Publications

4941

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 Background The Toll signaling pathway is a key component of insect innate immunity, yet its role in hemipteran vectors and their interactions with plant pathogens remains poorly understood. The potato psyllid ( Bactericera cockerelli ), an important agricultural pest in the USA, transmits two haplotypes of ‘Candidatus Liberibacter solanacearum ’ (LsoA and LsoB) that cause severe diseases in solanaceous crops. Methods and Results We characterized Toll pathway components in the B. cockerelli genome and examined their transcriptional responses during pathogen acquisition. Six Toll receptors and ten pathway-associated genes were identified in the psyllid genome. Gene expression was evaluated using RT-qPCR following exposure to the entomopathogenic fungus Beauveria bassiana (Bb-490) and infection with Lso haplotypes. The transcription of five Toll receptors was significantly induced in response to B. bassiana , indicating their potential role in antifungal defense. In contrast, Lso infection elicited haplotype-specific transcriptional responses: two Toll receptors (Toll like-1 and Toll-7) were up-regulated during LsoB acquisition, whereas only Toll-7 responded to LsoA. In addition, Spätzle-3 was induced by LsoB, while Spätzle was induced by LsoA. Persistent Lso infection did not significantly alter the transcriptional expression of Toll receptors. Conclusions These findings provide new insights into immune regulation in B. cockerelli and highlight the potential of targeting Toll-mediated responses and fungal biocontrol strategies for sustainable disease management.

The phloem-colonizing bacterial pathogen ‘ Candidatus Liberibacter asiaticus’ (CLas) causes the devastating citrus huanglongbing (HLB). Despite the important roles of roots, how CLas affects roots remains largely unknown. Here, we dissected host responses in leaf and root at different time points after CLas inoculation. Our data showed that CLas triggers less reactive oxygen species (ROS) accumulation and phloem callose deposition, which are known to contribute to HLB symptom development in leaves, in roots than in leaves. Because photosynthetically active chloroplasts are abundant in leaf tissues, but absent in roots, and serve as major hubs of pathogen-induced ROS production, elevated chloroplast-associated ROS likely contributes to the greater ROS accumulation observed in leaves during CLas infection. qRT-PCR analyses in roots and leaves, together with root RNA-seq analysis and comparison with previously reported leaf transcriptomic datasets, showed global induction of immune-related pathways in both roots and leaves, whereas photosynthesis-related pathways were only downregulated in leaves, but not in roots, by CLas infection. The differential expression pattern related to immunity and photosynthesis in the leaf and root tissues suggests that chloroplasts might be involved in the differences in ROS accumulation and callose deposition. Root cell death was observed in HLB-symptomatic trees regardless of the CLas infection status of roots. Significantly more starch accumulation was first observed in CLas-positive leaves than in healthy controls, followed by starch depletion in CLas-positive roots later, suggesting a causative relationship between them. Taken together, root cell death caused by CLas infection may be caused by carbon starvation rather than via a similar mechanism as in leaves. Our data demonstrate that chloroplast-associated ROS accumulation plays a critical role in the tissue-specific host responses to CLas infection and provide a framework for future investigations into the mechanisms underlying HLB pathogenesis. Copyright © 2026 The Author(s). This is an open access article distributed under the CC BY-NC-ND 4.0 International license .

Chokecherry ( Prunus virginiana L.) is a native shrub that is widely found across North America and is a host of ‘ Candidatus Phytoplasma pruni’ associated with X-disease of stone fruits. This pathogen is of major concern to U.S. cherry and peach growers, with the most recent and ongoing outbreak in the Pacific Northwest causing major economic harm. To investigate the role of chokecherry as a potential reservoir and source of orchard-associated strains, we surveyed wild chokecherry trees across 12 U.S. states and compared the strains present with those in commercially cultivated Prunus species. Approximately 47% of chokecherry sampled tested positive for a ‘ Ca. P. pruni’-related strain and phylogenetic analysis revealed high strain diversity, including six distinct clades and four newly identified lineages. In contrast, ‘ Ca. P. pruni’ strains isolated from cultivated cherry and peach trees exhibited lower diversity and were infected by known disease outbreak-associated strains such as Northwestern-X. These data suggest limited transmission between wild and cultivated hosts in the western United States, with patterns indicating spillover from orchards into nearby chokecherry populations rather than the reverse. However, in states east of the Rocky Mountains, chokecherry likely functions as a reservoir for orchard infections.

