Exploring the limits of vector construction based on Citrus tristeza virus

Citrus phloem specific transcriptional profiling through the development of a citrus tristeza virus expressed translating ribosome affinity purification system

BACKGROUND

The analysis of translationally active mRNAs, or translatome, is a useful approach for monitoring cellular and plant physiological responses. One such method is the translating ribosome affinity purification (TRAP) system, which utilizes tagged ribosomal proteins to isolate ribosome-associated transcripts. This approach enables spatial and temporal gene expression analysis by driving the expression of tagged ribosomal proteins with tissue- or development-specific promoters. In plants, TRAP has enhanced our understanding of physiological responses to various biotic and abiotic factors. However, its utility is hampered by the necessity to generate transgenic plants expressing the tagged ribosomal protein, making this approach particularly challenging in perennial crops such as citrus.

RESULTS

This study involved the construction of a citrus tristeza virus (CTV) vector to express an immuno-tagged ribosome protein (CTV-hfRPL18). CTV, limited to the phloem, has been used for expressing marker and therapeutic sequences, making it suitable for analyzing citrus vascular tissue responses, including those related to huanglongbing disease. CTV-hfRPL18 successfully expressed a clementine-derived hfRPL18 peptide, and polysome purifications demonstrated enrichment for the hfRPL18 peptide. Subsequent translatome isolations from infected Nicotiana benthamiana and Citrus macrophylla showed enrichment for phloem-associated genes.

CONCLUSION

The CTV-hfRPL18 vector offers a transgene-free and rapid system for TRAP expression and translatome analysis of phloem tissues within citrus.

An Engineered Citrus Tristeza Virus (T36CA)-Based Vector Induces Gene-Specific RNA Silencing and Is Graft Transmissible to Commercial Citrus Varieties

A protein-expressing citrus tristeza virus-based vector construct, pT36CA-V1.3, obtained from a California isolate of the T36 strain (T36CA), was retooled into a virus-induced gene silencing system intended for use with studies of California citrus. Virus-induced gene silencing constructs engineered with a truncated Citrus macrophylla PHYTOENE DESATURASE (CmPDS) gene sequence in the sense or antisense orientation worked equally well to silence the endogenous CmPDS gene. In a parallel effort to optimize vector performance, two nonsynonymous nucleotides in open reading frame 1a of pT36CA-V1.3 were replaced with those conserved in the reference sequences from the T36CA cDNA library. The resulting viruses, T36CA-V1.4 (with one amino acid modification: D760N) and T36CA-V1.5 (with two amino acid modifications: D760N and P1174L), along with T36CA-V1.3, were individually propagated in Nicotiana benthamiana and C. macrophylla plants. Enzyme-linked immunosorbent assay (ELISA) measurements of extracts of the newly emerged leaves suggested that all three viruses accumulated to similar levels in N. benthamiana plants at 5 weeks postinoculation. ELISA values of T36CA-V1.4- and -V1.5-infected C. macrophylla samples were significantly higher than that of T36CA-V1.3-infected samples within an 8- to 12-month postinoculation window, suggesting a higher accumulation of T36CA-V1.4 and -V1.5 than T36CA-V1.3. However, at 36 months postinoculation, the ELISA values suggested that all three viruses accumulated to similar levels. When C. macrophylla plants infected with each of the three viruses were grafted to commercial citrus varieties, a limited number of receptor plants became infected, demonstrating a weak but nonetheless (the first) successful delivery of T36CA to California-grown commercial citrus.

Citrus-mediated gene silencing of cytochrome P450 suppresses insecticide resistance and increases mortality in Diaphorina citri

BACKGROUND

Asian citrus psyllid, Diaphorina citri, is a hemipteran that vectors the causal pathogen of citrus greening disease, or huanglongbing (HLB). HLB is a tree killing disease that has severely limited citrus production globally. Unfortunately, there is no cure for this disease, and mitigation depends on multiple insecticide applications to reduce vector populations. Silencing of cytochrome P450 expression associated with detoxification enzymes by RNA interference is known to increase susceptibility of D. citri to insecticides. However, dsRNA was previously introduced into psyllids by topical applications. The possible application of this technology for pest management will require effective field delivery of the dsRNA. Therefore, we evaluated a virus vector (Citrus tristeza virus; 'mild strain' T36) to deliver gene silencing directly to this sap-sucking insect via plant phloem. Citrus macrophylla plants inoculated with CTV expressing a truncated consensus sequence of CYP450 (CTV-tCYP450) constantly produced small interfering RNA in the plant phloem that targeted five cytochrome p540 (CYP450) genes in D. citri.

