An Intranuclear Sodalis-Like Symbiont and Spiroplasma Coinfect the Carrot Psyllid, Bactericera trigonica (Hemiptera, Psylloidea)

The Hidden Secrets of Psylloidea: Biology, Behavior, Symbionts, and Ecology

Psyllids constitute a diverse group of sap-feeding Sternorrhyncha that were relatively obscure until it was discovered that a handful of species transmit bacterial plant pathogens. Yet the superfamily Psylloidea is much richer than the sum of its crop-associated vectors, with over 4,000 described species exhibiting diverse life histories and host exploitation strategies. A growing body of research is uncovering fascinating insights into psyllid evolution, biology, behavior, and species interactions. This work has revealed commonalities and differences with better-studied Sternorrhyncha, as well as unique evolutionary patterns of lineage divergence and host use. We are also learning how psyllid evolution and foraging ecology underlie life history traits and the roles of psyllids in communities. At finer scales, we are untangling the web of symbionts across the psyllid family tree, linking symbiont and psyllid lineages, and revealing mechanisms underlying reciprocal exchange between symbiont and host. In this review, we synthesize and summarize key advances within these areas with a focus on free-living (nongalling) Psylloidea.

Pantoea bathycoeliae sp. nov and Sodalis sp. are core gut microbiome symbionts of the two-spotted stink bug

Stink bug species (Pentatomoidea superfamily) have developed an interdependence with obligate bacterial gut symbionts in specialized midgut crypts (M4 sub-region). Species of the Enterobacteriaceae family (predominantly Pantoea) are vertically transferred to their offspring and provide nutrients that cannot be obtained from plant sap food sources. However, the bacteria in the other gut compartments of stink bugs have rarely been investigated. The two-spotted stink bug, Bathycoelia distincta, is a serious pest of macadamias in South Africa. Nothing is currently known regarding its gut microbiome or how symbionts are transferred between insect generations. In this study, the consistency of B. distincta gut bacteria across geographic locations and life stages was determined with 16S rRNA metabarcoding, considering both the M4 and other gut compartments. A novel Pantoea species was found to be the primary M4 gut symbiont and is vertically transferred to the offspring. The other gut compartments had a low bacterial diversity and genera varied between stink bug populations but a Sodalis species was prominent in all populations. Sequence data of the M4 compartment were used to produce high-quality metagenome-assembled genomes (MAGs) for the Pantoea and Sodalis species. Functional analyses suggested a similar role in nutrient provision for the host, yet also unique metabolites produced by each species. The Sodalis sp. also had additional traits, such as secretion systems, that likely allowed it to establish itself in the host. The Pantoea species was described as Pantoea bathycoeliae sp. nov based on the rules of the SeqCode.

Crossing the Gateless Barriers: Factors Involved in the Movement of Circulative Bacteria Within Their Insect Vectors

Plant bacterial pathogens transmitted by hemipteran vectors pose a large threat to the agricultural industry worldwide. Although virus-vector relationships have been widely investigated, a significant gap exists in our understanding of the molecular interactions between circulative bacteria and their insect vectors, mainly leafhoppers and psyllids. In this review, we will describe how these bacterial pathogens adhere, invade, and proliferate inside their insect vectors. We will also highlight the different transmission routes and molecular factors of phloem-limited bacteria that maintain an effective relationship with the insect host. Understanding the pathogen-vector relationship at the molecular level will help in the management of vector-borne bacterial diseases.

The plant pathogenic bacterium Candidatus Liberibacter solanacearum induces calcium-regulated autophagy in midgut cells of its insect vector Bactericera trigonica

