Publications

4486

A new disease of potatoes, tentatively named zebra chip (ZC) because of the intermittent dark and light symptom pattern in affected tubers which is enhanced by frying, was first found in Mexico in 1994 and in the southwestern United States in 2000. The disease can cause severe economic losses in all market classes of potatoes. The cause of ZC has been elusive, and only recently has been associated with ‘Candidatus Liberibacter’ sp. Field samples of potato plants were collected from several locations in the United States, Mexico, and Guatemala to determine transmission to potato and tomato by grafting of ZC-infected scions and psyllid feeding. The disease was successfully transmitted, through up to three generations, by sequential top- and side-grafting ZC-infection scions to several potato cultivars and to tomato. The disease was also successfully transmitted to potato and tomato plants in greenhouse experiments by potato psyllids collected from potato plants naturally affected with ZC. Transmission electron microscopic observation of ZC-affected tissues revealed the presence of bacteria-like organisms (BLOs) in the phloem of potato and tomato plants inoculated by grafting and psyllid feeding. The BLOs were morphologically similar in appearance to BLOs associated with other plant diseases. Polymerase chain reaction (PCR) amplified 16S rDNA sequences from samples representing different geographic areas, including the United States, Mexico, and Guatemala, were almost identical to the 16S rDNA of ‘Ca. L. solanacearum’ previously reported from solanaceous plants in New Zealand and the United States. Two subclades were identified that differed in two single base-pair substitutions. New specific primers along with an innovative rapid PCR were developed. This test allows the detection of the bacteria in less than 90 min. These data confirm the association of ‘Ca. L. solanacearum’ with potatoes affected by ZC in the United States, Mexico, and Guatemala.

In North America, elm yellows, aster yellows (AY), and X-disease phytoplasmas have been detected in American grapevines (1), and recently, Bois noir was detected in Canadian vineyards from British Columbia (BC) and Ontario (ON) (2). Typical symptoms of grapevine yellows (GY) include leaf rolling and chlorosis, uneven or total lack of lignification of canes, flower abortion or berry withering, and stunting. In 2006 and 2007, independent surveys were conducted by the Canadian Food Inspection Agency (CFIA) and Agriculture and Agri-Food Canada (AAFC) to detect phytoplasmas in Canadian vineyards containing different cultivars in BC, ON, Québec (QC), Nova Scotia, New Brunswick, and Prince Edward Island. The CFIA collected and tested 651 fresh leaf samples from recently imported grapevines and older grapevines in the same or neighboring blocks displaying symptoms typical of those associated with disease caused by phytoplasmas. Many vineyards were surveyed only once. AAFC collected and tested 3,485 samples from symptomatic and asymptomatic grapevines from established vineyards in ON, BC, and QC. The same vineyards were sampled in ON and BC both years; QC vineyards were only sampled in 2007. AAFC-collected leaf samples were freeze dried and stored at –20°C before processing. CFIA samples were tested by a modified real-time PCR assay and TaqMan probe targeting the 16S ribosomal RNA gene that detects a wide range of known phytoplasmas (2). Positive samples were confirmed by conventional PCR using the phytoplasma-specific primers P1/P7 (3) and the resulting ~1,800-bp fragment was cloned and sequenced as previously described (2). DNA extracted by AAFC was amplified by nested PCR technology with universal phytoplasma specific primer pairs P1/P6 and R16R2/R16F2 (3) and the resulting 1,200-bp fragment was cloned and sequenced. Two plants, one located in ON in 2006 and the other in BC in 2007, were found to be infected with an AY-like phytoplasma by the CFIA. The phytoplasmas detected in both infected plants had a 99.9% nt sequence identity with AY phytoplasma sequences from GenBank (Accession Nos. AF222063 and AY665676, respectively), with the BC isolate also showing 100% identity to a strain of AY, ash witches'-broom phytoplasma (GenBank Accession No. AY566302). AAFC detected phytoplasma DNA in both years in a total of 17 symptomatic plants and 21 asymptomatic plants from different vine varieties in ON, BC, and QC. Positive samples were found to have a 99.0% nt sequence identity to AY subgroup 16SrI-A (GenBank Accession No. AY180956). Sequences were exchanged for confirmation of phytoplasma identity and were deposited in Genbank under Accession Nos. FJ659844 and FJ824597. Phytoplasma strains were identified for all plants in which phytoplasmas were detected. Results show that AY is present in vineyards in the provinces of ON, BC, and QC. To our knowledge, this is the first report of AY being detected in grapevines in Canada. References: (1) E. Boudon-Padieu. Bull. O I V, 79:299, 2003. (2) M. Rott et al. Plant Dis. 91:1682, 2007. (3) E. Tanne et al. Phytopathology 91:741, 2001.