RESULTS

Insecticide susceptible D. citri reared on citrus infected with CTV-tCYP450 were subsequently more susceptible to imidacloprid, fenpropathrin, carbaryl, and chlorpyrifos than those reared on citrus infected with wildtype CTV or non-infected negative controls. Additionally, nymph survival and adult lifespan were significantly reduced when psyllids were reared on CTV-tCYP450 citrus plants compared with controls. Interestingly, similar results were obtained after one and two generations of rearing. Finally, field-collected psyllids from areas with known broad-spectrum insecticide resistance were rendered more susceptible to imidacloprid and fenpropathrin after feeding on CTV-tCYP450 citrus trees as compared with those reared on controls.

CONCLUSION

The integration of citrus-mediated RNA inference targeting psyllid detoxification enzymes could function as a resistance management tool and reduce insecticide input in an integrated pest management program for HLB. © 2024 Society of Chemical Industry.

Intra-Host Citrus Tristeza Virus Populations during Prolonged Infection Initiated by a Well-Defined Sequence Variant in Nicotiana benthamiana

Due to the error-prone nature of viral RNA-dependent RNA polymerases, the replication of RNA viruses results in a diversity of viral genomes harboring point mutations, deletions, insertions, and genome rearrangements. Citrus tristeza virus (CTV), a causal agent of diseases of economically important citrus species, shows intrinsic genetic stability. While the virus appears to have some mechanism that limits the accumulation of single-nucleotide variants, the production of defective viral genomes (DVGs) during virus infection has been reported for certain variants of CTV. The intra-host diversity generated during plant infection with variant T36 (CTV-T36) remains unclear. To address this, we analyzed the RNA species accumulated in the initially infected and systemic leaves of Nicotiana benthamiana plants inoculated with an infectious cDNA clone of CTV-T36, which warranted that infection was initiated by a known, well-defined sequence variant of the virus. CTV-T36 limited the accumulation of single-nucleotide mutants during infection. With that, four types of DVGs-deletions, insertions, and copy- and snap-backs-were found in all the samples, with deletions and insertions being the most common types. Hot-spots across the genome for DVG recombination and short direct sequence repeats suggest that sequence complementarity could mediate DVG formation. In conclusion, our study illustrates the formation of diverse DVGs during CTV-T36 infection. To the best of our knowledge, this is the first study that has analyzed the genetic variability and recombination of a well-defined sequence variant of CTV in an herbaceous host.

Xanthomonas citri subsp. citri type III effector PthA4 directs the dynamical expression of a putative citrus carbohydrate-binding protein gene for canker formation

Xanthomonas citri subsp. citri (Xcc), the causal agent of citrus canker, elicits canker symptoms in citrus plants because of the transcriptional activator-like (TAL) effector PthA4, which activates the expression of the citrus susceptibility gene CsLOB1. This study reports the regulation of the putative carbohydrate-binding protein gene Cs9g12620 by PthA4-mediated induction of CsLOB1 during Xcc infection. We found that the transcription of Cs9g12620 was induced by infection with Xcc in a PthA4-dependent manner. Even though it specifically bound to a putative TAL effector-binding element in the Cs9g12620 promoter, PthA4 exerted a suppressive effect on the promoter activity. In contrast, CsLOB1 bound to the Cs9g12620 promoter to activate its expression. The silencing of CsLOB1 significantly reduced the level of expression of Cs9g12620, which demonstrated that Cs9g12620 was directly regulated by CsLOB1. Intriguingly, PhtA4 interacted with CsLOB1 and exerted feedback control that suppressed the induction of expression of Cs9g12620 by CsLOB1. Transient overexpression and gene silencing revealed that Cs9g12620 was required for the optimal development of canker symptoms. These results support the hypothesis that the expression of Cs9g12620 is dynamically directed by PthA4 for canker formation through the PthA4-mediated induction of CsLOB1.

