Publications

3530

Abstract Nitrospirales, including the genus Nitrospira, are environmentally widespread chemolithoautotrophic nitrite-oxidizing bacteria. These mostly uncultured microorganisms gain energy through nitrite oxidation, fix CO2, and thus play vital roles in nitrogen and carbon cycling. Over the last decade, our understanding of their physiology has advanced through several new discoveries, such as alternative energy metabolisms and complete ammonia oxidizers (comammox Nitrospira). These findings mainly resulted from studies of terrestrial species, whereas less attention has been given to marine Nitrospirales. In this study, we cultured three new marine Nitrospirales enrichments and one isolate. Three of these four NOB represent new Nitrospira species while the fourth represents a novel genus. This fourth organism, tentatively named “Ca. Nitronereus thalassa”, represents the first cultured member of a Nitrospirales lineage that encompasses both free-living and sponge-associated nitrite oxidizers, is highly abundant in the environment, and shows distinct habitat distribution patterns compared to the marine Nitrospira species. Partially explaining this, “Ca. Nitronereus thalassa” harbors a unique combination of genes involved in carbon fixation and respiration, suggesting differential adaptations to fluctuating oxygen concentrations. Furthermore, “Ca. Nitronereus thalassa” appears to have a more narrow substrate range compared to many other marine nitrite oxidizers, as it lacks the genomic potential to utilize formate, cyanate, and urea. Lastly, we show that the presumed marine Nitrospirales lineages are not restricted to oceanic and saline environments, as previously assumed.

AbstractDiaphorina citriis the primary vector of the bacterium, ‘CandidatusLiberibacter asiaticus’ (CLas) associated with the severe Asian form of huanglongbing.CLas-positiveD. citriare more fecund than theirCLas-negative counterparts and require extra energy expenditure. Therefore, understanding the molecular mechanisms linking metabolism and reproduction is of particular importance. In this present study, we found adipokinetic hormone (DcAKH) and its receptor (DcAKHR) were essential for increased lipid metabolism and fecundity in response toCLas infection inD. citri.Knockdown ofDcAKHandDcAKHRnot only resulted in triacylglycerol accumulation and a decline of glycogen, but also significantly decreased fecundity andCLas titer in ovaries. Combinedin vivoandin vitroexperiments showed that miR-34 suppressesDcAKHRexpression by binding to its 3’ untranslated region, whilst overexpression of miR-34 resulted in a decline ofDcAKHRexpression andCLas titer in ovaries and caused defects that mimickedDcAKHRknockdown phenotypes. In addition, knockdown ofDcAKHandDcAKHRsignificantly reduced juvenile hormone (JH) titer and JH signaling pathway genes in fat bodies and ovaries, including the JH receptor,methoprene-tolerant(DcMet), and the transcription factor,Krüppel homolog 1 (DcKr-h1), that acts downstream of it, as well as the egg development related genesvitellogenin 1-like(DcVg-1-like),vitellogenin A1-like(DcVg-A1-like) and the vitellogenin receptor (DcVgR). As a result,CLas hijacks AKH/AKHR-miR-34-JH signaling to improveD. citrilipid metabolism and fecundity, while simultaneously increasing the replication ofCLas, suggesting a mutualistic interaction inD. citriovaries withCLas.

The devastating citrus disease, Huanglongbing (HLB), is associated with ‘Candidatus Liberibacter sp.’ and transmitted by citrus psyllids. Unfortunately, HLB has no known sustainable cure yet. Herein, we proposed γ-aminobutyric acid (GABA) as a potential eco-friendly therapeutic solution to HLB. Herein, we used GC/MS-based targeted metabolomics combined with gene expression to investigate the role of GABA in citrus response against HLB and to better understand its relationship(s) with different phytohormones. GABA supplementation via root drench boosts the accumulation of endogenous GABA in the leaves of both healthy and ‘Ca. L. asiaticus’-infected trees. GABA accumulation benefits the activation of a multi-layered defensive system via modulating the phytohormone levels and regulating the expression of their biosynthesis genes and some pathogenesis-related proteins (PRs) in both healthy and ‘Ca. L. asiaticus’-infected plants. Moreover, our findings showed that GABA application stimulates auxin biosynthesis in ‘Ca. L. asiaticus’-infected plants via the activation of the indole-3-pyruvate (I3PA) pathway, not via the tryptamine (TAM)-dependent pathway, to enhance the growth of HLB-affected trees. Likewise, GABA accumulation was associated with the upregulation of SA biosynthesis genes, particularly the PAL-dependent route, resulting in higher SA levels that activated CsPR1, CsPR2, CsPR5, and CsWRKY70, which are prominent to activation of the SA-mediated pathway. Additionally, higher GABA levels were correlated with an enhanced JA profile and linked with both CsPR3 and CsPR4, which activates the JA-mediated pathway. Collectively, our findings suggest that exogenous GABA application might be a promising alternative and eco-friendly strategy that helps citrus trees battle HLB.

The anaerobic oxidation of fatty acids and alcohols occurs near the thermodynamic limit of life. This process is driven by syntrophic bacteria that oxidize fatty acids and/or alcohols, their syntrophic partners that consume the products of this oxidation, and the pathways for interspecies electron exchange via these products or direct interspecies electron transfer (DIET). Due to the interdependence of syntrophic microorganisms on each other’s metabolic activity, their isolation in pure cultures is almost impossible. Thus, little is known about their physiology, and the only available way to fill in the knowledge gap on these organisms is genomic and metabolic analysis of syntrophic cultures. Here we report the results of genome sequencing and analysis of an obligately syntrophic alkaliphilic bacterium ‘Candidatus Contubernalis alkaliaceticus’. The genomic data suggest that acetate oxidation is carried out by the Wood–Ljungdahl pathway, while a bimodular respiratory system involving an Rnf complex and a Na+-dependent ATP synthase is used for energy conservation. The predicted genomic ability of ‘Ca. C. alkaliaceticus’ to outperform interspecies electron transfer both indirectly, via H2 or formate, and directly, via pili-like appendages of its syntrophic partner or conductive mineral particles, was experimentally demonstrated. This is the first indication of DIET in the class Dethiobacteria.