Citrus tristeza virus: survival at the edge of the movement continuum

Complex interplay: The interactions between citrus tristeza virus and its host

Citrus tristeza virus (CTV) is one of the largest and most economically important RNA viruses infecting plants. CTV's interactions with various citrus hosts can result in three diseases: quick decline, stem pitting, or seedling yellows. Studying CTV poses several challenges owing to its significant genetic diversity and the highly specific occurrence of disease symptoms when different genotypes infect different citrus hosts. Considerable progress has been made to functionally characterize the virus-host interactions involved in the induction of CTV's three diseases, revealing that the four CTV ORFs (p33, p18, p13 and p23) play significant roles in determining the pathogenicity of CTV infections. These ORFs are unique to CTV and are not conserved among other members of the family Closteroviridae. This minireview aims to capture the complexity of the factors that have been shown to be involved in CTV disease induction and highlights recent work that provides novel insights into this pathosystem.

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.

Serological and Molecular Detection of Citrus Tristeza Virus: A Review

Citrus tristeza virus (CTV) is a globally pervasive and economically significant virus that negatively impacts citrus trees, leading to substantial reductions in fruit yield. CTV occurs within the phloem of infected plants, causing a range of disease phenotypes, such as stem pitting (SP), quick decline (QD), and other detrimental diseases. Research on CTV is challenging due to the large size of its RNA genome and the diversity of CTV populations. Comparative genomic analyses have uncovered genetic diversity in multiple regions of CTV isolates' genomes, facilitating the classification of the virus into distinct genotypes. Despite these challenges, notable advancements have been made in identifying and controlling CTV strains through serological and molecular methods. The following review concentrates on the techniques of nucleic acid identification and serological analysis for various CTV isolates, assisting in the comparison and evaluation of various detection methods, which are crucial for the effective management of CTV diseases, and so contributes to the innovation and development of CTV detection methods.

Areca palm velarivirus 1 infection caused disassembly of chloroplast and reduction of photosynthesis in areca palm

The expansion of betel palm cultivation is driven by rising demand for betel nut, yet this growth is accompanied by challenges such as decreased agricultural biodiversity and the spread of infectious pathogens. Among these, Yellow Leaf Disease (YLD) emerges as a prominent threat to betel palm plantation. Areca Palm Velarivirus 1 (APV1) has been identified as a primary causative agent of YLD, precipitating leaf yellowing, stunted growth, and diminished yield. However, the precise mechanisms underlying APV1-induced damage remain elusive. Our study elucidates that APV1 infiltrates chloroplasts, instigating severe damage and consequential reductions in chlorophyll a/b and carotene levels, alongside notable declines in photosynthetic efficiency. Moreover, APV1 infection exerts broad regulatory effects on gene expression, particularly suppressing key genes implicated in chloroplast function and photosynthesis. These disruptions correlate with growth retardation, yield diminishment, and compromised nut quality. Intriguingly, the paradoxical destruction of the host's photosynthetic machinery by APV1 prompts inquiry into its evolutionary rationale, given the virus's dependence on host resources for replication and proliferation. Our findings reveal that APV1-induced leaf yellowing acts as a beacon for transmission vectors, hinting at a nuanced "host-pathogen-vector co-evolutionary" dynamic.

Quick Decline and Stem Pitting Citrus tristeza virus Isolates Induce a Distinct Metabolomic Profile and Antioxidant Enzyme Activity in the Phloem Sap of Two Citrus Species

Susceptibility to the severe Citrus tristeza virus (CTV), T36, is higher for Citrus macrophylla (CM) than for C. aurantium (CA). How host-virus interactions are reflected in host physiology is largely unknown. In this study, the profile of metabolites and the antioxidant activity in the phloem sap of healthy and infected CA and CM plants were evaluated. The phloem sap of quick decline (T36) and stem pitting (T318A) infected citrus, and control plants was collected by centrifugation, and the enzymes and metabolites analyzed. The activity of the antioxidant enzymes, superoxide dismutase (SOD) and catalase (CAT), in infected plants increased significantly in CM and decreased in CA, compared to the healthy controls. Using LC-HRMS² a metabolic profile rich in secondary metabolites was assigned to healthy CA, compared to healthy CM. CTV infection of CA caused a drastic reduction in secondary metabolites, but not in CM. In conclusion, CA and CM have a different response to severe CTV isolates and we propose that the low susceptibility of CA to T36 may be related to the interaction of the virus with the host's metabolism, which reduces significantly the synthesis of flavonoids and antioxidant enzyme activity.

From the smallest to the largest subcellular plant pathogen: Citrus tristeza virus and its unique p23 protein

Knowledge on diseases caused by Citrus tristeza virus (CTV) has greatly increased in last decades after their etiology was demonstrated in the past seventies. Professor Ricardo Flores substantially contributed to these advances in topics like: i) improvement of virus purification to obtain biologically active virions, ii) sequencing mild CTV isolates for genetic comparisons with sequences of moderate or severe isolates and genetic engineering, iii) analysis of genetic variation of both CTV genomic RNA ends and features of the highly variable 5' end that allow accommodating this variation within a conserved secondary structure, iv) studies on the structure, subcellular localization and biological functions of the CTV-unique p23 protein, and v) potential use of p23 and other 3'-proximal regions of the CTV genome to develop transgenic citrus resistant to the virus. Here we review his main achievements on these topics and how they contributed to deeper understanding of CTV biology and to new potential measures for disease control.

Modeling 'Candidatus Liberibacter asiaticus' Movement Within Citrus Plants

The phloem-limited 'Candidatus Liberibacter asiaticus' (Las) causes huanglongbing, a destructive citrus disease. Graft-inoculated potted plants were used to assess Las speed of movement in phloem in the greenhouse, and the impacts of temperature on plant colonization in growth-chamber experiments. For assessment of Las speed, plants were inoculated at the main stem and assessed over time by quantitative PCR (qPCR) or symptoms at various distances from the inoculum. For colonization, the plants were inoculated in one of two opposite top branches, maintained at from 8 to 20°C, from 18 to 30°C, or from 24 to 38°C daily range, and assessed by qPCR of samples taken from noninoculated shoots. For all experiments, frequencies of Las-positive sites were submitted to analysis of variance and binomial generalized linear model and logistic regression analyses. Probabilities of detecting Las in greenhouse plants were functions of time and distance from the inoculation site, which resulted in 2.9 and 3.8 cm day^-1 average speed of movement. In growth chambers, the temperature impacted plant colonization by Las, new shoot emission, and symptom expression. After a 7-month exposure time, Las was absent in all new shoots in the cooler environment (average three per plant), and present in 70% at the milder environment (six shoots, severe symptoms) and 25% in the warmer environment (eight shoots, no visible symptoms). Temperature of 25.7°C was the optimum condition for plant colonization. This explains the higher impact and incidence of huanglongbing disease during the winter months or regions of milder climates in Brazil.

