The autophagy pathway participates in resistance to tomato yellow leaf curl virus infection in whiteflies

Dual Guardians of Immunity: FoRab10 and FoRab29 in Frankliniella occidentalis Confer Resistance to Tomato Spotted Wilt Orthotospovirus

Rab GTPase is critical for autophagy processes and is implicated in insect immunity against viruses. In this study, we aimed to investigate the role of FoRabs in the autophagic regulation of antiviral defense against tomato spotted wilt orthotospovirus (TSWV) in Frankliniella occidentalis. Transcriptome analysis revealed the downregulation of FoRabs in viruliferous nymph and adults of F. occidentalis in response to TSWV infection. Manipulation of autophagy levels with 3-MA and Rapa treatments resulted in a 5- to 15-fold increase and a 38-64% decrease in viral titers, respectively. Additionally, interference with FoRab10 in nymphs and FoRab29 in adults led to a 20-90% downregulation of autophagy-related genes, a decrease in ATG8-II (an autophagy marker protein), and an increase in the TSWV titers by 1.5- to 2.5-fold and 1.3- to 2.0-fold, respectively. In addition, the leaf disk and the living plant methods revealed increased transmission rates of 20.8-41.6 and 68.3-88.3%, respectively. In conclusion, FoRab10 and FoRab29 play a role in the autophagic regulation of the antiviral defense in F. occidentalis nymphs and adults against TSWV, respectively. These findings offer insights into the intricate immune mechanisms functional in F. occidentalis against TSWV, suggesting potential targeted strategies for F. occidentalis and TSWV management.

Functional role of geminivirus encoded proteins in the host: Past and present

During plant-pathogen interaction, plant exhibits a strong defense system utilizing diverse groups of proteins to suppress the infection and subsequent establishment of the pathogen. However, in response, pathogens trigger an anti-silencing mechanism to overcome the host defense machinery. Among plant viruses, geminiviruses are the second largest virus family with a worldwide distribution and continue to be production constraints to food, feed, and fiber crops. These viruses are spread by a diverse group of insects, predominantly by whiteflies, and are characterized by a single-stranded DNA (ssDNA) genome coding for four to eight proteins that facilitate viral infection. The most effective means to managing these viruses is through an integrated disease management strategy that includes virus-resistant cultivars, vector management, and cultural practices. Dynamic changes in this virus family enable the species to manipulate their genome organization to respond to external changes in the environment. Therefore, the evolutionary nature of geminiviruses leads to new and novel approaches for developing virus-resistant cultivars and it is essential to study molecular ecology and evolution of geminiviruses. This review summarizes the multifunctionality of each geminivirus-encoded protein. These protein-based interactions trigger the abrupt changes in the host methyl cycle and signaling pathways that turn over protein normal production and impair the plant antiviral defense system. Studying these geminivirus interactions localized at cytoplasm-nucleus could reveal a more clear picture of host-pathogen relation. Data collected from this antagonistic relationship among geminivirus, vector, and its host, will provide extensive knowledge on their virulence mode and diversity with climate change.

Temporal changes in the levels of virus and betasatellite DNA in B. tabaci feeding on CLCuD affected cotton during the growing season

Cotton, a key source of income for Pakistan, has suffered significantly by cotton leaf curl disease (CLCuD) since 1990. This disease is caused by a complex of phylogenetically-related begomovirus (genus Begomovirus, family Geminiviridae) species and a specific betasatellite (genus Betasatellite, family Tolecusatellitidae), cotton leaf curl Multan betasatellite. Additionally, another DNA satellite called alphasatellite (family Alphasatellitidae), is also frequently associated. All these virus components are vectored by a single species of whitefly (Bemisia tabaci). While many factors affect cotton productivity, including cotton variety, sowing time, and environmental cues such as temperature, humidity, and rainfall, CLCuD is a major biotic constraint. Although the understanding of begomoviruses transmission by whiteflies has advanced significantly over the past three decades, however, the in-field seasonal dynamics of the viruses in the insect vector remained an enigma. This study aimed to assess the levels of virus and betasatellite in whiteflies collected from cotton plants throughout the cotton growing season from 2014 to 2016. Notably, begomovirus levels showed no consistent pattern, with minimal variations, ranging from 0.0017 to 0.0074 ng.μg-1 of the genomic DNA in 2014, 0.0356 to 0.113 ng.μg-1 of the genomic DNA in 2015, and 0.0517 to 0.0791 ng.μg-1 of the genomic DNA in 2016. However, betasatellite levels exhibited a distinct pattern. During 2014 and 2015, it steadily increased throughout the sampling period (May to September). While 2016 showed a similar trend from the start of sampling (July) to September but a decline in October (end of sampling). Such a study has not been conducted previously, and could potentially provide valuable insights about the epidemiology of the virus complex causing CLCuD and possible means of controlling losses due to it.

Maintenance of persistent transmission of a plant arbovirus in its insect vector mediated by the Toll-Dorsal immune pathway

Throughout evolution, arboviruses have developed various strategies to counteract the host's innate immune defenses to maintain persistent transmission. Recent studies have shown that, in addition to bacteria and fungi, the innate Toll-Dorsal immune system also plays an essential role in preventing viral infections in invertebrates. However, whether the classical Toll immune pathway is involved in maintaining the homeostatic process to ensure the persistent and propagative transmission of arboviruses in insect vectors remain unclear. In this study, we revealed that the transcription factor Dorsal is actively involved in the antiviral defense of an insect vector (Laodelphax striatellus) by regulating the target gene, zinc finger protein 708 (LsZN708), which mediates downstream immune-related effectors against infection with the plant virus (Rice stripe virus, RSV). In contrast, an antidefense strategy involving the use of the nonstructural-protein (NS4) to antagonize host antiviral defense through competitive binding to Dorsal from the MSK2 kinase was employed by RSV; this competitive binding inhibited Dorsal phosphorylation and reduced the antiviral response of the host insect. Our study revealed the molecular mechanism through which Toll-Dorsal-ZN708 mediates the maintenance of an arbovirus homeostasis in insect vectors. Specifically, ZN708 is a newly documented zinc finger protein targeted by Dorsal that mediates the downstream antiviral response. This study will contribute to our understanding of the successful transmission and spread of arboviruses in plant or invertebrate hosts.

Plant virology in the 21st century in China: Recent advances and future directions

Plant viruses are a group of intracellular pathogens that persistently threaten global food security. Significant advances in plant virology have been achieved by Chinese scientists over the last 20 years, including basic research and technologies for preventing and controlling plant viral diseases. Here, we review these milestones and advances, including the identification of new crop-infecting viruses, dissection of pathogenic mechanisms of multiple viruses, examination of multilayered interactions among viruses, their host plants, and virus-transmitting arthropod vectors, and in-depth interrogation of plant-encoded resistance and susceptibility determinants. Notably, various plant virus-based vectors have also been successfully developed for gene function studies and target gene expression in plants. We also recommend future plant virology studies in China.

Reassortments in single-stranded DNA multipartite viruses: Confronting expectations based on molecular constraints with field observations

Single-stranded DNA multipartite viruses, which mostly consist of members of the genus Begomovirus, family Geminiviridae, and all members of the family Nanoviridae, partly resolve the cost of genomic integrity maintenance through two remarkable capacities. They are able to systemically infect a host even when their genomic segments are not together in the same host cell, and these segments can be separately transmitted by insect vectors from host to host. These capacities potentially allow such viruses to reassort at a much larger spatial scale, since reassortants could arise from parental genotypes that do not co-infect the same cell or even the same host. To assess the limitations affecting reassortment and their implications in genome integrity maintenance, the objective of this review is to identify putative molecular constraints influencing reassorted segments throughout the infection cycle and to confront expectations based on these constraints with empirical observations. Trans-replication of the reassorted segments emerges as the major constraint, while encapsidation, viral movement, and transmission compatibilities appear more permissive. Confronting the available molecular data and the resulting predictions on reassortments to field population surveys reveals notable discrepancies, particularly a surprising rarity of interspecific natural reassortments within the Nanoviridae family. These apparent discrepancies unveil important knowledge gaps in the biology of ssDNA multipartite viruses and call for further investigation on the role of reassortment in their biology.

Plant reoviruses hijack autophagy in insect vectors

Plant reoviruses, transmitted only by insect vectors, seriously threaten global cereal production. Understanding how insect vectors efficiently transmit the viruses is key to controlling the viral diseases. Autophagy commonly plays important roles in plant host defense against virus infection, but recent studies have shown that plant reoviruses can hijack the autophagy pathway in insect cells to enable their persistence in the insect and continued transmission to plants. Here, we summarize and discuss new insights on viral activation, evasion, regulation, and manipulation of autophagy within the insect vectors and the role of autophagy in virus survival in insect vectors. Deeper knowledge of the functions of autophagy in vectors may lead to novel strategies for blocking transmission of insect-borne plant viruses.

Potato psyllids mount distinct gut responses against two different 'Candidatus Liberibacter solanacearum' haplotypes

'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.