Autophagy plays an important role against pathogen infection in many organisms; however, little has been done with regard to vector-borne plant and animal pathogens, that sometimes replicate and cause deleterious effects in their vectors. Candidatus Liberibacter solanacearum (CLso) is a fastidious gram-negative phloem-restricted plant pathogen and vectored by the carrot psyllid, Bactericera trigonica. The plant disease caused by this bacterium is called carrot yellows and has recently gained much importance due to worldwide excessive economical losses. Here, we demonstrate that calcium ATPase, cytosolic calcium, and most importantly Beclin-1 have a role in regulating autophagy and its association with Liberibacter inside the psyllid. The presence of CLso generates reactive oxygen species and induces the expression of detoxification enzymes in the psyllid midguts, a main site for bacteria transmission. CLso also induces the expression of both sarco/endoplasmic reticulum Ca2+pump (SERCA) and 1,4,5-trisphosphate receptors (ITPR) in midguts, resulting in high levels of calcium in the cellular cytosol. Silencing these genes individually disrupted the calcium levels in the cytosol and resulted in direct effects on autophagy and subsequently on Liberibacter persistence and transmission. Inhibiting Beclin1-phosphorylation through different calcium-induced kinases altered the expression of autophagy and CLso titers and persistence. Based on our results obtained from the midgut, we suggest the existence of a direct correlation between cytosolic calcium levels, autophagy, and CLso persistence and transmission by the carrot psyllid. IMPORTANCE Plant diseases caused by vector-borne Liberibacter species are responsible for the most important economic losses in many agricultural sectors. Preventing these diseases relies mostly on chemical sprays against the insect vectors. Knowledge-based interference with the bacteria-vector interaction remains a promising approach as a sustainable solution. For unravelling how Liberibacter exploits molecular pathways in its insect vector for transmission, here, we show that the bacterium manipulates calcium levels on both sides of the endoplasmic reticulum membrane, resulting in manipulating autophagy. Silencing genes associated with these pathways disrupted the calcium levels in the cytosol and resulted in direct effects on autophagy and Liberibacter transmission. These results demonstrate major pathways that could be exploited for manipulating and controlling the disease transmission.

Association of endoplasmic reticulum associated degradation (ERAD) with the transmission of Liberibacter solanacearum by its psyllid vector

Candidatus Liberibacter solanacearum (CLso) is a complex of gram negative plant pathogenic and fastidious bacterial haplotypes restricted to the phloem and transmitted by several psyllid species. In Israel, the carrot psyllid Bactericera trigonica transmits CLso haplotype D in a persistent and propagative manner and causes the carrot yellows disease, inflicting significant economic losses in many countries. Understanding the transmission of CLso is fundamental to devising sustainable management strategies. Persistent transmission of vector-borne pathogens involves the critical steps of adhesion, cell invasion and replication inside the insect gut cells before passage to the hemolymph. Using microscopy and expression analyses, we have previously confirmed a role for the endoplasmic reticulum (ER) in inducing immune responses and subsequent molecular pathways resulting in programmed cell death (apoptosis) upon CLso-infection in the midgut. In the current study, we confirm that the ER-associated degradation (ERAD) machinery and its associated marker genes were upregulated in CLso infected insects, including Derlin-1, Selenoprotein-1 and Ubiquitin Ligase RNF-185. Silencing Derlin-1, which acts on the ER membrane by regulating the degradation of unfolded proteins upon ER stress, revealed its role in CLso persistence and transmission. Molecular pathways initiated in the ER membrane upon bacterial infection are well documented in human, animal and insect systems, and this study confirms the role of the ER in CLso-psyllid interactions.

Diversity and dynamics of endosymbionts in a single population of sweet potato weevil, Cylas formicarius (Coleoptera: Brentidae): a preliminary study

Endosymbionts live symbiotically with insect hosts and play important roles in the evolution, growth, development, reproduction, and environmental fitness of hosts. Weevils are one of the most abundant insect groups that can be infected by various endosymbionts, such as Sodalis, Nardonella, and Wolbachia. The sweet potato weevil, Cylas formicarius (Coleoptera: Brentidae), is a notorious pest in sweet potato (Ipomoea batatas L.) cultivation. Currently, little is known about the presence of endosymbionts in C. formicarius. Herein, we assessed the endosymbiont load of a single geographic population of C. formicarius. The results showed that Nardonella and Rickettsia could infect C. formicarius at different rates, which also varied according to the developmental stages of C. formicarius. The relative titer of Nardonella was significantly related to C. formicarius developmental stages. The Nardonella-infecting sweet potato weevils were most closely related to the Nardonella in Sphenophorus levis (Coleoptera, Curculionidae). The Rickettsia be identified in bellii group. These results preliminarily revealed the endosymbionts in C. formicarius and helped to explore the diversity of endosymbionts in weevils and uncover the physiological roles of endosymbionts in weevils.

Pesticide residue exposure provides different responses of the microbiomes of distinct cultures of the stored product pest mite Acarus siro

Background

The contribution of the microbiome to pesticide breakdown in agricultural pests remains unclear. We analyzed the effect of pirimiphos-methyl (PM) on four geographically different cultures of the stored product pest mite Acarus siro (6 L, 6Tu, 6Tk and 6Z) under laboratory experiments. The effect of PM on mite mortality in the impregnated filter paper test was compared.