Viromics unveils extraordinary genetic diversity of the family Closteroviridae in wild citrus

Our knowledge of citrus viruses is largely skewed toward virus pathology in cultivated orchards. Little is known about the virus diversity in wild citrus species. Here, we used a metatranscriptomics approach to characterize the virus diversity in a wild citrus habitat within the proposed center of the origin of citrus plants. We discovered a total of 44 virus isolates that could be classified into species Citrus tristeza virus and putative species citrus associated ampelovirus 1, citrus associated ampelovirus 2, and citrus virus B within the family Closteroviridae, providing important information to explore the factors facilitating outbreaks of citrus viruses and the evolutionary history of the family Closteroviridae. We found that frequent horizontal gene transfer, gene duplication, and alteration of expression strategy have shaped the genome complexity and diversification of the family Closteroviridae. Recombination frequently occurred among distinct Closteroviridae members, thereby facilitating the evolution of Closteroviridae. Given the potential emergence of similar wild-citrus-originated novel viruses as pathogens, the need for surveillance of their pathogenic and epidemiological characteristics is of utmost priority for global citrus production.

Closterovirids are principal plant pathogens for citrus trees and other plants, as they sometimes cause new or re-emerging diseases. However, the closterovirid diversity in natural plant hosts, especially citrus plants, is unclear. Here, we describe three novel species and Citrus tristeza virus within the family Closteroviridae that were sampled from wild citrus trees growing in their natural habitat in southwestern China. The presence of three different taxon classes of the family Closteroviridae indicates the geographical uniqueness of the sampling region for citrus closterovirid evolution. Our analysis shows that frequent horizontal gene transfer, gene duplication, alteration of expression strategy, and recombination have been important evolutionary processes in the diversification of the family Closteroviridae. Our study also shows the significance of natural reserves as potential sources of disease agents endangering cultivated crop plants.

Evidence That 'Candidatus Liberibacter asiaticus' Moves Predominantly Toward New Tissue Growth in Citrus Plants

'Candidatus Liberibacter asiaticus' (Las) is an unculturable, phloem-limited, insect-transmitted bacterium associated with the Asiatic form of huanglongbing (HLB), the most destructive citrus disease. In Asia and the Americas, it is transmitted by the Asian citrus psyllid (Diaphorina citri Kuwavama). Despite considerable research, little is known about the processes involved in plant infection and colonization by Las. This study was conducted to determine whether the basal portion (below girdling) of the plant is an important route for Las to move laterally from a point of inoculation on a branch to pathogen-free branches elsewhere in the canopy, and to quantify the influence of actively growing tissues on vertical upward (acropetally) or downward (basipetally) movement of Las. Nongirdled and fully or partially girdled stems of potted plants of 'Pera' sweet orange, graft-inoculated above or below girdling, were sampled in distinct regions and assessed by qPCR, 6 months postinoculation. Las invaded all regions of partially and nongirdled plants but remained restricted to the inoculated regions of fully girdled plants, evidence that in planta bacterium movement is limited to the phloem. In fully girdled plants, starch accumulated above the girdling site, probably because of changes in flow of phloem sap. To study the influence of actively growing tissues, inoculated 'Valencia' sweet orange plants were kept intact or were top- or root-pruned to force production of new tissues, and sampled at 15-day intervals. Las migrated rapidly and most predominantly toward newly developing root and leaf tissues. The rapid and predominant movement of Las to newly developed shoots and roots would explain failures of canopy heat treatments and pruning to cure HLB-affected trees, and reinforces the need to protect rapidly growing new shoots from feeding by D. citri in order to minimize transmission and spread of the pathogen by the vector within and between orchards.

Walking Together: Cross-Protection, Genome Conservation, and the Replication Machinery of Citrus tristeza virus

"Cross-protection", a nearly 100 years-old virological term, is suggested to be changed to "close protection". Evidence for the need of such change has accumulated over the past six decades from the laboratory experiments and field tests conducted by plant pathologists and plant virologists working with different plant viruses, and, in particular, from research on Citrus tristeza virus (CTV). A direct confirmation of such close protection came with the finding that "pre-immunization" of citrus plants with the variants of the T36 strain of CTV but not with variants of other virus strains was providing protection against a fluorescent protein-tagged T36-based recombinant virus variant. Under natural conditions close protection is functional and is closely associated both with the conservation of the CTV genome sequence and prevention of superinfection by closely similar isolates. It is suggested that the mechanism is primarily directed to prevent the danger of virus population collapse that could be expected to result through quasispecies divergence of large RNA genomes of the CTV variants continuously replicating within long-living and highly voluminous fruit trees. This review article provides an overview of the CTV cross-protection research, along with a discussion of the phenomenon in the context of the CTV biology and genetics.

Modulation of disease severity by plant positive-strand RNA viruses: The complex interplay of multifunctional viral proteins, subviral RNAs and virus-associated RNAs with plant signaling pathways and defense responses

Plant viruses induce a range of symptoms of varying intensity, ranging from severe systemic necrosis to mild or asymptomatic infection. Several evolutionary constraints drive virus virulence, including the dependence of viruses on host factors to complete their infection cycle, the requirement to counteract or evade plant antiviral defense responses and the mode of virus transmission. Viruses have developed an array of strategies to modulate disease severity. Accumulating evidence has highlighted not only the multifunctional role that viral proteins play in disrupting or highjacking plant factors, hormone signaling pathways and intracellular organelles, but also the interaction networks between viral proteins, subviral RNAs and/or other viral-associated RNAs that regulate disease severity. This review focusses on positive-strand RNA viruses, which constitute the majority of characterized plant viruses. Using well-characterized viruses with different genome types as examples, recent advances are discussed as well as knowledge gaps and opportunities for further research.

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.

The resistance of sour orange to Citrus tristeza virus is mediated by both the salicylic acid and RNA silencing defence pathways

Citrus tristeza virus (CTV) induces in the field the decline and death of citrus varieties grafted on sour orange (SO) rootstock, which has forced the use of alternative decline-tolerant rootstocks in affected countries, despite the highly desirable agronomic features of the SO rootstock. Declining citrus plants display phloem necrosis below the bud union. In addition, SO is minimally susceptible to CTV compared with other citrus varieties, suggesting partial resistance of SO to CTV. Here, by silencing different citrus genes with a Citrus leaf blotch virus-based vector, we have examined the implication of the RNA silencing and salicylic acid (SA) defence pathways in the resistance of SO to CTV. Silencing of the genes RDR1, NPR1 and DCL2/DCL4, associated with these defence pathways, enhanced virus spread and accumulation in SO plants in comparison with non-silenced controls, whereas silencing of the genes NPR3/NPR4, associated with the hypersensitive response, produced a slight decrease in CTV accumulation and reduced stunting of SO grafted on CTV-infected rough lemon plants. We also found that the CTV RNA silencing suppressors p20 and p23 also suppress the SA signalling defence, with the suppressor activity being higher in the most virulent isolates.