VDAC1 balances mitophagy and apoptosis in leafhopper upon arbovirus infection

Mitophagy is a form of autophagy that selectively removes damaged mitochondria and attenuates mitochondrial-dependent apoptosis during viral infection, but how arboviruses balance mitophagy and apoptosis to facilitate persistent viral infection in insect vectors without causing evident fitness cost remains elusive. Here, we identified mitochondrial VDAC1 (voltage-dependent anion channel 1) that could be hijacked by nonstructural protein Pns11 of rice gall dwarf virus (RGDV), a plant nonenveloped double-stranded RNA virus, to synergistically activate pro-viral extensive mitophagy and limited apoptosis in leafhopper vectors. The direct target of fibrillar structures constructed by Pns11 with VDAC1 induced mitochondrial degeneration. Moreover, the degenerated mitochondria were recruited into Pns11-induced phagophores to initiate mitophagy via interaction of VDAC1 with Pns11 and an autophagy protein, ATG8. Such mitophagy mediated by Pns11 and VDAC1 required the classical PRKN/Parkin-PINK1 pathway. VDAC1 regulates apoptosis by controlling the release of apoptotic signaling molecules through its pore, while the anti-apoptotic protein GSN (gelsolin) could bind to VDAC1 pore. We demonstrated that the interaction of Pns11 with VDAC1 and gelsolin decreased VDAC1 expression but increased GSN expression, which prevented the extensive apoptotic response in virus-infected regions. Meanwhile, virus-induced mitophagy also effectively prevented extensive apoptotic response to decrease apoptosis-caused insect fitness cost. The subsequent fusion of virus-loaded mitophagosomes with lysosomes is prevented, and thus such mitophagosomes are exploited for persistent spread of virions within insect bodies. Our results reveal a new strategy for arboviruses to balance and exploit mitophagy and apoptosis, resulting in an optimal intracellular environment for persistent viral propagation in insect vectors.Abbreviations: ATG: autophagy related; BNIP3: BCL2 interacting protein 3; CYCS/CytC: cytochrome c, somatic; dsGSN: double-stranded RNAs targeting GSN/gelsolin; dsGFP: double-stranded RNAs targeting green fluorescent protein; dsPRKN: double-stranded RNAs targeting PRKN; dsPns11: double-stranded RNAs targeting Pns11; dsRNA: double-stranded RNA; EC: epithelia cell; GST: glutathione S-transferase; LAMP1: lysosomal associated membrane protein 1; Mito: mitochondrion; Mmg: middle midgut; MP, mitophagosome; PG, phagophore. padp: post-first access to diseased plants; PINK1: PTEN induced kinase 1; RGDV: rice gall dwarf virus; SQSTM1: sequestosome 1; TOMM20: translocase of outer mitochondrial membrane 20; TUNEL: terminal deoxynucleotidyl transferase dUTP nick end labeling; VDAC1: voltage dependent anion channel 1.

Begomoviruses: what is the secret(s) of their success?

Begomoviruses constitute an extremely successful group of emerging plant viruses transmitted by whiteflies of the Bemisia tabaci complex. Hosts include important vegetable, root, and fiber crops grown in the tropics and subtropics. Factors contributing to the ever-increasing diversity and success of begomoviruses include their predisposition to recombine their genomes, interaction with DNA satellites recruited throughout their evolution, presence of wild plants as a virus reservoir and a source of speciation, and extreme polyphagia and continuous movement of the insect vectors to temperate regions. These features as well as some controversial issues (replication in the insect vector, putative seed transmission, transmission by insects other than B. tabaci, and expansion of the host range to monocotyledonous plants) will be analyzed in this review.

Zinc finger protein 330 regulates Ramie mosaic virus infection in the whitefly Bemisia tabaci MED

BACKGROUND

The whitefly, Bemisia tabaci (Gennadius) is one of the most economically important pests that cause serious damage to agricultural production by transmitting plant pathogenic viruses. Approximately 90% of the virus species transmitted by the whitefly are members of the genus begomovirus. Ramie mosaic virus (RaMoV) is a new bipartite begomovirus that causes severe damage to ramie and several other economic crops in China. In previous studies, we have demonstrated that RaMoV had no obvious direct or indirect effects on B. tabaci. However, whether B. tabaci affects RaMoV infection and the molecular mechanisms of their interaction remain unclear.

RESULTS

Here, we identified a zinc finger protein 330 (ZNF330) in B. tabaci MED interacted with the coat protein (CP) of RaMoV by the yeast two-hybrid assay. Then the interaction between ZNF330 and RaMoV CP was further verified by glutathione S-transferase (GST) pull-down assay. The expression of ZNF330 gene was continuously induced after RaMoV infection. ZNF330 negatively regulated RaMoV replication in the B. tabaci MED. Furthermore, the longevity and fecundity of RaMoV-infected female adults were significantly decreased after silencing of ZNF330.

CONCLUSIONS

Our results indicated that the ZNF330 protein was involved in the negative regulation of RaMoV replication in the B. tabaci MED. High viral accumulation caused by ZNF330 silencing is detrimental to fecundity and longevity of the B. tabaci MED. These findings provided a new insight into identifying the binding partners in whitefly with viral CP and fully understanding the complex interactions between begomoviruses and their whitefly vector. © 2023 Society of Chemical Industry.

GAPDH mediates plant reovirus-induced incomplete autophagy for persistent viral infection in leafhopper vector

Macroautophagy/autophagy is a conserved mechanism launched by host organisms to fight against virus infection. Double-membraned autophagosomes in arthropod vectors can be remodeled by arboviruses to accommodate virions and facilitate persistent viral propagation, but the underlying mechanism is unknown. Rice gall dwarf virus (RGDV), a plant nonenveloped double-stranded RNA virus, induces the formation of virus-containing double-membraned autophagosomes to benefit persistent viral propagation in leafhopper vectors. In this study, it was found that the capsid protein P2 of RGDV alone induced autophagy. P2 specifically interacted with GAPDH (glyceraldehyde-3-phosphate dehydrogenase) and ATG4B both in vitro and in vivo. Furthermore, the GAPDH-ATG4B complex could be recruited to virus-induced autophagosomes. Silencing of GAPDH or ATG4B expression suppressed ATG8 lipidation, autophagosome formation, and efficient viral propagation. Thus, P2 could directly recruit the GAPDH-ATG4B complex to induce the formation of initial autophagosomes. Furthermore, such autophagosomes were modified to evade fusion with lysosomes for degradation, and thus could be persistently exploited by viruses to facilitate efficient propagation. GAPDH bound to ATG14 and inhibited the interaction of ATG14 with SNAP29, thereby preventing ATG14-SNARE proteins from mediating autophagosome-lysosome fusion. Taken together, these results highlight how RGDV activates GAPDH to initiate autophagosome formation and block autophagosome degradation, finally facilitating persistent viral propagation in insect vectors. The findings reveal a positive regulation of immune response in insect vectors during viral infection.

A plant virus hijacks phosphatidylinositol-3,5-bisphosphate to escape autophagic degradation in its insect vector

Hosts can initiate macroautophagy/autophagy as an antiviral defense response, while viruses have developed multiple ways to evade the host autophagic degradation. However, little is known as to whether viruses can target lipids to subvert autophagic degradation. Here, we show that a low abundant signaling lipid, phosphatidylinositol 3,5-bisphosphate (PtdIns(3,5)P2), is required for rice black-streaked dwarf virus (RBSDV) to evade the autophagic degradation in the insect vector Laodelphax striatellus. RBSDV binds to PtdIns(3,5)P2 and elevates its level through its main capsid protein P10, leading to inhibited autophagy and promoted virus propagation. Furthermore, we show that PtdIns(3,5)P2 inhibits the autophagy pathway by preventing the fusion of autophagosomes and lysosomes through activation of Trpml (transient receptor potential cation channel, mucolipin), an effector of PtdIns(3,5)P2. These findings uncover a strategy whereby a plant virus hijacks PtdIns(3,5)P2 via its viral capsid protein to evade autophagic degradation and promote its survival in insects.

Genome-wide identification and expression analysis of wall-associated kinase (WAK) and WAK-like kinase gene family in response to tomato yellow leaf curl virus infection in Nicotiana benthamiana

BACKGROUND

Tomato yellow leaf curl virus (TYLCV) is a major monopartite virus in the family Geminiviridae and has caused severe yield losses in tomato and tobacco planting areas worldwide. Wall-associated kinases (WAKs) and WAK-like kinases (WAKLs) are a subfamily of the receptor-like kinase family implicated in cell wall signaling and transmitting extracellular signals to the cytoplasm, thereby regulating plant growth and development and resistance to abiotic and biotic stresses. Recently, many studies on WAK/WAKL family genes have been performed in various plants under different stresses; however, identification and functional survey of the WAK/WAKL gene family of Nicotiana benthamiana have not yet been performed, even though its genome has been sequenced for several years. Therefore, in this study, we aimed to identify the WAK/WAKL gene family in N. benthamiana and explore their possible functions in response to TYLCV infection.