Results

The mite sensitivity to PM decreased in the order of 6 L, 6Tu, 6Tk, and 6Z. Then, the mites were cultured on PM residues (0.0125 and 1.25 µg·g⁻¹), and population growth was compared to the control after 21 days of exposure. The comparison showed two situations: (i) increasing population growth for the most sensitive cultures (6 L and 6Tu), and (ii) no effect on mite population growth for tolerant cultures (6Z and 6Tk). The microbiome of mites was analyzed by quantification of 16S DNA copies based on quantitative polymerase chain reaction (qPCR) and by barcode sequencing of the V4 fragment of 16S DNA on samples of 30 individuals from the control and PM residues. The microbiome comprised primarily Solitalea-like organisms in all cultures, except for 6Z, followed by Bacillus, Staphylococcus, and Lactobacillus. The microbiomes of mite cultures did not change with increasing population density. The microbiome of cultures without any differences in population density showed differences in the microbiome composition. A Sodalis-like symbiont replaced Solitalea in the 1.25 µg·g⁻¹ PM in the 6Tk culture. Sodalis and Bacillus prevailed in the microbiomes of PM-treated mites of 6Z culture, while Solitalea was almost absent.

Conclusion

The results showed that the microbiome of A. siro differs in composition and in response to PM residues in the diet. The results indicate that Sodalis-like symbionts can help recover mites from pesticide-induced stress.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12866-022-02661-4.

Investigating the microbial community of Cacopsylla spp. as potential factor in vector competence of phytoplasma

Phytoplasmas are obligatory intracellular bacteria that colonize the phloem of many plant species and cause hundreds of plant diseases worldwide. In nature, phytoplasmas are primarily transmitted by hemipteran vectors. While all phloem-feeding insects could in principle transmit phytoplasmas, only a limited number of species have been confirmed as vectors. Knowledge about factors that might determine the vector capacity is currently scarce. Here, we characterized the microbiomes of vector and non-vector species of apple proliferation (AP) phytoplasma 'Candidatus Phytoplasma mali' to investigate their potential role in the vector capacity of the host. We performed high-throughput 16S rRNA metabarcoding of the two principal AP-vectors Cacopsylla picta and Cacopsylla melanoneura and eight Cacopsylla species, which are not AP-vectors but co-occur in apple orchards. The microbiomes of all species are dominated by Carsonella, the primary endosymbiont of psyllids and a second uncharacterized Enterobacteriaceae endosymbiont. Each Cacopsylla species harboured a species-specific phylotype of both symbionts. Moreover, we investigated differences between the microbiomes of AP-vector versus non-vector species and identified the predominant endosymbionts but also Wolbachia and several minor taxa as potential indicator species. Our study highlights the importance of considering the microbiome in future investigations of potential factors influencing host vector competence. We investigated the potential role of symbiotic bacteria in the acquisition and transmission of phytoplasma. By comparing the two main psyillid vector species of Apple proliferation (AP) phytoplasma and eight co-occurring species, which are not able to vector AP-phytoplasma, we found differences in the microbial communities of AP-vector and non-vector species, which appear to be driven by the predominant symbionts in both vector species and Wolbachia and several minor taxa in the non-vector species. In contrast, infection with AP-phytoplasma did not affect microbiome composition in both vector species. Our study provides new insights into the endosymbiont diversity of Cacopsylla spp. and highlights the importance of considering the microbiome when investigating potential factors influencing host vector competence.

Rational engineering of a synthetic insect-bacterial mutualism

Many insects maintain mutualistic associations with bacterial endosymbionts, but little is known about how they originate in nature. In this study, we describe the establishment and manipulation of a synthetic insect-bacterial symbiosis in a weevil host. Following egg injection, the nascent symbiont colonized many tissues, including prototypical somatic and germinal bacteriomes, yielding maternal transmission over many generations. We then engineered the nascent symbiont to overproduce the aromatic amino acids tyrosine and phenylalanine, which facilitate weevil cuticle strengthening and accelerated larval development, replicating the function of mutualistic symbionts that are widely distributed among weevils and other beetles in nature. Our work provides empirical support for the notion that mutualistic symbioses can be initiated in insects by the acquisition of environmental bacteria. It also shows that certain bacterial genera, including the Sodalis spp. used in our study, are predisposed to develop these associations due to their ability to maintain benign infections and undergo vertical transmission in diverse insect hosts, facilitating the partner-fidelity feedback that is critical for the evolution of obligate mutualism. These experimental advances provide a new platform for laboratory studies focusing on the molecular mechanisms and evolutionary processes underlying insect-bacterial symbiosis.