PAMPs, PRRs, effectors and R-genes associated with citrus-pathogen interactions

BACKGROUND

Recent application of molecular-based technologies has considerably advanced our understanding of complex processes in plant-pathogen interactions and their key components such as PAMPs, PRRs, effectors and R-genes. To develop novel control strategies for disease prevention in citrus, it is essential to expand and consolidate our knowledge of the molecular interaction of citrus plants with their pathogens.

SCOPE

This review provides an overview of our understanding of citrus plant immunity, focusing on the molecular mechanisms involved in the interactions with viruses, bacteria, fungi, oomycetes and vectors related to the following diseases: tristeza, psorosis, citrus variegated chlorosis, citrus canker, huanglongbing, brown spot, post-bloom, anthracnose, gummosis and citrus root rot.

Effect of Temperature on Wheat Streak Mosaic Disease Development in Winter Wheat

Temperature is one of the key factors that influence viral disease development in plants. In this study, temperature effect on Wheat streak mosaic virus (WSMV) replication and in planta movement was determined using a green fluorescent protein (GFP)-tagged virus in two winter wheat cultivars. Virus-inoculated plants were first incubated at 10, 15, 20, and 25°C for 21 days, followed by 27°C for 14 days; and, in a second experiment, virus-inoculated plants were initially incubated at 27°C for 3 days, followed by 10, 15, 20, and 25°C for 21 days. In the first experiment, WSMV-GFP in susceptible 'Tomahawk' wheat at 10°C was restricted at the point of inoculation whereas, at 15°C, the virus moved systemically, accompanied with mild symptoms, and, at 20 and 25°C, WSMV elicited severe WSMV symptoms. In resistant 'Mace' wheat (PI 651043), WSMV-GFP was restricted at the point of inoculation at 10 and 15°C but, at 20 and 25°C, the virus infected systemically with no visual symptoms. Some plants that were not systemically infected at low temperatures expressed WSMV-GFP in regrowth shoots when later held at 27°C. In the second experiment, Tomahawk plants (100%) expressed systemic WSMV-GFP after 21 days at all four temperature levels; however, systemic WSMV expression in Mace was delayed at the lower temperatures. These results indicate that temperature played an important role in WSMV replication, movement, and symptom development in resistant and susceptible wheat cultivars. This study also demonstrates that suboptimal temperatures impair WSMV movement but the virus rapidly begins to replicate and spread in planta under optimal temperatures.

Rootstock-scion interaction affecting citrus response to CTV infection: a proteomic view

Citrus tristeza virus (CTV) is the causal agent of various diseases with dramatic effects on citrus crops worldwide. Most Citrus species, grown on their own roots, are symptomless hosts for many CTV isolates. However, depending on different scion-rootstock combination, CTV infection should result in distinct syndromes, being 'tristeza' the more severe one, leading to a complete decline of the susceptible plants in a few weeks. Transcriptomic analyses revealed several genes involved either in defense response, or systemic acquired resistance, as well as transcription factors and components of the phosphorylation cascades, to be differentially regulated during CTV infection in Citrus aurantifolia species. To date little is known about the molecular mechanism of this host-pathogen interaction, and about the rootstock effect on citrus response to CTV infection. In this work, the response to CTV infection has been investigated in tolerant and susceptible scion-rootstock combinations by two-dimensional gel electrophoresis (2DE). A total of 125 protein spots have been found to be differently accumulated and/or phosphorylated between the two rootstock combinations. Downregulation in tolerant plants upon CTV infection was detected for proteins involved in reactive oxygen species (ROS) scavenging and defense response, suggesting a probable acclimation response able to minimize the systemic effects of virus infection. Some of these proteins resulted to be modulated also in absence of virus infection, revealing a rootstock effect on scion proteome modulation. Moreover, the phospho-modulation of proteins involved in ROS scavenging and defense response, further supports their involvement either in scion-rootstock crosstalk or in the establishment of tolerance/susceptibility to CTV infection.

Simultaneous visualization of two Citrus tristeza virus genotypes provides new insights into the structure of multi-component virus populations in a host

Complex Citrus tristeza virus (CTV) populations composed of mixtures of different strains of the virus are commonly found in citrus trees in the field. At present, little is known about how these populations are formed, maintained, and how they are structured within a host. Here we used a novel in situ hybridization approach allowing simultaneous visualization of two different RNA targets with high sensitivity and specificity to examine the distribution of two isolates, T36 and T68-1, representing phylogenetically distinct strains of CTV, in a citrus host in single and mixed infections. Remarkably, in doubly inoculated plants the two virus variants appeared to be well mixed within the infected tissue and showed no spatial segregation. In addition, both CTV variants were often found occupying the same cells. Possible mechanisms involved in shaping CTV populations and the biological significance of the observed lack of structural separation of the individual components are discussed.

Finding balance: Virus populations reach equilibrium during the infection process

Virus populations, mixtures of viral strains or species, are a common feature of viral infection, and influence many viral processes including infection, transmission, and the induction of disease. Yet, little is known of the rules that define the composition and structure of these populations. In this study, we used three distinct strains of Citrus tristeza virus (CTV) to examine the effect of inoculum composition, titer, and order, on the virus population. We found that CTV populations stabilized at the same equilibrium irrespective of how that population was introduced into a host. In addition, both field and experimental observations showed that these equilibria were relatively uniform between individual hosts of the same species and under the same conditions. We observed that the structure of the equilibria reached is determined primarily by the host, with the same inoculum reaching different equilibria in different species, and by the fitness of individual virus variants.

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.

Comparative analysis among the small RNA populations of source, sink and conductive tissues in two different plant-virus pathosystems

Background

In plants, RNA silencing plays a fundamental role as defence mechanism against viruses. During last years deep-sequencing technology has allowed to analyze the sRNA profile of a large variety of virus-infected tissues. Nevertheless, the majority of these studies have been restricted to a unique tissue and no comparative analysis between phloem and source/sink tissues has been conducted. In the present work, we compared the sRNA populations of source, sink and conductive (phloem) tissues in two different plant virus pathosystems. We chose two cucurbit species infected with two viruses very different in genome organization and replication strategy; Melon necrotic spot virus (MNSV) and Prunus necrotic ringspot virus (PNRSV).