Scanning the Horizon for Environmental Applications of Genetically Modified Viruses Reveals Challenges for Their Environmental Risk Assessment

The release of novel genetically modified (GM) virus applications into the environment for agricultural, veterinary, and nature-conservation purposes poses a number of significant challenges for risk assessors and regulatory authorities. Continuous efforts to scan the horizon for emerging applications are needed to gain an overview of new GM virus applications. In addition, appropriate approaches for risk assessment and management have to be developed. These approaches need to address pertinent challenges, in particular with regard to the environmental release of GM virus applications with a high probability for transmission and spreading, including transboundary movements and a high potential to result in adverse environmental effects. However, the current preparedness at the EU and international level to assess such GM virus application is limited. This study addresses some of the challenges associated with the current situation, firstly, by conducting a horizon scan to identify emerging GM virus applications with relevance for the environment. Secondly, outstanding issues regarding the environmental risk assessment (ERA) of GM virus applications are identified based on an evaluation of case study examples. Specifically, the limited scientific information available for the ERA of some applications and the lack of detailed and appropriate guidance for ERA are discussed. Furthermore, considerations are provided for future work that is needed to establish adequate risk assessment and management approaches.

Gene silencing of cathepsins B and L using CTV-based, plant-mediated RNAi interferes with ovarial development in Asian citrus psyllid (ACP), Diaphorina citri

Diaphorina citri Kuwayama (Hemiptera: Liviidae) is a vector of the bacteria Candidatus Liberibacter americanus (CLam) and Candidatus Liberibacter asiaticus (CLas), which are phloem-restricted and associated with the most important and destructive worldwide citrus disease, Huanglongbing (HLB). Currently, no cure for HLB has been described. Therefore, measures have focused on reducing D. citri populations. In these insects, cathepsin B (DCcathB) and L (DCcathL) enzymes play an important role in digestion, and are involved in embryogenesis, immune defense, and ecdysis. In this study, we used a CTV-based vector to deliver dsRNA (CTV-dsRNA) into Citrus macrophylla plants targeting DCcathB and DCcathL genes in D. citri that fed on the phloem of these CTV-RNAi infected plants. Subsequently, we evaluated expression of DCcathB and DCcathL genes as well as the Vitellogenin (Vg) gene by RT-qPCR in D. citri fed on CTV-dsRNA occurring in plant phloem. It was found that a defective phenotype in D. citri females as a result of knockdown of DCcathB and DCcathL genes mediated by CTV dsRNA. These results showed that Psyllids fed on plants treated with the CTV-dsRNA exhibited downregulation of the Vg gene, one of the most important genes associated with embryogenic and female development, which was associated with dsRNA-mediated silencing of the two cathepsin genes. Based on our findings, a CTV-based strategy for delivering RNAi via plants that targets DCcathB and DCcathL genes may represent a suitable avenue for development of dsRNA-based tools to manage D. citri that limits the spread of HLB.

Genetic transformation of 'Hamlin' and 'Valencia' sweet orange plants expressing the cry11A gene of Bacillus thuringiensis as another tool to the management of Diaphorina citri (Hemiptera: Liviidae)

The Asian citrus psyllid (ACP) Diaphorina citri Kuwayama (Hemiptera: Liviidae) is the vector of Candidatus Liberibacter spp., the bacteria associated with huanglongbing (HLB), the most devastating disease of citrus worldwide. HLB management has heavily counted on insecticide applications to control the ACP, although there are efforts towards more sustainable alternatives. In previous work, our group assessed the potential bioactivity of different strains of Bacillus thuringiensis (Eubacteriales: Bacillaceae) (Bt) containing cry/cyt genes as feasible tools to control ACP nymphs. Here, we report an attempt to use the cry11A gene from Bt to produce transgenic sweet orange plants using two promoters. For the genetic transformation, 'Hamlin' and 'Valencia' sweet orange seedlings were used as sources of explants. Transgenic plants were detected by polymerase chain reaction (PCR) with specific primers, and the transgene copy number was confirmed by Southern blot analyses. Transcript expression levels were determined by qPCR. Mortality assays of D. citri nymphs were carried out in a greenhouse, and the effect of the events tested ranged from 22 to 43% at the end of the five-day exposure period. To our knowledge, this is the first manuscript reporting the production of citrus plants expressing the Bt cry11A gene for the management of D. citri nymphs.

Better together: the use of virus-induced gene silencing technique to repress the expression of two endogenous citrus genes simultaneously

Virus-induced gene silencing is a promising technique for functional genomics studies. Citrus tristeza virus was employed successfully to create an infectious clone that was used to silence endogenous citrus genes. Phytoene desaturase (PDS) and delta (δ)-aminolevulinic acid dehydratase (ALAD) were targeted successfully in citrus. Silencing PDS usually results in a photo-bleached leaf phenotype while silencing ALAD causes discrete yellow spots in leaves. Silencing two or more genes simultaneously using the same infectious clone could be difficult due to the capacity of the plasmid and subsequent cloning. On the other hand, inoculating a new construct into a citrus plant pre-infected with another construct fails due to the superinfection exclusion phenomenon. Herein, I report our successful trials whereby we simultaneously graft-inoculate constructs targeting PDS and ALAD. The budwoods were graft-inoculated into the same tree but on two different branches. Interestingly, a new phenotype was produced because of the silencing of the two genes, which we called "color-breaking". The phenotype was observed in both branches. Gene expression analysis showed a significant reduction of PDS and ALAD transcripts. This finding suggests the possibility of targeting more than one gene using different constructs, however, the graft-inoculation must be at the same time.