Interaction of Liberibacter Solanacearum with Host Psyllid Vitellogenin and Its Association with Autophagy

Candidatus Liberibacter solanacearum (CLso) haplotype D, transmitted by the carrot psyllid Bactericera trigonica, is a major constraint for carrot production in Israel. Unveiling the molecular interactions between the psyllid vector and CLso can facilitate the development of nonchemical approaches for controlling the disease caused by CLso. Bacterial surface proteins are often known to be involved in adhesion and virulence; however, interactions of CLso with carrot psyllid proteins that have a role in the transmission process has remained unexplored. In this study, we used CLso outer membrane protein (OmpA) and flagellin as baits to screen for psyllid interacting proteins in a yeast two-hybrid system assay. We identified psyllid vitellogenin (Vg) to interact with both OmpA and flagellin of CLso. As Vg and autophagy are often tightly linked, we also studied the expression of autophagy-related genes to further elucidate this interaction. We used the juvenile hormone (JH-III) to induce the expression of Vg, thapsigargin for suppressing autophagy, and rapamycin for inducing autophagy. The results revealed that Vg negatively regulates autophagy. Induced Vg expression significantly suppressed autophagy-related gene expression and the levels of CLso significantly increased, resulting in a significant mortality of the insect. Although the specific role of Vg remains obscure, the findings presented here identify Vg as an important component in the insect immune responses against CLso and may help in understanding the initial molecular response in the vector against Liberibacter. IMPORTANCE Pathogen transmission by vectors involves multiple levels of interactions, and for the transmission of liberibacter species by psyllid vectors, much of these interactions are yet to be explored. Candidatus Liberibacter solanacearum (CLso) haplotype D inflicts severe economic losses to the carrot industry. Understanding the specific interactions at different stages of infection is hence fundamental and could lead to the development of better management strategies to disrupt the transmission of the bacteria to new host plants. Here, we show that two liberibacter membrane proteins interact with psyllid vitellogenin and also induce autophagy. Altering vitellogenin expression directly influences autophagy and CLso abundance in the psyllid vector. Although the exact mechanism underlying this interaction remains unclear, this study highlights the importance of immune responses in the transmission of this disease agent.

Competitive Interactions Between Whitefly- and Aphid-Transmitted Poleroviruses Within the Plant Host and the Insect Vectors

Pepper cultivation in Israel has been constrained by two sympatric poleroviruses, Pepper vein yellows virus-2 (PeVYV-2) and Pepper whitefly-borne vein yellows virus (PeWBVYV), which are transmitted specifically by aphids and whiteflies, respectively. This study investigated the interaction between PeVYV-2 and PeWBVYV inside the host plant and the insect vectors. Our results show that PeVYV-2 and PeWBVYV compete against each other inside the host plant and also inside aphids. PeWBVYV was the weaker competitor inside the host plant, with diminished transmission rates when inoculated simultaneously or successively after PeVYV-2 and could only be transmitted efficiently when inoculated first and then challenged by PeVYV-2. Successive inoculations of plants with viruliferous whiteflies with PeWBVYV followed by viruliferous aphids with PeVYV-2 led to a coinfection rate of 60%, but with severely reduced titers of PeWBVYV in the coinfected plants compared with singly infected plants. In contrast, PeVYV-2 was the weaker competitor inside the insect vector, with reduced quantities of the acquired virus and a reduced transmission rate by aphids when given prior acquisition on PeWBVYV. However, we also show that the transmission efficiency of PeVYV-2 and PeWBVYV from coinfected plants by whiteflies and aphids remained comparable to that from singly infected plants. This is likely attributable to the reduced titers of PeWBVYV inside coinfected plants causing lesser impact on transmission of PeVYV-2 by aphids and the stronger competitiveness of PeWBVYV inside the whitefly. Competitive interactions between PeVYV-2 and PeWBVYV inside the host plant and insect vector can thus be beneficial for their coexistence.