Results

Our findings showed, in both systems, an increase of the 21-nt total sRNAs together with a decrease of those with a size of 24-nt in all the infected tissues, except for the phloem where the ratio of 21/24-nt sRNA species remained constant. Comparing the vsRNAs, both PNRSV- and MNSV-infected plants share the same vsRNA size distribution in all the analyzed tissues. Similar accumulation levels of sense and antisense vsRNAs were observed in both systems except for roots that showed a prevalence of (+) vsRNAs in both pathosystems. Additionally, the presence of overrepresented discrete sites along the viral genome, hot spots, were identified and validated by stem-loop RT-PCR. Despite that in PNRSV-infected plants the presence of vsRNAs was scarce both viruses modulated the host sRNA profile.

Conclusions

We compare for the first time the sRNA profile of four different tissues, including source, sink and conductive (phloem) tissues, in two plant-virus pathosystems. Our results indicate that antiviral silencing machinery in melon and cucumber acts mainly through DCL4. Upon infection, the total sRNA pattern in phloem remains unchanged in contrast to the rest of the analyzed tissues indicating a certain tissue-tropism to this polulation. Independently of the accumulation level of the vsRNAs both viruses were able to modulate the host sRNA pattern.

Electronic supplementary material

The online version of this article (doi:10.1186/s12864-015-1327-5) contains supplementary material, which is available to authorized users.

With a little help from my friends: complementation as a survival strategy for viruses in a long-lived host system

In selective host species, the extent of Citrus tristeza virus (CTV) infection is limited through the prevention of long-distance movement. As CTV infections often contain a population of multiple strains, we investigated whether the members of a population were capable of interaction, and what effect this would have on the infection process. We found that the tissue-tropism limitations of strain T36 in selective hosts could be overcome through interaction with a second strain, VT, increasing titer of, and number of cells infected by, T36. This interaction was strain-specific: other strains, T30 and T68, did not complement T36, indicating a requirement for compatibility between gene-products of the strains involved. This interaction was also host-specific, suggesting a second requirement of compatibility between the provided gene-product and host. These findings provide insight into the 'rules' that govern interaction between strains, and suggest an important mechanism by which viruses survive in a changing environment.

Citrus tristeza virus: making an ally from an enemy

Virus diseases of perennial trees and vines have characteristics not amenable to study using small model annual plants. Unique disease symptoms such as graft incompatibilities and stem pitting cause considerable crop losses. Also, viruses in these long-living plants tend to accumulate complex populations of viruses and strains. Considerable progress has been made in understanding the biology and genetics of Citrus tristeza virus (CTV) and in developing it into a tool for crop protection and improvement. The diseases in tree and vine crops have commonalities for which CTV can be used to develop a baseline. The purpose of this review is to provide a necessary background of systems and reagents developed for CTV that can be used for continued progress in this area and to point out the value of the CTV-citrus system in answering important questions on plant-virus interactions and developing new methods for controlling plant diseases.

A viral protein mediates superinfection exclusion at the whole-organism level but is not required for exclusion at the cellular level

Superinfection exclusion (SIE), the ability of an established virus infection to interfere with a secondary infection by the same or a closely related virus, has been described for different viruses, including important pathogens of humans, animals, and plants. Citrus tristeza virus (CTV), a positive-sense RNA virus, represents a valuable model system for studying SIE due to the existence of several phylogenetically distinct strains. Furthermore, CTV allows SIE to be examined at the whole-organism level. Previously, we demonstrated that SIE by CTV is a virus-controlled function that requires the viral protein p33. In this study, we show that p33 mediates SIE at the whole-organism level, while it is not required for exclusion at the cellular level. Primary infection of a host with a fluorescent protein-tagged CTV variant lacking p33 did not interfere with the establishment of a secondary infection by the same virus labeled with a different fluorescent protein. However, cellular coinfection by both viruses was rare. The obtained observations, along with estimates of the cellular multiplicity of infection (MOI) and MOI model selection, suggested that low levels of cellular coinfection appear to be best explained by exclusion at the cellular level. Based on these results, we propose that SIE by CTV is operated at two levels--the cellular and the whole-organism levels--by two distinct mechanisms that could function independently. This novel aspect of viral SIE highlights the intriguing complexity of this phenomenon, further understanding of which may open up new avenues to manage virus diseases.

IMPORTANCE

Many viruses exhibit superinfection exclusion (SIE), the ability of an established virus infection to interfere with a secondary infection by related viruses. SIE plays an important role in the pathogenesis and evolution of virus populations. The observations described here suggest that SIE could be controlled independently at different levels of the host: the whole-organism level or the level of individual cells. The p33 protein of citrus tristeza virus (CTV), an RNA virus, was shown to mediate SIE at the whole-organism level, while it appeared not to be required for exclusion at the cellular level. SIE by CTV is, therefore, highly complex and appears to use mechanisms different from those proposed for other viruses. A better understanding of this phenomenon may lead to the development of new strategies for controlling viral diseases in human populations and agroecosystems.

A personal history of virus-based vector construction

The ability to express foreign genes or to silence endogenous genes in plants has revolutionized both basic and applied plant biology. Virus-based expression systems, in which the foreign mRNA is greatly amplified by virus replication, can produce very high levels of proteins or peptides in leaves and other tissues. Vectors have been available for about 25 years. They are commonplace as laboratory tools, but their initial commercial expectations have not been met for numerous reasons. Yet, applications of viral vectors are still evolving. This chapter focuses on our laboratory's involvement in developing virus-based vectors in plants. We created the first 'add-a-gene' vectors that were capable of replication and movement throughout plants. These vectors were based on tobacco mosaic virus. Through the evolution of several prototypes, stable vectors were developed that produced relatively large amounts of product in plants. Recently, we created similar vectors for citrus trees based on citrus tristeza virus. Even though the citrus vectors were created as laboratory tools for improving the crop, circumstances have changed the applications to protection and therapy of trees in the field.

Citrus leaf blotch virus invades meristematic regions in Nicotiana benthamiana and citrus

To invade systemically host plants, viruses need to replicate in the infected cells, spread to neighbouring cells through plasmodesmata and move to distal parts of the plant via sieve tubes to start new infection foci. To monitor the infection of Nicotiana benthamiana plants by Citrus leaf blotch virus (CLBV), leaves were agroinoculated with an infectious cDNA clone of the CLBV genomic RNA expressing green fluorescent protein (GFP) under the transcriptional control of a duplicate promoter of the coat protein subgenomic RNA. Fluorescent spots first appeared in agroinfiltrated leaves 11-12 days after infiltration, indicating CLBV replication. Then, after entering the phloem vascular system, CLBV was unloaded in the upper parts of the plant and invaded all tissues, including flower organs and meristems. GFP fluorescence was not visible in citrus plants infected with CLBV-GFP. Therefore, to detect CLBV in meristematic regions, Mexican lime (Citrus aurantifolia) plants were graft inoculated with CLBV, with Citrus tristeza virus (CTV), a virus readily eliminated by shoot-tip grafting in vitro, or with both simultaneously. Although CLBV was detected by hybridization and real-time reverse transcription-polymerase chain reaction (RT-PCR) in 0.2-mm shoot tips in all CLBV-inoculated plants, CTV was not detected. These results explain the difficulty in eliminating CLBV by shoot-tip grafting in vitro.

