Publications

4467

Citrus huanglongbing (HLB), caused by phloem-limited ‘Candidatus Liberibacter’ bacteria, is a destructive disease threatening the worldwide citrus industry. The mechanisms of pathogenesis are poorly understood and no efficient strategy is available to control HLB. Here, we used a comparative genomics screen to identify candidate microbe-associated molecular patterns (MAMPs) from ‘Ca. Liberibacter’ spp. We identified the core genome from multiple ‘Ca. Liberibacter’ pathogens, and searched for core genes with signatures of positive selection. We hypothesized that genes encoding putative MAMPs would evolve to reduce recognition by the plant immune system, while retaining their essential functions. To efficiently screen candidate MAMP peptides, we established a high-throughput microtiter plate-based screening assay, particularly for citrus, that measured reactive oxygen species (ROS) production, which is a common immune response in plants. We found that two peptides could elicit ROS production in Arabidopsis and Nicotiana benthamiana. One of these peptides elicited ROS production and defense gene expression in HLB-tolerant citrus genotypes, and induced MAMP-triggered immunity against the bacterial pathogen Pseudomonas syringae. Our findings identify MAMPs that boost immunity in citrus and could help prevent or reduce HLB infection.

Zebra chip (ZC) disease of potato (Solanum tuberosum) is associated with infection by ‘Candidatus Liberibacter solanacearum’ (Lso). Two haplotypes of Lso—A and B—occur in the United States. Lso haplotype B is more virulent than haplotype A, causing greater disease incidence in tubers, more severe symptoms, and greater loss in tuber yield. This study assessed whether tubers from infected plants generate new infected plants the following year. The effects of both Lso haplotypes A and B on tuber resprout were examined on five potato cultivars. When compared with noninfected tubers, overall plant emergence rate from Lso A- or B-infected tubers was lower, plants emerged slower, and plants generated lower daughter tuber yields in weight and quantity. Plants generally emerged poorly from Lso B-infected tubers and produced lower daughter tuber yields than Lso A-infected tubers. Regardless of Lso treatment, all daughter tubers were asymptomatic, and only 0.3% tested positive for Lso in experiments conducted over 2 years. This suggests that plants generated from Lso A- and Lso B-infected seed potatoes with severe ZC symptoms are likely not a significant source of Lso in potato fields.

Abstract The potato/tomato psyllid Bactericera cockerelli (Šulc) transmits ‘Candidatus Liberibacter solanacearum’ (Lso) (also known as ‘Candidatus Liberibacter psyllaurous’), the bacterium associated with zebra chip disease (ZC) in potato. When disease incidence is high, ZC causes large economic losses through reductions in potato yield and tuber quality. No commercial potato variety has been found totally resistant to the pathogen. We evaluated host acceptance behaviors using no-choice assays on three breeding clones derived from Solanum chacoense Bitter with putative tolerance to Lso and/or ZC as part of an effort to determine whether the disease tolerance observed in those breeding clones was related to effects on psyllid settling behavior. We also counted the number of eggs laid and nymphs hatched on the different genotypes to observe any differences in reproduction. The potato variety ‘Russet Burbank’ was used as a susceptible control. Probing frequency and female walking duration were greater on Russet Burbank than the other genotypes. Oviposition did not differ among genotypes. However, female psyllids on two of the Lso-tolerant genotypes displayed reduced fertility 18–24 d after confinement with a male, relative to females on Russet Burbank. These results suggest that although the germplasms display minor abiotic activity on psyllid fertility, tolerance to Lso may be more strongly linked with plant tolerance to the pathogen rather than effects on host acceptance behaviors.