Citrus Tristeza Virus: From Pathogen to Panacea

Citrus tristeza virus (CTV) is the most destructive viral pathogen of citrus. During the past century, CTV induced grave epidemics in citrus-growing areas worldwide that have resulted in a loss of more than 100 million trees. At present, the virus continues to threaten citrus production in many different countries. Research on CTV is accompanied by distinctive challenges stemming from the large size of its RNA genome, the narrow host range limited to slow-growing Citrus species and relatives, and the complexity of CTV populations. Despite these hurdles, remarkable progress has been made in understanding the CTV-host interactions and in converting the virus into a tool for crop protection and improvement. This review focuses on recent advances that have shed light on the mechanisms underlying CTV infection. Understanding these mechanisms is pivotal for the development of means to control CTV diseases and, ultimately, turn this virus into an ally.

Expected final online publication date for the Annual Review of Virology, Volume 9 is September 2022. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

Location matters: from changing a presumption about the Citrus tristeza virus tissue tropism to understanding the stem pitting disease

Stem pitting is a common virus-induced disease phenotype that tremendously impacts growth of perennial woody plants. How stem pitting develops in the infected trees remains unclear. Here, we assessed the development of stem pits upon infection of citrus by Citrus tristeza virus (CTV), which has been regarded as 'phloem-limited'. By taking advantage of a highly susceptible virus host - Citrus macrophylla - and a CTV isolate lacking a viral effector - the p33 protein, the development pattern of stem pitting was revealed via time-course observations and histological analyses. The stem pits result from the virus-colonized nonlignified 'gumming' malformations which are initiated by virus invasion into multiple spatially separated tissue layers - protophloem, metaphloem, and, unexpectedly, metaxylem. Notably, invasion of CTV into the unspecialized metaxylem cells interrupted the differentiation of the xylem tracheary elements. With the radial spread of CTV into the adjacent cells towards the stem periphery, the clusters of virus-colonized immature metaxylem cells extended in size, merging, at a certain stage, with virus-bearing cells in the protophloem and metaphloem layers. Collectively, our data provide a new insight into the process of the stem pitting development and the role of the xylem tissue in the virus pathogenicity.

Citrus Genetic Transformation: An Overview of the Current Strategies and Insights on the New Emerging Technologies

Citrus are among the most prevailing fruit crops produced worldwide. The implementation of effective and reliable breeding programs is essential for coping with the increasing demands of satisfactory yield and quality of the fruit as well as to deal with the negative impact of fast-spreading diseases. Conventional methods are time-consuming and of difficult application because of inherent factors of citrus biology, such as their prolonged juvenile period and a complex reproductive stage, sometimes presenting infertility, self-incompatibility, parthenocarpy, or polyembryony. Moreover, certain desirable traits are absent from cultivated or wild citrus genotypes. All these features are challenging for the incorporation of the desirable traits. In this regard, genetic engineering technologies offer a series of alternative approaches that allow overcoming the difficulties of conventional breeding programs. This review gives a detailed overview of the currently used strategies for the development of genetically modified citrus. We describe different aspects regarding genotype varieties used, including elite cultivars or extensively used scions and rootstocks. Furthermore, we discuss technical aspects of citrus genetic transformation procedures via Agrobacterium, regular physical methods, and magnetofection. Finally, we describe the selection of explants considering young and mature tissues, protoplast isolation, etc. We also address current protocols and novel approaches for improving the in vitro regeneration process, which is an important bottleneck for citrus genetic transformation. This review also explores alternative emerging transformation strategies applied to citrus species such as transient and tissue localized transformation. New breeding technologies, including cisgenesis, intragenesis, and genome editing by clustered regularly interspaced short palindromic repeats (CRISPR), are also discussed. Other relevant aspects comprising new promoters and reporter genes, marker-free systems, and strategies for induction of early flowering, are also addressed. We provided a future perspective on the use of current and new technologies in citrus and its potential impact on regulatory processes.