Viral and cellular factors involved in Phloem transport of plant viruses

Phloem transport of plant viruses is an essential step in the setting-up of a complete infection of a host plant. After an initial replication step in the first cells, viruses spread from cell-to-cell through mesophyll cells, until they reach the vasculature where they rapidly move to distant sites in order to establish the infection of the whole plant. This last step is referred to as systemic transport, or long-distance movement, and involves virus crossings through several cellular barriers: bundle sheath, vascular parenchyma, and companion cells for virus loading into sieve elements (SE). Viruses are then passively transported within the source-to-sink flow of photoassimilates and are unloaded from SE into sink tissues. However, the molecular mechanisms governing virus long-distance movement are far from being understood. While most viruses seem to move systemically as virus particles, some viruses are transported in SE as viral ribonucleoprotein complexes (RNP). The nature of the cellular and viral factors constituting these RNPs is still poorly known. The topic of this review will mainly focus on the host and viral factors that facilitate or restrict virus long-distance movement.

Virus-based transient expression vectors for woody crops: a new frontier for vector design and use

Virus-based expression vectors are commonplace tools for the production of proteins or the induction of RNA silencing in herbaceous plants. This review considers a completely different set of uses for viral vectors in perennial fruit and nut crops, which can be productive for periods of up to 100 years. Viral vectors could be used in the field to modify existing plants. Furthermore, with continually emerging pathogens and pests, viral vectors could express genes to protect the plants or even to treat plants after they become infected. As technologies develop during the life span of these crops, viral vectors can be used for adding new genes as an alternative to pushing up the crop and replanting with transgenic plants. Another value of virus-based vectors is that they add nothing permanently to the environment. This requires that effective and stable viral vectors be developed for specific crops from endemic viruses. Studies using viruses from perennial hosts suggest that these objectives could be accomplished.

Citrus tristeza virus-host interactions

Citrus tristeza virus (CTV) is a phloem-limited virus whose natural host range is restricted to citrus and related species. Although the virus has killed millions of trees, almost destroying whole industries, and continually limits production in many citrus growing areas, most isolates are mild or symptomless in most of their host range. There is little understanding of how the virus causes severe disease in some citrus and none in others. Movement and distribution of CTV differs considerably from that of well-studied viruses of herbaceous plants where movement occurs largely through adjacent cells. In contrast, CTV systemically infects plants mainly by long-distance movement with only limited cell-to-cell movement. The virus is transported through sieve elements and occasionally enters an adjacent companion or phloem parenchyma cell where virus replication occurs. In some plants this is followed by cell-to-cell movement into only a small cluster of adjacent cells, while in others there is no cell-to-cell movement. Different proportions of cells adjacent to sieve elements become infected in different plant species. This appears to be related to how well viral gene products interact with specific hosts. CTV has three genes (p33, p18, and p13) that are not necessary for infection of most of its hosts, but are needed in different combinations for infection of certain citrus species. These genes apparently were acquired by the virus to extend its host range. Some specific viral gene products have been implicated in symptom induction. Remarkably, the deletion of these genes from the virus genome can induce large increases in stem pitting (SP) symptoms. The p23 gene, which is a suppressor of RNA silencing and a regulator of viral RNA synthesis, has been shown to be the cause of seedling yellows (SY) symptoms in sour orange. Most isolates of CTV in nature are populations of different strains of CTV. The next frontier of CTV biology is the understanding how the virus variants in those mixtures interact with each other and cause diseases.

Citrus tristeza virus p23: a unique protein mediating key virus-host interactions

The large RNA genome of Citrus tristeza virus (CTV; ca. 20 kb) contains 12 open reading frames, with the 3′-terminal one corresponding to a protein of 209 amino acids (p23) that is expressed from an abundant subgenomic RNA. p23, an RNA-binding protein with a putative zinc-finger domain and some basic motifs, is unique to CTV because no homologs have been found in other closteroviruses, including the type species of the genus Beet yellows virus (despite both viruses having many homologous genes). Consequently, p23 might have evolved for the specific interaction of CTV with its citrus hosts. From a functional perspective p23 has been involved in many roles: (i) regulation of the asymmetrical accumulation of CTV RNA strands, (ii) induction of the seedling yellows syndrome in sour orange and grapefruit, (iii) intracellular suppression of RNA silencing, (iv) elicitation of CTV-like symptoms when expressed ectopically as a transgene in several Citrus spp., and (v) enhancement of systemic infection (and virus accumulation) in sour orange and CTV release from the phloem in p23-expressing transgenic sweet and sour orange. Moreover, transformation of Mexican lime with intron-hairpin constructs designed for the co-inactivation of p23 and the two other CTV silencing suppressors results in complete resistance against the homologous virus. From a cellular point of view, recent data indicate that p23 accumulates preferentially in the nucleolus, being the first closterovirus protein with such a subcellular localization, as well as in plasmodesmata. These major accumulation sites most likely determine some of the functional roles of p23.

Citrus tristeza virus: Evolution of Complex and Varied Genotypic Groups

Amongst the Closteroviridae, Citrus tristeza virus (CTV) is almost unique in possessing a number of distinct and characterized strains, isolates of which produce a wide range of phenotype combinations among its different hosts. There is little understanding to connect genotypes to phenotypes, and to complicate matters more, these genotypes are found throughout the world as members of mixed populations within a single host plant. There is essentially no understanding of how combinations of genotypes affect symptom expression and disease severity. We know little about the evolution of the genotypes that have been characterized to date, little about the biological role of their diversity and particularly, about the effects of recombination. Additionally, genotype grouping has not been standardized. In this study we utilized an extensive array of CTV genomic information to classify the major genotypes, and to determine the major evolutionary processes that led to their formation and subsequent retention. Our analyses suggest that three major processes act on these genotypes: (1) ancestral diversification of the major CTV lineages, followed by (2) conservation and co-evolution of the major functional domains within, though not between CTV genotypes, and (3) extensive recombination between lineages that have given rise to new genotypes that have subsequently been retained within the global population. The effects of genotype diversity and host-interaction are discussed, as is a proposal for standardizing the classification of existing and novel CTV genotypes.

