Aphids are agricultural pests that damage crops directly through feeding and indirectly by transmitting plant viruses. Central to their success are salivary effectors, secreted proteins that suppress plant defenses and facilitate nutrient uptake. Nonetheless, numerous salivary effectors remain uncharacterized to date. It is unclear whether effector gene expression patterns vary in response to factors such as feeding duration, aphid age, tissue type, and host plant. To address current knowledge gaps, we reared pea aphids, Acyrthosiphon pisum (Harris) (Hemiptera: Aphididae), and 2 host plants, barrel medic (Medicago truncatula Gaertn.) and broad bean (Vicia faba Linnaeus), under a controlled environment. Next, we quantified the gene expression patterns of 2 well-characterized (C002 and Armet) and 2 less studied aphid salivary effectors (Apolipophorin and ACYPI006346). The results show that salivary effector gene expression declines by 72 h when switching hosts (V. faba to M. truncatula), although some effectors display fluctuating patterns between specific time points (24 vs 48 h and 48 vs 72 h) when switching hosts (V. faba to M. truncatula) or not (M. truncatula to M. truncatula). One-day-old aphids consistently exhibit higher gene expression levels than older individuals when fed on a single host (V. faba to V. faba). These findings highlight the importance of considering temporal dynamics, developmental stage, and host context in studies of aphid effector biology. Our findings lay a methodological foundation that supports the design of more consistent and informative transcriptomic experiments on aphid-plant interactions.

‘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. 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. This study revealed haplotype-specific gene regulation potentially leading to LsoB being transmitted more efficiently by psyllid nymphs.

‘Candidatus Liberibacter solanacearum’ (Lso) is a devastating bacterial pathogen of crops transmitted by psyllids. Previously, a link between changes in ubiquitination and the progression of diseases caused by Lso was established. Among the Lso haplotypes, LsoA and LsoB infect solanaceous crops. LsoA induces chlorosis and stunting in tomato, while LsoB infection leads to more severe symptoms, including plant death. To better characterize the molecular basis of the differences in the diseases caused by these haplotypes, we cataloged the ubiquitinome of tomato leaves four weeks after LsoA and LsoB-infection and following infestation with Lso-free psyllids. Our analysis revealed haplotype-specific changes in the plant ubiquitinome. Gene Ontology (GO) terms related to glucose metabolism and proteasome-mediated ubiquitin-dependent protein catabolism were enriched among the differentially ubiquitinated proteins between LsoA-infected and Lso-free infested plants. Additionally, a KEGG pathway analysis determined an overrepresentation of proteins involved in carbon metabolism and fixation via the Calvin cycle. In contrast, the comparison of LsoB-infected and Lso-free infested plants revealed a broader range of enriched GO terms and KEGG pathways, including those associated with carbohydrate and energy metabolism, proteasome activity, and oxidative stress. These results contribute to our understanding of the development of Liberibacter-related diseases and reveal the importance of ubiquitin-related pathways in Lso infection. They also contribute to the expanding inventory of ubiquitinated proteins.

'Candidatus Liberibacter solanacearum' is a phloem-limited Gram-negative bacterium transmitted by psyllids. Several haplotypes have been reported worldwide infecting different plants including solanaceous and apiaceous crops. In the United States of America, haplotypes A and B severely impact tomato and potato production. While the molecular mechanisms at play during 'Ca. L. solanacearum' infection remain unclear, this pathogen might manipulate its host plants by secreting effector proteins. Effectors that induce or suppress plant immunity can provide insights into the pathogen-host plant interactions leading to plant infection, but to date, 'Ca. L. solanacearum' effectors inducing immunity remain poorly studied. HPE21 is a 'Ca. L. solanacearum' effector previously reported as inducing reactive oxygen species in plants. Here, it is confirmed that HPE21 induced H2O2 accumulation in Nicotiana benthamiana. It was also discovered that this haplotype B-specific effector induced cell death in N. benthamiana and interacted with tomato aldo-keto reductase 1 (SlAKR1). Furthermore, the expression of SlAKR1 decreased HPE21-induced H2O2 accumulation in N. benthamiana. Finally, it was demonstrated that H2O2 accumulates in a haplotype-dependent manner in 'Ca. L. solanacearum'-infected plants. These results suggest that oxidative stress might play a key role in disease progression in 'Ca. L. solanacearum'-infected plants, and the interaction between HPE21 and SlAKR1 can be involved in the pathogenicity differences between the 'Ca. L. solanacearum' haplotypes A and B.

