A Plant Bacterial Pathogen Manipulates Its Insect Vector's Energy Metabolism

The greening-causing agent alters the behavioral and electrophysiological responses of the Asian citrus psyllid to a putative sex pheromone

The Asian Citrus Psyllid (ACP), Diaphorina citri, is a vector of the pathological bacterium Candidatus Liberibacter asiaticus (CLas), which causes the most devastating disease to the citrus industry worldwide, known as greening or huanglongbing (HLB). Earlier field tests with an acetic acid-based lure in greening-free, 'Valencia' citrus orange groves in California showed promising results. The same type of lures tested in São Paulo, Brazil, showed unsettling results. During the unsuccessful trials, we noticed a relatively large proportion of females in the field, ultimately leading us to test field-collected males and females for Wolbachia and CLas. The results showed high rates of Wolbachia and CLas infection in field populations. We then compared the olfactory responses of laboratory-raised, CLas-free, and CLas-infected males to acetic acid. As previously reported, CLas-uninfected males responded to acetic acid at 1 µg. Surprisingly, CLas-infected males required 50 × higher doses of the putative sex pheromone, thus explaining the failure to capture CLas-infected males in the field. CLas infection was also manifested in electrophysiological responses. Electroantennogram responses from CLas-infected ACP males were significantly higher than those obtained with uninfected males. To the best of our knowledge, this is the first report of a pathogen infection affecting a vector's response to a sex attractant.

The effects of maternal care on the developmental transcriptome and metatranscriptome of a wild bee

Maternal care acts as a strong environmental stimulus that can induce phenotypic plasticity in animals and may also alter their microbial communities through development. Here, we characterize the developmental metatranscriptome of the small carpenter bee, Ceratina calcarata, across developmental stages and in the presence or absence of mothers. Maternal care had the most influence during early development, with the greatest number and magnitude of differentially expressed genes between maternal care treatments, and enrichment for transcription factors regulating immune response in motherless early larvae. Metatranscriptomic data revealed fungi to be the most abundant group in the microbiome, with Aspergillus the most abundant in early larvae raised without mothers. Finally, integrative analysis between host transcriptome and metatranscriptome highlights several fungi correlating with developmental and immunity genes. Our results provide characterizations of the influence of maternal care on gene expression and the microbiome through development in a wild bee.

Seasonal Patterns in the Frequency of Candidatus Liberibacter Asiaticus in Populations of Diaphorina citri (Hemiptera: Psyllidae) in Florida

Candidatus Liberibacter asiaticus (CLas) is one of the putative causal agents of huanglongbing, which is a serious disease in citrus production. The pathogen is transmitted by Diaphorina citri Kuwayama (Hemiptera: Psyllidae). As an observational study, six groves in central Florida and one grove at the southern tip of Florida were sampled monthly from January 2008 through February 2012 (50 months). The collected psyllids were sorted by sex and abdominal color. Disease prevalence in adults peaked in November, with a minor peak in February. Gray/brown females had the highest prevalence, and blue/green individuals of either sex had the lowest prevalence. CLas prevalence in blue/green females was highly correlated with the prevalence in other sexes and colors. Thus, the underlying causes for seasonal fluctuations in prevalence operated in a similar fashion for all psyllids. The pattern was caused by larger nymphs displacing smaller ones from the optimal feeding sites and immunological robustness in different sex-color morphotypes. Alternative hypotheses were also considered. Improving our understanding of biological interactions and how to sample them will improve management decisions. We agree with other authors that psyllid management is critical year-round.

Differential gene expression of Asian citrus psyllids infected with 'Ca. Liberibacter asiaticus' reveals hyper-susceptibility to invasion by instar fourth-fifth and teneral adult stages

The bacterial pathogen Candidatus Liberibacter asiaticus (CLas) is the causal agent of citrus greening disease. This unusual plant pathogenic bacterium also infects its psyllid host, the Asian citrus psyllid (ACP). To investigate gene expression profiles with a focus on genes involved in infection and circulation within the psyllid host of CLas, RNA-seq libraries were constructed from CLas-infected and CLas-free ACP representing the five different developmental stages, namely, nymphal instars 1-2, 3, and 4-5, and teneral and mature adults. The Gbp paired-end reads (296) representing the transcriptional landscape of ACP across all life stages and the official gene set (OGSv3) were annotated based on the chromosomal-length v3 reference genome and used for de novo transcript discovery resulting in 25,410 genes with 124,177 isoforms. Differential expression analysis across all ACP developmental stages revealed instar-specific responses to CLas infection, with greater overall responses by nymphal instars, compared to mature adults. More genes were over-or under-expressed in the 4-5th nymphal instars and young (teneral) adults than in instars 1-3, or mature adults, indicating that late immature instars and young maturing adults were highly responsive to CLas infection. Genes identified with potential for direct or indirect involvement in the ACP-CLas circulative, propagative transmission pathway were predominantly responsive during early invasion and infection processes and included canonical cytoskeletal remodeling and endo-exocytosis pathway genes. Genes with predicted functions in defense, development, and immunity exhibited the greatest responsiveness to CLas infection. These results shed new light on ACP-CLas interactions essential for pathogenesis of the psyllid host, some that share striking similarities with effector protein-animal host mechanisms reported for other culturable and/or fastidious bacterial- or viral- host pathosystems.

