Publications

4367

Huanglongbing (HLB) is currently the largest threat to global citrus production. We examined the effect of HLB pathogen ‘Candidatus Liberibacter asiaticus’ infection or infestation by its vector, Diaphorina citri, on ‘Valencia’ sweet orange leaf pigments using high-performance liquid chromatography, followed by gene expression analysis for 46 involved genes in carotenoid and chlorophyll biosynthesis pathways. Both ‘Ca. L. asiaticus’ and D. citri alter the total citrus leaf pigment balance with a greater impact by ‘Ca. L. asiaticus’. Although zeaxanthin was accumulated in ‘Ca. L. asiaticus’-infected leaves, chlorophyllide a was increased in D. citri-infested plants. Our findings support the idea that both ‘Ca. L. asiaticus’ and D. citri affect the citrus pigments and promote symptom development but using two different mechanisms. ‘Ca. L. asiaticus’ promotes chlorophyll degradation but accelerates the biosynthesis of carotenoid pigments, resulting in accumulation of abscisic acid and its precursor, zeaxanthin. Zeaxanthin also has a photoprotective role. By contrast, D. citri induced the degradation of most carotenoids and accelerated chlorophyll biosynthesis, leading to chlorophyllide a accumulation. Chlorophyllide a might have an antiherbivory role. Accordingly, we suggest that citrus plants try to defend themselves against ‘Ca. L. asiaticus’ or D. citri using multifaceted defense systems, based on the stressor type. These findings will help in better understanding the tritrophic interactions among plant, pathogen, and vector.

Host acquisition of a new microbial species can readily perturb the dynamics of preexisting microbial associations. Molecular cross talk between microbial associates may be necessary for efficient resource allocation and enhanced survival. Classic examples involve quorum sensing (QS), which detects population densities and is both used and coopted to control expression of bacterial genes, including host adaptation factors. We report that a 56-amino-acid repressor protein made by the resident psyllid endosymbiont Wolbachia can enter cells of Liberibacter crescens , a cultured proxy for the uncultured psyllid endosymbiont “ Ca . Liberibacter asiaticus” and repress “ Ca . Liberibacter asiaticus” phage lytic cycle genes. Such repression in “ Ca . Liberibacter asiaticus” may be critical to survival of both endosymbionts, since phage-mediated lysis would likely breach the immunogenic threshold of the psyllid, invoking a systemic and nonspecific innate immune reaction.

AbstractGenital mycoplasmas, which can be vertically transmitted, have been implicated in preterm birth, neonatal infections, and chronic lung disease of prematurity. Our prior work uncovered 16S rRNA genes belonging to a novel, as-yet-uncultivated mycoplasma (lineage ‘Mnola’) in the oral cavity of a premature neonate. Here, we characterize the organism’s associated community, growth status, metabolic potential, and population diversity. Sequencing of genomic DNA from the infant’s saliva yielded 1.44 Gbp of high-quality, non-human read data, from which we recovered three essentially complete (including ‘Mnola’) and three partial draft genomes (including Trichomonas vaginalis). The completed 629,409-bp ‘Mnola’ genome (Candidatus Mycoplasma girerdii str. UC-B3) was distinct at the strain level from its closest relative, vaginally-derived Ca. M. girerdii str. VCU-M1, which is also associated with T. vaginalis. Replication rate measurements indicated growth of str. UC-B3 within the infant. Genes encoding surface-associated proteins and restriction-modification systems were especially diverse within and between strains. In UC-B3, the population genetic underpinnings of phase variable expression were evident in vivo. Unique among mycoplasmas, Ca. M. girerdii encodes pyruvate-ferredoxin oxidoreductase and may be sensitive to metronidazole. This study reveals a metabolically unique mycoplasma colonizing a premature neonate, and establishes the value of genome-resolved metagenomics in tracking phase variation.

AbstractFree-living amoebae are well known for their role in controlling microbial community composition through grazing, but some groups, namely Acanthamoeba species, also frequently serve as hosts for bacterial symbionts. Here we report the first identification of a bacterial symbiont in the testate amoeba Cochliopodium. The amoeba was isolated from a cooling tower water sample and identified as C. minus. Fluorescence in situ hybridization and transmission electron microscopy revealed intracellular symbionts located in vacuoles. 16S rRNA-based phylogenetic analysis identified the endosymbiont as member of a monophyletic group within the family Coxiellaceae (Gammaprotebacteria; Legionellales), only moderately related to known amoeba symbionts. We propose to tentatively classify these bacteria as ‘Candidatus Cochliophilus cryoturris’. Our findings add both, a novel group of amoeba and a novel group of symbionts, to the growing list of bacteria-amoeba relationships.