The Actin Cytoskeleton Mediates Transmission of "Candidatus Liberibacter solanacearum" by the Carrot Psyllid

Several vector-borne plant pathogens have evolved mechanisms to exploit and to hijack vector host cellular, molecular, and defense mechanisms for their transmission. In the past few years, Liberibacter species, which are transmitted by several psyllid vectors, have become an economically important group of pathogens that have devastated the citrus industry and caused tremendous losses to many other important crops worldwide. The molecular mechanisms underlying the interactions of Liberibacter species with their psyllid vectors are poorly studied. "Candidatus Liberibacter solanacearum," which is associated with important vegetable diseases, is transmitted by the carrot psyllid Bactericera trigonica in a persistent manner. Here, we elucidated the role of the B. trigonica Arp2/3 protein complex, which plays a major role in regulation of the actin cytoskeleton, in the transmission of "Ca Liberibacter solanacearum." "Ca Liberibacter solanacearum" colocalized with ArpC2, a key protein in this complex, and this colocalization was strongly associated with actin filaments. Silencing of the psyllid ArpC2 disrupted the colocalization and the dynamics of F-actin. Silencing of RhoGAP21 and Cdc42, which act in the signaling cascade leading to upregulation of Arp2/3 and F-actin bundling, showed similar results. On the other hand, silencing of ArpC5, another component of the complex, did not induce any significant effects on F-actin formation. Finally, ArpC2 silencing caused a 73.4% reduction in "Ca Liberibacter solanacearum" transmission by psyllids, strongly suggesting that transmission of "Ca Liberibacter solanacearum" by B. trigonica is cytoskeleton dependent and "Ca Liberibacter solanacearum" interacts with ArpC2 to exploit the intracellular actin nucleation process for transmission. Targeting this unique interaction could lead to the development of a novel strategy for the management of Liberibacter-associated diseases.IMPORTANCE Plant diseases caused by vector-borne pathogens are responsible for tremendous losses and threaten some of the most important agricultural crops. A good example is the citrus greening disease, which is caused by bacteria of the genus Liberibacter and is transmitted by psyllids; it has devastated the citrus industry in the United States, China, and Brazil. Here, we show that psyllid-transmitted "Candidatus Liberibacter solanacearum" employs the actin cytoskeleton of psyllid gut cells, specifically the ArpC2 protein in the Arp2/3 complex of this system, for movement and transmission in the vector. Silencing of ArpC2 dramatically influenced the interaction of "Ca Liberibacter solanacearum" with the cytoskeleton and decreased the bacterial transmission to plants. This system could be targeted to develop a novel approach for the control of Liberibacter-associated diseases.

Spatial and morphological reorganization of endosymbiosis during metamorphosis accommodates adult metabolic requirements in a weevil

Bacterial intracellular symbiosis (endosymbiosis) is widespread in nature and impacts many biological processes. In holometabolous symbiotic insects, metamorphosis entails a complete and abrupt internal reorganization that creates a constraint for endosymbiont transmission from larvae to adults. To assess how endosymbiosis copes-and potentially evolves-throughout this major host-tissue reorganization, we used the association between the cereal weevil Sitophilus oryzae and the bacterium Sodalis pierantonius as a model system. S. pierantonius are contained inside specialized host cells, the bacteriocytes, that group into an organ, the bacteriome. Cereal weevils require metabolic inputs from their endosymbiont, particularly during adult cuticle synthesis, when endosymbiont load increases dramatically. By combining dual RNA-sequencing analyses and cell imaging, we show that the larval bacteriome dissociates at the onset of metamorphosis and releases bacteriocytes that undergo endosymbiosis-dependent transcriptomic changes affecting cell motility, cell adhesion, and cytoskeleton organization. Remarkably, bacteriocytes turn into spindle cells and migrate along the midgut epithelium, thereby conveying endosymbionts to midgut sites where future mesenteric caeca will develop. Concomitantly, endosymbiont genes encoding a type III secretion system and a flagellum apparatus are transiently up-regulated while endosymbionts infect putative stem cells and enter their nuclei. Infected cells then turn into new differentiated bacteriocytes and form multiple new bacteriomes in adults. These findings show that endosymbiosis reorganization in a holometabolous insect relies on a synchronized host-symbiont molecular and cellular "choreography" and illustrates an adaptive feature that promotes bacteriome multiplication to match increased metabolic requirements in emerging adults.