Metabolic changes and potential biomarkers in "Candidatus Liberibacter solanacearum"-infected potato psyllids: implications for psyllid-pathogen interactions

Psyllid yellows, vein-greening (VG), and zebra chip (ZC) diseases, which are primarily transmitted by potato psyllid (PoP) carrying Candidatus Liberibacter solanacearum (CLso), have caused significant losses in solanaceous crop production worldwide. Pathogens interact with their vectors at the organic and cellular levels, while the potential changes that may occur at the biochemical level are less well reported. In this study, the impact of CLso on the metabolism of PoP and the identification of biomarkers from infected psyllids were examined. Using ultra-performance liquid chromatography tandem mass spectrometry (UPLC-MS/MS) analysis, metabolomic changes in CLso-infected psyllids were compared to uninfected ones. A total of 34 metabolites were identified as potential biomarkers of CLso infection, which were primarily related to amino acid, carbohydrate, and lipid metabolism. The significant increase in glycerophospholipids is thought to be associated with CLso evading the insect vector's immune defense. Matrix-assisted Laser Desorption Ionization Mass Spectrometry Imaging (MALDI-MSI) was used to map the spatial distribution of these biomarkers, revealing that 15-keto-Prostaglandin E2 and alpha-D-Glucose were highly expressed in the abdomen of uninfected psyllids but down-regulated in infected psyllids. It is speculated that this down-regulation may be due to CLso evading surveillance by immune suppression in the PoP midgut. Overall, valuable biochemical information was provided, a theoretical basis for a better understanding of psyllid-pathogen interactions was offered, and the findings may aid in breaking the transmission cycle of these diseases.

Investigation of the Association between the Energy Metabolism of the Insect Vector Laodelphax striatellus and Rice Stripe Virus (RSV)

Viruses, as intracellular parasites, rely on the host organism to complete their life cycle. Although over 70% of plant viruses are transmitted by insect vectors, the role of vector energy metabolism on the infection process of insect-borne plant viruses is unclear. In this study, full-length cDNAs of three energy metabolism-related genes (LsATPase, LsMIT13 and LsNADP-ME) were obtained from the small brown planthopper (SBPH, Laodelphax striatellus), which transmits the Rice stripe virus (RSV). Expression levels of LsATPase, LsMIT13 and LsNADP-ME increased by 105%, 1120% and 259%, respectively, due to RSV infection. The repression of LsATPase, LsMIT13 or LsNADP-ME by RNAi had no effect on RSV nucleocapsid protein (NP) transcripts or protein levels. The repression of LsATPase caused a significant increase in LsMIT13 and LsNADP-ME transcript levels by 230% and 217%, respectively, and the repression of LsMIT13 caused a significant increase in LsNADP-ME mRNA levels. These results suggested that the silencing of LsATPase induced compensatory upregulation of LsMIT13 and LsNADP-ME, and silencing LsMIT13 induced compensatory upregulation of LsNADP-ME. Further study indicated that the co-silencing of LsATPase, LsMIT13 and LsNADP-ME in viruliferous SBPHs increased ATP production and RSV loads by 182% and 117%, respectively, as compared with nonviruliferous SBPHs. These findings indicate that SBPH energy metabolism is involved in RSV infection and provide insight into the association between plant viruses and energy metabolism in the insect vector.

Bioprospecting Microbiome for Soil and Plant Health Management Amidst Huanglongbing Threat in Citrus: A Review

Microorganisms have dynamic and complex interactions with their hosts. Diverse microbial communities residing near, on, and within the plants, called phytobiome, are an essential part of plant health and productivity. Exploiting citrus-associated microbiomes represents a scientific approach toward sustained and environment-friendly module of citrus production, though periodically exposed to several threats, with Huanglongbing (HLB) predominantly being most influential. Exploring the composition and function of the citrus microbiome, and possible microbial redesigning under HLB disease pressure has sparked renewed interest in recent times. A concise account of various achievements in understanding the citrus-associated microbiome, in various niche environments viz., rhizosphere, phyllosphere, endosphere, and core microbiota alongside their functional attributes has been thoroughly reviewed and presented. Efforts were also made to analyze the actual role of the citrus microbiome in soil fertility and resilience, interaction with and suppression of invading pathogens along with native microbial communities and their consequences thereupon. Despite the desired potential of the citrus microbiota to counter different pathogenic diseases, utilizing the citrus microbiome for beneficial applications at the field level is yet to be translated as a commercial product. We anticipate that advancement in multiomics technologies, high-throughput sequencing and culturing, genome editing tools, artificial intelligence, and microbial consortia will provide some exciting avenues for citrus microbiome research and microbial manipulation to improve the health and productivity of citrus plants.

Not Just a Cycle: Three gab Genes Enable the Non-Cyclic Flux Toward Succinate via GABA Shunt in 'Candidatus Liberibacter asiaticus'-Infected Citrus

