Publications

4994

‘ Candidatus Liberibacter solanacearum’ (Lso), haplotype D, is an insect-transmitted, phloem-limited bacterium that induces developmental abnormalities in carrots, including witches’ broom and hairy root symptoms. We hypothesize that these symptoms result from Lso-induced hormonal imbalances. To investigate this, we analyzed the spatial and temporal distribution of Lso in carrot plants and assessed its effects on hormone-related gene expression and phytohormone levels. Our findings revealed that Lso first accumulates in the shoot apical meristem before spreading to root tissues, aligning with phloem flow dynamics. Transcriptomic and biochemical analyses indicated that cytokinin (CK) biosynthesis and response genes were upregulated, whereas gibberellin biosynthesis genes were downregulated. In contrast, no significant changes were observed in auxin biosynthesis or signaling. Hormone quantification further demonstrated increased CK levels in lateral roots and decreased CK levels in the root meristem of infected plants, with no detectable changes in auxin levels. Additionally, salicylic acid and jasmonic acid were significantly elevated following Lso infection, suggesting a persistent plant defense response. To validate the role of CK and auxin in symptom development, we applied synthetic growth regulators to infected and uninfected plants. CK treatment exacerbated witches’ broom symptoms, whereas auxin application mitigated this phenotype but enhanced lateral root formation. These results suggest that Lso manipulates phytohormone homeostasis to induce disease symptoms, offering a potential avenue for symptom mitigation through targeted hormone applications. Our findings provide novel insights into the role of plant hormones in Lso pathogenesis and highlight new strategies for managing carrot yellows disease.

ABSTRACTDespite recent genomic studies and increased molecular data, epitheliocystis remains an enigmatic fish disease with no experimental in vitro or in vivo models to aid the advancement of research. In this study, we revert to a classical microscopical approach and screen with the electron microscope the epitheliocystis lesions caused by a Ca. Parilichlamydia sp., infecting mucus cells in Greater amberjack. We report distinct morphological features of this bacterial family, characterised by Intermediate Bodies that closely resemble those of previously described Candidatus similchlamydia, and Elementary Bodies that exhibit morphological similarities to Chlamydia trachomatis. We describe the characteristics of a novel Chlamydial Inclusion Membrane (IM) type, with abundant interdigitations, possibly shaped by fusion of the IM with cytoplasmic vesicles, and moreover discuss the presence of multivesicular bodies in the infected cell. Our observation of immune cells in the infected areas indicates an interaction of macrophages with infected cells, a role for granular cells as pathogens reservoirs and an active phagoptosis process in the nearby areas, overall shedding light on cellular immune processes characterising these infections in fish hosts.

Abstract Huanglongbing (HLB), a devastating citrus disease caused by Candidatus Liberibacter asiaticus (CLas), triggers persistent immune activation marked by excessive callose deposition and reactive oxygen species (ROS) accumulation, which impairs phloem function. This maladaptive response has led to HLB being described as a “pathogen-triggered immune disease”. Overexpression of the Arabidopsis (Arabidopsis thaliana) NONEXPRESSOR OF PATHOGENESIS-RELATED GENES1 (AtNPR1) gene, a master regulator of systemic acquired resistance (SAR), confers robust HLB tolerance in susceptible citrus varieties, with transgenic lines exhibiting minimal or no disease symptoms following CLas infection. However, the mechanism underlying this tolerance remains unclear. In this study, we demonstrate that AtNPR1 restores immune homeostasis in CLas-infected trees by suppressing callose and ROS hyperaccumulation, thereby alleviating HLB symptom development. Similarly, silencing the Citrus sinensis homolog of NPR3 (CsNPR3), a negative regulator of SAR, mitigates CLas-induced immune overactivation and enhances HLB tolerance. Notably, salicylic acid (SA) levels are lower in AtNPR1-overexpressing citrus plants than wild-type controls after CLas infection, consistent with NPR1's role in negative feedback regulation of pathogen-induced SA accumulation. In Arabidopsis, overexpression of AtNPR1 or disruption of AtNPR3/AtNPR4 also attenuates pathogen-induced callose and ROS responses. Together, these findings reveal conserved roles for NPR1/NPR3/NPR4 in immune regulation across species and suggest that HLB susceptibility in commercial citrus varieties stems from a diminished capacity to maintain immune balance.

