Publications

4527

The oxidative (H2O2) burst is a seminal feature of the basal plant defense response to attempted pathogen invasions. In ‘Candidatus Liberibacter asiaticus’ UF506, expression of the SC2 prophage-encoded secreted peroxidase (F489_gp15) increases bacterial fitness and delays symptom progression in citrus. Two chromosomal 1-Cys peroxiredoxin genes, CLIBASIA_RS00940 (Lasprx5) and CLIBASIA_RS00445 (Lasbcp), are conserved among all sequenced ‘Ca. L. asiaticus’ strains, including those lacking prophages. Both LasBCP and LasdPrx5 have only a single conserved peroxidatic Cys (CP/SH) and lack the resolving Cys (CR/SH). Lasprx5 appeared to be a housekeeping gene with similar moderate transcript abundance in both ‘Ca. L. asiaticus’–infected psyllids and citrus. By contrast, Lasbcp was expressed only in planta, similar to the expression of the SC2 peroxidase. Since ‘Ca. L. asiaticus’ is uncultured, Lasbcp and Lasprx5 were functionally validated in a cultured surrogate species, Liberibacter crescens, and both genes significantly increased oxidative stress tolerance and cell viability in culture. LasBCP was nonclassically secreted and, in L. crescens, conferred 214-fold more resistance to tert-butyl hydroperoxide (tBOOH) than wild type. Transient overexpression of Lasbcp in tobacco suppressed H2O2-mediated transcriptional activation of RbohB, the key gatekeeper of the systemic plant defense signaling cascade. Lasbcp expression did not interfere with the perception of ‘Ca. L. asiaticus’ flagellin (flg22Las) but interrupted the downstream activation of RbohB and stereotypical deposition of callose in tobacco. Critically, LasBCP also protected against tBOOH-induced peroxidative degradation of lipid membranes in planta, preventing subsequent accumulation of antimicrobial oxylipins that can also trigger the localized hypersensitive cell death response.

AbstractRickettsialesare a lineage of obligatorily intracellularAlphaproteobacteria, encompassing important human pathogens, manipulators of host reproduction, and mutualists. Here we report the discovery of a novelRickettsialesbacterium associated withParamecium, displaying a unique extracellular lifestyle, including the ability to replicate outside host cells. Genomic analyses show that the bacterium possesses a higher capability to synthesize amino acids, compared to all investigatedRickettsiales. Considering these observations, phylogenetic and phylogenomic reconstructions, and re-evaluating the different means of interaction ofRickettsialesbacteria with eukaryotic cells, we propose an alternative scenario for the evolution of intracellularity inRickettsiales. According to our reconstruction, theRickettsialesancestor would have been an extracellular and metabolically versatile bacterium, while obligate intracellularity and genome reduction would have evolved later in parallel and independently in different sub-lineages. The proposed new scenario could impact on the open debate on the lifestyle of the last common ancestor of mitochondria withinAlphaproteobacteria.

AbstractHuanglongbing (HLB), the most destructive citrus disease, is caused by three species of phloem-limited Candidatus Liberibacter. Chemical control is a critical short-term strategy against Candidatus Liberibacter asiaticus (Las). Currently, application of antibiotics in agricultural practices is limited due to public concerns regarding emergence of antibiotic-resistant bacteria and potential side effects in humans. The present study screened 39 antimicrobials (non-antibiotics) for effectiveness against Las using an optimized graft-based screening system. Results of principal component, hierarchical clustering and membership function analyses demonstrated that 39 antimicrobials were clustered into three groups: “effective” (Group I), “partly effective” (Group II), and “ineffective” (Group III). Despite different modes of action, 8 antimicrobials (aluminum hydroxide, D,L-buthionine sulfoximine, nicotine, surfactin from Bacillus subtilis, SilverDYNE, colloidal silver, EBI-601, and EBI-602), were all as highly effective at eliminating or suppressing Las, showing both the lowest Las infection rates and titers in treated scions and inoculated rootstock. The ineffective group, which included 21 antimicrobials, did not eliminate or suppress Las, resulting in plants with increased titers of Candidatus Liberibacter. The other 10 antimicrobials partly eliminated/suppressed Las in treated and graft-inoculated plants. These effective antimicrobials are potential candidates for HLB control either via rescuing infected citrus germplasms or restricted field application.

Metagenomic sequencing of fracture fluid from South Africa recovered a nearly complete “ Candidatus Bathyarchaeota” archaeon genome. The metagenome-assembled genome of BE326-BA-RLH contains genes involved in methane metabolism and dissimilatory nitrate reduction.

AbstractThe symbiosis between a marine alga and a N2-fixing cyanobacterium (UCYN-A) is geographically widespread in the oceans and is important in the marine N cycle. UCYN-A is uncultivated, and is an unusual unicellular cyanobacterium because it lacks many metabolic functions, including oxygenic photosynthesis and carbon fixation, which are typical in cyanobacteria. It is now presumed to be an obligate symbiont of haptophytes closely related to Braarudosphaera bigelowii. N2-fìxing cyanobacteria use different strategies to avoid inhibition of N2 fixation by the oxygen evolved in photosynthesis. Most unicellular cyanobacteria temporally separate the two incompatible activities by fixing N2 only at night, but surprisingly UCYN-A appears to fix N2 during the day. The goal of this study was to determine how the unicellular UCYN-A coordinates N2 fixation and general metabolism compared to other marine cyanobacteria. We found that UCYN-A has distinct daily cycles of many genes despite the fact that it lacks two of the three circadian clock genes found in most cyanobacteria. We also found that transcription patterns in UCYN-A are most similar to marine cyanobacteria that are capable of aerobic N2 fixation in the light such as Trichodesmium and heterocyst-forming cyanobacteria, rather than Crocosphaera or Cyanothece species, which are more closely related to unicellular marine cyanobacteria evolutionarily. Our findings suggest that the symbiotic interaction has resulted in a shift of transcriptional regulation to coordinate UCYN-A metabolism with the phototrophic eukaryotic host, thus allowing efficient coupling of N2 fixation (by the cyanobacterium) to the energy obtained from photosynthesis (by the eukaryotic unicellular alga) in the light.ImportanceThe symbiotic N2-fixing cyanobacterium UCYN-A and its eukaryotic algal host, which is closely related to Braarudosphaera bigelowii, have been shown to be globally distributed and important in open ocean N2 fixation. These unique cyanobacteria have reduced metabolic capabilities, even lacking genes for oxygenic photosynthesis and carbon fixation. Cyanobacteria generally use energy from photosynthesis for nitrogen fixation, but require mechanisms for avoiding inactivation of the oxygen-sensitive nitrogenase enzyme by ambient oxygen (O2) or the O2 evolved through photosynthesis. This study shows that the symbiosis between the N2-fixing cyanobacterium UCYN-A and its eukaryotic algal host has led to adaptation of its daily gene expression pattern in order to enable daytime aerobic N2 fixation, which is likely more energetically efficient than fixing N2 at night, as in other unicellular marine cyanobacteria.