The candidate phyla radiation is a large monophyletic lineage comprising unculturable bacterial taxa with small cell and genome sizes, mostly known from genomes obtained from environmental sources without cultivation. Here, we present the closed complete genome of a member of the superphylum Microgenomates obtained from the metagenome of a deep subsurface thermal aquifer. Phylogenetic analysis indicates that the new bacterium, designated Ch65, represents a novel phylum-level lineage within the Microgenomates group, sibling to the candidate phylum Collierbacteria. The Ch65 genome has a highly unusual nucleotide composition with one strand of highly enriched in cytosine versus guanine throughout the whole length. Such nucleotide composition asymmetry, also detected in the members of Ca. Collierbacteria and Ca. Beckwithbacteria, suggests that most of the Ch65 chromosome is replicated in one direction. A genome analysis predicted that the Ch65 bacterium has fermentative metabolism and could produce acetate and lactate. It lacks respiratory capacity, as well as complete pathways for the biosynthesis of lipids, amino acids, and nucleotides. The Embden–Meyerhof glycolytic pathway and nonoxidative pentose phosphate pathway are mostly complete, although glucokinase, 6-phosphofructokinase, and transaldolase were not found. The Ch65 bacterium lacks secreted glycoside hydrolases and conventional transporters for importing sugars and amino acids. Overall, the metabolic predictions imply that Ch65 adopts the lifestyle of a symbiont/parasite, or a scavenger, obtaining resources from the lysed microbial biomass. We propose the provisional taxonomic assignment ‘Candidatus Chazhemtobacterium aquaticus’, genus ‘Chazhemtobacterium‘, family ‘Chazhemtobacteraceae‘ in the Microgenomates group.

Abstract Background: Candidatus Phytoplasma ulmi' is the agent associated with elm yellows and has been categorised in the European Union as a quarantine pathogen. For central and northern European countries, information on the occurrence and distribution of the pathogen and its impact on elms is scarce, so a survey of native elm trees has been conducted in Germany. Results: About 6,500 samples from Ulmus minor , Ulmus laevis and Ulmus glabra , were collected nationwide. Phytoplasma detection was performed by applying a universal 16Sr DNA-based quantitative PCR (qPCR) assay and a novel ' Ca. P. ulmi' specific qPCR assay targeting the 16S-23S spacer region. Both assays revealed that 28% of the samples were infected by ‘ Ca. P. ulmi’, but infection rates of the elm species and regional incidences differed. The phytoplasma presence in the trees was not correlated to disease-specific symptoms. The survey identified a regional disparity of infection which was high in east, south and central Germany, whereas only a few infected sites were found in the western and northern parts of the country. Monitoring the seasonal titre of ‘Ca. P. ulmi’ in an infected tree by qPCR revealed a high colonisation in all parts of the tree throughout the year. Conclusions: ‘ Ca. P. ulmi’ is widely present in elms in Germany. The rare occurrence of symptoms indicates either a high degree of tolerance in elm populations or a low virulence of pathogen strains enabling high infection rates in a long-living host.

Abstract BACKGROUND Rickettsia spp. are human pathogens that cause a number of diseases and are transmitted by arthropods, including ixodid ticks. Estonia contributes a region, where the distribution area of two exophilic tick species of known medical importance, Ixodes persulcatus and I. ricinus, overlap. The presence of the nidicolous rodent-associated I. trianguliceps has recently been shown for Estonia. Although there is no Estonian data available on human disease caused by tick-borne Rickettsia spp., the presence of three Rickettsia species in non-nidicolous ticks, albiet at very dissimilar rates, was also previously reported. The aim of this studywas to screen, identify and characterize Rickettsia species in nidicolous and non-nidicolous ticks attached to rodents. RESULTS Nymphs and larvae of I. ricinus ( n = 1004), I . persulcatus ( n = 75) and I. trianguliceps ( n = 117) attached to rodents and shrews caught in different parts of Estonia were studied for the presence of Rickettsia spp. by nested PCR. Ticks were removed from 314 small animals of 5 species (bank voles Myodes glareolus , yellow necked mice Apodemus flavicollis , striped field mice A. agrarius, pine voles M. subterranius and common shrews S. araneus) . Rickettsial DNA was detected in 8,7% (103/1186) 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 in west of the Ural.