Comparative analysis identifies amino acids critical for citrus tristeza virus (T36CA) encoded proteins involved in suppression of RNA silencing and differential systemic infection in two plant species

Complementary (c)DNA clones corresponding to the full-length genome of T36CA (a Californian isolate of Citrus tristeza virus with the T36 genotype), which shares 99.1% identity with that of T36FL (a T36 isolate from Florida), were made into a vector system to express the green fluorescent protein (GFP). Agroinfiltration of two prototype T36CA-based vectors (pT36CA) to Nicotiana benthamiana plants resulted in local but not systemic GFP expression/viral infection. This contrasted with agroinfiltration of the T36FL-based vector (pT36FL), which resulted in both local and systemic GFP expression/viral infection. A prototype T36CA systemically infected RNA silencing-defective N. benthamiana lines, demonstrating that a genetic basis for its defective systemic infection was RNA silencing. We evaluated the in planta bioactivity of chimeric pT36CA-pT36FL constructs and the results suggested that nucleotide variants in several open reading frames of the prototype T36CA could be responsible for its defective systemic infection. A single amino acid substitution in each of two silencing suppressors, p20 (S107G) and p25 (G36D), of prototype T36CA facilitated its systemic infectivity in N. benthamiana (albeit with reduced titre relative to that of T36FL) but not in Citrus macrophylla plants. Enhanced virus accumulation and, remarkably, robust systemic infection of T36CA in N. benthamiana and C. macrophylla plants, respectively, required two additional amino acid substitutions engineered in p65 (N118S and S158L), a putative closterovirus movement protein. The availability of pT36CA provides a unique opportunity for comparative analysis to identify viral coding and noncoding nucleotides or sequences involved in functions that are vital for in planta infection.

Citrus tristeza virus P33 Protein is Required for Efficient Transmission by the Aphid Aphis (Toxoptera) citricidus (Kirkaldy)

Plant viruses are threatening many valuable crops, and Citrus tristeza virus (CTV) is considered one of the most economically important plant viruses. CTV has destroyed millions of citrus trees in many regions of the world. Consequently, understanding of the transmission mechanism of CTV by its main vector, the brown citrus aphid, Aphis (Toxoptera) citricidus (Kirkaldy), may lead to better control strategies for CTV. The objective of this study was to understand the CTV-vector relationship by exploring the influence of viral genetic diversity on virus transmission. We built several infectious clones with different 5'-proximal ends from different CTV strains and assessed their transmission by the brown citrus aphid. Replacement of the 5'- end of the T36 isolate with that of the T30 strain (poorly transmitted) did not increase the transmission rate of T36, whereas replacement with that of the T68-1 isolate (highly transmitted) increased the transmission rate of T36 from 1.5 to 23%. Finally, substitution of p33 gene of the T36 strain with that of T68 increased the transmission rate from 1.5% to 17.8%. Although the underlying mechanisms that regulate the CTV transmission process by aphids have been explored in many ways, the roles of specific viral proteins are still not explicit. Our findings will improve our understanding of the transmission mechanisms of CTV by its aphid vector and may lead to the development of control strategies that interfere with its transmission by vector.

Plant Virus-Derived Vectors: Applications in Agricultural and Medical Biotechnology

Major advances in our understanding of plant viral genome expression strategies and the interaction of a virus with its host for replication and movement, induction of disease, and resistance responses have been made through the generation of infectious molecules from cloned viral sequences. Autonomously replicating viral vectors derived from infectious clones have been exploited to express foreign genes in plants. Applications of virus-based vectors include the production of human/animal therapeutic proteins in plant cells and the specific study of plant biochemical processes, including those that confer resistance to pathogens. Additionally, virus-induced gene silencing, which is RNA mediated and triggered through homology-dependent RNA degradation mechanisms, has been exploited as an efficient method to study the functions of host genes in plants and to deliver small RNAs to insects. New and exciting strategies for vector engineering, delivery, and applications of plant virus-based vectors are the subject of this review.