RESULTS

Thirty-eight putative WAK/WAKL genes were identified and named according to their locations in N. benthamiana. Phylogenetic analysis showed that NbWAK/WAKLs are clustered into five groups. The protein motifs and gene structure compositions of NbWAK/WAKLs appear to be highly conserved among the phylogenetic groups. Numerous cis-acting elements involved in phytohormone and/or stress responses were detected in the promoter regions of NbWAK/WAKLs. Moreover, gene expression analysis revealed that most of the NbWAK/WAKLs are expressed in at least one of the examined tissues, suggesting their possible roles in regulating the growth and development of plants. Virus-induced gene silencing and quantitative PCR analyses demonstrated that NbWAK/WAKLs are implicated in regulating the response of N. benthamiana to TYLCV, ten of which were dramatically upregulated in locally or systemically infected leaves of N. benthamiana following TYLCV infection.

CONCLUSIONS

Our study lays an essential base for the further exploration of the potential functions of NbWAK/WAKLs in plant growth and development and response to viral infections in N. benthamiana.

Autophagy mediates a direct synergistic interaction during co-transmission of two distinct arboviruses by insect vectors

Multiple viral infections in insect vectors with synergistic effects are common in nature, but the underlying mechanism remains elusive. Here, we find that rice gall dwarf reovirus (RGDV) facilitates the transmission of rice stripe mosaic rhabdovirus (RSMV) by co-infected leafhopper vectors. RSMV nucleoprotein (N) alone activates complete anti-viral autophagy, while RGDV nonstructural protein Pns11 alone induces pro-viral incomplete autophagy. In co-infected vectors, RSMV exploits Pns11-induced autophagosomes to assemble enveloped virions via N-Pns11-ATG5 interaction. Furthermore, RSMV could effectively propagate in Sf9 cells. Expression of Pns11 in Sf9 cells or leafhopper vectors causes the recruitment of N from the ER to Pns11-induced autophagosomes and inhibits N-induced complete autophagic flux, finally facilitating RSMV propagation. In summary, these results demonstrate a previously unappreciated role of autophagy in the regulation of the direct synergistic interaction during co-transmission of two distinct arboviruses by insect vectors and reveal the functional importance of virus-induced autophagosomes in rhabdovirus assembly.

A plant nonenveloped double-stranded RNA virus activates and co-opts BNIP3-mediated mitophagy to promote persistent infection in its insect vector

Mitophagy that selectively eliminates damaged mitochondria is an essential mitochondrial quality control mechanism. Recently, mitophagy has been shown to be induced in host cells infected by a few animal viruses. Here, we report that southern rice black-streaked dwarf virus (SRBSDV), a plant nonenveloped double-stranded RNA virus, can also trigger mitophagy in its planthopper vector to prevent mitochondria-dependent apoptosis and promote persistent viral propagation. We find that the fibrillar structures constructed by the nonstructural protein P7-1 of SRBSDV directly target mitochondria via interaction with the mitophagy receptor BNIP3 (BCL2 interacting protein 3), and these mitochondria are then sequestered within autophagosomes to form mitophagosomes. Moreover, SRBSDV infection or P7-1 expression alone can promote BNIP3 dimerization on the mitochondria, and induce autophagy via the P7-1-ATG8 interaction. Furthermore, SRBSDV infection stimulates the phosphorylation of AMP-activated protein kinase (AMPK), resulting in BNIP3 phosphorylation via the AMPKα-BNIP3 interaction. Together, P7-1 induces BNIP3-mediated mitophagy by promoting the formation of phosphorylated BNIP3 dimers on the mitochondria. Silencing of ATG8, BNIP3, or AMPKα significantly reduces virus-induced mitophagy and viral propagation in insect vectors. These data suggest that in planthopper, SRBSDV-induced mitophagosomes are modified to accommodate virions and facilitate persistent viral propagation. In summary, our results demonstrate a previously unappreciated role of a viral protein in the induction of BNIP3-mediated mitophagy by bridging autophagosomes and mitochondria and reveal the functional importance of virus-induced mitophagy in maintaining persistent viral infection in insect vectors.Abbreviations: AMPK: AMP-activated protein kinase; ATG: autophagy related; BNIP3: BCL2 interacting protein 3; CASP3: caspase 3; dsRNA: double strand RNA; ER: endoplasmic reticulum; FITC: fluorescein isothiocyanate; FKBP8: FKBP prolyl isomerase 8; FUNDC1: FUN14 domain containing 1; GFP: green fluorescent protein; GST: glutathione S-transferase; padp: post-first access to diseased plants; Phos-tag: Phosphate-binding tag; PINK1: PTEN induced kinase 1; Sf9: Spodoptera frugiperda; SQSTM1: sequestosome 1; SRBSDV: southern rice black-streaked dwarf virus; STK11/LKB1: serine/threonine kinase 11; TOMM20: translocase of outer mitochondrial membrane 20; RBSDV: rice black-streaked dwarf virus; TUNEL: terminal deoxynucleotidyl dUTP nick end labeling; ULK1: unc-51 like autophagy activating kinase 1; VDAC1: voltage dependent anion channel 1.

Treatment of Rapamycin and Evaluation of an Autophagic Response in the Gut of Bactericera cockerelli (Sulč)

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.

Southern rice black-streaked dwarf virus induces incomplete autophagy for persistence in gut epithelial cells of its vector insect

Autophagy plays an important role in virus infection of the host, because viral components and particles can be degraded by the host's autophagy and some viruses may be able to hijack and subvert autophagy for its benefit. However, details on the mechanisms that govern autophagy for immunity against viral infections or benefit viral survival remain largely unknown. Plant reoviruses such as southern rice black-streaked dwarf virus (SRBSDV), which seriously threaten crop yield, are only transmitted by vector insects. Here, we report a novel mechanism by which SRBSDV induces incomplete autophagy by blocking autophagosome-lysosome fusion, resulting in viral accumulation in gut epithelial cells of its vector, white-backed planthopper (Sogatella furcifera). SRBSDV infection leads to stimulation of the c-Jun N-terminal kinase (JNK) signaling pathway, which further activates autophagy. Mature and assembling virions were found close to the edge7 of the outer membrane of autophagosomes. Inhibition autophagy leads to the decrease of autophagosomes, which resulting in impaired maturation of virions and the decrease of virus titer, whereas activation of autophagy facilitated virus titer. Further, SRBSDV inhibited fusion of autophagosomes and lysosomes by interacting with lysosomal-associated membrane protein 1 (LAMP1) using viral P10. Thus, SRBSDV not only avoids being degrading by lysosomes, but also further hijacks these non-fusing autophagosomes for its subsistence. Our findings reveal a novel mechanism of reovirus persistence, which can explain why SRBSDV can be acquired and transmitted rapidly by its insect vector.

Autophagy Is Required to Sustain Increased Intestinal Cell Proliferation during Phenotypic Plasticity Changes in Honey Bee (Apis mellifera)

Tissue phenotypic plasticity facilitates rapid adaptation of organisms to biotic and/or abiotic pressure. The reproductive capacity of honey bee workers (Apis mellifera) is plastic and responsive to pheromones produced by broods and the queen. Egg laying workers (ELWs), which could reactivate their ovaries and lay haploid eggs upon queen lost, have been commonly discussed from many aspects. However, it remains unclear whether midgut homeostasis in ELWs is affected during plastic changes. Here, we found that the expression of nutrition- and autophagy-related genes was up-regulated in the midguts of ELWs, compared with that in nurse workers (NWs) by RNA-sequencing. Furthermore, the area and number of autophagosomes were increased, along with significantly increased cell death in the midguts of ELWs. Moreover, cell cycle progression in the midguts of ELWs was increased compared with that in NWs. Consistent with the up-regulation of nutrition-related genes, the body and midgut sizes, and the number of intestinal proliferation cells of larvae reared with royal jelly (RJ) obviously increased more than those reared without RJ in vitro. Finally, cell proliferation was dramatically suppressed in the midguts of ELWs when autophagy was inhibited. Altogether, our data suggested that autophagy was induced and required to sustain cell proliferation in ELWs' midguts, thereby revealing the critical role of autophagy played in the intestines during phenotypic plasticity changes.

Vector acquisition and co-inoculation of two plant viruses influences transmission, infection, and replication in new hosts

This study investigated the role of vector acquisition and transmission on the propagation of single and co-infections of tomato yellow leaf curl virus (TYLCV,) and tomato mottle virus (ToMoV) (Family: Geminiviridae, Genus: Begomovirus) by the whitefly vector Bemisia tabaci MEAM1 (Gennadius) in tomato. The aim of this research was to determine if the manner in which viruses are co-acquired and co-transmitted changes the probability of acquisition, transmission and new host infections. Whiteflies acquired virus by feeding on singly infected plants, co-infected plants, or by sequential feeding on singly infected plants. Viral titers were also quantified by qPCR in vector cohorts, in artificial diet, and plants after exposure to viruliferous vectors. Differences in transmission, infection status of plants, and titers of TYLCV and ToMoV were observed among treatments. All vector cohorts acquired both viruses, but co-acquisition/co-inoculation generally reduced transmission of both viruses as single and mixed infections. Co-inoculation of viruses by the vector also altered virus accumulation in plants regardless of whether one or both viruses were propagated in new hosts. These findings highlight the complex nature of vector-virus-plant interactions that influence the spread and replication of viruses as single and co-infections.