Although the mitochondria retain all required enzymes for an intact tricarboxylic acid (TCA) cycle, plants might shift the cyclic flux from the TCA cycle to an alternative noncyclic pathway via γ-aminobutyric acid (GABA) shunt under specific physiological conditions. We hypothesize that several genes may ease this noncyclic flux and contribute to the citrus response to the phytopathogenic bacterium 'Candidatus Liberibacter asiaticus', the causal agent of Huanglongbing in citrus. To test this hypothesis, we used multiomics techniques (metabolomics, fluxomics, and transcriptomics) to investigate the potential roles of putative gab homologies from Valencia sweet orange (Citrus sinensis). Our findings showed that 'Ca. L. asiaticus' significantly increased the endogenous GABA and succinate content but decreased ketoglutarate in infected citrus plants. Citrus genome harbors three putative gab genes, including amino-acid permease (also known as GABA permease; CsgabP), GABA transaminase (CsgabT), and succinate-semialdehyde dehydrogenase (also known as GABA dehydrogenase; CsgabD). The transcript levels of CsgabP, CsgabT, and CsgabD were upregulated in citrus leaves upon the infection with 'Ca. L. asiaticus' and after the exogenous application of GABA or deuterium-labeled GABA isotope (GABA-D₆). Moreover, our finding showed that exogenously applied GABA is quickly converted to succinate and fed into the TCA cycle. Likewise, the fluxomics study showed that GABA-D₆ is rapidly metabolized to succinate-D₄. Our work proved that GABA shunt and three predicated gab genes from citrus, support the upstream noncyclic flux toward succinate rather than an intact TCA cycle and contribute to citrus defense responses to 'Ca. L. asiaticus'.[Formula: see text] Copyright © 2022 The Author(s). This is an open access article distributed under the CC BY-NC-ND 4.0 International license.

Transcriptome Profiling of 'Candidatus Liberibacter asiaticus' in Citrus and Psyllids

'Candidatus Liberibacter asiaticus' (Las) is an emergent bacterial pathogen that is associated with the devastating citrus huanglongbing (HLB). Vectored by the Asian citrus psyllid, Las colonizes the phloem tissue of citrus, causing severe damage to infected trees. So far, cultivating pure Las culture in axenic media has not been successful, and dual-transcriptome analyses aiming to profile gene expression in both Las and its hosts have a low coverage of the Las genome because of the low abundance of bacterial RNA in total RNA extracts from infected tissues. Therefore, a lack of understanding of the Las transcriptome remains a significant knowledge gap. Here, we used a bacterial cell enrichment procedure and confidently determined the expression profiles of approximately 84% of the Las genes. Genes that exhibited high expression in citrus include transporters, ferritin, outer membrane porins, specific pilins, and genes involved in phage-related functions, cell wall modification, and stress responses. We also found 106 genes to be differentially expressed in citrus versus Asian citrus psyllids. Genes related to transcription or translation and resilience to host defense response were upregulated in citrus, whereas genes involved in energy generation and the flagella system were expressed to higher levels in psyllids. Finally, we determined the relative expression levels of potential Sec-dependent effectors, which are considered as key virulence factors of Las. This work advances our understanding of HLB biology and offers novel insight into the interactions of Las with its plant host and insect vector.

Metagenomic Survey of the Highly Polyphagous Anastrepha ludens Developing in Ancestral and Exotic Hosts Reveals the Lack of a Stable Microbiota in Larvae and the Strong Influence of Metamorphosis on Adult Gut Microbiota

We studied the microbiota of a highly polyphagous insect, Anastrepha ludens (Diptera: Tephritidae), developing in six of its hosts, including two ancestral (Casimiroa edulis and C. greggii), three exotic (Mangifera indica cv. Ataulfo, Prunus persica cv. Criollo, and Citrus x aurantium) and one occasional host (Capsicum pubescens cv. Manzano), that is only used when extreme drought conditions limit fruiting by the common hosts. One of the exotic hosts (“criollo” peach) is rife with polyphenols and the occasional host with capsaicinoids exerting high fitness costs on the larvae. We pursued the following questions: (1) How is the microbial composition of the larval food related to the composition of the larval and adult microbiota, and what does this tell us about transience and stability of this species’ gut microbiota? (2) How does metamorphosis affect the adult microbiota? We surveyed the microbiota of the pulp of each host fruit, as well as the gut microbiota of larvae and adult flies and found that the gut of A. ludens larvae lacks a stable microbiota, since it was invariably associated with the composition of the pulp microbiota of the host plant species studied and was also different from the microbiota of adult flies indicating that metamorphosis filters out much of the microbiota present in larvae. The microbiota of adult males and females was similar between them, independent of host plant and was dominated by bacteria within the Enterobacteriaceae. We found that in the case of the “toxic” occasional host C. pubescens the microbiota is enriched in potentially deleterious genera that were much less abundant in the other hosts. In contrast, the pulp of the ancestral host C. edulis is enriched in several bacterial groups that can be beneficial for larval development. We also report for the first time the presence of bacteria within the Arcobacteraceae family in the gut microbiota of A. ludens stemming from C. edulis. Based on our findings, we conclude that changes in the food-associated microbiota dictate major changes in the larval microbiota, suggesting that most larval gut microbiota is originated from the food.

The Mechanism of Citrus Host Defense Response Repression at Early Stages of Infection by Feeding of Diaphorina citri Transmitting Candidatus Liberibacter asiaticus

