Publications

4398

Abstract Background ‘ Candidatus Liberibacter solanacearum’ (Lso) is a phloem-limited bacterial pathogen causing significant diseases in solanaceous crops. In the United States, haplotypes A and B are transmitted by the potato psyllid Bactericera cockerelli . We previously identified differences in their acquisition and transmission between adults and nymphs. The present study characterized the dynamics of LsoA and LsoB acquisition and transmission by nymphs and examined the transcriptional responses of the nymphal gut upon their acquisition. Methods and results Nymphs were exposed to LsoA- or LsoB-infected plants for 1, 3, 5, or 7 days to measure the bacterial accumulation and for 8 days to assess the transmission efficiency following sequential inoculation of tomato plants. Quantitative PCR showed that LsoB accumulated to higher levels than LsoA after 3 days of acquisition. Following the sequential inoculation, LsoB was transmitted earlier than LsoA indicating a shorter latency period. RNA-seq analysis of the guts following a 1- and 5-day acquisition access periods revealed a greater transcriptional regulation at 5 days than at 1 day. Furthermore, the responses were haplotype-specific: LsoA primarily affected genes involved in protein translation, ER stress, and cell cycle regulation, whereas LsoB regulated genes involved in autophagy, apoptosis, and immune pathways. Conclusions This study revealed haplotype-specific gene regulation potentially leading to LsoB being transmitted more efficiently by psyllid nymphs.

Abstract ‘Candidatus Mycoplasma haemolamae’ (CMhl) is a hemotropic bacterium infecting South American Camelids (SAC), whose epidemiology and clinical significance are still not fully elucidated. This study investigated CMhl by qPCR in blood samples of alpacas ( Vicugna pacos ) and llamas ( Lama glama ) in Italy in 2021–2024, characterizing CMhl haplotypes based on partial 16 S rRNA sequences and defining haematological findings in a subset of CMhl-infected animals. Statistical analysis was performed to detect potential risk factors associated with CMhl positivity. Out of 206 animals (200 alpacas and 6 llamas), CMhl was detected by qPCR in 88 (42.7%) animals. CMhl haplotype #1 and #2 were detected in 42.2% and 0.5% of the animals, respectively. Binary logistic regression analysis showed that higher CMhl positivity was significantly associated with larger herd size ( P  = 0.011), Northern Italy area of origin ( P  = 0.031) and warmer seasons ( P  = 0.003). The detection of two CMhl haplotypes confirms the high diffusion of CMhl in SAC farmed in Italy. Results confirm that CMhl usually causes subclinical infections, suggesting that antimicrobial treatment is likely not necessary based on qPCR positivity alone. Results focusing on the association of CMhl positivity with warm temperatures and large herd size suggest the need of further studies on the genetic features, epidemiology and immune response in CMhl-positive SAC from Italy, with emphasis on understanding potential vector transmission.

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.

‘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.”

The X-disease phytoplasma (‘ Candidatus Phytoplasma pruni’) is an obligate pathogen that is capable of infection, persistence, and pathogenicity in both its major plant host ( Prunus spp.) and leafhopper vector ( Colladonus spp.) species. How ‘ Ca. P. pruni’ interacts with its plant and insect hosts, and how it alters its gene expression to do so, is unknown. Therefore, in this study, we conducted comparative RNA sequencing and differential gene expression analysis on ‘ Ca. P. pruni’-infected Prunus avium and Colladonus reductus samples. We found that the phytoplasma altered the expression of approximately 32% of its annotated protein-coding and pseudogenes, including intercellular transporters, proteolytic activity, and membrane structure, as well as upregulating genes associated with potential mobile units when in insect tissues versus in plant tissues. Most notably, differential expression was observed in genes that were identified by in silico analysis as being putative secreted effectors that may play a role in allowing phytoplasma infection and survival in these two very different host systems, as well as inducing X-disease in Prunus spp., which offers targets for control of this damaging pathogen by disrupting phytoplasma–host interactions. [Formula: see text] Copyright © 2026 The Author(s). This is an open access article distributed under the CC BY-NC-ND 4.0 International license .