Publications

4719

Plant diseases caused by vector-borne pathogens are responsible for tremendous losses and threaten some of the most important agricultural crops. A good example is the citrus greening disease, which is caused by bacteria of the genus Liberibacter and is transmitted by psyllids; it has devastated the citrus industry in the United States, China, and Brazil.

Huanglongbing (HLB) is the most destructive, yet incurable disease of citrus. Finding sources of genetic resistance to HLB-associated ‘CandidatusLiberibacter asiaticus’ (Las) becomes strategic to warrant crop sustainability, but no resistantCitrusgenotypes exist. SomeCitrusrelatives of the family Rutaceae, subfamily Aurantioideae, were described as full-resistant to Las, but they are phylogenetically far, thus incompatible withCitrus. Partial resistance was indicated for certain cross-compatible types. Moreover, other genotypes from subtribe Citrinae, sexually incompatible but graft-compatible withCitrus, may provide new rootstocks able to restrict bacterial titer in the canopy. Use of seedlings from monoembryonic species and inconsistencies in previous reports likely due to Las recalcitrance encouraged us to evaluate more accurately theseCitrusrelatives. We tested for Las resistance a diverse collection of graft-compatible Citrinae species using an aggressive and consistent challenge-inoculation and evaluation procedure. Most Citrinae species examined were either susceptible or partially resistant to Las. However,Eremocitrus glaucaand Papua/New GuineaMicrocitrusspecies as well as their hybrids and those withCitrusarose here for the first time as full-resistant, opening the way for using these underutilized genotypes as Las resistance sources in breeding programs or attempting using them directly as possible new Las-resistantCitrusrootstocks or interstocks.

“Candidatus Nitrosocaldaceae” are globally distributed in neutral or slightly alkaline hot springs and geothermally heated soils. Despite their essential role in the nitrogen cycle in high-temperature ecosystems, they remain poorly understood because they have never been isolated in pure culture, and very few genomes are available. In the present study, a metagenomics approach was employed to obtain “Ca. Nitrosocaldaceae” metagenomic-assembled genomes (MAGs) from hot spring samples collected from India and China. Phylogenomic analysis placed these MAGs within “Ca. Nitrosocaldaceae.” Average nucleotide identity and average amino acid identity analysis suggested the new MAGs represent two novel species of “Candidatus Nitrosocaldus” and a novel genus, herein proposed as “Candidatus Nitrosothermus.” Key genes responsible for chemolithotrophic ammonia oxidation and a thaumarchaeal 3HP/4HB cycle were detected in all MAGs. Furthermore, genes coding for urea degradation were only present in “Ca. Nitrosocaldus,” while biosynthesis of the vitamins, biotin, cobalamin, and riboflavin were detected in almost all MAGs. Comparison of “Ca. Nitrosocaldales/Nitrosocaldaceae” with other AOA revealed 526 specific orthogroups. This included genes related to thermal adaptation (cyclic 2,3-diphosphoglycerate, and S-adenosylmethionine decarboxylase), indicating their importance for life at high temperature. In addition, these MAGs acquired genes from members from archaea (Crenarchaeota) and bacteria (Firmicutes), mainly involved in metabolism and stress responses, which might play a role to allow this group to adapt to thermal habitats.

Abstract BACKGROUND: Rickettsia spp. are human pathogens that cause a number of diseases and are transmitted by arthropods, such as ixodid ticks. Estonia is one of few regions where the distribution area of two medically important tick species, Ixodes persulcatus and I. ricinus, overlaps. The presence of the nidicolous rodent-associated I. trianguliceps has also recently been shown in Estonia. Although there is no data available in Estonia on human disease caused by tick-borne Rickettsia spp., the presence of three Rickettsia species in non-nidicolous ticks was also previously reported. The aim of this study was to detect, identify and partially characterize Rickettsia species in nidicolous and non-nidicolous ticks attached to rodents.RESULTS: Larvae and nymphs of I. ricinus (n = 1004), I. persulcatus (n = 75) and I. trianguliceps (n = 117) removed from rodents and shrews caught in different parts of Estonia were studied for the presence of Rickettsia spp. by nested PCR. Ticks were collected from 314 small animals of 5 species (Myodes glareolus (bank voles), Apodemus flavicollis (yellow necked mice), A. agrarius (striped field mice), Microtus subterranius (pine voles) and Sorex araneus (common shrews)). Rickettsial DNA was detected in 8.7% (103/1186) of the studied ticks. In addition to R. helvetica previously found in questing ticks, this study reports the first identification of the recently described I. trianguliceps-associated Candidatus R. uralica west of the Ural Mountains.

iaphorina citri is considered the most dangerous citrus pest because it transmits Candidatus Liberibacter asiaticus, the causal agent of Huanglongbing (HLB). Like other hemiptera insects, D. citri has developed mutualistic interactions with prokaryotic organisms known as endosymbionts. This symbiosis can be obligatory, when the interaction occurs with a primary endosymbiont, or facultative, when it is a secondary endosymbiont. Symbiosis is essential for various physiological functions, but some endosymbionts can adversely affect the psyllid's abilities. D. citri is associated with a great diversity of endosymbionts, with Candidatus Carsonella ruddii, Candidatus Profftella armatura, Candidatus Wolbachia spp. and Candidatus Liberibacter spp standing out. The aim o symbiotic relationship, prokaryotes, Candidatus Wolbachia, transmission of Candidatus Liberibacter asiaticus.

Sugarcane Grassy Shoot (SCGS) disease is known to be related to Rice Yellow Dwarf (RYD) phytoplasmas (16SrXI-B group) which are found predominantly in sugarcane growing areas of the Indian subcontinent and South-East Asia. The 16S rRNA gene sequences of SCGS phytoplasma strains belonging to the 16SrXI-B group share 98.07 % similarity with ‘Ca. Phytoplasma cynodontis’ strain BGWL-C1 followed by 97.65 % similarity with ‘Ca. P. oryzae’ strain RYD-J. Being placed distinctly away from both the phylogenetically related species, the taxonomic identity of SCGS phytoplasma is unclear and confusing. We attempted to resolve the phylogenetic positions of SCGS phytoplasma based on the phylogenetic analysis of 16S rRNA gene (>1500 bp), nine housekeeping genes (>3500 aa), core genome phylogeny (>10 000 aa) and OGRI values. The draft genome sequences of SCGS phytoplasma (strain SCGS) and Bermuda Grass White leaf (BGWL) phytoplasma (strain LW01), closely related to ‘Ca. P. cynodontis’, were obtained. The SCGS genome was comprised of 29 scaffolds corresponding to 505 173 bp while LW01 assembly contained 21 scaffolds corresponding to 483 935 bp with the fold coverages over 330× and completeness over 90 % for both the genomes. The G+C content of SCGS was 19.86 % while that of LW01 was 20.46 %. The orthoANI values for the strain SCGS against strains LW01 was 79.42 %, and dDDH values were 22. Overall analysis reveals that SCGS phytoplasma forms a distant clade in RYD group of phytoplasmas. Based on phylogenetic analyses and OGRI values obtained from the genome sequences, a novel taxon ‘Candidatus Phytoplasma sacchari’ is proposed.