Citrus Huanglongbing (HLB) is the most devastating disease of citrus, presumably caused by "Candidatus Liberibacter asiaticus" (CaLas). Although transcriptomic profiling of HLB-affected citrus plants has been studied extensively, the initial steps in pathogenesis have not been fully understood. In this study, RNA sequencing (RNA-seq) was used to compare very early transcriptional changes in the response of Valencia sweet orange (VAL) to CaLas after being fed by the vector, Diaphorina citri (Asian citrus psyllid, or ACP). The results suggest the existence of a delayed defense reaction against the infective vector in VAL, while the attack by the healthy vector prompted immediate and substantial transcriptomic changes that led to the rapid erection of active defenses. Moreover, in the presence of CaLas-infected psyllids, several downregulated differentially expressed genes (DEGs) were identified on the pathways, such as signaling, transcription factor, hormone, defense, and photosynthesis-related pathways at 1 day post-infestation (dpi). Surprisingly, a burst of DEGs (6,055) was detected at 5 dpi, including both upregulated and downregulated DEGs on the defense-related and secondary metabolic pathways, and severely downregulated DEGs on the photosynthesis-related pathways. Very interestingly, a significant number of those downregulated DEGs required ATP binding for the activation of phosphate as substrate; meanwhile, abundant highly upregulated DEGs were detected on the ATP biosynthetic and glycolytic pathways. These findings highlight the energy requirement of CaLas virulence processes. The emerging picture is that CaLas not only employs virulence strategies to subvert the host cell immunity, but the fast-replicating CaLas also actively rewires host cellular metabolic pathways to obtain the necessary energy and molecular building blocks to support virulence and the replication process. Taken together, the very early response of citrus to the CaLas, vectored by infective ACP, was evaluated for the first time, thus allowing the changes in gene expression relating to the primary mechanisms of susceptibility and host-pathogen interactions to be studied, and without the secondary effects caused by the development of complex whole plant symptoms.

Candidatus Liberibacter asiaticus manipulates the expression of vitellogenin, cytoskeleton, and endocytotic pathway-related genes to become circulative in its vector, Diaphorina citri (Hemiptera: Psyllidae)

Citrus greening disease or huanglongbing (HLB) caused by Candidatus Liberibacter asiaticus (CLas) limits citrus production worldwide. CLas is transmitted by the Asian citrus psyllid (ACP), Diaphorina citri (Hemiptera: Psyllidae) in a persistent-propagative manner. Understanding the molecular interaction between CLas and ACP and interrupting the interrelationship can provide an alternative to insecticides for managing citrus greening disease. Transcriptome analysis of ACP in response to CLas showed differential expression of 3911 genes (2196 upregulated, and 1715 downregulated) including the key genes of ACP involved in cytoskeleton synthesis and nutrition-related proteins, such as vitellogenins, extensin, laminin, tropomyosin, troponin C, and flightin. Majority of the differentially expressed genes were categorized under molecular functions followed by cellular components and biological processes. KEGG pathway analysis showed differential regulation of carbohydrate, nucleotide, and energy metabolic pathways, the endocytotic pathway, and the defense-related pathways. Differential regulation of genes associated with the key pathways might favour CLas to become systemic and propagate in its insect vector. The study provides an understanding of genes involved in circulation of CLas in ACP. The candidate genes involved in key physiological processes and CLas transmission by ACP would be potential targets for sustainable management of ACP and CLas.

SUPPLEMENTARY INFORMATION

The online version contains supplementary material available at 10.1007/s13205-021-02641-x.

Melatonin Is Involved in Citrus Response to the Pathogen Huanglongbing via Modulation of Phytohormonal Biosynthesis

Huanglongbing (HLB) is a devastating citrus disease worldwide that is putatively caused by Candidatus Liberibacter asiaticus and transmitted by Diaphorina citri Melatonin is a ubiquitously distributed auxin-like metabolite found in both prokaryotes and eukaryotes. In this study, we used integrative metabolomic and transcriptomic approaches to investigate the potential role of melatonin in citrus response against HLB and to understand the relationships between melatonin and the stress-associated phytohormones at molecular and metabolic levels. Melatonin was detected in the leaves of Valencia sweet orange (Citrus sinensis) after derivatization with N-methyl-N-trimethylsilyltrifluoroacetamide using a targeted gas chromatography-mass spectrometry running in selective ion monitoring mode-based method. Ca. L. asiaticus infection and D. citri infestation significantly increased endogenous melatonin levels in Valencia sweet orange leaves and upregulated the expression of its biosynthetic genes (CsTDC, CsT5H, CsSNAT, CsASMT, and CsCOMT). However, infection with Ca. L. asiaticus had a greater effect than did infestation with D. citri Melatonin induction was positively correlated with salicylic acid content, but not that of trans-jasmonic acid. Moreover, melatonin supplementation enhanced the endogenous contents of the stress-associated phytohormones (salicylates, auxins, trans-jasmonic acid, and abscisic acid) and the transcript levels of their biosynthetic genes. Furthermore, melatonin supplementation diminished the Ca. L. asiaticus titer within the infected leaves, which suggests that melatonin might play an antibacterial role against this bacterium and gram-negative bacteria in general. These findings provide a better understanding of the melatonin-mediated defensive response against HLB via modulation of multiple hormonal pathways. Understanding the role of melatonin in citrus defense to HLB may provide a novel therapeutic strategy to mitigate the disease.

Development on Citrus medica infected with 'Candidatus Liberibacter asiaticus' has sex-specific and -nonspecific impacts on adult Diaphorina citri and its endosymbionts

Huanglongbing (HLB) is a deadly, incurable citrus disease putatively caused by the unculturable bacterium, 'Candidatus Liberibacter asiaticus' (CLas), and transmitted by Diaphorina citri. Prior studies suggest D. citri transmits CLas in a circulative and propagative manner; however, the precise interactions necessary for CLas transmission remain unknown, and the impact of insect sex on D. citri-CLas interactions is poorly understood despite reports of sex-dependent susceptibilities to CLas. We analyzed the transcriptome, proteome, metabolome, and microbiome of male and female adult D. citri reared on healthy or CLas-infected Citrus medica to determine shared and sex-specific responses of D. citri and its endosymbionts to CLas exposure. More sex-specific than shared D. citri responses to CLas were observed, despite there being no difference between males and females in CLas density or relative abundance. CLas exposure altered the abundance of proteins involved in immunity and cellular and oxidative stress in a sex-dependent manner. CLas exposure impacted cuticular proteins and enzymes involved in chitin degradation, as well as energy metabolism and abundance of the endosymbiont 'Candidatus Profftella armatura' in both sexes similarly. Notably, diaphorin, a toxic Profftella-derived metabolite, was more abundant in both sexes with CLas exposure. The responses reported here resulted from a combination of CLas colonization of D. citri as well as the effect of CLas infection on C. medica. Elucidating these impacts on D. citri and their endosymbionts contributes to our understanding of the HLB pathosystem and identifies the responses potentially critical to limiting or promoting CLas acquisition and propagation in both sexes.