“Candidatus Liberibacter solanacearum” (Lso) is a phloem-limited bacterial pathogen transmitted by psyllid. In the USA, the haplotypes A and B are transmitted by the tomato psyllid, Bactericera cockerelli, and are responsible for economic damage to the solanaceous crop industry. In tomato, haplotypes A and B cause diseases with differing degrees of severity, yet their underlying mechanisms are not well understood. The role that abiotic stress plays in disease development has not been characterized. In the present study, we evaluated whether water stress influenced the development of symptoms associated with these haplotypes in tomato (Solanum lycopersicum “Moneymaker”). First, pot-grown tomato plants were subjected to either a control or a water-stress regime, then they were assigned to one of three infection treatments (Lso-free, LsoA, or LsoB) under laboratory-controlled conditions. Symptom development was monitored for up to 8 weeks following infection. Results of this experiment revealed that water stress significantly reduced the plant water potential and was the primary driver of reduced height, with infection only having an effect in later weeks. Water stress also increased the detectability of Lso throughout the plant and across three timepoints (weeks 4, 6, and 8 after infection). This increase in Lso-positive tissues suggests that water stress creates a more homogenous distribution of Lso throughout the plant. More noticeably, water stress significantly reduced the severity of the disease associated with haplotype A but not B. Overall, these findings revealed the existence of haplotype-specific interactions between Lso infection and water stress on the tomato health. This work highlights the influence of abiotic factors on plant health.

"Candidatus Liberibacter solanacearum" (Lso) is a highly destructive plant pathogen within the alpha-proteobacteria group. Multiple Lso haplotypes occur worldwide, each uniquely associated with a specific psyllid vector. Haplotypes A and B, found in the United States, cause serious damage to solanaceous crops and are transmitted by Bactericera cockerelli, known as the potato psyllid in a circulative and persistent manner. The psyllid gut is the first organ encountered by Lso and may act as a barrier to its transmission; however, the immune response of the gut to Lso infection remains largely uncharacterized. In this study, we examined autophagic responses in the gut of potato psyllids at early, mid, and late infection stages. Based on gene expression analyses, microscopic observations, and Western blotting, we found no clear evidence of autophagy induction despite Lso infection. Nevertheless, during mid and late phases of LsoB infection, we observed downregulation of the mechanistic target of rapamycin and decreased accumulation of ATG8-II. Notably, inducing autophagy with rapamycin significantly reduced LsoA titers in the psyllid guts after a 5-day acquisition access period and lowered transmission efficiency, while LsoB remained unaffected. These findings suggest that modulating autophagy in the gut of potato psyllids could be a promising strategy to limit LsoA acquisition and transmission, while highlighting LsoA and LsoB possess distinct molecular regulatory mechanisms in the psyllid gut.IMPORTANCELiberibacters are devastating plant pathogens transmitted by psyllids. Because these bacteria are fastidious, the study of the molecular mechanisms involved in plant infection and transmission is difficult. Here, we determined that inducing autophagy in the potato psyllid can affect the acquisition and transmission of Lso haplotype A but not haplotype B. Comparing the host and vector responses to different liberibacters can help identify their transmission and infection mechanisms and find targets to disrupt these processes.

The spotted lanternfly (SLF), Lycorma delicatula, is a polyphagous invasive species in the United States known for causing significant damage to grapevines. This phloem-feeding insect secretes saliva containing effector proteins at the time of feeding, allowing for the ingestion of the phloem’s nutrients. The mechanisms that SLF uses to modulate plant defenses and feed efficiently remain unknown and understudied. Methods and Results: We identified LdEP01, an SLF predicted secreted effector protein with a FK506 binding protein (FKBP)-type domain and two EF-hand Ca2+ binding domains. LdEP01 is highly expressed in the salivary glands compared to the SLF’s full body. LdEP01 modulates plant defenses by suppressing cytosolic calcium accumulation and suppressing hypersensitive response (HR). Functional analysis confirmed that LdEP01 binds to calcium but does not undergo conformational change when bound to calcium. Conclusions: These findings suggest that LdEP01 plays a crucial role in modulating plant defenses, allowing SLF to feed efficiently on plants.