Developing an understanding of cross-protection by Citrus tristeza virus

Citrus tristeza virus (CTV) causes two citrus diseases that have caused devastating losses in global citrus production. The first disease is quick decline of trees propagated on the sour orange rootstock. The second disease is stem pitting, which severely affects a number of economically important citrus varieties regardless of the rootstock used and results in reduced tree growth and vigor as well as in reduced fruit size and quality. Both diseases continue to invade new areas. While quick decline could be effectively managed by the use of resistant and/or tolerant rootstocks, the only means to protect commercial citrus against endemic stem pitting isolates of CTV has been cross-protection with mild isolates of the virus. In some citrus areas cross-protection has been successful and allowed production of certain citrus cultivars despite the presence of severe stem pitting isolates in those regions. However, many other attempts to find isolates that would provide sustained protection against aggressive isolates of the virus had failed. In general, there has been no understanding why some mild isolates were effective and others failed to protect. We have been working on the mechanism of cross-protection by CTV. Recent considerable progress has significantly advanced our understanding of how cross-protection may work in the citrus/CTV pathosystem. As we demonstrated, only isolates that belong to the same strain of the virus cross protect against each other, while isolates from different strains do not. We believe that the results of our research could now make finding protecting isolates relatively straightforward. This review discusses some of the history of CTV cross-protection along with the recent findings and our "recipe" for selection of protecting isolates.

Citrus tristeza virus p23: determinants for nucleolar localization and their influence on suppression of RNA silencing and pathogenesis

Citrus tristeza virus (CTV) encodes a singular protein (p23, 209 amino acids) with multiple functions, including RNA silencing suppression (RSS). Confocal laser-scanning microscopy of green fluorescent protein (GFP)-p23 agroexpressed in Nicotiana benthamiana revealed its accumulation in the nucleolus, Cajal bodies, and plasmodesmata. To dissect the nucleolar localization signal (NoLS) typically associated with basic motifs, seven truncated and 10 point-mutated versions of p23 were assayed. Deletion mutants showed that regions 50 to 86 and 100 to 157 (excluding fragment 106 to 114), both with basic motifs and the first with a zinc-finger, contain the (bipartite) NoLS. Alanine substitutions delimited this signal to three cysteines of the Zn-finger and some basic amino acids. RSS activity of p23 in N. benthamiana was abolished by essentially all mutants, indicating that it involves most p23 regions. The necrotic-inducing ability of p23 when launched in N. benthamiana from Potato virus X was only retained by deletion mutant 158-209 and one substitution mutant, showing that the Zn-finger and flanking basic motifs form part of the pathogenic determinant. Ectopic expression of p23 and some deletion mutants in transgenic Mexican lime demarcated a similar determinant, suggesting that p23 affects related pathways in citrus and N. benthamiana. Both RSS activity and pathogenicity of p23 appear related to its nucleolar localization.

Enhancement or attenuation of disease by deletion of genes from Citrus tristeza virus

Stem pitting is a common virus-induced disease of perennial woody plants induced by a range of different viruses. The phenotype results from sporadic areas of the stem in which normal xylem and phloem development is prevented during growth of stems. These alterations interfere with carbohydrate transport, resulting in reduced plant growth and yield. Citrus tristeza virus (CTV), a phloem-limited closterovirus, induces economically important stem-pitting diseases of citrus. CTV has three nonconserved genes (p33, p18, and p13) that are not related to genes of other viruses and that are not required for systemic infection of some species of citrus, which allowed us to examine the effect of deletions of these genes on symptom phenotypes. In the most susceptible experimental host, Citrus macrophylla, the full-length virus causes only very mild stem-pitting symptoms. Surprisingly, we found that certain deletion combinations (p33 and p18 and/or p13) induced greatly increased stem-pitting symptoms, while other combinations (p13 or p13 plus p18) resulted in reduced stem pitting. These results suggest that the stem-pitting phenotype, which is one of more economically important disease phenotypes, can result not from a specific sequence or protein but from a balance between the expression of different viral genes. Unexpectedly, using green fluorescent protein-tagged full-length virus and deletion mutants (CTV9Δp33 and CTV9Δp33Δp18Δp13), the virus was found at pitted areas in abnormal locations outside the normal ring of phloem. Thus, increased stem pitting was associated not only with a prevention of xylem production but also with a proliferation of cells that supported viral replication, suggesting that at random areas of stems the virus can elicit changes in cellular differentiation and development.

Transformation of Mexican lime with an intron-hairpin construct expressing untranslatable versions of the genes coding for the three silencing suppressors of Citrus tristeza virus confers complete resistance to the virus

Citrus tristeza virus (CTV), the causal agent of the most devastating viral disease of citrus, has evolved three silencing suppressor proteins acting at intra- (p23 and p20) and/or intercellular level (p20 and p25) to overcome host antiviral defence. Previously, we showed that Mexican lime transformed with an intron-hairpin construct including part of the gene p23 and the adjacent 3' untranslated region displays partial resistance to CTV, with a fraction of the propagations from some transgenic lines remaining uninfected. Here, we transformed Mexican lime with an intron-hairpin vector carrying full-length, untranslatable versions of the genes p25, p20 and p23 from CTV strain T36 to silence the expression of these critical genes in CTV-infected cells. Three transgenic lines presented complete resistance to viral infection, with all their propagations remaining symptomless and virus-free after graft inoculation with CTV-T36, either in the nontransgenic rootstock or in the transgenic scion. Accumulation of transgene-derived siRNAs was necessary but not sufficient for CTV resistance. Inoculation with a divergent CTV strain led to partially breaking the resistance, thus showing the role of sequence identity in the underlying mechanism. Our results are a step forward to developing transgenic resistance to CTV and also show that targeting simultaneously by RNA interference (RNAi) the three viral silencing suppressors appears critical for this purpose, although the involvement of concurrent RNAi mechanisms cannot be excluded.

Superinfection exclusion is an active virus-controlled function that requires a specific viral protein

Superinfection exclusion, a phenomenon in which a preexisting viral infection prevents a secondary infection with the same or a closely related virus, has been described for various viruses, including important pathogens of humans, animals, and plants. The phenomenon was initially used to test the relatedness of plant viruses. Subsequently, purposeful infection with a mild isolate has been implemented as a protective measure against virus isolates that cause severe disease. In the medical and veterinary fields, superinfection exclusion was found to interfere with repeated applications of virus-based vaccines to individuals with persistent infections and with the introduction of multicomponent vaccines. In spite of its significance, our understanding of this phenomenon is surprisingly incomplete. Recently, it was demonstrated that superinfection exclusion of Citrus tristeza virus (CTV), a positive-sense RNA closterovirus, occurs only between isolates of the same strain, but not between isolates of different strains of the virus. In this study, I show that superinfection exclusion by CTV requires production of a specific viral protein, the p33 protein. Lack of the functional p33 protein completely eliminated the ability of the virus to exclude superinfection by the same or a closely related virus. Remarkably, the protein appeared to function only in a homology-dependent manner. A cognate protein from a heterologous strain failed to confer the exclusion, suggesting the existence of precise interactions of the p33 protein with other factors involved in this complex phenomenon.

