Publications

4368

Huanglongbing, or citrus greening, threatens the global citrus industry. The presumptive pathogens, ‘Candidatus Liberibacter asiaticus’ and ‘Ca. L. americanus’ can be transferred from citrus to more easily studied experimental hosts by using holoparasitic dodder plants. However, the interaction between ‘Candidatus Liberibacter’ spp. and the dodder has not been studied. We combined quantitative polymerase chain reaction with electron microscopy to show that only 65% of tendrils of Cuscuta indecora grown on ‘Ca. Liberibacter’ spp.-infected host plants had detectable levels of the pathogen. Among tendrils that were colonized by Liberibacter in at least one 2 cm segment, most were not colonized in all segments. Furthermore, the estimated population levels of the pathogen present in serial 2 cm segments of dodder tendrils varied widely and without any consistent pattern. Thus, there was generally not a concentration gradient of the pathogen from the source plant towards the recipient and populations of the pathogen were sometimes found in the distal segments of the dodder plant but not in the proximal or middle segments. Populations of the pathogens ranged from 2 × 102 to 3.0 × 108 cells per 2 cm segment. On a fresh weight basis, populations as high as 1.4 × 1010 cells per g of tissue were observed demonstrating that ‘Ca. Liberibacter’ spp. multiplies well in Cuscuta indecora. However, 55% of individual stem segments did not contain detectable levels of the pathogen, consistent with a pattern of nonuniform colonization similar to that observed in the much more anatomically complex citrus tree. Colonization of dodder by the pathogen is also nonuniform at the ultrastructural level, with adjacent phloem vessel elements being completely full of the pathogen or free of the pathogen. We also observed bacteria in the phloem vessels that belonged to two distinct size classes based on the diameters of cross sections of cells. In other sections from the same tendrils we observed single bacterial cells that were apparently in the process of differentiating between the large and round forms to the long and thin forms (or vice versa). The process controlling this morphological differentiation of the pathogen is not known. The highly reduced and simplified anatomy of the dodder plant as well as its rapid growth rate compared with citrus, and the ability of the plant to support multiplication of the pathogen to high levels, makes it an interesting host plant for further studies of host–pathogen interactions.

ABSTRACT Previously isolated dissimilatory perchlorate-reducing bacteria (DPRB) have been primarily affiliated with the Betaproteobacteria . Enrichments from the cathodic chamber of a bioelectrical reactor (BER) inoculated from creek water in Berkeley, CA, yielded a novel organism most closely related to a previously described strain, WD (99% 16S rRNA gene identity). Strain VDY T has 96% 16S rRNA gene identity to both Magnetospirillum gryphiswaldense and Magnetospirillum magnetotacticum , and along with strain WD, distinguishes a clade of perchlorate-reducing Magnetospirillum species in the Alphaproteobacteria . In spite of the phylogenetic location of VDY T , attempted PCR for the key magnetosome formation genes mamI and mamL was negative. Strain VDY T was motile, non-spore forming, and, in addition to perchlorate, could use oxygen, chlorate, nitrate, nitrite, and nitrous oxide as alternative electron acceptors with acetate as the electron donor. Transient chlorate accumulation occurred during respiration of perchlorate. The organism made use of fermentation end products, such as acetate and ethanol, as carbon sources and electron donors for heterotrophic growth, and in addition, strain VDY T could grow chemolithotrophically with hydrogen serving as the electron donor. VDY T contains a copy of the RuBisCo cbbM gene, which was expressed under autotrophic but not heterotrophic conditions. DNA-DNA hybridization with strain WD confirmed VDY T as a separate species (46.2% identity), and the name Magnetospirillum bellicus sp. nov. (DSM 21662, ATCC BAA-1730) is proposed.

Prunus persica (L.) Bastch (family Rosaceae) is currently represented by 83 accessions at the Canadian Clonal Genebank. Approximately 3,200 ha are devoted to peach cultivation in Canada where Ontario Province accounts for 82% of the national production. The clonal peach accessions, also located in Ontario, are monitored routinely for symptoms of phytoplasma infection, including rosette-like symptoms (3) that are characterized by new shoots with very short internodes, loss of older shoot leaves leaving only bunches of young leaves on the tips of naked shoots, and flowers that rarely set fruit. From June to August 2009, peach accessions PRU0382 and PRU0445 showed typical peach rosette symptoms, while another 14 accessions exhibited either short internodes or no symptoms. Leaf midrib samples were collected from 16 peach accessions, including 17 symptomatic (from which 8 corresponded to accession PRU0382, 6 for PRU0445, 1 for PRU0335, 1 for PRU0179, and 1 for PRU0451) and 16 asymptomatic (from which 5 corresponded to a representative of each accession PRU0382, PRU0445, PRU0335, PRU0179, and PRU0451 and 11 to other peach accessions). Total DNA was extracted (DNeasy Plant Extraction Mini Kit, QIAGEN, Valencia, CA) from 100 mg of each sample and used as a template in a nested PCR with phytoplasma universal primers R16mF2/R1 (1) and fU5/rU3 (2). Nested PCR products of the expected size (~880 bp) were obtained from all symptomatic samples (14 of 14) of accessions PRU0382 (peach-almond cv. Kando from the Czech Republic) and PRU0445 (peach cv. HW271 from Canada) only. All other plants with or without symptoms yielded no PCR products. Amplicons were purified (Wizard PCR Clean-up, Promega, Madison, WI), cloned in pGEM-T Easy Vector (Promega), and sequenced (Robarts Institute, London, Canada). The resulting 16S rDNA sequences were identical; one of each was archived in GenBank as Accession No. GU223904. BLAST analysis determined that the P. persica phytoplasma sequence shared 99% identity with 16S rDNA sequences of ‘Candidatus Phytoplasma asteris’-related strains. This relationship was also supported by restriction fragment length polymorphism analysis (RFLP) of rDNA amplicons using AluI, RsaI, and MseI endonucleases that yielded fragment profiles indicative of phytoplasmas belonging to group 16SrI (Aster Yellows), subgroup B (16SrI-B). Among phytoplasma diseases, those attributed to group 16SrI strains are most numerous and affect the widest plant host range. They include peach rosette in the United States and Europe (3) as well as diseases of various horticultural crops in Canada, including grapevine (4). To our knowledge, this is the first report of a subgroup 16SrI-B phytoplasma affecting peach in Canada. Early detection of phytoplasmas by PCR in accessions with both European and Canadian origins underscores the importance of prompt identification of infected plants for subsequent thermotherapy treatment to maintain the health of the collection and prevent further disease spread. References: (1) D. E Gundersen and I.-M. Lee. Phytopathol. Mediterr. 35:1441, 1996. (2) K. H. Lorenz et al. Phytopathology 85:771, 1995. (3) C. Marcone et al. Acta Hortic. 386:471, 1995. (4) C. Y. Olivier et al. Plant Dis. 93:669, 2009.