Development of a full-genome cDNA clone of Citrus leaf blotch virus and infection of citrus plants

Identification and Characterization of a Novel Quanzhou Mulberry Virus from Mulberry (Morus alba)

In this study, we discovered a new virus named Quanzhou mulberry virus (QMV), which was identified from the leaves of an ancient mulberry tree. This tree is over 1300 years old and is located at Fujian Kaiyuan Temple, a renowned cultural heritage site in China. We obtained the complete genome sequence of QMV using RNA sequencing followed by rapid amplification of complementary DNA ends (RACE). The QMV genome is 9256 nucleotides (nt) long and encodes five open reading frames (ORFs). Its virion was made of icosahedral particles. Phylogenetic analysis suggests that it belongs to the unclassified Riboviria. An infectious clone for QMV was generated and agroinfiltrated into Nicotiana benthamiana and mulberry, resulting in no visible disease symptoms. However, systemic movement of the virus was only observed in mulberry seedlings, suggesting that it has a host-specific pattern of movement. Our findings provide a valuable reference for further studies on QMV and related viruses, contributing to the understanding of viral evolution and biodiversity in mulberry.

Complete genome and molecular characterization of a putative novel citrivirus from Rudbeckia sp

We identified a tentative novel positive-strand RNA virus from Rudbeckia sp., namely, Rudbeckia citrivirus A (RuCVA). The complete genome sequence of the novel virus was 8821 nucleotides in length, excluding the poly(A) tail. It has three open reading frames (ORFs): a putative polyprotein, a movement protein, and a coat protein. Phylogenetic analysis showed that the virus was more closely related to Citrus leaf blotch virus isolates and unassigned citriviruses. The sequence identity of the virus with other citriviruses was lower than 56.9% at the complete nucleotide sequence level. For each ORF, the sequence identity was lower than 64.2% at the nucleotide level and 67.8% at the amino acid level. These results satisfied the species demarcation criteria for Betaflexiviridae. Therefore, we suggest that RuCVA is a novel member of the genus Citrivirus.

Autophagy Inhibits Intercellular Transport of Citrus Leaf Blotch Virus by Targeting Viral Movement Protein

Autophagy is an evolutionarily conserved cellular-degradation mechanism implicated in antiviral defense in plants. Studies have shown that autophagy suppresses virus accumulation in cells; however, it has not been reported to specifically inhibit viral spread in plants. This study demonstrated that infection with citrus leaf blotch virus (CLBV; genus Citrivirus, family Betaflexiviridae) activated autophagy in Nicotiana benthamiana plants as indicated by the increase of autophagosome formation. Impairment of autophagy through silencing of N. benthamiana autophagy-related gene 5 (NbATG5) and NbATG7 enhanced cell-to-cell and systemic movement of CLBV; however, it did not affect CLBV accumulation when the systemic infection had been fully established. Treatment using an autophagy inhibitor or silencing of NbATG5 and NbATG7 revealed that transiently expressed movement protein (MP), but not coat protein, of CLBV was targeted by selective autophagy for degradation. Moreover, we identified that CLBV MP directly interacted with NbATG8C1 and NbATG8i, the isoforms of autophagy-related protein 8 (ATG8), which are key factors that usually bind cargo receptors for selective autophagy. Our results present a novel example in which autophagy specifically targets a viral MP to limit the intercellular spread of the virus in plants.