Pathogens have evolved mechanisms to manipulate their hosts to facilitate effective colonization and infection. One such mechanism involves the secretion of effectors that interfere with the host immune response. Effectors are typically secreted by dedicated secretion machinery that delivers the protein to the host cell. Liberibacters are intracellular bacterial pathogens that do not encode the typical secretion systems employed to secrete effectors; therefore, other mechanisms might be at play to allow Liberibacters to infect their hosts, such as the secretion of effectors via the sec-secretion system or via non-classical secretion systems. In this study, we datamined the genomes of five Liberibacter pathogens and identified from 66 to 102 putative non-classical secreted proteins encoded. Then, we focused on two predicted non-classical secreted proteins encoded by ‘Candidatus Liberibacter solanacearum’ haplotype B, CKC_05770 and CKC_00930, which showed similarities to non-classical secreted effectors from other Liberibacter species. We evaluated their secretion using alkaline phosphatase assays, if they suppressed programmed cell death or reactive oxygen species accumulation in Nicotiana benthamiana upon transient expression, and whether they could interact with tomato ascorbate peroxidases. We also evaluated if CKC_05770 had a peroxidase activity. Our results suggest that CKC_05770 interacted with tomato ascorbate peroxidases in two in vivo assays but not in vitro. Further, CKC_05770 did not suppress plant immunity nor did it have a peroxidase activity as the ‘Candidatus Liberibacter asiaticus’ homologs did. Therefore, Liberibacter pathogens encode non-classical secreted proteins that could be effectors, but the roles of these proteins need to be validated in each pathosystem.

Solenopsis invicta queens experience significant behavioral and physiological changes after mating, which are essential for their reproductive success. We investigated differences in ovary gene expression in virgin alate queens, newly mated queens, and mated queens to identify candidate genes associated with their physiological transition to mature egg-laying queens. Virgin queens and mated queens were obtained from field colonies and newly mated queens were collected from the ground immediately after their mating flight. Whole ovaries of virgin alate queens, and germaria and vitellaria from the ovaries of newly mated and mature mated queens were dissected. Pools of each of these five organs/tissues were used for RNAseq and RT-qPCR analyses. Principal component analyses revealed a distinct transcriptomic profile among alate virgin ovaries, germaria of newly mated, and germaria of mated queens, highlighting the effect of mating driving significant differences in global gene expression. Mating did not have such a differentiating effect among libraries of newly mated and mated queen vitellaria. Differentially expressed genes (DEGs) were identified between whole ovary transcriptome of virgin alate queens and germaria of newly mated and mated queens, as well as vitellaria of newly mated and mated queens. There were 22 gene ontology terms enriched among the DEGs in the germaria analysis, of note were those enriched in development and phosphorylation. In the vitellarium, terms related to nucleobase-containing molecule processes and fatty acid metabolism were enriched. Sixty-one DEGs were shared between germaria and vitellaria libraries, mainly linked to immunity, lipid metabolism, development, and transcriptional regulation. Phenoloxidase was highly expressed in mated queens in both ovarian regions, suggesting a role in immunity and choriogenesis. Vg3, one S. invicta vitellogenin gene, was upregulated in the vitellaria of mated queens, reinforcing its role in vitellogenesis. Transcripts of the prostaglandin E2 receptor showed ovary region-specific regulation, suggesting a significant role in immunity, oocyte development and potentially in the release of egg-laying behavior. Insulin-related genes were up-regulated in mated queens, reflecting the metabolic demands for egg production. This study advances our understanding of immunity and mating and other key signaling pathways in fire ant reproduction.