ABSTRACT Huanglongbing (HLB) is a devastating and incurable citrus disease associated with the gram-negative, phloem-limited, and unculturable bacterium “ Candidatus Liberibacter asiaticus” ( C Las), transmitted by the psyllid Diaphorina citri . Our goal was to identify high-performing microbial candidates from a previously established citrus-associated microbial collection for the management of HLB and associated root pathogens. Candidate selection was based on bioactivity, genomic, and metabolomic profiles. Eighteen Bacillus isolates were screened for antagonistic activity against Liberibacter crescens , C Las, and citrus-associated Phytophthora and Fusarium spp. Phylogenetic analysis grouped the isolates into four species groups: Bacillus velezensis , B. subtilis , B. pumilus , and B. cereus . Among these, B. velezensis exhibited the strongest and broadest inhibitory activity, followed by B. subtilis . This enhanced antagonistic potential is likely associated with the greater number and diversity of biosynthetic gene clusters (BGCs) identified. Notably, C Las-inhibitory isolates included B. velezensis (CB0021 and CB0909) and B. altitudinis (CB0086). Bacilysin and surfactin BGCs were conserved across all C Las-inhibitory isolates, suggesting a potential role in C Las suppression. Accordingly, surfactins were detected in bioactive fractions of B. velezensis (CB0027 and CB0909) and B. subtilis (CB0893), supporting their likely contribution to bioactivity and inhibiting L. crescens growth in vitro , highlighting their potential as candidate compounds for C Las management. Overall, this study underscores the importance of microbial discovery, screening, and characterization as an effective strategy for identifying sustainable solutions to manage HLB. IMPORTANCE Huanglongbing (HLB) is a severe disease of citrus that has altered the citrus landscape worldwide and currently has no cure. It is caused by the bacterium “ Candidatus Liberibacter asiaticus” ( C Las) and transmitted by the psyllid Diaphorina citri . Currently, the most effective control strategy for this disease relies on trunk injection of antibiotics oxytetracycline and streptomycin, and there is a need to find alternative solutions to complement or replace chemical control. Here, we aimed to select high-performing candidates from our previously established citrus-associated microbial collection to manage HLB and root-associated pathogens. We screened 18 Bacillus isolates for antagonistic activity against Liberibacter crescens , C Las, and citrus-associated Phytophthora and Fusarium spp., and we generated their genomic and metabolomic profiles. Our research centers on selecting microbial candidates for their bioactivity and antimicrobial metabolite production, aiming to develop effective plant-protective bioinoculants for mitigation of HLB.