Polyphosphate-accumulating organisms (PAOs) play a crucial role in enhanced biological phosphorus removal (EBPR) processes. In addition to biosynthesis, they rely on phosphate for energy generation. However, Candidatus Accumulibacter, a model PAO, has been shown to adapt to low phosphate conditions by switching to a glycogen-accumulating metabolism (GAM), with variable success across genus members and experiments. This study aimed to explore the metabolic shift of several Accumulibacter species subjected to low-phosphate concentration in different operating conditions using metatranscriptomics analysis. Furthermore, the study enabled a comparison of the transcriptomic profiles of Accumulibacter with those of Propionivibrio , a glycogen-accumulating organism typically found in EBPR plants. Two sequencing batch reactors were operated with different carbon sources to enrich for different populations of Accumulibacter . After decreasing the influent phosphate concentration, carbon removal performance was maintained while anaerobic phosphate release dropped dramatically, suggesting a shift from a phosphate-accumulating to a glycogen-accumulating metabolism. Analysis of metatranscriptomics data indicated that Accumulibacter regalis (type I) and Propionivibrio aalborgensis remained the most abundant species after the phosphate decrease in the reactor with acetate-propionate and allylthiourea, while Accumulibacter delftensis (type I) and Accumulibacter phosphatis (type II) remained active in the reactor with acetate-glucose and no allylthiourea. Transcription of the genes from the ethylmalonyl-CoA pathway involved in the production of propionyl-CoA and regulation of the anaerobic redox balance was enhanced under low-phosphate conditions, especially for type I Accumulibacter . Conversely, the transcription of the methylmalonyl-CoA pathway was enhanced under low-phosphate conditions in Propionivibrio and type II Accumulibacter .

Huanglongbing (HLB), or citrus greening, remains one of the most destructive diseases affecting citrus production globally. Associated with the phloem-limited bacterium Candidatus Liberibacter asiaticus ( C Las) and vectored by Diaphorina citri , HLB leads to canopy decline, fibrous root loss, and reductions in fruit yield and quality. Recently, the systemic delivery of oxytetracycline (OTC) via trunk injection was approved in Florida as a targeted therapy to reduce C Las titers and improve tree health. In parallel, Individual Protective Covers (IPCs) have been adopted to delay C Las infection in newly planted citrus trees by vector exclusion. This study investigates the combined use of IPCs and trunk injection of OTC for post-IPC therapy. ‘Valencia’ sweet orange trees grafted on US-812 and US-942 rootstocks were planted in December 2020 under HLB-endemic conditions in southwest Florida. IPCs were installed at planting and removed after 18 months. The first OTC injection was performed in May 2023, 10 months after IPC removal. A second injection was performed in May 2024. A 2 × 2 × 2 factorial experimental design evaluated the effects of infection history (early-infected and late-infected), rootstock cultivar (US-812 and US-942), and injection treatment (OTC-injected and non-injected) on tree responses over two consecutive production seasons. In year 1, infection history significantly influenced tree size, fruit yield, total soluble solids (TSS), TSS/titratable acidity ratio, and peel color. Late-infected trees outperformed early-infected trees, regardless of injection treatment and rootstock cultivar. In year 2, OTC-injected trees exhibited significantly higher yields, improved juice quality, and enhanced canopy health regardless of infection history and rootstock cultivar. Fibrous root microbiome analyses based on 16S rRNA sequencing revealed no significant effects of OTC injection on bacterial alpha or beta diversity, with stable community structure observed across treatments and time points. This suggests that targeted vascular delivery of OTC may not cause any major disruption to the root endorhizosphere microbiome. Together, the results from this study demonstrate the efficacy of integrating preventative (use of IPCs) and therapeutic (OTC vascular delivery) strategies for sustainable HLB management while preserving microbial integrity and offering a model for citrus production in parts of the world where HLB is prevalent.

Abstract Background Phytoplasma research encounters limitations due to the lack of availability of pure cultures of these microorganisms. In this study a culture-independent approach was employed to investigate the genome and pathogenic mechanisms of phytoplasma responsible for witches’ broom disease in Trema levigata (Yunnan province, China). The phytoplasma genome was assembled using Illumina sequencing data and the Phytoassembly pipeline based on mixed samples. Results Nested PCR analysis identified a 16Sr group I, ‘ Candidatus Phytoplasma asteris’ strain in Trema levigate showing witches’ broom disease. Comparative study between infected and healthy plants resulted in an 849.7 kb draft genome with 94.1% coverage, 27.6% GC content, encoding 1356 predicted genes, of which 587 were functionally annotated. Multilocus phylogenetic analysis showed that this phytoplasma is closely related to ‘ Ca. P. asteris’. The genome possesses complete pathways for glycolysis and pyruvate metabolism pathways and harbors specific transporters (spermidine/putrescine, lysine), indicating dependency on host metabolites. Thirty-one putative secreted proteins, including TENGU and SAP05/54-like effectors were identified along with three potential mobile units carrying replication/recombination genes. Conclusions This approach demonstrates the effectiveness of pathogen purification-free analysis for phytoplasma studies in naturally infected host species, thereby enhancing the understanding of phytoplasma-host interactions.