Vitellogenin (Vg) has been recognized as a co-opted gene involved in social colonies where it is associated with reproduction in the queen and task transitions in workers. The Solenopsis invicta genome harbours four Vg genes, Vg1, Vg2, Vg3 and Vg4, and this study explored the regional expression of these genes in the brain and head of the worker caste in different subcaste and social contexts. Transcriptomic analyses of the worker brain while in the presence or absence of brood revealed differential expression of genes involved with social behaviours. Among these, we found higher expression of Vg2 and Vg3 in the brains of workers in the absence of brood. The influence of brood on expression of Vg between worker castes was also investigated. RT-qPCR revealed higher expression of Vg in the head of workers in the absence of brood. Further studies demonstrated that Vg was also differentially expressed in the heads of workers conducting specific tasks and differing between subcastes. Nurses maintained higher Vg expression than foragers. We conclude worker Vg expression may be linked to the brood and/or nutritional state. Vg expression changes when workers are deprived of protein suggesting it is regulated by nutritional signalling and the presence of brood.

Division of labor is a hallmark characteristic of social insect colonies. While it is understood that worker differentiation is regulated through either the queen or her brood, the understanding of the physiology behind task regulation varies within social species. Studies in eusocial insects have shown that juvenile hormone (JH) is associated with division of labor and the onset of foraging tasks. Although, outside of a few key species, this interaction has yet to be elucidated in the red imported fire ant, Solenopsis invicta . In this study, we evaluated the role of a JH analog, S-hydroprene in worker task transition in Solenopsis invicta . S-hydroprene was applied to nurses to observe behavioral changes. S-hyroprene application to nurses did not affect phototaxis, but there was a shift in behavior from internal, nest-based behaviors to external, foraging-based behaviors. These results show that JH may be implicated in worker task transition in S. invicta and may function similarly as it does in other eusocial insects.

‘Candidatus Liberibacter solanacearum’ (Lso) is a plant pathogenic bacterium transmitted by psyllids that causes significant agricultural damage. Several Lso haplotypes have been reported. Among them, LsoA and LsoB are transmitted by the potato psyllid Bactericera cockerelli and infect solanaceous crops, and LsoD is transmitted by the carrot psyllid B. trigonica and infects apiaceous crops. Several studies evaluated the transmission of these haplotypes by adult psyllids. However, fewer data are available on the transmission of different Lso haplotypes by psyllid nymphs. In this study, we investigated the transmission of these three haplotypes by psyllid nymphs to expand our basic understanding of Lso transmission. Specifically, the objective was to determine if the haplotypes differed in their transmission rates by nymphs and if LsoA and LsoB accumulated at different rates in the guts of nymphs as it occurs in adults. First, we quantified LsoA and LsoB titers in the guts of third- and fifth-instar potato psyllid nymphs. We found similar LsoA titers in the two nymphal stages, while LsoB titer was lower in the gut of the third-instar nymphs compared to fifth-instar nymphs. Second, we assessed the transmission efficiency of LsoA and LsoB by third-instar nymphs to tomato plants, revealing that LsoA was transmitted earlier and with higher efficiency than LsoB. Finally, we examined the transmission of LsoD by carrot psyllid nymphs to celery plants and demonstrated an age-related difference in the transmission rate. These findings provide valuable insights into the transmission dynamics of different Lso haplotypes by nymphal vectors, shedding light on their epidemiology and interactions with their psyllid vectors.

'Candidatus Liberibacter solanacearum' (Lso) is a bacterial pathogen that causes 'zebra chip' disease in potato. Vectored by the potato psyllid Bactericera cockerelli, Lso inflicts severe economic losses annually in solanaceous and other crops. Conventional management strategies have been insufficient to control the pathogen. As an environmentally friendly phytosanitary technique, electron beam (eBeam) irradiation has been shown to drastically suppress psyllid growth and development. However, whether this chemical-free technology negatively impacts the pathogen transmission remains to be investigated. In this study, we irradiated newly emerged adult psyllids and measured abundance of Lso they carried. An eBeam dose of 100 Gy significantly suppressed Lso in irradiated psyllids, although a rebound was detected on day 7 post irradiation. This resumed bacterial growth was not observed in psyllids irradiated at 250 or 500 Gy. Electrical penetration graphs revealed that eBeam substantially impacted psyllid feeding activity. No significant difference was detected in actin cytoskeleton structure and nucleus integrity of its midgut cells 1 or 7 days post eBeam irradiation at 500 Gy. Furthermore, infection of tomato plants was drastically reduced when fed upon by psyllids exposed to 250 Gy and undetectable at 500 Gy. Thus, eBeam significantly lowered psyllid's transmission efficiency and even abolished this capacity. Taken together, eBeam irradiation curtailed Lso titers in potato psyllids and impaired its bacterial transmission. Our results have demonstrated the real potential of irradiation technology in management of insect-vectored diseases.

