Publications

4372

AbstractThe epidemiology of ‘Candidatus Phytoplasma prunorum’ was studied in Austria and Hungary from 2014 to 2018. Testing of root samples showed average infections rates of 61 and 40% of the Austrian Prunus spinosa and Prunus domestica spp. insititia samples, respectively. In Hungary, on average 21% of the P. spinosa and 13% of the feral Prunus cerasifera samples were infected. The pathogen was found in 18 out of 19 apricot orchards and PCR positive Cacopsylla pruni were observed at 11 out of 17 sampling locations in both countries. In cage experiments with C. pruni remigrants successful pathogen transmission to Prunus armeniaca, P. domestica and P. spinosa seedlings in budding and foliated developmental stages was recorded, an inoculation access period of 4 hr was sufficient for transmission. A field experiment with ungrafted apricot seedlings planted in 2012 and 2014 indicated a prominent role of the insect vectors for disease spread. In 2017, 40 and 28% of the trees planted in 2012 and 2014, respectively, were infected. Molecular characterisation based on the genes aceF and imp allowed the discrimination between 10 phytoplasma types in apricots. Around 70% of the phytoplasma types in apricots were also common in P. spinosa, in P. domestica spp. insititia and in remigrant C. pruni pointing to a possible pathogen exchange by insects between wild and cultivated Prunus spp. For disease control, vector management over the entire flight period of the remigrants seems necessary; when selecting active compounds, the short inoculation access period of not more than 4 hr should be considered.

A novel actinomycete, designated strain GLM-1T, was isolated from arbuscular mycorrhizal fungal spores from Funneliformis mosseae RYA08, collected from Aquilaria crassna Pierre ex Lec. rhizosphere soil in Klaeng, Rayong Province, Thailand. Morphological characteristics of this strain included long chains of rod-like cells and squarish elements. The cell-wall composition of this novel isolate contained meso-diaminopimelic acid. The whole-cell diagnostic sugars were arabinose and galactose. The predominant menaquinone was MK-9(H4). The major fatty acids were iso-C16 : 0 and iso-C15 : 0. Only phosphatidylethanolamine was detected as a polar lipid. Phylogenetic analysis based on 16S rRNA gene sequences showed that strain GLM-1T was closely related to Amycolatopsis rhabdoformis SB026T (99.11 %) with a low DNA–DNA hybridization value of 22.6–34.7 %. Genome sequencing revealed a genome size of 10 Mbp. There were obvious distinctions in the average nucleotide identity values between stain GLM-1T and its closely related strains at around 86–93 % (ANIb) and 89–94 % (ANIm). The digital DNA–DNA hybridization values between strain GLM-1T and type strains of phylogenetically related species were 34–55 %. The G+C content of the genomic DNA was 71.8 mol%. Based on these data, strain GLM-1T is considered to represent a novel species of the genus Amycolatopsis , for which the name Amycolatopsis eburnea sp. nov. is proposed. The type strain is GLM-1T (=TBRC 9315T=NBRC 113658T)

AbstractBois noir (BN), the most prevalent disease of the grapevine yellows complex, causes considerable yield loss in vineyards. BN is associated with phytoplasma strains of the species ‘Candidatus Phytoplasma solani’ (taxonomic subgroup 16SrXII‐A). In Europe, the BN phytoplasma is transmitted to grapevine mainly by Hyalesthes obsoletus, a polyphagous cixiid completing its life cycle on stinging nettle and field bindweed. As a result of the complexity of BN epidemiology, no effective control strategies have been developed. In previous studies conducted in the eastern Mediterranean coast of Israel, chaste tree (Vitex agnus‐castus) was found to be the preferred host plant of H. obsoletus but did not harbour BN phytoplasma. Thus, a ‘push and pull’ strategy was suggested based on the fact that chaste tree plants located at vineyard borders was an effective trap plant for H. obsoletus adults. However, in other studies carried out in the eastern Adriatic coast of Montenegro, chaste tree was found to be a key source plant for BN phytoplasma transmission to grapevine. This study aimed to investigate (i) the interaction between chaste tree and H. obsoletus through survival, attractiveness and oviposition experiments conducted comparing the behaviour of H. obsoletus in chaste tree versus stinging nettle and grapevine and (ii) the capability of chaste tree to harbor ‘Ca. P. solani’ in northern Italy through transmission trials. H. obsoletus adults were found to survive on chaste tree and grapevine over a 1 week period and prefer chaste tree to grapevine. Moreover, H. obsoletus produced eggs and overwintered as nymphs on chaste tree, even if at a lesser extent than on stinging nettle. H. obsoletus originating from nettle was found able to transmit ‘Ca. P. solani’ to chaste tree (2 plants of 16 were found infected by the BN phytoplasma strain St5 identified in H. obsoletus specimens). These results increased our knowledge about the role of Vitex agnus‐castus as host plant of H. obsoletus and BN phytoplasma in northern Italy and do not recommend considering chaste tree as trap plant at vineyard borders.

Phytoplasmas are the causative agent of numerous diseases of plant species all over the world, including important food crops. The mode by which phytoplasmas multiply and behave in their host is poorly understood and often based on genomic data. We used yeast two-hybrid screening to find new protein–protein interactions between the causal agent of apple proliferation ‘Candidatus Phytoplasma mali’ and its host plant. Here, we report that the ‘Ca. P. mali’ strain PM19 genome encodes a protein PM19_00185 that interacts with at least six different ubiquitin-conjugating enzymes (UBC; E2) of Arabidopsis thaliana. An in vitro ubiquitination assay showed that PM19_00185 is enzymatically active as E3 ligase with A. thaliana E2 UBC09 and Malus domestica E2 UBC10. We show that a nonhost bacteria (Pseudomonas syringae pv. tabaci) can grow in transgenic A. thaliana plant lines expressing PM19_00185. A connection of phytoplasma effector proteins with the proteasome proteolytic pathway has been reported before. However, this is, to our knowledge, the first time that a phytoplasma effector protein with E3 ligase activity has been reported.

Abstract The most abundant aquatic microbes are small in cell and genome size. Genome-streamlining theory predicts gene loss caused by evolutionary selection driven by environmental factors, favouring superior competitors for limiting resources. However, evolutionary histories of such abundant, genome-streamlined microbes remain largely unknown. Here we reconstruct the series of steps in the evolution of some of the most abundant genome-streamlined microbes in freshwaters (“Ca. Methylopumilus”) and oceans (marine lineage OM43). A broad genomic spectrum is visible in the family Methylophilaceae (Betaproteobacteria), from sediment microbes with medium-sized genomes (2–3 Mbp genome size), an occasionally blooming pelagic intermediate (1.7 Mbp), and the most reduced pelagic forms (1.3 Mbp). We show that a habitat transition from freshwater sediment to the relatively oligotrophic pelagial was accompanied by progressive gene loss and adaptive gains. Gene loss has mainly affected functions not necessarily required or advantageous in the pelagial or is encoded by redundant pathways. Likewise, we identified genes providing adaptations to oligotrophic conditions that have been transmitted horizontally from pelagic freshwater microbes. Remarkably, the secondary transition from the pelagial of lakes to the oceans required only slight modifications, i.e., adaptations to higher salinity, gained via horizontal gene transfer from indigenous microbes. Our study provides first genomic evidence of genome reduction taking place during habitat transitions. In this regard, the family Methylophilaceae is an exceptional model for tracing the evolutionary history of genome streamlining as such a collection of evolutionarily related microbes from different habitats is rare in the microbial world.