Cotton leaf curl Multan virus differentially regulates innate antiviral immunity of whitefly (Bemisia tabaci) vector to promote cryptic species-dependent virus acquisition

Begomoviruses represent the largest group of economically important, highly pathogenic, DNA plant viruses that contribute a substantial amount of global crop disease burden. The exclusive transmission of begomoviruses by whiteflies (Bemisia tabaci) requires them to interact and efficiently manipulate host responses at physiological, biological and molecular scales. However, the molecular mechanisms underlying complex begomovirus-whitefly interactions that consequently substantiate efficient virus transmission largely remain unknown. Previously, we found that whitefly Asia II 7 cryptic species can efficiently transmit cotton leaf curl Multan virus (CLCuMuV) while MEAM1 cryptic species is a poor carrier and incompetent vector of CLCuMuV. To investigate the potential mechanism/s that facilitate the higher acquisition of CLCuMuV by its whitefly vector (Asia II 7) and to identify novel whitefly proteins that putatively interact with CLCuMuV-AV1 (coat protein), we employed yeast two-hybrid system, bioinformatics, bimolecular fluorescence complementation, RNA interference, RT-qPCR and bioassays. We identified a total of 21 Asia II 7 proteins putatively interacting with CLCuMuV-AV1. Further analyses by molecular docking, Y2H and BiFC experiments validated the interaction between a whitefly innate immunity-related protein (BTB/POZ) and viral AV1 (coat protein). Gene transcription analysis showed that the viral infection significantly suppressed the transcription of BTB/POZ and enhanced the accumulation of CLCuMuV in Asia II 7, but not in MEAM1 cryptic species. In contrast to MEAM1, the targeted knock-down of BTB/POZ substantially reduced the ability of Asia II 7 to acquire and accumulate CLCuMuV. Additionally, antiviral immune signaling pathways (Toll, Imd, Jnk and Jak/STAT) were significantly suppressed following viral infection of Asia II 7 whiteflies. Taken together, the begomovirus CLCuMuV potentiates efficient virus accumulation in its vector B. tabaci Asia II 7 by targeting and suppressing the transcription of an innate immunity-related BTB/POZ gene and other antiviral immune responses in a cryptic species-specific manner.

F-actin dynamics in midgut cells enables virus persistence in vector insects

Hemipteran insects that transmit plant viruses in a persistent circulative manner acquire, retain and transmit viruses for their entire life. The mechanism enabling this persistence has remained unclear for many years. Here, we determined how wheat dwarf virus (WDV) persists in its leafhopper vector Psammotettix alienus. We found that WDV caused the up-regulation of actin-depolymerizing factor (ADF) at the mRNA and protein levels in the midgut cells of leafhoppers after experiencing a WDV acquisition access period (AAP) of 6, 12 or 24 h. Experimental inhibition of F-actin depolymerization by jasplakinolide and dsRNA injection led to lower virus accumulation levels and transmission efficiencies, suggesting that depolymerization of F-actin regulated by ADF is essential for WDV invasion of midgut cells. Exogenous viral capsid protein (CP) inhibited ADF depolymerization of actin filaments in vitro and in Spodoptera frugiperda 9 (Sf9) cells because the CP competed with actin to bind ADF and then blocked actin filament disassembly. Interestingly, virions colocalized with ADF after a 24-h AAP, just as actin polymerization occurred, indicating that the binding of CP with ADF affects the ability of ADF to depolymerize F-actin, inhibiting WDV entry. Similarly, the luteovirus barley yellow dwarf virus also induced F-actin depolymerization and then polymerization in the gut cells of its vector Schizaphis graminum. Thus, F-actin dynamics are altered by nonpropagative viruses in midgut cells to enable virus persistence in vector insects.

A balance between vector survival and virus transmission is achieved through JAK/STAT signaling inhibition by a plant virus

Viruses pose a great threat to animal and plant health worldwide, with many being dependent on insect vectors for transmission between hosts. While the virus-host arms race has been well established, how viruses and insect vectors adapt to each other remains poorly understood. Begomoviruses comprise the largest genus of plant-infecting DNA viruses and are exclusively transmitted by the whitefly Bemisia tabaci. Here, we show that the vector Janus kinase/signal transducer and activator of transcription (JAK/STAT) pathway plays an important role in mediating the adaptation between the begomovirus tomato yellow leaf curl virus (TYLCV) and whiteflies. We found that the JAK/STAT pathway in B. tabaci functions as an antiviral mechanism against TYLCV infection in whiteflies as evidenced by the increase in viral DNA and coat protein (CP) levels after inhibiting JAK/STAT signaling. Two STAT-activated effector genes, BtCD109-2 and BtCD109-3, mediate this anti-TYLCV activity. To counteract this vector immunity, TYLCV has evolved strategies that impair the whitefly JAK/STAT pathway. Infection of TYLCV is associated with a reduction of JAK/STAT pathway activity in whiteflies. Moreover, TYLCV CP binds to STAT and blocks its nuclear translocation, thus, abrogating the STAT-dependent transactivation of target genes. We further show that inhibition of the whitefly JAK/STAT pathway facilitates TYLCV transmission but reduces whitefly survival and fecundity, indicating that this JAK/STAT-dependent TYLCV-whitefly interaction plays an important role in keeping a balance between whitefly fitness and TYLCV transmission. This study reveals a mechanism of plant virus-insect vector coadaptation in relation to vector survival and virus transmission.

The Role of Mosquito Hemocytes in Viral Infections

Insect hemocytes are the only immune cells that can mount a humoral and cellular immune response. Despite the critical involvement of hemocytes in immune responses against bacteria, fungi, and parasites in mosquitoes, our understanding of their antiviral potential is still limited. It has been shown that hemocytes express humoral factors such as TEP1, PPO, and certain antimicrobial peptides that are known to restrict viral infections. Insect hemocytes also harbor the major immune pathways, such as JAK/STAT, TOLL, IMD, and RNAi, which are critical for the control of viral infection. Recent research has indicated a role for hemocytes in the regulation of viral infection through RNA interference and autophagy; however, the specific mechanism by which this regulation occurs remains uncharacterized. Conversely, some studies have suggested that hemocytes act as agonists of arboviral infection because they lack basal lamina and circulate throughout the whole mosquito, likely facilitating viral dissemination to other tissues such as salivary glands. In addition, hemocytes produce arbovirus agonist factors such as lectins, which enhance viral infection. Here, we summarize our current understanding of hemocytes’ involvement in viral infections.

Identification of a Novel Densovirus in Aphid, and Uncovering the Possible Antiviral Process During Its Infection

Densoviruses (DVs) are single-stranded DNA viruses and exclusively happen in invertebrates. Most of DVs reported in insects are pathogenic to their native hosts, however, no pathogenic effect of them has been examined in vertebrates. Hence, DVs are the potential agents used in pest managements. Aphids are the primary vectors of plant viruses. In this study, we identified a novel DV in Chinese Sitobion miscanthi population, provisionally named “Sitobion miscanthi densovirus” (SmDV). Taxonomically, SmDV belongs to genus Hemiambidensovirus. In S. miscanthi, SmDV is hosted in diverse cells and can be horizontally transmitted via wheat feeding. Subject to SmDV, aphids activate their intrinsic antiviral autophagy pathway. Grouped with ascorbate and aldarate metabolism, chlorophyll metabolism, p450 related drug metabolism, and retinoid metabolism, aphids form a complex immune network response to the infection of SmDV. Obviously, it works as elder aphids still alive even they contain the highest examined concentration of SmDV. This study provides a foundation for the identifications of novel DVs, and further improves the understanding of the molecular interactions between insects and DVs.

Silencing of the Prophenoloxidase Gene BtPPO1 Increased the Ability of Acquisition and Retention of Tomato chlorosis virus by Bemisia tabaci

Tomato chlorosis virus (ToCV) has seriously impacted tomato production around the world. ToCV is semi-persistently transmitted by the whitefly, Bemisia tabaci, which is a serious agricultural pest in the world. However, the interaction mechanism between ToCV and its whitefly vector is still poorly understood. Our previous transcriptome analysis demonstrated that the expression level of an immune-related gene, prophenoloxidase (PPO), in B. tabaci increased after ToCV acquisition, which indicates that the PPO may be involved in the interaction mechanism between the ToCV and its vector. To determine the role of the PPO in the acquisition and retention of ToCV by B. tabaci, we cloned the complete Open Reading Frames (ORF) of the BtPPOs (BtPPO1 and BtPPO2), and then structure and phylogenetic analyses were performed. BtPPOs were closely related to the PPO genes of Hemiptera insects. Spatial-temporal expression detection was qualified by using reverse transcription quantitative PCR (RT-qPCR), and this revealed that BtPPOs were expressed in all tissues and developmental stages. We found that only BtPPO1 was significantly upregulated after B. tabaci acquired ToCV for 12 and 24 h. According to the paraffin-fluorescence probe-fluorescence in situ hybridization (FISH) experiment, we verified that ToCV and BtPPO1 were co-located in the thorax of B. tabaci, which further revealed the location of their interaction. Finally, the effects of the BtPPOs on ToCV acquisition and retention by B. tabaci were determined using RNA interference (RNAi). The results showed that the RNAi of the responsive gene (BtPPO1) significantly increased the titer of ToCV in B. tabaci. These results demonstrate that BtPPO1 participates in ToCV acquisition and retention by B. tabaci.