A Non-Conserved p33 Protein of Citrus Tristeza Virus Interacts with Multiple Viral Partners

The RNA genome of citrus tristeza virus (CTV), one of the most damaging viral pathogens of citrus, contains 12 open reading frames resulting in production of at least 19 proteins. Previous studies on the intraviral interactome of CTV revealed self-interaction of the viral RNA-dependent RNA polymerase, the major coat protein (CP), p20, p23, and p33 proteins, while heterologous interactions between the CTV proteins have not been characterized. In this work, we examined interactions between the p33 protein, a nonconserved protein of CTV, which performs multiple functions in the virus infection cycle and is needed for virus ability to infect the extended host range, with other CTV proteins shown to mediate virus interactions with its plant hosts. Using yeast two-hybrid, bimolecular fluorescence complementation, and coimmunoprecipitation assays, we demonstrated that p33 interacts with three viral proteins, i.e., CP, p20, and p23, in vivo and in planta. Coexpression of p33, which is an integral membrane protein, resulted in a shift in the localization of the p20 and p23 proteins toward the subcellular crude-membrane fraction. Upon CTV infection, the four proteins colocalized in the CTV replication factories. In addition, three of them, CP, p20, and p23, were found in the p33-formed membranous structures. Using bioinformatic analyses and mutagenesis, we found that the N-terminus of p33 is involved in the interactions with all three protein partners. A potential role of these interactions in virus ability to infect the extended host range is discussed.

A Long Non-Coding RNA of Citrus tristeza virus: Role in the Virus Interplay with the Host Immunity

During infection, Citrus tristeza virus (CTV) produces a non-coding subgenomic RNA referred to as low-molecular-weight tristeza 1 (LMT1), which for a long time has been considered as a by-product of the complex CTV replication machinery. In this study, we investigated the role of LMT1 in the virus infection cycle using a CTV variant that does not produce LMT1 (CTV-LMT1d). We showed that lack of LMT1 did not halt virus ability to replicate or form proper virions. However, the mutant virus demonstrated significantly reduced invasiveness and systemic spread in Nicotiana benthamiana as well as an inability to establish infection in citrus. Introduction of CTV-LMT1d into the herbaceous host resulted in elevation of the levels of salicylic acid (SA) and SA-responsive pathogenesis-related genes beyond those upon inoculation with wild-type (WT) virus (CTV-WT). Further analysis showed that the LMT1 RNA produced by CTV-WT or via ectopic expression in the N. benthamiana leaves suppressed SA accumulation and up-regulated an alternative oxidase gene, which appeared to mitigate the accumulation of reactive oxygen species. To the best of our knowledge, this is the first report of a plant viral long non-coding RNA being involved in counter-acting host response by subverting the SA-mediated plant defense.

Toxicity of Bacillus thuringiensis-Derived Pesticidal Proteins Cry1Ab and Cry1Ba against Asian Citrus Psyllid, Diaphorina citri (Hemiptera)

The Asian citrus psyllid (ACP), Diaphorina citri Kuwayama (Hemiptera), is an important pest of citriculture. The ACP vectors a bacterium that causes huanglongbing (HLB), a devastating and incurable disease of citrus. The bacterium Bacillus thuringiensis (Bt) produces multiple toxins with activity against a diverse range of insects. In efforts to provide additional control methods for the ACP vector of HLB, we identified pesticidal proteins derived from Bt for toxicity against ACP. The trypsin proteolytic profiles of strain-derived toxins were characterized. Strain IBL-00200, one of six strains with toxins shown to have basal activity against ACP was selected for liquid chromatography-mass spectrometry (LC-MS/MS) identification of the individual Cry toxins expressed. Toxicity assays with individual toxins derived from IBL-00200 were then performed. The activated form of the Cry toxins Cry1Ab and Cry1Ba were toxic to ACP with LC₅₀ values of approximately 120 µg/mL. Disruption of the midgut epithelium was associated with the toxicity of both the IBL-00200-derived toxin mixture, and with Cry1Ba. With further optimization of the efficacy of Cry1Ab and Cry1Ba, these toxins may have practical utility against ACP. Bt toxins with activity against ACP may provide an additional tool for management of ACP and the associated HLB disease, thereby providing a more sustainable and environmentally benign approach than repeated application of broad-spectrum insecticides.

The p33 protein of Citrus tristeza virus affects viral pathogenicity by modulating a host immune response

Accumulation of reactive oxygen species (ROS) is a general plant basal defense strategy against viruses. In this study, we show that infection by Citrus tristeza virus (CTV) triggered ROS burst in Nicotiana benthamiana and in the natural citrus host, the extent of which was virus-dose dependent. Using Agrobacterium-mediated expression of CTV-encoded proteins in N. benthamiana, we found that p33, a unique viral protein, contributed to the induction of ROS accumulation and programmed cell death. The role of p33 in CTV pathogenicity was assessed based on gene knockout and complementation in N. benthamiana. In the citrus-CTV pathosystem, deletion of the p33 open reading frame in a CTV variant resulted in a significant decrease in ROS production, compared to that of the wild type CTV, which correlated with invasion of the mutant virus into the immature xylem tracheid cells and abnormal differentiation of the vascular system. By contrast, the wild type CTV exhibited phloem-limited distribution with a minor effect on the vasculature. We conclude that the p33 protein is a CTV effector that negatively affects virus pathogenicity and suggest that N. benthamiana recognizes p33 to activate the host immune response to restrict CTV into the phloem tissue and minimize the disease syndrome.