Ectopic expression of the p23 silencing suppressor of Citrus tristeza virus differentially modifies viral accumulation and tropism in two transgenic woody hosts

Citrus tristeza virus (CTV), a phloem-restricted closterovirus infecting citrus, encodes three different silencing suppressors (p25, p20 and p23), one of which (p23) is a pathogenicity determinant that induces aberrations resembling CTV symptoms when expressed ectopically in transgenic citrus hosts. In this article, the effect of p23 ectopic expression on virus infection was examined in sweet orange (SwO), a highly susceptible host, and sour orange (SO), which severely restricts CTV cell-to-cell movement. Transgenic plants of both species ectopically expressing p23, or transformed with an empty vector, were graft inoculated with the mild CTV isolate T385 or with CTV-BC1/GFP, a clonal strain derived from the severe isolate T36 carrying the gene for the green fluorescent protein (GFP). CTV distribution in infected tissues was assessed by direct tissue blot immunoassay and fluorescence emission, and virus accumulation was estimated by quantitative real-time reverse transcriptase-polymerase chain reaction. CTV accumulation in p23-expressing and control SwO plants was similar, whereas the viral load in transgenic SO expressing p23 was 10-10(5) times higher than in the cognate control plants. Although few infection foci composed of a single cell were observed in the phloem of CTV-infected control SO, the number of foci in p23-expressing plants was higher and usually comprised two to six cells, indicating viral cell-to-cell movement. CTV was detected in mesophyll protoplasts and cells from infected SO and SwO expressing p23, but not in similar protoplasts and cells from infected control plants. Our results show that the ectopic expression of p23 enables CTV to escape from the phloem and, in addition, facilitates systemic infection of the resistant SO host. This is the first report of a viral-encoded protein that enhances virus accumulation and distribution in woody hosts.

The evolutionary rate of citrus tristeza virus ranks among the rates of the slowest RNA viruses

Citrus tristeza virus (CTV) has been studied intensively at the molecular level. However, knowledge regarding the dynamics of its evolution is practically non-existent. In the past, diverse authors have referred to CTV as a highly variable virus, implying rapid evolution. Others have, in recent times, referred to CTV as an exceptionally slowly evolving virus. In this work, we used the capsid protein (CP) gene to estimate the rate of evolution. This was obtained from a large set of heterochronous CP gene sequences using a bayesian coalescent approach. The best-fitting evolutionary and population models pointed to an evolutionary rate of 1.58×10(-4) nt per site year(-1) (95 % highest posterior density, 1.73×10(-5)-3.16×10(-4) nt per site year(-1)). For an unbiased comparison with other plant and animal viruses, the evolutionary rate of synonymous substitutions was considered. In a series of 88 synonymous evolutionary rates, ranging from 5.2×10(-6) to 6.2×10(-2) nt per site year(-1), CTV ranks in the 10th percentile, embedded among the slowest animal RNA viruses. At the time of citrus dissemination to Europe and the New World, the major clades that led to the current phylogenetic groups were already defined, which may explain the absence nowadays of geographical speciation.

A plant virus evolved by acquiring multiple nonconserved genes to extend its host range

Viruses have evolved as combinations of genes whose products interact with cellular components to produce progeny virus throughout the plants. Some viral genes, particularly those that are involved in replication and assembly, tend to be relatively conserved, whereas other genes that have evolved for interactions with the specific host for movement and to counter host-defense systems tend to be less conserved. Closteroviridae encode 1-5 nonconserved ORFs. Citrus tristeza virus (CTV), a Closterovirus, possesses nonconserved p33, p18, and p13 genes that are expendable for systemic infection of the two laboratory hosts, Citrus macrophylla and Mexican lime. In this study, we show that the extended host range of CTV requires these nonconserved genes. The p33 gene was required to systemically infect sour orange and lemon trees, whereas either the p33 or the p18 gene was sufficient for systemic infection of grapefruit trees and the p33 or the p13 gene was sufficient for systemic infection of calamondin plants. Thus, these three genes are required for systemic infection of the full host range of CTV, but different genes were specific for different hosts. Remarkably, either of two genes was sufficient for infection of some citrus hybrids. These findings suggest that CTV acquired multiple nonconserved genes (p33, p18, and p13) and, as a result, gained the ability to interact with multiple hosts, thus extending its host range during the course of evolution. These results greatly extend the complexity of known virus-plant interactions.

Citrus tristeza virus infection induces the accumulation of viral small RNAs (21-24-nt) mapping preferentially at the 3'-terminal region of the genomic RNA and affects the host small RNA profile

To get an insight into the host RNA silencing defense induced by Citrus tristeza virus (CTV) and into the counter defensive reaction mediated by its three silencing suppressors (p25, p20 and p23), we have examined by deep sequencing (Solexa-Illumina) the small RNAs (sRNAs) in three virus-host combinations. Our data show that CTV sRNAs: (i) represent more than 50% of the total sRNAs in Mexican lime and sweet orange (where CTV reaches relatively high titers), but only 3.5% in sour orange (where the CTV titer is significantly lower), (ii) are predominantly of 21-22-nt, with a biased distribution of their 5' nucleotide and with those of (+) polarity accumulating in a moderate excess, and (iii) derive from essentially all the CTV genome (ca. 20 kb), as revealed by its complete reconstruction from viral sRNA contigs, but adopt an asymmetric distribution with a prominent hotspot covering approximately the 3'-terminal 2,500 nt. These results suggest that the citrus homologues of Dicer-like (DCL) 4 and 2 most likely mediate the genesis of the 21 and 22 nt CTV sRNAs, respectively, and show that both ribonucleases act not only on the genomic RNA but also on the 3' co-terminal subgenomic RNAs and, particularly, on their double-stranded forms. The plant sRNA profile, very similar and dominated by the 24-nt sRNAs in the three mock-inoculated controls, was minimally affected by CTV infection in sour orange, but exhibited a significant reduction of the 24-nt sRNAs in Mexican lime and sweet orange. We have also identified novel citrus miRNAs and determined how CTV influences their accumulation.