Effects of heat stress on virus transmission and virus-mediated apoptosis in whitefly Bemisia tabaci

Tomato yellow leaf curl virus (TYLCV), a plant DNA virus of the genus Begomovirus, is transmitted by whiteflies of the Bemisia tabaci species complex in a persistent manner. Our previous study indicated that activation of the apoptosis pathway in whiteflies could facilitate TYLCV accumulation and transmission. Considering that temperature change can influence the spread of insect-borne plant viruses, we focused on plant virus induced-apoptosis to investigate the underlying mechanism of temperature regulation on plant virus transmission via an insect vector. We found that heat stress (40°C) on whiteflies could facilitate TYLCV accumulation and increase transmission to tomato plants. Despite upregulation of caspase-1 and caspase-3 gene expression, heat stress failed to induce an increase in the activation of cleaved caspase-3 and DNA fragmentation in TYLCV-infected whiteflies. However, our data failed to determine the role of heat stress in apoptosis modulation of insect-plant virus interplay while still providing clues to understand insect vectors and their transmitted plant viruses.

A nonstructural protein encoded by a rice reovirus induces an incomplete autophagy to promote viral spread in insect vectors

Viruses can hijack autophagosomes as the nonlytic release vehicles in cultured host cells. However, how autophagosome-mediated viral spread occurs in infected host tissues or organs in vivo remains poorly understood. Here, we report that an important rice reovirus, rice gall dwarf virus (RGDV) hijacks autophagosomes to traverse multiple insect membrane barriers in the midgut and salivary gland of leafhopper vector to enhance viral spread. Such virus-containing double-membraned autophagosomes are prevented from degradation, resulting in increased viral propagation. Mechanistically, viral nonstructural protein Pns11 induces autophagy and embeds itself in the autophagosome membranes. The autophagy-related protein 5 (ATG5)-ATG12 conjugation is essential for initial autophagosome membrane biogenesis. RGDV Pns11 specifically interacts with ATG5, both in vitro and in vivo. Silencing of ATG5 or Pns11 expression suppresses ATG8 lipidation, autophagosome formation, and efficient viral propagation. Thus, Pns11 could directly recruit ATG5-ATG12 conjugation to induce the formation of autophagosomes, facilitating viral spread within the insect bodies. Furthermore, Pns11 potentially blocks autophagosome degradation by directly targeting and mediating the reduced expression of N-glycosylated Lamp1 on lysosomal membranes. Taken together, these results highlight how RGDV remodels autophagosomes to benefit viral propagation in its insect vector.

Numerous plant viruses replicate inside the cells of their insect vectors. Here, we demonstrate that the progeny virions of rice gall dwarf virus in leafhopper vector are engulfed within virus-induced double-membraned autophagosomes. Such autophagosomes are modified to evade degradation, thus can be persistently exploited by viruses to safely transport virions across multiple insect membrane barriers. Viral nonstructural protein Pns11 induces the formation of autophagosomes via interaction with ATG5, and potentially blocks autophagosome degradation via mediating the reduced expression of N-glycosylated Lamp1 on lysosomal membranes. For the first time, we reveal that a nonstructural protein encoded by a persistent plant virus can induce an incomplete autophagy to benefit viral propagation in its insect vectors.

Silencing the Autophagy-Related Genes ATG3 and ATG9 Promotes SRBSDV Propagation and Transmission in Sogatella furcifera

Autophagy plays diverse roles in the interaction among pathogen, vector, and host. In the plant virus and insect vector system, autophagy can be an antiviral/pro-viral factor to suppress/promote virus propagation and transmission. Here, we report the antiviral role of autophagy-related genes ATG3 and ATG9 in the white-backed planthopper (Sogatella furcifera) during the process of transmitting the southern rice black-streaked dwarf virus (SRBSDV). In this study, we annotated two autophagy-related genes, SfATG3 and SfATG9, from the female S. furcifera transcriptome. The cDNA of SfATG3 and SfATG9 comprised an open reading frame (ORF) of 999 bp and 2295 bp that encodes a protein of 332 and 764 amino acid residues, respectively. SfATG3 has two conserved domains and SfATG9 has one conserved domain. In S. furcifera females exposed to SRBSDV, expression of autophagy-related genes was significantly activated and shared similar temporal patterns to those of SRBSDV S9-1 and S10, all peaking at 4 d post viral exposure. Silencing the expression of SfATG3 and SfATG9 promoted SRBSDV propagation and transmission. This study provides evidence for the first time that S. furcifera autophagy-related genes ATG3 and ATG9 play an antiviral role to suppress SRBSDV propagation and transmission.

Autophagy in plant viral infection

Autophagy is a conserved degradation pathway that delivers dysfunctional cellular organelles or other cytosol components to degradative vesicular structures (vacuoles in plants and yeasts, lysosomes in mammals) for degradation and recycling. Viruses are intracellular parasites that hijack their host to live. Research on regulation of the trade-off between plant cells and viruses has indicated that autophagy is an integral part of the host responses to virus infection. Meanwhile, plants have evolved a diverse array of defense responses to counter pathogenic viruses. In this review, we focus on the roles of autophagy in plant virus infection and offer a glimpse of recent advances about how plant viruses evade autophagy or manipulate host autophagy pathways to complete their replication cycle.

A spotlight on non-host resistance to plant viruses

Plant viruses encounter a range of host defenses including non-host resistance (NHR), leading to the arrest of virus replication and movement in plants. Viruses have limited host ranges, and adaptation to a new host is an atypical phenomenon. The entire genotypes of plant species which are imperceptive to every single isolate of a genetically variable virus species are described as non-hosts. NHR is the non-specific resistance manifested by an innately immune non-host due to pre-existing and inducible defense responses, which cannot be evaded by yet-to-be adapted plant viruses. NHR-to-plant viruses are widespread, but the phenotypic variation is often not detectable within plant species. Therefore, molecular and genetic mechanisms of NHR need to be systematically studied to enable exploitation in crop protection. This article comprehensively describes the possible mechanisms of NHR against plant viruses. Also, the previous definition of NHR to plant viruses is insufficient, and the main aim of this article is to sensitize plant pathologists to the existence of NHR to plant viruses and to highlight the need for immediate and elaborate research in this area.

PEBP balances apoptosis and autophagy in whitefly upon arbovirus infection

Apoptosis and autophagy are two common forms of programmed cell death (PCD) used by host organisms to fight against virus infection. PCD in arthropod vectors can be manipulated by arboviruses, leading to arbovirus-vector coexistence, although the underlying mechanism is largely unknown. In this study, we find that coat protein (CP) of an insect-borne plant virus TYLCV directly interacts with a phosphatidylethanolamine-binding protein (PEBP) in its vector whitefly to downregulate MAPK signaling cascade. As a result, apoptosis is activated in the whitefly increasing viral load. Simultaneously, the PEBP4-CP interaction releases ATG8, a hallmark of autophagy initiation, which reduces arbovirus levels. Furthermore, apoptosis-promoted virus amplification is prevented by agonist-induced autophagy, whereas the autophagy-suppressed virus load is unaffected by manipulating apoptosis, suggesting that the viral load is predominantly determined by autophagy rather than by apoptosis. Our results demonstrate that a mild intracellular immune response including balanced apoptosis and autophagy might facilitate arbovirus preservation within its whitefly insect vector.

Arbovirus has co-evolved with its insect vector, enabling efficient and persistent transmission by vectors. Here, the authors reveal an immune homeostatic mechanism shaped by apoptosis and autophagy that facilitates arbovirus preservation within its whitefly vector.

Identification of Autophagy-Related Genes in the Potato Psyllid, Bactericera cockerelli and Their Expression Profile in Response to 'Candidatus Liberibacter Solanacearum' in the Gut

Simple Summary

In North America, the bacterial plant pathogen ‘Candidatus Liberibacter solanacearum’ (Lso) infects solanaceous plants. Currently, Lso haplotypes, LsoA and LsoB are transmitted by potato psyllid, Bactericera cockerelli (Šulc). Because these bacteria are transmitted in a circulative and persistent manner, the gut of the psyllid is the first organ they encounter and could be a barrier to its transmission. Therefore, it is important to understand the molecular mechanisms involved in Lso acquisition and transmission. This study explored if an autophagic response was triggered in response to LsoA and/or LsoB in the gut of the adult potato psyllid. The results showed that Lso may induce the autophagic response in the adult psyllid gut since the majority of autophagy-related genes (ATGs) are sensitive and responsive to the exposure or infection of both LsoA and LsoB. Therefore, this study represents a stepping-stone towards understanding the molecular mechanisms involved in Lso transmission.