Nucleotide heterogeneity at the terminal ends of the genomes of two California Citrus tristeza virus strains and their complete genome sequence analysis

Background

The non-translated regions at the genome ends of RNA viruses serve diverse functions and can exhibit various levels of nucleotide (nt) heterogeneity. However, the extent of nt heterogeneity at the extreme termini of Citrus tristeza virus (CTV) genomes has not been comprehensively documented. This study aimed to characterize two widely prevalent CTV genotypes, T36-CA and T30-CA, from California that have not been sequenced or analyzed substantially. The information obtained will be used in our ongoing effort to construct the infectious complementary (c) DNA clones of these viruses.

Methods

The terminal nts of the viral genomes were identified by sequencing cDNA clones of the plus- and/or minus-strand of the viral double-stranded (ds) RNAs generated using 5′ and 3′ rapid amplification of cDNA ends. Cloned cDNAs corresponding to the complete genome sequences of both viruses were generated using reverse transcription-polymerase chain reactions, sequenced, and subjected to phylogenetic analysis.

Results

Among the predominant terminal nts identified, some were identical to the consensus sequences in GenBank, while others were different or unique. Remarkably, one of the predominant 5′ nt variants of T36-CA contained the consensus nts “AATTTCAAA” in which a highly conserved cytidylate, seen in all other full-length T36 sequences, was absent. As expected, but never systematically verified before, unique variants with additional nt (s) incorporated upstream of the 5′ terminal consensus nts of T36-CA and T30-CA were also identified. In contrast to the extreme 5′ terminal nts, those at the extreme 3′ termini of T36-CA and T30-CA were more conserved compared to the reference sequences, although nt variants were also found. Notably, an additional thymidylate at the extreme 3′ end was identified in many T36-CA sequences. Finally, based on pairwise comparisons and phylogenetic analysis with multiple reference sequences, the complete sequences of both viruses were found to be highly conserved with those of the respective genotypes.

Conclusions

The extreme terminal nts in the T36-CA and T30-CA genomes were identified, revealing new insights on the heterogeneity of these CTV genomic regions. T36-CA and T30-CA were the first and the second genotypes, respectively, of CTV originating from California to be completely sequenced and analyzed.

Electronic supplementary material

The online version of this article (10.1186/s12985-018-1041-4) contains supplementary material, which is available to authorized users.

Effects of δ-aminolevulinic acid dehydratase silencing on the primary and secondary metabolisms of citrus

δ-aminolevulinic acid dehydratase (ALAD) is an important enzyme in tetrapyrrole synthesis. ALAD combines two δ-aminolevulinic acid (δ-ALA) molecules to form the pyrrole molecule, porphobilinogen, an important precursor for plant pigments involved in photosynthesis, respiration, and nutrient uptake. In this study, we investigated the effects of silencing of ALAD gene on citrus leaf pigments and metabolites. The ALAD enzyme was inhibited using virus-induced gene silencing (VIGS) technology using citrus tristeza virus (CTV). δ-ALA accumulated in citrus plants inoculated with the recombinant virus (CTV-tALAD) to silence ALAD and resulted in discrete yellow spots (yellow islands) and necrosis in leaves and stems. The levels of chlorophylls, starch, sucrose, trans- and cis-violaxanthin, and α- and β-cryptoxanthin were reduced in CTV-tALAD plants, whereas zeaxanthin was increased. The increase in zeaxanthin and the decrease in its precursors indicated that the reduction in chlorophylls resulted in light damage. Salicylic acid and jasmonic acid levels, as well as emission of (E)-α-bergamotene and (E)-β-farnesene, increased in CTV-tALAD plants indicating these plants were under stress. Our results showed that silencing of ALAD induces stress in plants and that VIGS using mild CTV strains is a promising technique to study biological function of citrus genes.