Efficient and stable expression of GFP through Wheat streak mosaic virus-based vectors in cereal hosts using a range of cleavage sites: formation of dense fluorescent aggregates for sensitive virus tracking

A series of Wheat streak mosaic virus (WSMV)-based expression vectors were developed by engineering a cycle 3 GFP (GFP) cistron between P1 and HC-Pro cistrons with several catalytic/cleavage peptides at the C-terminus of GFP. WSMV-GFP vectors with the Foot-and-mouth disease virus 1D/2A or 2A catalytic peptides cleaved GFP from HC-Pro but expressed GFP inefficiently. WSMV-GFP vectors with homologous NIa-Pro heptapeptide cleavage sites did not release GFP from HC-Pro, but efficiently expressed GFP as dense fluorescent aggregates. However, insertion of one or two spacer amino acids on either side of NIb/CP heptapeptide cleavage site or deletion in HC-Pro cistron improved processing by NIa-Pro. WSMV-GFP vectors were remarkably stable in wheat for seven serial passages and for 120 days postinoculation. Mite transmission efficiencies of WSMV-GFP vectors correlated with the amount of free GFP produced. WSMV-GFP vectors infected the same range of cereal hosts as wild-type virus, and GFP fluorescence was detected in most wheat tissues.

Infection with strains of Citrus tristeza virus does not exclude superinfection by other strains of the virus

Superinfection exclusion or homologous interference, a phenomenon in which a primary viral infection prevents a secondary infection with the same or closely related virus, has been observed commonly for viruses in various systems, including viruses of bacteria, plants, and animals. With plant viruses, homologous interference initially was used as a test of virus relatedness to define whether two virus isolates were "strains" of the same virus or represented different viruses, and subsequently purposeful infection with a mild isolate was implemented as a protective measure against isolates of the virus causing severe disease. In this study we examined superinfection exclusion of Citrus tristeza virus (CTV), a positive-sense RNA closterovirus. Thirteen naturally occurring isolates of CTV representing five different virus strains and a set of isolates originated from virus constructs engineered based on an infectious cDNA clone of T36 isolate of CTV, including hybrids containing sequences from different isolates, were examined for their ability to prevent superinfection by another isolate of the virus. We show that superinfection exclusion occurred only between isolates of the same strain and not between isolates of different strains. When isolates of the same strain were used for sequential plant inoculation, the primary infection provided complete exclusion of the challenge isolate, whereas isolates from heterologous strains appeared to have no effect on replication, movement or systemic infection by the challenge virus. Surprisingly, substitution of extended cognate sequences from isolates of the T68 or T30 strains into T36 did not confer the ability of resulting hybrid viruses to exclude superinfection by those donor strains. Overall, these results do not appear to be explained by mechanisms proposed previously for other viruses. Moreover, these observations bring an understanding of some previously unexplained fundamental features of CTV biology and, most importantly, build a foundation for the strategy of selecting mild isolates that would efficiently exclude severe virus isolates as a practical means to control CTV diseases.

The pathogenicity determinant of Citrus tristeza virus causing the seedling yellows syndrome maps at the 3'-terminal region of the viral genome

Citrus tristeza virus (CTV) (genus Closterovirus, family Closteroviridae) causes some of the more important viral diseases of citrus worldwide. The ability to map disease-inducing determinants of CTV is needed to develop better diagnostic and disease control procedures. A distinctive phenotype of some isolates of CTV is the ability to induce seedling yellows (SY) in sour orange, lemon and grapefruit seedlings. In Florida, the decline isolate of CTV, T36, induces SY, whereas a widely distributed mild isolate, T30, does not. To delimit the viral sequences associated with the SY syndrome, we created a number of T36/T30 hybrids by substituting T30 sequences into different regions of the 3' half of the genome of an infectious cDNA of T36. Eleven T36/T30 hybrids replicated in Nicotiana benthamiana protoplasts. Five of these hybrids formed viable virions that were mechanically transmitted to Citrus macrophylla, a permissive host for CTV. All induced systemic infections, similar to that of the parental T36 clone. Tissues from these C. macrophylla source plants were then used to graft inoculate sour orange and grapefruit seedlings. Inoculation with three of the T30/T36 hybrid constructs induced SY symptoms identical to those of T36; however, two hybrids with T30 substitutions in the p23-3' nontranslated region (NTR) (nucleotides 18 394-19 296) failed to induce SY. Sour orange seedlings infected with a recombinant non-SY p23-3' NTR hybrid also remained symptomless when challenged with the parental virus (T36), demonstrating the potential feasibility of using engineered constructs of CTV to mitigate disease.

Accumulation of transgene-derived siRNAs is not sufficient for RNAi-mediated protection against Citrus tristeza virus in transgenic Mexican lime

Mexican lime plants transformed with the 3'-terminal 549 nucleotides of the Citrus tristeza virus (CTV) genome in sense, antisense and intron-hairpin formats were analysed for transgene-derived transcript and short interfering RNA (siRNA) accumulation, and for CTV resistance. Propagations from all sense, antisense and empty-vector transgenic lines were susceptible to CTV, except for a single sense-line plant with a complex transgene integration pattern that showed transgene-derived siRNAs in association with low levels of the transgene-derived transcript. In contrast, nine of 30 intron-hairpin lines showed CTV resistance, with 9%-56% of bud-propagated plants, depending on the line, remaining uninfected on graft inoculation, and the others being susceptible. Although resistance was always associated with the presence of transgene-derived siRNAs, their level in different sense and intron-hairpin transformants was variable irrespective of the response to CTV infection. In intron-hairpin lines with single transgene integration, CTV resistance was correlated with low accumulation of the transgene-derived transcript rather than with high accumulation of transgene-derived siRNAs.

Genetic diversity and evidence for recent modular recombination in Hawaiian Citrus tristeza virus

The Hawaiian Islands are home to a widespread and diverse population of Citrus tristeza virus (CTV), an economically important pathogen of citrus. In this study, we quantified the genetic diversity of two CTV genes and determined the complete genomic sequence for two strains of Hawaiian CTV. The nucleotide diversity was estimated to be 0.0565 + or - 0.0022 for the coat protein (CP) gene (n = 137) and 0.0822 + or - 0.0033 for the p23 gene (n = 30). The genome size and organization of CTV strains HA18-9 and HA16-5 were similar to other fully sequenced strains of CTV. The 3'-terminal halves of their genomes were nearly identical (98.5% nucleotide identity), whereas the 5'-terminal halves were more distantly related (72.3% nucleotide identity), suggesting a possible recombination event. Closer examination of strain HA16-5 indicated that it arose through recent recombination between the movement module of an HA18-9 genotype, and the replication module of an undescribed CTV genotype.