Publications

4486

‘Candidatus Liberibacter asiaticus’ (CLas), the causal agent of citrus huanglongbing, is transmitted by the Asian citrus psyllid Diaphorina citri. While CLas-positive (CLas+) females exhibit increased fecundity and metabolic demands, their neuroendocrine regulation remains unclear. We propose CLas manipulates dopamine (DA) signaling to enhance psyllid fecundity and CLas proliferation. Metabolomics revealed elevated DA in CLas+ females. Silencing DA synthesis genes and receptor DcDop2 via RNAi reduced lipid reserves, fecundity, and ovarian CLas titers. Through combined in vivo and in vitro experiments, we demonstrated that the microRNA miR-31a suppresses DcDop2 expression by binding to its 3’ untranslated region. Overexpression of miR-31a resulted in decreased DcDop2 expression and CLas titers in the ovaries, eliciting phenotypic defects akin to DcDop2 knockdown. Furthermore, DcDop2 knockdown and miR-31a overexpression reduced juvenile hormone (JH) levels and adipokinetic hormone (AKH) signaling in fat bodies and ovaries. Consequently, CLas hijacks the DA/DcDop2-miR-31a-AKH-JH signaling cascade to improve D. citri lipid metabolism and fecundity, while simultaneously promoting its replication. These findings suggest a mutualistic interaction between CLas proliferation and ovarian development within the insect host that enrich our understanding of the molecular interplay between plant pathogens and vector insects and offer novel targets and strategies for the field management of HLB.

Candidatus Liberibacter spp. can infect most citrus plants and rely entirely on phloem sieve tube cells of the host plant for survival. Candidatus Liberibacter primarily contains Ca. L. asiaticus (CLas), Ca. L. africanus (CLaf), and Ca. L. americanus (CLam). Among these, CLas is the most harmful and widely distributed and is the primary pathogen of the devastating citrus disease Huanglongbing (HLB). Effectors are among the core weapons secreted by pathogens into plant cells to attack the plant immune system. In this study, we focused on CLas-specific effectors and those that are highly expressed during the infection stage to identify essential virulence effectors. Using secretion signal peptide prediction analysis, 40 candidate effectors with potential secretory capabilities were identified. Transient expression of these candidate effectors in Nicotiana benthamiana revealed their impact on pattern-triggered immunity, including INF-induced cell death and microbial pattern-induced reactive oxygen species (ROS) bursts, and the resistance of N. benthamiana to the bacterial pathogen Pst DC3000. 10 candidate effectors capable of suppressing plant immunity were identified. The stable expression of these candidate effectors in Arabidopsis showed that several candidate effectors enhanced plant susceptibility to Pst DC3000 and inhibited flg22-induced ROS production and MAPK activation. Among the three candidate effectors that significantly suppressed ROS burst, one effector, E3 (CLIBASIA_03085), interacts with the plant NADPH oxidase RbohD, a key enzyme responsible for ROS production. This suggests that E3 likely inhibits ROS accumulation by directly targeting RbohD. Here, we identified multiple candidate effectors capable of suppressing microbial pattern-triggered immunity that may be essential virulence factors for CLas infection, enhancing our understanding of CLas pathogenesis.

Abstract Methanogens are strictly anaerobic archaea capable of energy conservation by methane production, yet their presence in oxic and arid environments challenges existing paradigms. In this study, we enriched and genomically characterized seven methanogenic cultures from desert biocrusts, affiliated with the genera Methanobacterium, Methanosarcina, and Methanocella. Six of these new enrichment cultures represent new species. Nonetheless, phylogenomic analyses revealed close genetic relationships with organisms from anoxic environments, indicating the absence of an evolutionary distinction. Comparative genomics exposed diverse though non-unique repertories of antioxidant (e.g. catalase, superoxide dismutase and desulfoferrodoxin), and desiccation-resistance genes (including genes for maintaining osmotic pressure and repair of cell wall and membrane), with Methanobacterium spp possessing the lowest gene abundance and diversity for oxygen and desiccation tolerance. Nevertheless, the occurrence of a Class I methanogen such as Methanobacterium in arid soils challenges the notion that members of this class are less oxygen tolerant than Class II. Pangenome analysis further uncovered unique genes enriched in membrane-associated functions and potentially non-functional stress-related genes. Via a global metagenomic survey we find that methanogens are underdetected in dryland soils, likely due to sequencing depth limitations. Our findings highlight previously overlooked methanogen diversity and ecological plasticity in oxic and desiccated habitats, and emphasize the need for further studies to elucidate their survival strategies.

