Publications

4861

Temperature has been shown to have a significant effect on development of liberibacter species associated with citrus Huanglongbing disease. ‘Candidatus Liberibacter africanus’ and ‘Ca. L. americanus’ are both heat sensitive, whereas ‘Ca. L. asiaticus’ is heat tolerant. The recently described ‘Ca. L. solanacearum’ is associated with zebra chip (ZC), a newly emerging and economically important disease of potato worldwide. This psyllid-transmitted liberibacter species severely affects several other solanaceous crops and carrot. Experiments were conducted to evaluate effects of temperature on development of ‘Ca. L. solanacearum’ and ZC disease. Potato plants were inoculated with ‘Ca. L. solanacearum’ by briefly exposing them to liberibacter-infective potato psyllids at various temperatures under laboratory conditions. Following insect exposure, the plants were maintained at selected temperature regimes in growth chambers, monitored for ZC symptom development, and later tested for liberibacter by polymerase chain reaction to confirm infection. Results indicated that temperatures below 17°C appear to slow development of ‘Ca. L. solanacearum’ and ZC symptoms, whereas temperatures above 32°C are detrimental to this liberibacter. Compared to Huanglongbing liberibacters, ‘Ca. L. solanacearum’ appears heat sensitive. The sensitivity of this bacterium and its insect vector to temperature may partially explain incidence, severity, and distribution of ZC in affected regions.

The crystal structure of biomineralized magnetic nanocrystals depends on environmental and genetic factors.

ABSTRACT Members of the noncultured clade of Frankia enter into root nodule symbioses with actinorhizal species from the orders Cucurbitales and Rosales. We report the genome sequence of a member of this clade originally from Pakistan but obtained from root nodules of the American plant Datisca glomerata without isolation in culture.

A novel phytoplasma, designated strain SoyST1c1, associated with a newly emerging disease in soybean (Glycine max), known as soybean stunt (SoyST), was found in 2002 in a soybean plantation in Alajuela Province, Costa Rica. The same phytoplasma, or a very closely related strain, also infected sweet pepper (Capsicum annuum) with purple vein syndrome (SwPPV) and passion fruit vine (Passiflora edulis) with bud proliferation disease (PasFBP) in the same region. Sequence analysis of cloned 16S rRNA gene sequences (GenBank accession nos FJ226068–FJ226073 and HQ225624–HQ225635) indicated that all three affected plants were infected by phytoplasmas that shared <97.5 % sequence similarity with previously described phytoplasmas. The SoyST-causing phytoplasma represents a new taxon, most closely related to phytoplasma group 16SrI and 16SrXII strains. Virtual RFLP analysis indicated that the SoyST-causing phytoplasma and its closely related strains represent a novel 16Sr group, designated 16SrXXXI. Phylogenetic analysis of 16S rRNA gene sequences from the new phytoplasma strains, those previously described as ‘Candidatus Phytoplasma spp.’ and other distinct, as yet unnamed, phytoplasmas indicated that the SoyST-causing phytoplasma represents a distinct lineage within the aster yellows/stolbur branch on the phylogenetic tree. On the basis of its unique 16S rRNA gene sequence and biological properties, strain SoyST1c1 represents a novel taxon, for which the name ‘Candidatus Phytoplasma costaricanum’ is proposed with SoyST1c1 as the reference strain.

Summary In this study, members of a specific group of thin (6–14 µm filament diameter), vacuolated Beggiatoa‐ like filaments from six different hypersaline microbial mats were morphologically and phylogenetically characterized. Therefore, enrichment cultures were established, filaments were stained with fluorochromes to show intracellular structures and 16S rRNA genes were sequenced. Morphological characteristics of Beggiatoa‐ like filaments, in particular the presence of intracellular vacuoles, and the distribution of nucleic acids were visualized. In the intracellular vacuole nitrate reached concentrations of up to 650 mM. Fifteen of the retrieved 16S rRNA gene sequences formed a monophyletic cluster and were phylogenetically closely related (≥ 94.4% sequence identity). Sequences of known filamentous sulfide‐oxidizing genera Beggiatoa and Thioploca that comprise non‐vacuolated and vacuolated filaments from diverse habitats clearly delineated from this cluster. The novel monophyletic cluster was furthermore divided into two sub‐clusters: one contained sequences originating from Guerrero Negro (Mexico) microbial mats and the other comprised sequences from five distinct Spanish hypersaline microbial mats from Ibiza, Formentera and Lake Chiprana. Our data suggest that Beggiatoa ‐like filaments from hypersaline environments displaying a thin filament diameter contain nitrate‐storing vacuoles and are phylogenetically separate from known Beggiatoa . Therefore, we propose a novel genus for these organisms, which we suggest to name ‘ Candidatus Allobeggiatoa’.

The aim of this work was to assess the effects of a combined inoculum of a rhizobacterium and an arbuscular mycorrhizal (AM) fungus on plant responses to phytoplasma infection, and on phytoplasma multiplication and viability in Chrysanthemum carinatum plants infected by chrysanthemum yellows phytoplasma (CY). Combined inoculation with Glomus mosseae BEG12 and Pseudomonas putida S1Pf1Rif resulted in some resistance to phytoplasma infection (about 30%), delayed symptom expression in nonresistant plants, improved growth of the aerial part of the infected plants (+68·1%), and altered root morphology (root tip number: +49·9%; branching degree: +82·8%). Combined inoculation with the two beneficial microorganisms did not alter CY multiplication and viability. In inoculated and infected plants, phytoplasma morphology was typical of senescent cells. A more active and efficient root system in double‐inoculated plants probably mediated the effects of the two rhizospheric microorganisms in the infected plants. The practical application of rhizospheric microorganisms for mitigating phytoplasma damage, following evaluation under field conditions, represents an additional tool for the integrated management of phytoplasmosis.