Publications

3886

‘Candidatus Liberibacter asiaticus’ (CLas), the causal agent of citrus huanglongbing (HLB), colonizes inside the phloem and is naturally transmitted by the Asian citrus psyllid (ACP). Here, we investigated spatiotemporal CLas colonization in different tissues after ACP transmission. Of the nine plants successfully infected via ACP transmission, CLas was detected in the roots of all trees at 75 days postremoval of ACPs (DPR) but in the mature leaf of only one tree; this finding is consistent with the model that CLas moves passively from source to sink tissues. At 75 and 365 DPR, CLas was detected in 11.1 and 43.1% of mature leaves not fed on by ACPs during transmission, respectively, unveiling active movement to the source tissue. The difference in colonization timing of sink and source tissues indicates that CLas is capable of both passive and active movement, with passive movement being dominant. At 225 DPR, leaves fed on by ACPs during the young stage showed the highest ratio of HLB symptomatic leaves and the highest CLas titer, followed by leaves that emerged after ACP removal and mature leaves not fed on by ACPs. Importantly, our data showed that ACPs were unable to transmit CLas via feeding on mature leaves. It is estimated that it takes 3 years at most for CLas to infect the whole tree. Overall, spatiotemporal detection of CLas in different tissues after ACP transmission helps visualize the infection process of CLas in planta and subsequent HLB symptom development and provides evidence showing that young leaves should be the focus of HLB management.

AbstractDichloromethane (DCM; CH2Cl2) is a toxic groundwater pollutant that also has a detrimental effect on atmospheric ozone levels. As a dense non-aqueous phase liquid, DCM migrates vertically through groundwater to low redox zones, yet information on anaerobic microbial DCM transformation remains scarce due to a lack of cultured organisms. We report here the characterisation of DCMF, the dominant organism in an anaerobic enrichment culture (DFE) capable of fermenting DCM to the environmentally benign product acetate. Stable carbon isotope experiments demonstrated that the organism assimilated carbon from DCM and bicarbonate via the Wood–Ljungdahl pathway. DCMF is the first anaerobic DCM-degrading population also shown to metabolise non-chlorinated substrates. It appears to be a methylotroph utilising the Wood–Ljungdahl pathway for metabolism of methyl groups from methanol, choline, and glycine betaine. The flux of these substrates from subsurface environments may either directly (DCM, methanol) or indirectly (choline, glycine betaine) affect the climate. Community profiling and cultivation of cohabiting taxa in culture DFE without DCMF suggest that DCMF is the sole organism in this culture responsible for substrate metabolism, while the cohabitants persist via necromass recycling. Genomic and physiological evidence support placement of DCMF in a novel genus within the Peptococcaceae family, ‘Candidatus Formimonas warabiya’.

Abstract Phytoplasmas are wall-less parasitic bacteria living exclusively in plant phloem as consequence of transmission by sap-sucking insect vectors (Lee et al., 2000); they have been associated with several hundred plant diseases. 'Candidatus Phytoplasma phoenicium' (CaPphoe), subgroup 16SrIX-B, is the aetiological agent of almond witches'-broom (AlmWB), a severe disease affecting almond, peach and nectarine trees in Lebanon and Iran. The first epidemics of AlmWB occurred in almond trees in Lebanon in the early 1990s and in Iran in 1995. In Lebanon, the disease rapidly spread from coastal to high mountainous areas, killing almost 150,000 trees over a period of 15 years. CaPphoe was first added to the EPPO Alert List in 2001 and removed from the list in 2006. The more recent rapid spread of CaPphoe in peach and nectarine orchards and in other plant hosts, along with the identification of efficient insect vectors, increased the alarm about the risk it poses for stone fruit production in the Middle East and in all the countries of the Mediterranean basin. Thus it was re-inserted in the EPPO Alert List in 2015.

The cyanobacterial genus Nostoc is an important contributor to carbon and nitrogen bioavailability in terrestrial ecosystems and a frequent partner in symbiotic relationships with non-diazotrophic organisms. However, since this currently is a polyphyletic genus, the diversity of Nostoc -like cyanobacteria is considerably underestimated at this moment. While reviewing the phylogenetic placement of previously isolated Nostoc -like cyanobacteria originating from Brazilian Amazon, Caatinga and Atlantic forest samples, we detected 17 strains isolated from soil, freshwater, rock and tree surfaces presenting patterns that diverged significantly from related strains when ecological, morphological, molecular and genomic traits were also considered. These observations led to the identification of the evaluated strains as representative of three novel nostocacean genera and species: Amazonocrinis nigriterrae gen. nov., sp. nov.; Atlanticothrix silvestris gen. nov., sp. nov.; and Dendronalium phyllosphericum gen. nov., sp. nov., which are herein described according to the rules of the International Code of Nomenclature for algae, fungi and plants. This finding highlights the great importance of tropical and equatorial South American ecosystems for harbouring an unknown microbial diversity in the face of the anthropogenic threats with which they increasingly struggle.

Abstract 'Ca. Phytoplasma pini' is a member of phytoplasma 16S rRNA gene RFLP group XXI, subgroup XXI-A. It has been identified in a number of European countries, including Germany, Poland, Lithuania, Spain, Czech Republic and Croatia. Outside of Europe it has been identified in China and Mozambique. A related strain has also been identified in Maryland, USA. In the USA, favourable climatic conditions and wide availability of potential host plants of the phytoplasma, suggest that the potential for spread of 'Ca. Phytoplasma pini' could be significant. Host plants include Pinus sylvestris, P. halepensis, P. mugo, P. banksiana, P. nigra, P. tabuliformis, Abies procera and Tsuga canadensis. Symptoms include the formation of ball-like growths containing dwarfed needles, yellowed or reddish needles and the loss of needles. It is transmitted by insect vectors that are currently unknown.