‘Candidatus Phytoplasma solani’ of the 16SrXII group is an emerging vector-borne pathogen in European crop production. The cixiid planthopper Hyalesthes obsoletus transmits 16SrXII-A stolbur phytoplasmas that are associated with diseases in grapevine, potato, and various weeds. While 16SrXII-P genomes transmitted by Pentastiridius leporinus are available, no genome of an H. obsoletus-transmissible 16SrXII-A phytoplasma has been reported from Germany. Here, we present insights into the phylogenetic position and pathogen–host interactions through the functional reconstruction of the complete 832,614 bp genome of the H. obsoletus transmissible ‘Ca. P. solani’ 16SrXII-A strain POT from a potato field. Phylogenetic analyses highlight the heterogeneity within the stolbur group using whole-genome alignment and a BUSCO-based core gene analysis approach. The POT chromosome shares highest average nucleotide identity with Italian bindweed-associated genomes and displays strong synteny with the c5 strain. Consistent with the typical phytoplasma architecture, the POT genome combines mobile-element-driven instability with a conserved core metabolism. Virulence factors include transposon-linked effectors but lack pathogenicity island organisation. POT further differs from other 16SrXII-group phytoplasmas through unique collagen-like proteins that could contribute to virulence. These findings provide a robust genomic framework that improves diagnostics, enables strain-level resolution and supports the assessment of breeding materials under stolbur phytoplasma pressure, thereby refining our understanding of stolbur phytoplasma diversity and highlighting the evolutionary divergence within the 16SrXII subgroup.

Huanglongbing (HLB), caused by Candidatus Liberibacter asiaticus (CLas), is the most destructive citrus disease worldwide, severely reducing yield and fruit quality. Although no naturally resistant cultivars are available, citrus plants overexpressing Arabidopsis NPR1 ( At NPR1) display enhanced tolerance, yet the molecular mechanisms underlying this phenotype remain insufficiently understood. To uncover early transcriptional events associated with tolerance, we performed comparative RNA-seq and phytohormone profiling of susceptible wild-type (WT) and tolerant At NPR1-overexpressing ( At NPR1-OE) Duncan grapefruit at 0, 6, and 24 hours post infection (hpi). Before infection, At NPR1 plants downregulated genes involved in cytoskeleton organization, cell wall biogenesis, and receptor signaling, suggesting a primed basal defense state. After CLas exposure, At NPR1 plants exhibited stronger and earlier transcriptional reprogramming, with substantially more differentially expressed genes at 6 hpi than WT. At 24 hpi, At NPR1 plants showed suppression of callose synthase genes and selective induction of β-1,3-glucanases, indicating more controlled phloem callose regulation. Concurrently, attenuated expression of respiratory burst oxidase homologs and ROS-associated genes suggested a moderated and less damaging oxidative burst. At NPR1 plants maintained stable levels of salicylic acid, and gibberellins while preventing the CLas-induced induction of abscisic acid observed in WT. Collectively, these findings reveal that At NPR1 overexpression enhances HLB tolerance by integrating early transcriptional reprogramming with balanced structural, oxidative, and hormonal responses. This study provides a mechanistic framework for understanding NPR1-mediated tolerance to CLas during the initial stages of infection.

Abstract Nitrobacter strain NHB1 is a nitrite-oxidizing bacterium previously demonstrated to form a consortium capable of nitrification under acidic conditions when cocultivated with a neutrophilic ammonia-oxidizing bacterium. Here, we characterize the growth of isolated NHB1 under different pH and nitrite (NO2−) concentrations, as well as its influence on the activity of obligately acidophilic soil ammonia-oxidizing archaea (AOA) isolated from acidic soils when grown in coculture. NHB1 is acidotolerant with optimal growth at pH 6.0 (range: 5.0–7.5) at an initial NO2− concentration of 500 μM. However, at lower NO2− concentrations, closer to those found in soil, its pH optimum decreases to 5.0 and with detectable growth extended to pH 3.5. In coculture, NHB1 enhances the growth of the acidophilic AOA Nitrosotalea devaniterrae Nd1 and Nitrosotalea sinensis Nd2, which are highly sensitive to NO2-derived compounds and typically oxidize only ~200 to 300 μM ammonia (NH3) when grown in batch cultures as isolates. However, in coculture with NHB1, both strains oxidized up to ~3 mM NH3, limited only by the buffering capacity of the medium, and their pH range was also extended downward by ~0.5 units. NHB1 also possesses a cyanase, enabling reciprocal cross-feeding through cyanate-derived NH3 production while utilizing AOA-derived NO2−. These findings suggest that NO2− removal is essential for ammonia oxidizer growth in acidic soils and emphasize the importance of considering substrate and metabolic product concentrations when characterizing ecophysiology. Genome analysis reveals that NHB1 is distinct from validated species, and we propose the name “Nitrobacter laanbroekii.”