Abstract

Autophagy, also known as type II programmed cell death, is a cellular mechanism of “self-eating”. Autophagy plays an important role against pathogen infection in numerous organisms. Recently, it has been demonstrated that autophagy can be activated and even manipulated by plant viruses to facilitate their transmission within insect vectors. However, little is known about the role of autophagy in the interactions of insect vectors with plant bacterial pathogens. ‘Candidatus Liberibacter solanacearum’ (Lso) is a phloem-limited Gram-negative bacterium that infects crops worldwide. Two Lso haplotypes, LsoA and LsoB, are transmitted by the potato psyllid, Bactericera cockerelli and cause damaging diseases in solanaceous plants (e.g., zebra chip in potatoes). Both LsoA and LsoB are transmitted by the potato psyllid in a persistent circulative manner: they colonize and replicate within psyllid tissues. Following acquisition, the gut is the first organ Lso encounters and could be a barrier for transmission. In this study, we annotated autophagy-related genes (ATGs) from the potato psyllid transcriptome and evaluated their expression in response to Lso infection at the gut interface. In total, 19 ATGs belonging to 17 different families were identified. The comprehensive expression profile analysis revealed that the majority of the ATGs were regulated in the psyllid gut following the exposure or infection to each Lso haplotype, LsoA and LsoB, suggesting a potential role of autophagy in response to Lso at the psyllid gut interface.

Autophagy regulates whitefly-symbiont metabolic interactions

Nutritional symbionts are restricted to specialized host cells called bacteriocytes in various insect orders. These symbionts can provide essential nutrients to the host. However, the cellular mechanisms underlying the regulation of these insect-symbiont metabolic associations remain largely unclear. The whitefly, Bemisia tabaci MEAM1, hosts Portiera and Hamiltonella bacteria in the same bacteriocyte. In this study, the induction of autophagy by chemical treatment and gene silencing decreased symbiont titers, and essential amino acid (EAA) and B vitamin contents. In contrast, the repression of autophagy in bacteriocytes via Atg8 silencing increased symbiont titers, and amino acid and B vitamin contents. Furthermore, dietary supplementation with non-EAAs or B vitamins alleviated autophagy in whitefly bacteriocytes, elevated TOR (target of rapamycin) expression and increased symbiont titers. TOR silencing restored symbiont titers in whiteflies after dietary supplementation with B vitamins. These data suggest that Portiera and Hamiltonella evade autophagy of the whitefly bacteriocytes by activating the TOR pathway via providing essential nutrients. Taken together, we demonstrated that autophagy plays a critical role in regulating the metabolic interactions between the whitefly and two intracellular symbionts. Therefore, this study reveals that autophagy is an important cellular basis for bacteriocyte evolution and symbiosis persistence in whiteflies. The whitefly symbiosis unravels the interactions between cellular and metabolic functions of bacteriocytes. Importance Nutritional symbionts, which are restricted to specialized host cells called bacteriocytes, can provide essential nutrients for many hosts. However, the cellular mechanisms of regulation of animal-symbiont metabolic associations have been largely unexplored. Here, using the whitefly-Portiera/Hamiltonella endosymbiosis, we demonstrate autophagy regulates the symbiont titers, and thereby alters the essential amino acid and B vitamin contents. For persistence in the whitefly bacteriocytes, Portiera and Hamiltonella alleviate autophagy by activating the TOR (target of rapamycin) pathway through providing essential nutrients. Therefore, we demonstrate that autophagy plays a critical role in regulating the metabolic interactions between the whitefly and two intracellular symbionts. This study also provides insight into the cellular basis of bacteriocyte evolution and symbiosis persistence in the whitefly. The mechanisms underlying the role of autophagy in whitefly symbiosis could be widespread in many insect nutritional symbioses. These findings provide new avenue for whitefly control via regulating autophagy in the future.

Transovarial Transmission of Dolichos Yellow Mosaic Virus by Its Vector, Bemisia tabaci Asia II 1

The cultivation of dolichos bean [Lablab purpureus (L.) Sweet] has been severely affected by dolichos yellow mosaic virus (DoYMV, Begomovirus) transmitted by whitefly, Bemisia tabaci (Hemiptera: Aleyrodidae). We tested the transovarial transmission of DoYMV in next-generation B. tabaci by PCR, real-time PCR, Southern blot hybridization, and biological transmission. The eggs, laid by DoYMV-exposed B. tabaci, carry the virus in a unique pattern. Only the eggs laid in between 3 and 6 days post virus acquisition by a parent B. tabaci were DoYMV positive. When tested individually in real-time PCR, around 31-53% of the eggs carried the virus. The presence of DoYMV in ovaries and F1 eggs was further substantiated by the hybridization of a Cy3-conjugated nucleic acid probe complementary to the viral strand of DoYMV. Viral DNA was also detected in F1 adults and F2 eggs. B. tabaci progenies carried not only the DoYMV DNA but were also infective. The F1 adults transmitted DoYMV to all tested plants and produced strong yellow mosaic symptoms. An increase in viral copies from egg to nymphal stage indicated propagation of DoYMV in B. tabaci. However, the increase was for a short period and decreased thereafter. The present study provides the first evidence of transovarial transmission and propagation of a bipartite begomovirus in its vector, B. tabaci Asia II 1. The transovarial transmission and replication of DoYMV in B. tabaci have great epidemiological relevance as B. tabaci can serve as a major host of the virus to bridge the gap between the cropping seasons.

First evidence for thermal tolerance benefits of the bacterial symbiont Cardinium in an invasive whitefly, Bemisia tabaci

BACKGROUD

Cardinium symbiont is a maternally inherited bacterial endosymbiont and widely spreads in arthropods including Bemisia tabaci (Hemiptera: Aleyrodidae). However, the potential role of Cardinium played in the biology of their hosts is largely unknown. In two genetic lines (i.e. LS and SG lines) of B. tabaci MED, collected from different locations in China, we tested the effects of Cardinium on the performance of the host whitefly under a constant high temperature (31 °C) using the age-stage two-sex life table method, and explored the genes influenced by Cardinium-infection by RNA-sequencing.

RESULTS

We found that Cardinium did provide protection of B. tabaci against heat stress under 31 °C. However, there was a significant connection between Cardinium-infection and whitefly genetic backgrounds. Performance revealed that Cardinium infection can increase the longevity of both female and male adults and oviposition periods in both lines, but it also conferred benefits of fecundity and pre-adult period to LS line. Additionally, the population parameters such as intrinsic rate of increase (r), finite rate of increase (λ) and mean generation time (T) demonstrated that Cardinium infection conferred fitness benefits to LS line but not to SG line. Transcriptome analysis indicated that several genes related to homeostasis and metamorphosis such as ubiquitin-related genes were highly expressed in Cardinium-infected B. tabaci.

CONCLUSION

The research provided the first evidence that Cardinium can increase the thermal tolerance of whitefly, which may be associated with host genetic background.

Factors Determining Transmission of Persistent Viruses by Bemisia tabaci and Emergence of New Virus-Vector Relationships

Many plant viruses depend on insect vectors for their transmission and dissemination. The whitefly Bemisia tabaci (Hemiptera: Aleyrodidae) is one of the most important virus vectors, transmitting more than four hundred virus species, the majority belonging to begomoviruses (Geminiviridae), with their ssDNA genomes. Begomoviruses are transmitted by B. tabaci in a persistent, circulative manner, during which the virus breaches barriers in the digestive, hemolymph, and salivary systems, and interacts with insect proteins along the transmission pathway. These interactions and the tissue tropism in the vector body determine the efficiency and specificity of the transmission. This review describes the mechanisms involved in circulative begomovirus transmission by B. tabaci, focusing on the most studied virus in this regard, namely the tomato yellow leaf curl virus (TYLCV) and its closely related isolates. Additionally, the review aims at drawing attention to the recent knowhow of unorthodox virus—B. tabaci interactions. The recent knowledge of whitefly-mediated transmission of two recombinant poleroviruses (Luteoviridae), a virus group with an ssRNA genome and known to be strictly transmitted with aphids, is discussed with its broader context in the emergence of new whitefly-driven virus diseases.

Rice black-streaked dwarf virus P10 promotes phosphorylation of GAPDH (glyceraldehyde-3-phosphate dehydrogenase) to induce autophagy in Laodelphax striatellus

Macroautophagy/autophagy is an important innate and adaptive immune response that can clear microbial pathogens through guiding their degradation. Virus infection in animals and plants is also known to induce autophagy. However, how virus infection induces autophagy is largely unknown. Here, we provide evidence that the early phase of rice black-streaked dwarf virus (RBSDV) infection in Laodelphax striatellus can also induce autophagy, leading to suppression of RBSDV invasion and accumulation. We have determined that the main capsid protein of RBSDV (P10) is the inducer of autophagy. RBSDV P10 can specifically interact with GAPDH (glyceraldehyde-3-phosphate dehydrogenase), both in vitro and in vivo. Silencing of GAPDH in L. striatellus could significantly reduce the activity of autophagy induced by RBSDV infection. Furthermore, our results also showed that both RBSDV infection and RBSDV P10 alone can promote phosphorylation of AMP-activated protein kinase (AMPK), resulting in GAPDH phosphorylation and relocation of GAPDH from the cytoplasm into the nucleus in midgut cells of L. striatellus or Sf9 insect cells. Once inside the nucleus, phosphorylated GAPDH can activate autophagy to suppress virus infection. Together, these data illuminate the mechanism by which RBSDV induces autophagy in L. striatellus, and indicate that the autophagy pathway in an insect vector participates in the anti-RBSDV innate immune response.