In August 2008, unusual symptoms were observed in Mexican lime trees (Citrus aurantifolia (Christm.) Swing) in the municipality of Luperón, province of Puerto Plata on the north coast of the Dominican Republic. Symptoms observed in young and old trees included blotchy mottle on leaves, healthy-appearing larger branches with smaller side branches that displayed chlorotic leaves, abscised and lopsided fruit, and branch dieback, all symptoms similar to those of citrus huanglongbing associated with ‘Candidatus Liberibacter’ spp. (1). Symptoms were observed in an area of ~100 ha surrounding Luperón, where Mexican lime trees were grown as seedlings and no commercial plantings of other citrus were present. Symptomatic leaves were collected from 16 trees in September 2008, and DNA was extracted from petioles and midveins with a DNeasy kit (Qiagen, Gaithersburg, MD) or with chloroform/isoamyl alcohol (24:1). Real-time PCR with the16S rDNA primer/probe set specific to ‘Ca. Liberibacter asiaticus’ and performed as described (2) gave Ct values comparable with the positive control for five samples. Conventional PCR with the forward (5′-tcgagcgcgtatgcaatacg-3′) and reverse (5′-ctacctttttctacgggataacgc-3′) primers used in real-time PCR (2) amplified a 75-bp product from these five samples. Eleven to twelve clones were sequenced from each sample and BLAST analysis of a consensus sequence for each sample (GenBank Accession Nos. FJ489643–FJ489647) showed 98% (e.g., EU921622, EU921618) to 100% identity (e.g., FJ236554, FJ263702) with 16S rDNA sequences of ‘Ca. Liberibacter asiaticus’. A larger 912-bp portion of the 16S rDNA gene was amplified from each sample by conventional PCR with sense (5′-gagcctaccaaggctacgat-3′) and antisense (5′-gcgttatcccgtagaaaaaggtag-3′) primers designed from the sequence of ‘Ca. Liberibacter asiaticus’ strain LJZ-4730 (GenBank Accession No. FJ263700). Nine to twelve clones were sequenced for each sample and BLAST analysis of a consensus sequence for each sample (GenBank Accession Nos. FJ811891–FJ811895) showed 100% identity to ‘Ca. Liberibacter asiaticus’ (DQ471900, DQ4719010) from Florida and Brazil, respectively. To confirm this identification, 693 bp of the outer membrane protein gene (omp) were amplified from each of the five positive samples by conventional PCR using sense (5′-gtgattctgagggtgagcg-3′) and antisense (5′-cgaactcactgagaactgatc-3′) primers designed from nucleotides 15 to 33 and 687 to 707, respectively, of the omp gene from ‘Ca. Liberibacter asiaticus’ strain MZ (GenBank Accession No. EF580135). The consensus sequences from 12 clones from each sample (GenBank Accession Nos. FJ489638–FJ489642) showed 99% (e.g., FJ236564, AY842429) to 100% (e.g., FJ236566, EF580135) nucleotide identity and the predicted translation product showed 100% amino acid identity (e.g., AAX47433, AAX47431) with the omp gene from ‘Ca. Liberibacter asiaticus’. These results confirm the presence of and to our knowledge, this is the first report of ‘Ca. Liberibacter asiaticus’ in the Dominican Republic. Huanglongbing is a destructive disease of citrus (1) and its spread is expected to adversely affect citrus production in the Dominican Republic with subsequent negative effects predicted for the employment of thousands of people. References: (1) J. M. Bové, J. Plant Pathol. 88:7, 2006. (2) W. Li et al. J. Microbiol. Methods 66:104, 2006.

Summary Multicellular magnetotactic prokaryotes (MMPs) represent highly organized, spherical and motile aggregates of 10–40 bacterial cells containing magnetosomes. Although consisting of different cells, each with its own magnetosomes and flagellation, MMPs orient themselves within a magnetic field and exhibit magnetotaxis. So far, MMPs have only been found in several North and South American coastal lagoons and salt marshes. In the present study, a novel type of MMP was discovered in coastal tidal sand flats of the North Sea. High‐resolution scanning electron microscopy revealed the presence of bullet‐shaped magnetosomes which were aligned in several parallel chains. Within each aggregate, the magnetosome chains of individual cells were oriented in the same direction. Energy dispersive X‐ray (EDX) analysis showed that the magnetosomes are composed of iron sulfide. This particular morphology and arrangement of magnetosomes has previously not been reported for other MMPs. 16S rRNA gene sequence analysis revealed a single phylotype which represented a novel phylogenetic lineage with ≥ 4% sequence divergence to all previously described MMP sequences and was related to the dissimilatory sulfate‐reducing Desulfosarcina variabilis within the family Desulfobacteraceae of the subphylum Deltaproteobacteria . Fluorescence in situ hybridization with a specific oligonucleotide probe revealed that all MMPs in the tidal flat sediments studied belonged to the novel phylotype. Within each MMP, all bacterial cells showed a hybridization signal, indicating that the aggregates are composed of cells of the same phylotype. Genes for dissimilatory sulfite reductase ( dsr AB) and dissimilatory adenosine‐5′‐phosphate reductase ( apr A) could be detected in purified MMP samples, suggesting that MMPs are capable of sulfate reduction. Chemotaxis assays with 41 different test compounds yielded strong responses towards acetate and propionate, whereas other organic acids, alcohols, sugars, sugar alcohols or sulfide did not elicit any response. By means of its coordinated magnetotaxis and chemotaxis, the novel type of MMP is well adapted to the steep chemical gradients which are characteristic for intertidal marine sediments.

ABSTRACT The characterization of transcriptional start sites of 14 genes encoded by the extremely AT-rich genome of “ Candidatus Blochmannia floridanus” revealed a high degree of conservation with the RpoD promoter consensus sequence of the free-living relative Escherichia coli . Moreover, in agreement with the presence of the alternative heat shock sigma factor RpoH in “ Ca . Blochmannia,” typical RpoH-dependent promoters were identified. However, no heat shock response resembling that of E. coli could be detected in “ Ca . Blochmannia.”