Abstract Phytoplasmas are cell-wall-less plant pathogenic bacteria of the class Mollicutes, which inhabit the phloem sieve tubes of plants and have been associated with several hundred diseases affecting economically important crops. Over the past few decades 'Candidatus Phytoplasma solani', belonging to the 16SrXII-A ribosomal subgroup, has been found to cause a range of plant diseases in different agro-ecosystems in many countries in Europe and the eastern Mediterranean area and a number of others all over the world. It is thought likely that it has always been present, at least in its European range, but has only been noticed in recent years. Diseases caused include bois noir in grapevines, stolbur in tomatoes, potatoes and other wild and cultivated plants, maize redness, lavender decline, and yellowing, reddening, decline, dwarfism, leaf malformation and degeneration diseases of other plants. 'Ca. P. solani' is usually transmitted from plant to plant by the polyphagous insect vector Hyalesthes obsoletus (Cixiidae) which, although it can complete its life cycle on only a small number of plant species, feeds on a much wider range. Recent studies have demonstrated the presence of additional insect vectors of this phytoplasma in Europe, such as Reptalus panzeri in Serbia, possibly R. quinquecostatus in Serbia and France, and Anaceratagallia ribauti in Austria. This scenario highlights the extreme complexity of the ecology of both 'Ca. Phytoplasma solani' and its insect vectors, underlying the difficulty in studying the epidemiology of diseases associated with this pathogen and in developing efficient control strategies. 'Ca. Phytoplasma solani' is also transmitted by parasitic plants and by grafting and vegetative propagation of infected host plants; it can be spread when host plants are transported by people. In the European Union it is listed as a harmful organism necessitating restrictions on the import of plants in the family Solanaceae.

A phytoplasma was initially detected in Dypsis poivriana by nested and real-time PCR from the botanical gardens in Cairns, Queensland, Australia in 2017. Further surveys in the Cairns region identified phytoplasma infections in eight additional dying ornamental palm species (Euterpe precatoria, Cocos nucifera, Verschaffeltia splendida, Brassiophoenix drymophloeodes, Burretiokentia hapala, Cyrtostachys renda, Reinhardtia gracilis, Carpoxylon macrospermum), a Phoenix species, a Euterpe species and two native palms (Archontophoenix alexandrae). Analysis of 16S rRNA gene sequences showed that this phytoplasma is distinct as it shared less than 97.5 % similarity with all other ‘Candidatus Phytoplasma’ species. At 96.3 % similarity, the most closely related formally described member of the provisional 'Ca. Phytoplasma' genus was 'Ca. Phytoplasma noviguineense', a novel taxon from the island of New Guinea found in monocotyledonous plants. It was slightly more closely related (96.6–96.8 %) to four palm-infecting strains from the Americas, which belong to strain group 16SrIV and which have not been assigned to a formal 'Candidatus Phytoplasma’ species taxon. Phylogenetic analysis of the 16S rRNA gene and ribosomal protein genes of the phytoplasma isolate from a dying coconut palm revealed that the phytoplasma represented a distinct lineage within the phytoplasma clade. As the nucleotide identity with other phytoplasmas is less than 97.5 % and the phylogenetic analyses show that it is distinct, a novel taxon 'Candidatus Phytoplasma dypsidis' is proposed for the phytoplasma found in Australia. Strain RID7692 (GenBank accession no. MT536195) is the reference strain. The impact and preliminary aspects of the epidemiology of the disease outbreak associated with this novel taxon are described.

The actinobacterial strain 15G-AUS-rotT was isolated from an artificial pond located near Salzburg, Austria. The strain showed 16S rRNA gene sequence similarities of 98.7 % to Candidatus Aquiluna rubra and of 96.6 and 96.7 % to the two validly described species of the genus Rhodoluna . Phylogenetic reconstructions based on 16S rRNA gene sequences and genome-based on amino acid sequences of 118 single copy genes referred strain 15G-AUS-rotT to the family Microbacteriaceae and therein to the so-called subcluster Luna-1. The genome-based phylogenetic tree showed that the new strain represents a putative new genus. Cultures of strain 15G-AUS-rotT were light red pigmented and comprised very small, rod-shaped cells. They metabolized a broad variety of substrates. Major fatty acids (>10 %) of cells were iso-C16 : 0, antiso-C15 : 0 and iso-C14 : 0. The major respiratory quinone was MK-11 and a minor component was MK-10. The peptidoglycan structure belonged to an unusual B type. The closed genome sequence of the strain was very small (1.4 Mbp) and had a DNA G+C content of 54.8 mol%. An interesting feature was the presence of genes putatively encoding the complete light-driven proton pumping actinorhodopsin/retinal system, which were located at three different positions of the genome. Based on the characteristics of the strain, a new genus and a new species termed Aquiluna borgnonia is proposed for strain 15G-AUS-rotT (=DSM 107803T=JCM 32974T).

Pumpkin plants showing symptoms of witches’ broom (PuWB) were observed in Xinjiang Uyghur autonomous region, China, in September 2018. A phytoplasma was detected in symptomatic plants by PCR amplifying portions of the 16S ribosomal and tuf genes. In addition, the phylogeny based on these genes sequencing indicated that the PuWB strain clusters with ‘Candidatus Phytoplasma solani’ (subgroup 16SrXII-A). Furthermore, based on in silico and in vitro restriction fragment length polymorphism analyses, the PuWB phytoplasma was confirmed as a ‘Ca. P. solani’-related strain. This was the first record of the occurrence of phytoplasma presence in pumpkins in China, and the first record of 16SrXII phytoplasma infecting pumpkins in the world.