Molecular characterization of SlATG18f in response to Tomato leaf curl New Delhi virus infection in tomato and development of a CAPS marker for leaf curl disease tolerance

Analysis of autophagy-related genes in tomato shows the involvement of SlATG18f in leaf curl disease tolerance and a CAPS marker developed from this gene demonstrates its usefulness in marker-assisted selection. Autophagy is a highly conserved catabolic process regulating cellular homeostasis and adaptation to different biotic and abiotic stress. Several autophagy-related proteins (ATGs) are reported to be involved in autophagic processes, and considering their importance in regulating growth and stress adaptation, these proteins have been identified and characterized in several plant species. However, there is no information available on the role of autophagy-related proteins regulating the tolerance of tomato to tomato leaf curl disease (ToLCD). Given this, the present genome-wide study identified thirty ATG-encoding genes (SlATG) in tomato, followed by their functional characterization. Expression profiling of the SlATG genes in contrasting tomato cultivars subjected to virus infection showed a 4.5-fold upregulation of SlATG18f in the tolerant cultivar. Further, virus-induced gene silencing of SlATG18f in the tolerant cultivar conferred disease susceptibility, which suggested the role of this gene in Tomato leaf curl New Delhi virus tolerance. Comparison of the gene sequence of both tolerant and susceptible cultivars along with the 5' upstream regions identified an SNP (A/T) at -2916 upstream of the start codon. A cleaved amplified polymorphic sequence (CAPS) marker was developed targeting this region, which showed a significant association with the tolerance characteristics in the tomato germplasm (R² = 0.1787). Altogether, the study identified a potential gene that could be used to develop ToLCNDV tolerant tomato cultivars using transgene-based or marker-assisted breeding-based approaches.

Laodelphax striatellus Atg8 facilitates Rice stripe virus infection in an autophagy-independent manner

Rice stripe virus (RSV) is the causative agent of rice stripe disease and is completely dependent on insect vectors for its plant-to-plant transmission. Laodelphax striatellus is the major insect vector for RSV. In this study, we explored the interactions between RSV infection and L. striatellus autophagy, a potential intrinsic antiviral mechanism in insects. We found that L. striatellus autophagic activity did not affect RSV infection; however, the autophagy-related-8 (Atg8) gene significantly enhanced virus infection. During RSV initial infection within the L. striatellus midgut, silencing of Atg8 expression significantly decreased the phosphorylation of c-Jun N-terminal kinase (p-JNK); however, when RSV infection is absent, silencing of Atg8 did not alter p-JNK levels. These results indicated that Atg8 might activate the JNK machinery by allowing more virus infection into cells. We further revealed that Atg8-deficiency significantly decreased RSV accumulation on the surface of the insect midgut epithelial cells, suggesting a receptor trafficking function of the γ-aminobutyric acid receptor-associated protein family. Using the RSV ovary entry as a model, in which vitellogenin receptor (VgR) mediates RSV cell entry, we clarified that Atg8-deficiency decreased the abundance of VgR localizing on the cytomembrane and disturbed the attachment of RSV in the germarium zones. Collectively, these results revealed an autophagy-independent function of L. striatellus Atg8 that enhances RSV initial infection by increasing virus attachment on the infection sites.

Vector Transmission of Tomato Yellow Leaf Curl Thailand Virus by the Whitefly Bemisia tabaci: Circulative or Propagative?

Viruses that cause tomato yellow leaf curl disease are part of a group of viruses of the genus Begomovirus, family Geminiviridae. Tomato-infecting begomoviruses cause epidemics in tomato crops in tropical, subtropical, and Mediterranean climates, and they are exclusively transmitted by Bemisia tabaci in the field. The objective of the present study was to examine the transmission biology of the tomato yellow leaf curl Thailand virus (TYLCTHV) by B. tabaci, including virus-infected tissues, virus translocation, virus replication, and transovarial transmission. The results demonstrated that the virus translocates from the alimentary gut to the salivary glands via the hemolymph, without apparent replication when acquired by B. tabaci. Furthermore, the virus was detected in 10% of the first-generation progeny of viruliferous females, but the progeny was unable to cause the viral infection of host plants. There was no evidence of transovarial transmission of TYLCTHV in B. tabaci. When combined with the current literature, our results suggest that B. tabaci transmits TYLCTHV in a persistent-circulative mode. The present study enhances our understanding of virus-vector interaction and the transmission biology of TYLCTHV in B. tabaci.

Alfalfa leaf curl virus is efficiently acquired by its aphid vector Aphis craccivora but inefficiently transmitted

Alfalfa leaf curl virus (ALCV) is the first geminivirus for which aphid transmission was reported. Transmission by Aphis craccivora was determined previously to be highly specific and circulative. Using various complementary techniques, the transmission journey of ALCV was monitored from its uptake from infected plant tissues up to the head of its vector. ALCV was shown to be restricted to phloem tissues using fluorescence in situ hybridization (FISH) and electropenetrography (EPG) monitoring of virus acquisition. Furthermore, the virus is heterogeneously distributed in phloem tissues, as revealed by FISH and quantitative PCR of viral DNA acquired by EPG-monitored aphids. Despite the efficient ingestion of viral DNA, about 106 viral DNA copies per insect in a 15 h feeding period on ALCV-infected plants, the individual maximum transmission rate was 12 %. Transmission success was related to a critical viral accumulation, around 1.6×107 viral DNA copies per insect, a threshold that generally needed more than 48 h to be reached. Moreover, whereas the amount of acquired virus did not decrease over time in the whole aphid body, it declined in the haemolymph and heads. ALCV was not detected in progenies of viruliferous aphids and did not affect aphid fitness. Compared to geminiviruses transmitted by whiteflies or leafhoppers, or to luteoviruses transmitted by aphids, the transmission efficiency of ALCV by A. craccivora is low. This result is discussed in relation to the aphid vector of this geminivirus and the agroecological features of alfalfa, a hardy perennial host plant.

Insect Transmission of Plant Single-Stranded DNA Viruses

Of the approximately 1,200 plant virus species that have been described to date, nearly one-third are single-stranded DNA (ssDNA) viruses, and all are transmitted by insect vectors. However, most studies of vector transmission of plant viruses have focused on RNA viruses. All known plant ssDNA viruses belong to two economically important families, Geminiviridae and Nanoviridae, and in recent years, there have been increased efforts to understand whether they have evolved similar relationships with their respective insect vectors. This review describes the current understanding of ssDNA virus-vector interactions, including how these viruses cross insect vector cellular barriers, the responses of vectors to virus circulation, the possible existence of viral replication within insect vectors, and the three-way virus-vector-plant interactions. Despite recent breakthroughs in our understanding of these viruses, many aspects of plant ssDNA virus transmission remain elusive. More effort is needed to identify insect proteins that mediate the transmission of plant ssDNA viruses and to understand the complex virus-insect-plant three-way interactions in the field during natural infection.

Liberibacter, A Preemptive Bacterium: Apoptotic Response Repression in the Host Gut at the Early Infection to Facilitate Its Acquisition and Transmission

“Candidatus Liberibacter solanacearum” (Lso) is a phloem-limited Gram-negative bacterium that infects crops worldwide. In North America, two haplotypes of Lso (LsoA and LsoB) are transmitted by the potato psyllid, Bactericera cockerelli (Šulc), in a circulative and persistent manner. Both haplotypes cause damaging plant diseases (e.g., zebra chip in potatoes). The psyllid gut is the first organ Lso encounters and could be a barrier for its transmission. However, little is known about the psyllid gut immune responses triggered upon Lso infection. In this study, we focused on the apoptotic response in the gut of adult potato psyllids at the early stage of Lso infection. We found that there was no evidence of apoptosis induced in the gut of the adult potato psyllids upon infection with either Lso haplotype based on microscopic observations. However, the expression of the inhibitor of apoptosis IAPP5.2 gene (survivin-like) was significantly upregulated during the period that Lso translocated into the gut cells. Interestingly, silencing of IAPP5.2 gene significantly upregulated the expression of two effector caspases and induced apoptosis in the psyllid gut cells. Moreover, RNA interference (RNAi) of IAPP5.2 significantly decreased the Lso titer in the gut of adult psyllids and reduced their transmission efficiency. Taken together, these observations suggest that Lso might repress the apoptotic response in the psyllid guts by inducing the anti-apoptotic gene IAPP5.2 at an early stage of the infection, which may favor Lso acquisition in the gut cells and facilitate its transmission by potato psyllid.