Efficient Protein Expression and Virus-Induced Gene Silencing in Plants Using a Crinivirus-Derived Vector

Plant virus-based vectors are valuable tools for recombinant gene expression and functional genomics for both basic and applied research. In this study, Lettuce infectious yellows virus (LIYV) of the genus Crinivirus was engineered into a virus vector that is applicable for efficient protein expression and virus-induced gene silencing (VIGS) in plants. We examined gene replacement and “add a gene” strategies to develop LIYV-derived vectors for transient expression of the green fluorescent protein (GFP) reporter in Nicotiana benthamiana plants. The latter yielded higher GFP expression and was further examined by testing the effects of heterologous controller elements (CEs). A series of five vector constructs with progressively extended LIYV CP sgRNA CEs were tested, the longest CE gave the highest GFP expression but lower virus accumulation. The whitefly transmissibility of the optimized vector construct to other host plants, and the capability to accommodate and express a larger gene, a 1.8 kb β-glucuronidase (GUS) gene, were confirmed. Furthermore, the LIYV vector was also validated VIGS by silencing the endogenous gene, phytoene desaturase (PDS) in N. benthamiana plants, and the transgene GFP in N. benthamiana line 16c plants. Therefore, LIYV-derived vectors could provide a technical reference for developing vectors of other economically important criniviruses.

Functional diversification upon leader protease domain duplication in the Citrus tristeza virus genome: Role of RNA sequences and the encoded proteins

Viruses from the family Closteroviridae show an example of intra-genome duplications of more than one gene. In addition to the hallmark coat protein gene duplication, several members possess a tandem duplication of papain-like leader proteases. In this study, we demonstrate that domains encoding the L1 and L2 proteases in the Citrus tristeza virus genome underwent a significant functional divergence at the RNA and protein levels. We show that the L1 protease is crucial for viral accumulation and establishment of initial infection, whereas its coding region is vital for virus transport. On the other hand, the second protease is indispensable for virus infection of its natural citrus host, suggesting that L2 has evolved an important adaptive function that mediates virus interaction with the woody host.

Self-interaction of Citrus tristeza virus p33 protein via N-terminal helix

Citrus tristeza virus (CTV), the most economically important viral pathogen of citrus, encodes a unique protein, p33. CTV p33 shows no similarity with other known proteins, yet plays an important role in viral pathogenesis: it extends the virus host range and mediates virus ability to exclude superinfection by other variants of the virus. Previously we demonstrated that p33 is an integral membrane protein and appears to share characteristics of viral movement proteins. In this study, we show that the p33 protein self-interacts in vitro and in vivo using co-immunoprecipitation, yeast two hybrid, and bimolecular fluorescence complementation assays. Furthermore, a helix located at the N-terminus of the protein is required and sufficient for the protein self-interaction.

Membrane association of a nonconserved viral protein confers virus ability to extend its host range

Citrus tristeza virus (CTV), the largest and most complex member of the family Closteroviridae, encodes a unique protein, p33, which shows no homology with other known proteins, however, plays an important role in virus pathogenesis. In this study, we examined some of the characteristics of p33. We show that p33 is a membrane-associated protein that is inserted into the membrane via a transmembrane helix formed by hydrophobic amino acid residues at the C-terminal end of the protein. Removal of this transmembrane domain (TMD) dramatically altered the intracellular localization of p33. Moreover, the TMD alone was sufficient to confer membrane localization of an unrelated protein. Finally, a CTV variant that produced a truncated p33 lacking the TMD was unable to infect sour orange, one of the selected virus hosts, which infection requires p33, suggesting that membrane association of p33 is important for the ability of CTV to extend its host range.

Grapevine leafroll disease and associated viruses: a unique pathosystem

Grapevine leafroll is the most complex and intriguing viral disease of grapevine (Vitis spp.). Several monopartite closteroviruses (family Closteroviridae) from grapevines have been molecularly characterized, yet their role in disease etiology is not completely resolved. Hence, these viruses are currently designated under the umbrella term of Grapevine leafroll-associated viruses (GLRaVs). This review examines our current understanding of the genetically divergent GLRaVs and highlights the emerging picture of several unique aspects of the leafroll disease pathosystem. A systems biology approach using contemporary technologies in molecular biology, -omics, and cell biology aids in exploring the comparative molecular biology of GLRaVs and deciphering the complex network of host-virus-vector interactions to bridge the gap between genomics and phenomics of leafroll disease. In addition, grapevine-infecting closteroviruses have a great potential as designer viruses to pursue functional genomics and for the rational design of novel disease intervention strategies in this agriculturally important perennial fruit crop.