Among social insects, task allocation within its group members remains as one of the paramount pillars of social functionality. Division of labor in many eusocial insects is maintained by behavioral flexibility that can shift according to the needs of the colony they reside in. Workers typically, over time as they age, shift from intranidal nurses to extranidal foragers. If the needs of the colony change, either from the needs of the adults or the brood therein, workers shift their behavior in order to compensate for the need of a particular task to be done. This shift, either accelerating towards a behavior associated with an older worker, or regressing back into the nest, is not clearly understood in social insects outside of honeybees. In this study, evaluated how brood type affected the red imported fire ant, Solenopsis invicta, worker task reversion and acceleration. Through observation of worker behaviors performed over multiple time-points per day, we discovered that worker task reversion and acceleration does occur within this ant species. Furthermore, the type of brood influenced the rate at which this occurred, with larvae having the strongest effect of all types. Finally, there was a propensity for workers to maintain their new behavior throughout the experiment. This study shows that the needs of brood within a social insect colony can influence the behavior workers perform, reversing the age polyethism that is common among social insect species.

Citrus tristeza virus (CTV) can be transmitted by several aphid species in a semi-persistent mode, with Toxoptera citricida being the most efficient vector. In Brazil, mild CTV isolates are used for pre-immunization of citrus trees against severe isolates. We aimed to determine the capacity and efficiency of T. citricida in separating the viral complex into haplotypes from three well-characterized CTV isolates (PIAC, CS1, and Forte Rolândia). Single-aphid transmission assays were conducted to determine CTV transmission efficiency. The results showed that T. citricida transmitted only haplotypes from the PIAC and CS1 isolates, with efficiencies of 8 and 4%, respectively. Both isolates caused mild CTV symptoms in Brazil. However, isolate Forte Rolândia, which causes severe symptoms in citrus trees, was not transmitted by T. citricida. The detection of CTV haplotypes from PIAC and CS1 isolates in sweet orange (Citrus sinensis) plants after a single aphid transmission occurred at different time points. The first haplotype observed was from PIAC 150 days after citrus was challenged by a single aphid, followed by CS1 at day 210 after transmission. In addition, differences in the single-strand conformation polymorphism patterns between the CTV isolates and CTV haplotypes were determined, suggesting that an aphid can acquire and transmit only one CTV haplotype to citrus plants during phloem feeding. The study of the mechanism of transmission by the vector can increase our knowledge of the interactions among hosts, vectors, and pathogens, which are often neglected.

Autophagy is a catabolic process that results in the autophagosomic-lysosomal degradation of bulk cytoplasmic content, abnormal protein aggregates, and excess of/or damaged organelles to promote cell survival. Autophagy is also a component of innate immunity in insects and is involved in the clearance of pathogens, including bacteria. The potato psyllid, Bactericera cockerelli, transmits the plant bacterial pathogen 'Candidatus Liberibacter solanacearum' (Lso) in the Americas and causes serious damage to solanaceous crops. Our previous studies showed that autophagy could be involved in the psyllid response to Lso and could affect pathogen acquisition. However, the tools to evaluate this response have not been validated in psyllids. To this end, the effect of rapamycin, a commonly used autophagy inducer, on potato psyllid survival and the expression of autophagy-related genes was evaluated. Further, the autophagic activity was assessed via microscopy and by measuring the autophagic flux. Artificial diet-feeding assays using rapamycin resulted in significant psyllid mortality, an increase in the autophagic flux, as well as an increase in the amount of autolysosomes. This study represents a stepping stone in determining the role of autophagy in psyllid immunity.