ABSTRACT Citrus greening disease, associated with the bacterium Candidatus Liberibacter asiaticus ( C Las), has severely impacted citrus yields on a global scale. C Las is vectored by the Asian citrus psyllid, Diaphorina citri Kuwayama, in a circulative and propagative manner. Extensive application of chemical insecticides for suppression of psyllid populations has resulted in widespread resistance. Despite considerable effort and expenditure, citrus greening disease continues to impair citrus production. For transmission to occur, C Las must bind and infect the psyllid vector via the gut epithelium, but little is known of the specific molecular interactions involved. By screening a phage display library, we selected ten psyllid gut binding peptides, a subset of which have sequence similarity to C Las surface proteins. Peptide binding to the surface of the psyllid gut epithelium was confirmed by identification of gut proteins bound by specific peptides. We evaluated whether these peptides could interfere with the interaction between C Las and the gut epithelium of D. citri , thereby impeding pathogen transmission. Peptide DcNy2 significantly reduced C Las acquisition and inoculation by D. citri compared to control treatments. Nymphs fed specific peptides exhibited reduced C Las acquisition and infection of both the alimentary canal and salivary glands without negatively impacting psyllid fecundity, development, or survival. These results support peptide-mediated disruption of C Las transmission by D. citri as a potential component of integrated strategies to manage citrus greening disease. IMPORTANCE Citrus greening disease, or huanglongbing, causes billions of dollars in losses and represents one of the greatest challenges facing global citrus production. Despite significant investment on multiple fronts, effective disease management strategies are lacking. This study shows for the first time that short peptides that bind to the surface of the gut of the Asian citrus psyllid vector interfere with C Las association with the gut. This critical first gut binding step is required for C Las to infect and propagate within the psyllid vector. The downstream consequence of this peptide-mediated interference is reduced acquisition and transmission of the citrus pathogen. This work provides the foundation for new transgenic or paratransgenic tools for use in the management of citrus greening disease.

Abstract The majority of marine nitrite-oxidising bacteria (NOB) belong to three uncultured genera within the Nitrospinaceae family and thus the ecophysiology of these main nitrate producing microorganisms in the ocean remains poorly resolved. Here, we report on the environmental characterization of one of these genera, which we name Candidatus Nitromaricola. A single species within this genus, Candidatus Nitromaricola pontica, comprises >95% of the nitrite-oxidising community in the permanently stratified Black Sea. This provided a unique opportunity to study this globally abundant genus in situ, across pronounced environmental gradients of oxygen and inorganic nitrogen concentrations. We show that Ca. Nitromaricola pontica is capable of oxidising nitrite at similar rates in both oxic and apparently anoxic waters, although it had a higher abundance in the more oxygenated waters of the Black Sea. The genome of Ca. Nitromaricola pontica indicates that like other NOB, it is a chemolithoautotroph, oxidising nitrite, and fixing inorganic carbon. However, Ca. Nitromaricola pontica and other members of the Ca. Nitromaricola genus lack canonical high affinity terminal oxidases found in some other NOB genera. Single-cell analyses confirmed active autotrophic growth and showed that ammonium and urea, rather than nitrite, were the main nitrogen sources for biomass production. Our findings identify Ca. Nitromaricola as a globally abundant marine NOB and highlight its cosmopolitan lifestyle.

A novel phytoplasma species, ‘ Candidatus Phytoplasma zeae’, is proposed based on ecological distinctiveness, vector specificity, whole-genome comparisons and community consensus. This phytoplasma is associated with maize bushy stunt (MBS) disease in corn ( Zea mays ) and is transmitted exclusively by Dalbulus maidis and Dalbulus elimatus , two leafhopper species endemic to the Americas that has been reported in Brazil, Colombia, Mexico, Peru and several U.S. states. Here, we sequenced and assembled the genome of MBS phytoplasma (MBSP) strains from Brazil and the USA to describe and propose this new species. Although the 16S rRNA gene sequence of the proposed reference strain, MBSP-BR RS , shares >99% identity with that of ‘ Candidatus Phytoplasma asteris’, key nucleotide polymorphisms distinguish ‘ Ca . P. zeae’ from other 16SrI-related phytoplasma species. Genome-wide average nucleotide identity between ‘ Ca . P. zeae’ and all available ‘ Ca . P. asteris’-related strains genomes was between 92.6 and 98.4%. Comparative genomic analyses revealed unique gene clusters in ‘ Ca . P. zeae’ associated with amino acid transport, defence mechanisms and protein turnover, which may contribute to its specialization in corn. The ecological profile of ‘ Ca . P. zeae’, including its narrow host and vector range and restricted geographic distribution, supports its recognition as a novel species under Rule c of the IRPCM guidelines. The designation ‘ Ca . P. zeae’ is, therefore, proposed by members of the research community who have studied this pathogen for over a decade, with the MBSP-Brazil RS strain serving as the reference.