Publications

3867

Apple proliferation (AP), pear decline (PD) and European stone fruit yellows (ESFY) are among the most economically important plant diseases that are caused by phytoplasmas. Phylogenetic analyses revealed that the 16S rDNA sequences of strains of each of these pathogens were identical or nearly identical. Differences between the three phytoplasmas ranged from 1·0 to 1·5 % of nucleotide positions and were thus below the recommended threshold of 2·5 % for assigning species rank to phytoplasmas under the provisional status ‘Candidatus’. However, supporting data for distinguishing the AP, PD and ESFY agents at the species level were obtained by examining other molecular markers, including the 16S–23S rDNA spacer region, protein-encoding genes and randomly cloned DNA fragments. The three phytoplasmas also differed in serological comparisons and showed clear differences in vector transmission and host-range specificity. From these results, it can be concluded that the AP, PD and ESFY phytoplasmas are coherent but discrete taxa that can be distinguished at the putative species level, for which the names ‘Candidatus Phytoplasma mali’, ‘Candidatus Phytoplasma pyri’ and ‘Candidatus Phytoplasma prunorum’, respectively, are proposed. Strains AP15R, PD1R and ESFY-G1R were selected as reference strains. Examination of available data on the peach yellow leaf roll (PYLR) phytoplasma, which clusters with the AP, PD and ESFY agents, confirmed previous results showing that it is related most closely to the PD pathogen. The two phytoplasmas share 99·6 % 16S rDNA sequence similarity. Significant differences were only observed in the sequence of a gene that encodes an immunodominant membrane protein. Until more information on this phytoplasma is available, it is proposed that the PYLR phytoplasma should be regarded as a subtype of ‘Candidatus Phytoplasma pyri’.

Clover proliferation phytoplasma (CPR) is designated as the reference strain for the CP phylogenetic group or subclade, on the basis of molecular analyses of genomic DNA, the 16S rRNA gene and the 16S–23S spacer region. Other strains related to CPR include alfalfa witches'-broom (AWB), brinjal little leaf (BLL), beet leafhopper-transmitted virescence (BLTV), Illinois elm yellows (ILEY), potato witches'-broom (PWB), potato yellows (PY), tomato big bud in California (TBBc) and phytoplasmas from Fragaria multicipita (FM). Phylogenetic analysis of the 16S rRNA gene sequences of BLL, CPR, FM and ILEY, together with sequences from 16 other phytoplasmas that belong to the ash yellows (AshY), jujube witches'-broom (JWB) and elm yellows (EY) groups that were available in GenBank, produced a tree on which these phytoplasmas clearly clustered as a discrete group. Three subgroups have been classified on the basis of sequence homology and the collective RFLP patterns of amplified 16S rRNA genes. AWB, BLTV, PWB and TBBc are assigned to taxonomic subgroup CP-A, FM belongs to subgroup CP-B and BLL and ILEY are assigned to subgroup CP-C. Genetic heterogeneity between different isolates of AWB, CPR and PWB has been observed from heteroduplex mobility assay analysis of amplified 16S rRNA genes and the 16S–23S spacer region. Two unique signature sequences that can be utilized to distinguish the CP group from others were present. On the basis of unique properties of the DNA from clover proliferation phytoplasma, the name ‘Candidatus Phytoplasma trifolii’ is proposed for the CP group.

Bermuda grass white leaf (BGWL) is a destructive, phytoplasmal disease of Bermuda grass (Cynodon dactylon). The causal pathogen, the BGWL agent, differs from other phytoplasmas that cluster in the same major branch of the phytoplasma phylogenetic clade in <2·5 % of 16S rDNA nucleotide positions, the threshold for assigning species rank to phytoplasmas under the provisional status ‘Candidatus’. Thus, the objective of this work was to examine homogeneity of BGWL isolates and to determine whether there are, in addition to 16S rDNA, other markers that support delineation of the BGWL agent at the putative species level. Phylogenetic analyses revealed that the 16S rDNA sequences of BGWL strains were identical or nearly identical. Clear differences that support separation of the BGWL agent from related phytoplasmas were observed within the 16S–23S rDNA spacer sequence, by serological comparisons, in vector transmission and in host-range specificity. From these results, it can be concluded that the BGWL phytoplasma is a discrete taxon at the putative species level, for which the name ‘Candidatus Phytoplasma cynodontis' is proposed. Strain BGWL-C1 was selected as the reference strain. Phytoplasmas that are associated with brachiaria white leaf, carpet grass white leaf and diseases of date palms showed 16S rDNA and/or 16S–23S rDNA spacer sequences that were identical or nearly identical to those of the BGWL phytoplasmas. However, the data available do not seem to be sufficient for a proper taxonomic assignment of these phytoplasmas.

Aster yellows (AY) group (16SrI) phytoplasmas are associated with over 100 economically important diseases worldwide and represent the most diverse and widespread phytoplasma group. Strains that belong to the AY group form a phylogenetically discrete subclade within the phytoplasma clade and are related most closely to the stolbur phytoplasma subclade, based on analysis of 16S rRNA gene sequences. AY subclade strains are related more closely to their culturable relatives, Acholeplasma spp., than any other phytoplasmas known. Within the AY subclade, six distinct phylogenetic lineages were revealed. Congruent phylogenies obtained by analyses of tuf gene and ribosomal protein (rp) operon gene sequences further resolved the diversity among AY group phytoplasmas. Distinct phylogenetic lineages were identified by RFLP analysis of 16S rRNA, tuf or rp gene sequences. Ten subgroups were differentiated, based on analysis of rp gene sequences. It is proposed that AY group phytoplasmas represent at least one novel taxon. Strain OAY, which is a member of subgroups 16SrI-B, rpI-B and tufI-B and is associated with evening primrose (Oenothera hookeri) virescence in Michigan, USA, was selected as the reference strain for the novel taxon ‘Candidatus Phytoplasma asteris’. A comprehensive database of diverse AY phytoplasma strains and their geographical distribution is presented.

SummaryTwo phylogenetically distinct marine strains producing transparent exopolymers (TEP), designated HTCC2155T and HTCC2160, were cultivated from Oregon coast seawater by dilution to extinction in a high throughput culturing format. When cultured in low‐nutrient seawater media, these strains copiously produced Alcian Blue‐stainable viscous TEP. Growing cells were attached to each other by the TEP in a three dimensional network. Polymerase chain reaction employing 16S rDNA primers specific for the novel isolates indicated that they are indigenous to the water column of the Atlantic and Pacific oceans. The abundance of the isolates as determined by 16S rRNA dot blots, however, indicated that they are less than 1% of the total bacterial community. In phylogenetic analyses, the strains consistently formed a new phylum‐level lineage within the domain Bacteria, together with members of the candidate phylum VadinBE97, which consists of Victivallis, the first cultured genus in the candidate phylum, and 16S rRNA gene clones from DNA extracted from marine or anaerobic terrestrial habitats. Five putative subgroups were delineated within this phylum‐level lineage, including a marine group and an anaerobic group. The isolates are Gram negative, strictly aerobic, chemoheterotrophic, and facultatively oligotrophic sphere‐shaped bacteria. The DNA G+C content of strain HTCC2155T was 48.3 mol% and the genome size was 2.9 mb. It is proposed from these observations that the strains be placed into a new genus and a new species named Lentisphaera araneosa (type strain HTCC2155T = ATCC BAA‐859T = KCTC 12141T) gen. nov., sp. nov., the cultured marine representative of the Lentisphaerae phyl. nov., and the phylum be divided into two novel orders named the Lentisphaerales ord. nov. and the Victivallales ord. nov.