'Candidatus Liberibacter solanacearum' (Lso) is a bacterial pathogen infecting several crops and causing damaging diseases. Several Lso haplotypes have been identified. Among the seven haplotypes present in North America, LsoA and LsoB are transmitted by the potato psyllid, Bactericera cockerelli (Šulc), in a circulative and persistent manner. The gut, which is the first organ pathogen encounters, could be a barrier for Lso transmission. However, the molecular interactions between Lso and the psyllid vector at the gut interface remain largely unknown. In this study, we investigated the global transcriptional responses of the adult psyllid gut upon infection with two Lso haplotypes (LsoA and LsoB) using Illumina sequencing. The results showed that each haplotype triggers a unique transcriptional response, with most of the distinct genes elicited by the highly virulent LsoB. The differentially expressed genes were mainly associated with digestion and metabolism, stress response, immunity, detoxification as well as cell proliferation and epithelium renewal. Importantly, distinct immune pathways were triggered by LsoA and LsoB in the gut of the potato psyllid. The information in this study will provide an understanding of the molecular basis of the interactions between the potato psyllid gut and Lso, which may lead to the discovery of novel molecular targets for the control of these pathogens.

The red imported fire ant Solenopsis invicta is an invasive pest in the USA, eastern Asia, and Australia that causes billions worth of damage where it has been introduced. In the insect colony, workers perform tasks based on their age as well as the subcaste, where the younger workers tend to remain in the nest and tend to the brood while the older workers leave the nest to perform activities such as foraging. In eusocial insects, juvenile hormone has been identified to be a catalyst for behavioral changes among the worker caste, but the involvement of this hormone in S. invicta task allocation has not been investigated. Here, we conducted RNA-seq analyses to identify genes associated with worker division of labor. We compared the expression profiles of foragers and nurses and found 816 differentially expressed genes. We also identified 100 differentially expressed genes between nurses treated with acetone and nurses treated with a juvenile hormone analog. For this study, we focused on the differentially expressed genes between foragers and nurses that were associated with energy metabolism, glycolysis, juvenile hormone synthesis, metabolism, and immunity because these pathways have been identified as differentially expressed between foragers and nurses in different social insects. We also identified changes in gene expression induced by the juvenile hormone analog, some of which were as expected if the juvenile hormone is involved in regulating the shift from nursing to foraging in S. invicta workers. Overall, our results support a potential role of juvenile hormone in S. invicta task transition probably in association with other factors such as insect age or nutritional status which were not controlled in this experiment.

'Candidatus Liberibacter solanacearum' (Lso) is a phloem-limited pathogen associated with devastating diseases in members of the Solanaceae and Apiaceae and vectored by several psyllid species. Different Lso haplotypes have been identified, and LsoA and LsoB are responsible for diseases in Solanaceae crops. Our efforts are aimed at identifying pathogenicity factors used by this bacterium to thrive in different hosts. Bacterial secreted proteins can play a role in host colonization or the manipulation of the host immune responses; these proteins are called effectors. In this study, we identified six LsoB-specific proteins with a conserved secretion motif as well as a conserved N-terminal domain in the mature protein. These proteins had different expression and secretion patterns but a similar subcellular localization in Nicotiana benthamiana leaves, suggesting that they play different roles regardless of their conserved secretion motif. One of these proteins, CKC_04425, was expressed at high levels in the insect vector and the host plant, indicating that it could play a role in both the plant and insect hosts, whereas the others were mainly expressed in the plant. One protein, CKC_05701, was able to efficiently suppress programmed cell death and reactive oxygen species production, suggesting that it may have a virulence role in LsoB-specific pathogenesis.

‘Candidatus Liberibacter solanacearum’ (Lso) is a phloem-limited bacterial plant pathogen infecting solanaceous plants in the Americas and New Zealand and is associated with diseases of apiaceous crops in Europe, Northern Africa, and the Middle East. This pathogen is also related to other Liberibacter species that infect other crops. In the USA, two haplotypes of Lso, LsoA and LsoB, are predominant and responsible for diseases in potato and tomato. Tobacco, Nicotiana tabacum, a model species to study plant defenses, is a host for Lso; therefore, the interaction between Lso and this host plant could be used to study Liberibacter−plant interactions. In this study, we characterized the infection associated with LsoA and LsoB in tobacco. Under laboratory conditions, LsoB caused more severe symptoms than LsoA, and LsoA and LsoB titers were dynamic during the 7 weeks of the experiment. We also measured SA and other metabolites, including oxylipins, at an early point of infection and found that SA was accumulated in plants infected with LsoB but not with LsoA; whereas ABA levels were reduced in LsoA- but not in LsoB-infected plants.