A Transcriptomic and Proteomic Analysis of the Diaphorina citri Salivary Glands Reveals Genes Responding to Candidatus Liberibacter asiaticus

The Asian citrus psyllid (ACP), Diaphorina citri Kuwayama, is the principal vector of the Candidatus Liberibacter asiaticus (CLas) bacterium that causes Huanglongbing (HLB) disease. The D. citri salivary glands (SG) is an important barrier to the transmission of CLas. Despite its importance, the transcriptome and proteome of SG defense against CLas are unstudied in D. citri. In the present study, we generated a comparative transcriptome dataset of the SG in infected and uninfected D. citri using an Illumina RNA-Seq technology. We obtained 407 differentially expressed genes (DEGs), including 159 upregulated DEGs and 248 downregulated DEGs. Functional categories showed that many DEGs were associated with the ribosome, the insecticide resistance, the immune response and the digestion in comparison with CLas-infected SG and CLas-free SG. Gene Ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG) databases confirmed that metabolism and immunity were important functions in the SG. Among the DEGs, 68 genes (35 upregulated and 33 downregulated) encoding putative-secreted proteins were obtained with a signal peptide, suggesting that these genes may play important roles in CLas infection. A total of 673 SG proteins were identified in uninfected D. citri by liquid chromatography-mass spectrometry/mass spectrometry (LC-MS/MS) analysis, and 30 DEGs (15 upregulated and 15 downregulated) were found using the local tBLASTP programs. Among the 30 DEGs, many DEGs mainly involved in the metabolism and cellular processes pathways. This study provides basic transcriptome and proteome information for the SG in D. citri, and helps illuminate the molecular interactions between CLas and D. citri.

Development on Infected Citrus over Generations Increases Vector Infection by 'Candidatus Liberibacter Asiaticus in Diaphorina citri'

'Candidatus Liberibacter asiaticus' (CLas) is a major causal agent of citrus Huanglongbing (HLB), which is transmitted by Asian citrus psyllid (ACP), Diaphorina citri, causing severe losses in various regions of the world. Vector efficiency is higher when acquisition occurs by ACP immature stages and over longer feeding periods. In this context, our goal was to evaluate the progression of CLas population and infection rate over four ACP generations that continuously developed on infected citrus plants. We showed that the frequency of CLas-positive adult samples increased from 42% in the parental generation to 100% in the fourth generation developing on CLas-infected citrus. The bacterial population in the vector also increased over generations. This information reinforces the importance of HLB management strategies, such as vector control and eradication of diseased citrus trees, to avoid the development of CLas-infected ACP generations with higher bacterial loads and, likely, a higher probability of spreading the pathogen in citrus orchards.

Dynamics of Insect-Microbiome Interaction Influence Host and Microbial Symbiont

Insects share an intimate relationship with their gut microflora and this symbiotic association has developed into an essential evolutionary outcome intended for their survival through extreme environmental conditions. While it has been clearly established that insects, with very few exceptions, associate with several microbes during their life cycle, information regarding several aspects of these associations is yet to be fully unraveled. Acquisition of bacteria by insects marks the onset of microbial symbiosis, which is followed by the adaptation of these bacterial species to the gut environment for prolonged sustenance and successful transmission across generations. Although several insect-microbiome associations have been reported and each with their distinctive features, diversifications and specializations, it is still unclear as to what led to these diversifications. Recent studies have indicated the involvement of various evolutionary processes operating within an insect body that govern the transition of a free-living microbe to an obligate or facultative symbiont and eventually leading to the establishment and diversification of these symbiotic relationships. Data from various studies, summarized in this review, indicate that the symbiotic partners, i.e., the bacteria and the insect undergo several genetic, biochemical and physiological changes that have profound influence on their life cycle and biology. An interesting outcome of the insect-microbe interaction is the compliance of the microbial partner to its eventual genome reduction. Endosymbionts possess a smaller genome as compared to their free-living forms, and thus raising the question what is leading to reductive evolution in the microbial partner. This review attempts to highlight the fate of microbes within an insect body and its implications for both the bacteria and its insect host. While discussion on each specific association would be too voluminous and outside the scope of this review, we present an overview of some recent studies that contribute to a better understanding of the evolutionary trajectory and dynamics of the insect-microbe association and speculate that, in the future, a better understanding of the nature of this interaction could pave the path to a sustainable and environmentally safe way for controlling economically important pests of crop plants.

