Publications

4774

‘Candidatus Liberibacter solanacearum’ is a plant pathogen affecting the families Solanaceae and Apiaceae in different parts of the world. ‘Ca. L. solanacearum’ is a Gram-negative, fastidious α-proteobacterium that is vectored by different psyllid species. Plant-pathogenic bacteria are known for interfering with the host physiology or defense mechanisms, often by secreting bacterial effectors. Effector proteins are critical for virulence; therefore, the identification of effectors could help with disease management. In this study, we characterized the Sec-translocon-dependent ‘Ca. L. solanacearum’–hypothetical protein effector 1 (Lso-HPE1). We compared this protein sequence in the different ‘Ca. L. solanacearum’ haplotypes. We predicted the signal peptide and validated its function using Escherichia coli’s alkaline phosphatase fusion assay. Agrobacterium tumefaciens-mediated transient expression in Nicotiana benthamiana demonstrated that Lso-HPE1 from ‘Ca. L. solanacearum’ haplotypes A and B were able to inhibit the induction of cell death in plants. We also compared gene expression of the Lso-HPE1- transcripts in ‘Ca. L. solanacearum’ haplotypes A and B in tomato and in the vector Bactericera cockerelli. This work validates the identification of a Sec-translocon-dependent ‘Ca. L. solanacearum’ protein possibly involved in suppression of plant cell death.

‘Candidatus Liberibacter asiaticus’ (Las) is an unculturable α-proteobacterium associated with citrus huanglongbing (HLB), a devastating disease currently threatening the citrus industry worldwide. Here, we present the genome sequence of Las strain TaiYZ2 from an HLB-affected pomelo tree in Hat Yai district, Songkhla Province, Thailand. The TaiYZ2 genome is composed of 1,230,623 bp with G+C content of 36.4%. This is the first Las genome sequence from Thailand, which will enrich current Las genome resource and facilitate HLB research and management.

‘Candidatus Liberibacter asiaticus’ is the unculturable causative agent of citrus huanglongbing disease. Here, we report the first citrus root metagenome sequence containing the draft genome of ‘Ca. L. asiaticus’ strain AHCA17, obtained from a pummelo tree in California. The assembled genome was 1.2 Mbp and resulted in 37 contigs (N50 = 158.7 kbp) containing 1,057 predicted open reading frames and 45 RNA-coding genes. This draft genome will provide a valuable resource in further study of ‘Ca. L. asiaticus’ genome diversity and pathogen epidemiology.

A novel actinobacterial strain, designated K81G1T, was isolated from a soil sample collected in Kantulee peat swamp forest, Surat Thani Province, Thailand, and its taxonomic position was determined using a polyphasic approach. Optimal growth of strain K81G1Toccurred at 28–30 °C, at pH 5.0–6.0 and without NaCl. Strain K81G1Thad cell-wall chemotype IV (meso-diaminopimelic acid as the diagnostic diamino acid, and arabinose and galactose as diagnostic sugars) and phospholipid pattern type II, characteristic of the genusAmycolatopsis. It contained MK-9(H4) as the predominant menaquinone, iso-C16 : 0, C17 : 0cyclo and C16 : 0as the major cellular fatty acids, and phospholipids consisting of phosphatidylglycerol, diphosphatidylglycerol, phosphatidylethanolamine, hydroxyphosphatidylethanolamine, phosphatidylinositol and two unidentified phospholipids. Based on 16S rRNA gene sequence similarity and phylogenetic analyses, strain K81G1Twas most closely related toAmycolatopsis rhizosphaeraeTBRC 6029T(97.8 % similarity),Amycolatopsis acidiphilaJCM 30562T(97.8 %) andAmycolatopsis bartoniaeDSM 45807T(97.6 %). Strain K81G1Texhibited low average nucleotide identity and digital DNA–DNA hybridization values withA. rhizosphaeraeTBRC 6029T(76.4 %, 23.0 %),A. acidiphilaJCM 30562T(77.9 %, 24.6 %) andA. bartoniaeDSM 45807T(77.8 %, 24.3 %). The DNA G+C content of strain K81G1Twas 69.7 mol%. Based on data from this polyphasic study, strain K81G1Trepresents a novel species of the genusAmycolatopsis, for which the nameAmycolatopsis acidicolasp. nov. is proposed. The type strain is K81G1T(=TBRC 10047T=NBRC 113896T).

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.

Despite the detection ofNeorickettsiaspecies DNA sequences in various trematode species and their hosts, only threeNeorickettsiaspecies have been cell culture isolated and whole-genome sequenced and are known to infect mammals and/or cause disease. The molecular mechanisms that enable the obligatory intracellular bacteriumNeorickettsiato colonize trematodes and to horizontally transmit from trematodes to mammals, as well as the virulence factors associated with specific mammalian hosts, are unknown. Potomac horse fever (PHF) is a severe and acute systemic infectious disease of horses, with clinical signs that include diarrhea.Neorickettsia risticiiis the only known bacterial species that causes PHF. Ingestion of insects harboringN. risticii-infected trematodes by horses leads to PHF. Our discovery of a newNeorickettsiaspecies that causes PHF and whole-genome sequence analysis of this bacterium will improve laboratory diagnosis and vaccine development for PHF and will contribute to our understanding ofNeorickettsiaecology, pathogenesis, and biology.

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.