New insights into the transovarial transmission of the symbiont Rickettsia in whiteflies

Endosymbiont transmission via eggs to future host generations has been recognized as the main strategy for its persistence in insect hosts; however, the mechanisms for transmission have yet to be elucidated. Here, we describe the dynamic locations of Rickettsia in the ovarioles and eggs during oogenesis and embryogenesis in a globally significant pest whitefly Bemisia tabaci. Field populations of the whitefly have a high prevalence of Rickettsia, and in all Rickettsia-infected individuals, the bacterium distributes in the body cavity of the host, especially in the midgut, fat body, hemocytes, hemolymph, and near bacteriocytes. The distribution of Rickettsia was subjected to dynamic changes in the ovary during oogenesis, and our ultrastructural observations indicated that the bacteria infect host ovarioles during early developmental stages via two routes: (i) invasion of the tropharium by endocytosis and then transmission into vitellarium via nutritive cord and (ii) entry into vitellarium by hijacking bacteriocyte translocation. Most of the Rickettsia are degraded in the oocyte cytoplasm in late-stage oogenesis. However, a few reside beneath the vitelline envelope of mature eggs, spread into the embryo, and proliferate during embryogenesis to sustain high-fidelity transmission to the next generation. Our findings provide novel insights into the maternal transmission underpinning the persistence and spread of insect symbionts.

Apoptosis in a Whitefly Vector Activated by a Begomovirus Enhances Viral Transmission

Of the approximately 1,100 known plant viruses, about one-third are DNA viruses that are vectored by insects. Plant virus infections often induce cellular and molecular responses in their insect vectors, which can, in many cases, affect the spread of viruses. However, the mechanisms underlying vector responses that affect virus accumulation and transmission are poorly understood. Here, we examined the role of virus-induced apoptosis in the transmission of begomoviruses, a group of single-stranded plant DNA viruses that are transmitted by whiteflies and cause extensive damage to many crops worldwide. We demonstrated that virus infection can induce apoptosis in the insect vector conferring protection to the virions from degradation, leading to enhanced viral accumulation and transmission to host plants. Our findings provide valuable clues for designing new strategies to block the transmission of insect-vectored plant viruses, particularly plant DNA viruses.

Apoptosis is generally considered the first line of defense against viral infection. However, the role of apoptosis in the interactions between plant viruses and their insect vectors has rarely been investigated. By studying plant DNA viruses of the genus Begomovirus within the family Geminiviridae, which are transmitted by whiteflies of the Bemisia tabaci species complex in a persistent manner, we revealed that virus-induced apoptosis in insect vectors can facilitate viral accumulation and transmission. We found that infection with tomato yellow leaf curl virus activated the apoptosis pathway in B. tabaci. Suppressing apoptosis by inhibitors or silencing caspase-3 significantly reduced viral accumulation, while the activation of apoptosis increased viral accumulation in vivo. Moreover, the positive effect of whitefly apoptosis on virus accumulation and transmission was not due to its cross talk with the autophagy pathway that suppresses begomovirus infection in whiteflies. We further showed that viral replication, rather than the viral coat protein, is likely the critical factor in the activation of apoptosis by the virus. These novel findings indicate that similarly to many animal and a few plant RNA viruses, plant DNA viruses may activate apoptosis in their insect vectors leading to enhanced viral accumulation and transmission.

IMPORTANCE Of the approximately 1,100 known plant viruses, about one-third are DNA viruses that are vectored by insects. Plant virus infections often induce cellular and molecular responses in their insect vectors, which can, in many cases, affect the spread of viruses. However, the mechanisms underlying vector responses that affect virus accumulation and transmission are poorly understood. Here, we examined the role of virus-induced apoptosis in the transmission of begomoviruses, a group of single-stranded plant DNA viruses that are transmitted by whiteflies and cause extensive damage to many crops worldwide. We demonstrated that virus infection can induce apoptosis in the insect vector conferring protection to the virions from degradation, leading to enhanced viral accumulation and transmission to host plants. Our findings provide valuable clues for designing new strategies to block the transmission of insect-vectored plant viruses, particularly plant DNA viruses.

Transmission of the Bean-Associated Cytorhabdovirus by the Whitefly Bemisia tabaci MEAM1

The knowledge of genomic data of new plant viruses is increasing exponentially; however, some aspects of their biology, such as vectors and host range, remain mostly unknown. This information is crucial for the understanding of virus–plant interactions, control strategies, and mechanisms to prevent outbreaks. Typically, rhabdoviruses infect monocot and dicot plants and are vectored in nature by hemipteran sap-sucking insects, including aphids, leafhoppers, and planthoppers. However, several strains of a potentially whitefly-transmitted virus, papaya cytorhabdovirus, were recently described: (i) bean-associated cytorhabdovirus (BaCV) in Brazil, (ii) papaya virus E (PpVE) in Ecuador, and (iii) citrus-associated rhabdovirus (CiaRV) in China. Here, we examine the potential of the Bemisia tabaci Middle East-Asia Minor 1 (MEAM1) to transmit BaCV, its morphological and cytopathological characteristics, and assess the incidence of BaCV across bean producing areas in Brazil. Our results show that BaCV is efficiently transmitted, in experimental conditions, by B. tabaci MEAM1 to bean cultivars, and with lower efficiency to cowpea and soybean. Moreover, we detected BaCV RNA in viruliferous whiteflies but we were unable to visualize viral particles or viroplasm in the whitefly tissues. BaCV could not be singly isolated for pathogenicity tests, identification of the induced symptoms, and the transmission assay. BaCV was detected in five out of the seven states in Brazil included in our study, suggesting that it is widely distributed throughout bean producing areas in the country. This is the first report of a whitefly-transmitted rhabdovirus.

Tomato Yellow Leaf Curl Virus: Impact, Challenges, and Management

Tomato yellow leaf curl virus (TYLCV) is one of the most studied plant viral pathogens because it is the most damaging virus for global tomato production. In order to combat this global threat, it is important that we understand the biology of TYLCV and devise management approaches. The prime objective of this review is to highlight management strategies for efficiently tackling TYLCV epidemics and global spread. For that purpose, we focus on the impact TYLCV has on worldwide agriculture and the role of recent advances for our understanding of TYLCV interaction with its host and vector. Another important focus is the role of recombination and mutations in shaping the evolution of TYLCV genome and geographical distribution.

Proteomic Analyses of Whitefly-Begomovirus Interactions Reveal the Inhibitory Role of Tumorous Imaginal Discs in Viral Retention

In nature, plant viruses are mostly transmitted by hemipteran insects, such as aphids, leafhoppers, and whiteflies. However, the molecular mechanisms underlying the interactions between virus and insect vector are poorly known. Here, we investigate the proteomic interactions between tomato yellow leaf curl virus (TYLCV, genus Begomovirus, family Geminiviridae), a plant virus, and its vector whitefly (Bemisia tabaci) species complex. First, using a yeast two-hybrid system, we identified 15 candidate whitefly proteins interacting with the coat protein of TYLCV. GO and KEGG pathway analysis implicated that these 15 whitefly proteins are of different biological functions/processes mainly including metabolic process, cell motility, signal transduction, and response to stimulus. We then found that the whitefly protein tumorous imaginal discs (Tid), one of the 15 whitefly proteins identified, had a stable interaction with TYLCV CP in vitro, and the DnaJ_C domain of Tid_301−499aa may be the viral binding site. During viral retention, the expression of whitefly protein Tid was observed to increase at the protein level, and feeding whiteflies with dsRNA or antibody against Tid resulted in a higher quantity of TYLCV in the whitefly body, suggesting the role of Tid in antiviral infection. Our data indicate that the induction of Tid following viral acquisition is likely a whitefly immune response to TYLCV infection.

A plant DNA virus replicates in the salivary glands of its insect vector via recruitment of host DNA synthesis machinery

Viruses pose a great threat to animal and plant health worldwide. Whereas most plant viruses only replicate in plant hosts, some also replicate in their animal (insect) vector. A detailed knowledge of host expansion will give a better understanding of virus evolution, and identification of virus and host components involved in this process can lead to new strategies to combat virus spread. Here, we reveal that a plant DNA virus has evolved to induce and recruit insect DNA synthesis machinery to support its replication in vector salivary glands. Our study sheds light on the understanding of TYLCV–whitefly interactions and provides insights into how a plant virus may evolve to infect and replicate in an insect vector.

Whereas most of the arthropod-borne animal viruses replicate in their vectors, this is less common for plant viruses. So far, only some plant RNA viruses have been demonstrated to replicate in insect vectors and plant hosts. How plant viruses evolved to replicate in the animal kingdom remains largely unknown. Geminiviruses comprise a large family of plant-infecting, single-stranded DNA viruses that cause serious crop losses worldwide. Here, we report evidence and insight into the replication of the geminivirus tomato yellow leaf curl virus (TYLCV) in the whitefly (Bemisia tabaci) vector and that replication is mainly in the salivary glands. We found that TYLCV induces DNA synthesis machinery, proliferating cell nuclear antigen (PCNA) and DNA polymerase δ (Polδ), to establish a replication-competent environment in whiteflies. TYLCV replication-associated protein (Rep) interacts with whitefly PCNA, which recruits DNA Polδ for virus replication. In contrast, another geminivirus, papaya leaf curl China virus (PaLCuCNV), does not replicate in the whitefly vector. PaLCuCNV does not induce DNA-synthesis machinery, and the Rep does not interact with whitefly PCNA. Our findings reveal important mechanisms by which a plant DNA virus replicates across the kingdom barrier in an insect and may help to explain the global spread of this devastating pathogen.