Probing Behavior of Diaphorina citri (Hemiptera: Liviidae) on Valencia Orange Influenced by Sex, Color, and Size

Candidatus Liberibacter asiaticus Jagoueix, Bové, and Garnier (Rhizobiales: Rhizobiaceae) is transmitted by the psyllid Diaphorina citri Kuwayama and putatively causes Huanglongbing disease in citrus. Huanglongbing has reduced yields by 68% relative to pre-disease yields in Florida. Disease management is partly through vector control. Understanding vector biology is essential in this endeavor. Our goal was to document differences in probing behavior linked to sex. Based on both a literature review and our results, we conclude that there is either no effect of sex or that identifying such an effect requires a sample size at least four times larger than standard methodologies. Including both color and sex in statistical models did not improve model performance. Both sex and color are correlated with body size, and body size has not been considered in previous studies on sex in D. citri in terms of probing behavior. An effect of body size was found wherein larger psyllids took longer to reach ingestion behaviors and larger individuals spent more time-ingesting phloem, but these relationships explained little of the variability in these data. We suggest that the effects of sex can be ignored when running EPG experiments on healthy psyllids.

The Role of Plant-Associated Microbes in Mediating Host-Plant Selection by Insect Herbivores

There is increasing evidence that plant-associated microorganisms play important roles in shaping interactions between plants and insect herbivores. Studies of both pathogenic and beneficial plant microbes have documented wide-ranging effects on herbivore behavior and performance. Some studies, for example, have reported enhanced insect-repellent traits or reduced performance of herbivores on microbe-associated plants, while others have documented increased herbivore attraction or performance. Insect herbivores frequently rely on plant cues during foraging and oviposition, suggesting that plant-associated microbes affecting these cues can indirectly influence herbivore preference. We review and synthesize recent literature to provide new insights into the ways pathogenic and beneficial plant-associated microbes alter visual, olfactory, and gustatory cues of plants that affect host-plant selection by insect herbivores. We discuss the underlying mechanisms, ecological implications, and future directions for studies of plant-microbial symbionts that indirectly influence herbivore behavior by altering plant traits.

The Power of Electropenetrography in Enhancing Our Understanding of Host Plant-Vector Interactions

The invasive Asian citrus psyllid, Diaphorina citri (Hemiptera: Liviidae), is the primary vector of the phloem-infecting bacterium, Candidatus Liberibacter asiaticus. Candidatus L. asiaticus is the putative causal agent of Huanglongbing (HLB) disease, a destructive disease of Citrus. While many Citrus species are susceptible to D. citri probing and HLB disease, there are marked behavioral differences in D. citri probing responses and Ca. Liberibacter asiaticus infection severity among Citrus species. Using four mandarin hybrid selections and pummelo plants variably resistant to D. citri probing, oviposition, and survival, we explored probing differences using electropenetrography (EPG), conducted an oviposition and survival study, and determined host plant metabolites using gas-chromatography mass-spectroscopy (GC-MS). We found thirty-seven D. citri probing variables to be significantly different among tested mandarin selections and pummelo, in addition to differential oviposition and survivorship abilities on tested plants. We found sixty-three leaf metabolites with eight being significantly different among tested mandarin selections and pummelo. Detailed analysis of probing behavior, oviposition, survivorship, and host plant metabolite concentrations reveals the complex, layered resistance mechanisms utilized by resistant Citrus against D. citri probing. EPG is a powerful technology for screening Asian citrus psyllid resistant Citrus to elucidate host plant-vector interactions, with an aim to minimize vector probing and eliminate the spread of the bacterial pathogen, Ca. L. asiaticus.

Physiochemical changes mediated by "Candidatus Liberibacter asiaticus" in Asian citrus psyllids

Plant pathogenic bacteria interact with their insect host(s)/vector(s) at the cellular and molecular levels. This interaction may alter the physiology of their insect vector, which may also promote the growth and transmission of the bacterium. Here we studied the effect of “Candidatus Liberibacter asiaticus” (“Ca. L. asiaticus”) on physiochemical conditions within its insect vector, the Asian citrus psyllid (ACP), and whether these changes were beneficial for the pathogen. The local microenvironments inside ACPs were quantified using microelectrodes. The average hemolymph pH was significantly higher in infected ACPs (8.13 ± 0.21) than in “Ca. L. asiaticus”-free ACPs (7.29 ± 0.15). The average hemolymph oxygen tension was higher in “Ca. L. asiaticus”-free ACPs than in infected ACPs (67.13% ± 2.11% vs. 35.61% ± 1.26%). Oxygen tension reduction and pH increase were accompanied by “Ca. L. asiaticus” infection. Thus, oxygen tension of the hemolymph is an indicator of infection status, with pH affected by the severity of the infection.

The Probing Behavior Component of Disease Transmission in Insect-Transmitted Bacterial Plant Pathogens

Insects can be effective vectors of plant diseases and this may result in billions of dollars in lost agricultural productivity. New, emerging or introduced diseases will continue to cause extensive damage in afflicted areas. Understanding how the vector acquires the pathogen and inoculates new hosts is critical in developing effective management strategies. Management may be an insecticide applied to kill the vector or a host plant resistance mechanism to make the host plant less suitable for the vector. In either case, the tactic must act before the insect performs the key behavior(s) resulting in either acquisition or transmission. This requires knowledge of the timing of behaviors the insect uses to probe the plant and commence ingestion. These behaviors are visualized using electropenetrography (EPG), wherein the plant and insect become part of an electrical circuit. With the tools to define specific steps in the probing process, we can understand the timing of acquisition and inoculation. With that understanding comes the potential for more relevant testing of management strategies, through insecticides or host plant resistance. The primary example will be Candidatus Liberibacter asiaticus transmitted by Diaphorina citri Kuwayama in the citrus agroecosystem, with additional examples used as appropriate.

The infection of its insect vector by bacterial plant pathogen "Candidatus Liberibacter solanacearum" is associated with altered vector physiology

Many bacterial and viral plant pathogens are transmitted by insect vectors, and pathogen-mediated alterations of plant physiology often influence insect vector behavior and fitness. It remains largely unknown for most plant pathogens whether, and how, they might directly alter the physiology of their insect vectors in ways that promote pathogen transmission. Here we examined whether the presence of "Candidatus Liberibacter solanacearum" ("Ca. L. solanacearum"), an obligate bacterial pathogen of plants and of its psyllid vector alters the physiochemical environment within its insect vector, the potato psyllid (Bactericera cockerelli). Microelectrodes were used to measure the local pH and oxygen tension within the abdomen of "Ca. L. solanacearum"-free psyllids and those infected with "Ca. L. solanacearum". The hemolymph of infected psyllids had higher pH at 9.09 ± 0.12, compared to "Ca. L. solanacearum"-free psyllids (8.32 ± 0.11) and a lower oxygen tension of 33.99% vs. 67.83%, respectively. The physicochemical conditions inside "Ca. L. solanacearum"-free and -infected psyllids body differed significantly with the infected psyllids having a higher hemolymph pH and lower oxygen tension than "Ca. L. solanacearum"-free psyllids. Notably, the bacterial titer increased under conditions of higher pH and lower oxygen tension values. This suggests that the vector's physiology is altered by the presence of the pathogen, potentially, resulting in a more conducive environment for "Ca. L. solanacearum" survival and subsequent transmission.

Infection with phytopathogenic bacterium inhibits melatonin biosynthesis, decreases longevity of its vector, and suppresses the free radical-defense

Vector-borne phytopathogenic bacteria may alter the reproductive fitness, survival, behavior, and metabolism of their vectors. Candidatus Liberibacter asiaticus (CLas) is associated with the Huanglongbing (also known as citrus greening disease), one of the most destructive citrus diseases worldwide, and transmitted by Asian citrus psyllid, Diaphorina citri (Insecta, Hemiptera, Liviidae). The genome sequencing of CLas revealed that it does not have the ability to synthesize tryptophan, the precursor of melatonin, and it must acquire it from its host plant or insect vector to achieve its biologic processes, such as growth and multiplication. Herein, we aimed to develop a GC-MS-SIM-based method to detect the endogenous melatonin from small insects such as D. citri, and to explore the hidden relationship between melatonin content and D. citri-adult survival. Then, we studied the ability of exogenous melatonin supplementation to reverse the negative effects of CLas-infection. Our findings showed that CLas-infection reduced the levels of melatonin and its biosynthetic genes (DcTPHs, DcAAAD, DcSNAT, and DcASMT) of D. citri compared to uninfected insects. In addition, CLas decreased the longevity of its vector, D. citri via the suppression of the free radical-defense associated genes (SODs, GSTs, PODs, and PHGPXs). On the other hand, melatonin supplementation could reverse the negative effects of CLas-infection. Melatonin supplementation enhanced the endogenous melatonin content, melatonin biosynthetic genes, free radical-defense associated genes, and the longevity of both healthy and CLas-infected D. citri. Furthermore, melatonin supplementation decreased the CLas bacterial population within the D. citri psyllids. Based on these findings, we hypothesize that melatonin plays multi-layered defensive roles in D. citri. These roles include acting as a natural antioxidant or as an antibacterial compound.

Diaphorina citri Nymphs Are Resistant to Morphological Changes Induced by "Candidatus Liberibacter asiaticus" in Midgut Epithelial Cells

"Candidatus Liberibacter asiaticus" is the causative bacterium associated with citrus greening disease. "Ca Liberibacter asiaticus" is transmitted by Diaphorina citri more efficiently when it is acquired by nymphs rather than adults. Why this occurs is not known. We compared midguts of D. citri insects reared on healthy or "Ca Liberibacter asiaticus"-infected citrus trees using quantitative PCR, confocal microscopy, and mitochondrial superoxide staining for evidence of oxidative stress. Consistent with its classification as propagative, "Ca Liberibacter asiaticus" titers were higher in adults than in nymphs. Our previous work showed that adult D. citri insects have basal levels of karyorrhexis (fragmentation of the nucleus) in midgut epithelial cells, which is increased in severity and frequency in response to "Ca Liberibacter asiaticus." Here, we show that nymphs exhibit lower levels of early-stage karyorrhexis than adults and are refractory to the induction of advanced karyorrhexis by "Ca Liberibacter asiaticus" in the midgut epithelium. MitoSox Red staining showed that guts of infected adults, particularly males, experienced oxidative stress in response to "Ca Liberibacter asiaticus." A positive correlation between the titers of "Ca Liberibacter asiaticus" and the Wolbachia endosymbiont was observed in adult and nymph midguts, suggesting an interplay between these bacteria during development. We hypothesize that the resistance of the nymph midgut to late-stage karyorrhexis through as yet unknown molecular mechanisms benefits "Ca Liberibacter asiaticus" for efficient invasion of midgut epithelial cells, which may be a factor explaining the developmental dependency of "Ca Liberibacter asiaticus" acquisition by the vector.

Concomitant Loss of the Glyoxalase System and Glycolysis Makes the Uncultured Pathogen "Candidatus Liberibacter asiaticus" an Energy Scavenger

Methylglyoxal (MG) is a cytotoxic, nonenzymatic by-product of glycolysis that readily glycates proteins and DNA, resulting in carbonyl stress. Glyoxalase I and II (GloA and GloB) sequentially convert MG into d-lactic acid using glutathione (GSH) as a cofactor. The glyoxalase system is essential for the mitigation of MG-induced carbonyl stress, preventing subsequent cell death, and recycling GSH for maintenance of cellular redox poise. All pathogenic liberibacters identified to date are uncultured, including "Candidatus Liberibacter asiaticus," a psyllid endosymbiont and causal agent of the severely damaging citrus disease "huanglongbing." In silico analysis revealed the absence of gloA in "Ca Liberibacter asiaticus" and all other pathogenic liberibacters. Both gloA and gloB are present in Liberibacter crescens, the only liberibacter that has been cultured. L. crescens GloA was functional in a heterologous host. Marker interruption of gloA in L. crescens appeared to be lethal. Key glycolytic enzymes were either missing or significantly downregulated in "Ca Liberibacter asiaticus" compared to (cultured) L. crescens Marker interruption of sut, a sucrose transporter gene in L. crescens, decreased its ability to take up exogenously supplied sucrose in culture. "Ca Liberibacter asiaticus" lacks a homologous sugar transporter but has a functional ATP/ADP translocase, enabling it to thrive both in psyllids and in the sugar-rich citrus phloem by (i) avoiding sucrose uptake, (ii) avoiding MG generation via glycolysis, and (iii) directly importing ATP from the host cell. MG detoxification enzymes appear to be predictive of "Candidatus" status for many uncultured pathogenic and environmental bacteria.IMPORTANCE Discovered more than 100 years ago, the glyoxalase system is thought to be present across all domains of life and fundamental to cellular growth and viability. The glyoxalase system protects against carbonyl stress caused by methylglyoxal (MG), a highly reactive, mutagenic and cytotoxic compound that is nonenzymatically formed as a by-product of glycolysis. The uncultured alphaproteobacterium "Ca Liberibacter asiaticus" is a well-adapted endosymbiont of the Asian citrus psyllid, which transmits the severely damaging citrus disease "huanglongbing." "Ca Liberibacter asiaticus" lacks a functional glyoxalase pathway. We report here that the bacterium is able to thrive both in psyllids and in the sugar-rich citrus phloem by (i) avoiding sucrose uptake, (ii) avoiding (significant) MG generation via glycolysis, and (iii) directly importing ATP from the host cell. We hypothesize that failure to culture "Ca Liberibacter asiaticus" is at least partly due to its dependence on host cells for both ATP and MG detoxification.

A plant pathogenic bacterium exploits the tricarboxylic acid cycle metabolic pathway of its insect vector

Huanglongbing in citrus is caused by a phloem-limited, uncultivable, gram-negative α-proteobacterium, Candidatus Liberibacter asiaticus (CLas). CLas is transmitted by the phloem-sucking insect, Diaphorina citri (Hemiptera: Liviidae), in a persistent, circulative, and propagative manner. In this study, we investigated the metabolomic and respiration rates changes in D. citri upon infection with CLas using gas chromatography-mass spectrometry (GC-MS) and gas exchange analysis. The level of glycine, _L-serine, _L-threonine, and gamma-amino butyric acid were higher in CLas-infected D. citri, while _L-proline, _L-aspartic acid, and _L-pyroglutamic acid were lower in CLas-infected D. citri compared with the control. Citric acid was increased in CLas-infected D. citri, whereas malic and succinic acids were reduced. Interestingly, most of the reduced metabolites such as malate, succinate, aspartate, and _L-proline are required for the growth of CLas. The increase in citric acid, serine, and glycine indicated that CLas induced glycolysis and the tricarboxylic acid cycle (TCA) in its vector. In agreement with the GC-MS results, the gene expression results also indicated that glycolysis and TCA were induced in CLas-infected D. citri and this was accompanied with an increases in respiration rate. Phosphoric acid and most of the sugar alcohols were higher in CLas-infected D. citri, indicating a response to the biotic stress or cell damage. Only slight increases in the levels of few sugars were observed in CLas-infected D. citri, which indicated that sugars are tightly regulated by D. citri. Our results indicated that CLas induces nutrient and energetic stress in its host insect. This study may provide some insights into the mechanism of colonization of CLas in its vector.

Molecular mechanisms behind the accumulation of ATP and H2O2 in citrus plants in response to 'Candidatus Liberibacter asiaticus' infection

Candidatus Liberibacter asiaticus (Las) is a fastidious, phloem-restricted pathogen with a significantly reduced genome, and attacks all citrus species with no immune cultivars documented to date. Like other plant bacterial pathogens, Las deploys effector proteins into the organelles of plant cells, such as mitochondria and chloroplasts to manipulate host immunity and physiology. These organelles are responsible for the synthesis of adenosine triphosphate (ATP) and have a critical role in plant immune signaling during hydrogen peroxide (H₂O₂) production. In this study, we investigated H₂O₂ and ATP accumulation in relation to citrus huanglongbing (HLB) in addition to revealing the expression profiles of genes critical for the production and detoxification of H₂O₂ and ATP synthesis. We also found that as ATP and H₂O₂ concentrations increased in the leaf, so did the severity of the HLB symptoms, a trend that remained consistent among the four different citrus varieties tested. Furthermore, the upregulation of ATP synthase, a key enzyme for energy conversion, may contribute to the accumulation of ATP in infected tissues, whereas downregulation of the H₂O₂ detoxification system may cause oxidative damage to plant macromolecules and cell structures. This may explain the cause of some of the HLB symptoms such as chlorosis or leaf discoloration. The findings in this study highlight important molecular and physiological mechanisms involved in the host plants' response to Las infection and provide new targets for interrupting the disease cycle.