The Citrus yellow mosaic badnavirus ORFI functions as a RNA-silencing suppressor

Citrus yellow mosaic badnavirus (CMBV) causes mosaic disease in all economically important citrus cultivars of India, with losses reaching up to 70%. CMBV belongs to the genus Badnavirus, family Caulimoviridae, possessing a circular double-stranded (ds) DNA genome with six open reading frames (ORFs I to VI), whose functions are yet to be deciphered. The RNA-silencing suppressor (RSS) activity has not been assigned to any CMBV ORF as yet. In the present study, it was found that ORFI exhibited RSS activity among all the six CMBV ORFs tested. Studies were done by employing the well-established Agrobacterium-mediated transient assay based on the transgenic Nicotiana benthamiana 16c plant line expressing the green fluorescent protein (GFP). The RSS activity of ORFI was confirmed by the analysis of the GFP visual expression in the agroinfiltrated leaves, further supported by quantification of GFP expression by RT-PCR. Based on the GFP visual expression, the CMBV ORFI was a weak RSS when compared to the p19 protein of tomato bushy stunt virus. In contrast, the ORFII, ORFIV, ORFV, ORFVI, and CP gene did not exhibit any RSS activity. Hence, ORFI is the first ORF of CMBV to be identified with RNA-silencing suppression activity.

IDA (INFLORESCENCE DEFICIENT IN ABSCISSION)-like peptides and HAE (HAESA)-like receptors regulate corolla abscission in Nicotiana benthamiana flowers

BACKGROUND

Abscission is an active, organized, and highly coordinated cell separation process enabling the detachment of aerial organs through the modification of cell-to-cell adhesion and breakdown of cell walls at specific sites on the plant body known as abscission zones. In Arabidopsis thaliana, abscission of floral organs and cauline leaves is regulated by the interaction of the hormonal peptide INFLORESCENCE DEFICIENT IN ABSCISSION (IDA), a pair of redundant receptor-like protein kinases, HAESA (HAE) and HAESA-LIKE2 (HSL2), and SOMATIC EMBRYOGENESIS RECEPTOR-LIKE KINASE (SERK) co-receptors. However, the functionality of this abscission signaling module has not yet been demonstrated in other plant species.

RESULTS

The expression of the pair of NbenIDA1 homeologs and the receptor NbenHAE.1 was supressed at the base of the corolla tube by the inoculation of two virus-induced gene silencing (VIGS) constructs in Nicotiana benthamiana. These gene suppression events arrested corolla abscission but did not produce any obvious effect on plant growth. VIGS plants retained a higher number of corollas attached to the flowers than control plants, an observation related to a greater corolla breakstrength. The arrest of corolla abscission was associated with the preservation of the parenchyma tissue at the base of the corolla tube that, in contrast, was virtually collapsed in normal corollas. In contrast, the inoculation of a viral vector construct that increased the expression of NbenIDA1A at the base of the corolla tube negatively affected the growth of the inoculated plants accelerating the timing of both corolla senescence and abscission. However, the heterologous ectopic overexpression of citrus CitIDA3 and Arabidopsis AtIDA in N. benthamiana did not alter the standard plant phenotype suggesting that the proteolytic processing machinery was unable to yield active peptides.

CONCLUSION

Here, we demonstrate that the pair of NbenIDA1 homeologs encoding small peptides of the IDA-like family and the receptor NbenHAE.1 control cellular breakdown at the base of the corolla tube awhere an adventitious AZ should be formed and, therefore, corolla abscission in N. benthamiana flowers. Altogether, our results provide the first evidence supporting the notion that the IDA-HAE/HSL2 signaling module is conserved in angiosperms.

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.

New Plant Breeding Techniques in Citrus for the Improvement of Important Agronomic Traits. A Review

New plant breeding techniques (NPBTs) aim to overcome traditional breeding limits for fruit tree species, in order to obtain new varieties with improved organoleptic traits and resistance to biotic and abiotic stress, and to maintain fruit quality achieved over centuries by (clonal) selection. Knowledge on the gene(s) controlling a specific trait is essential for the use of NPBTs, such as genome editing and cisgenesis. In the framework of the international scientific community working on fruit tree species, including citrus, NPBTs have mainly been applied to address pathogen threats. Citrus could take advantage of NPBTs because of its complex species biology (seedlessness, apomixis, high heterozygosity, and long juvenility phase) and aptitude for in vitro manipulation. To our knowledge, genome editing in citrus via transgenesis has successful for induced resistance to Citrus bacterial canker in sweet orange and grapefruit using the resistance gene CsLOB1. In the future, NPBTs will also be used to improve fruit traits, making them healthier. The regeneration of plants following the application of NPBTs is a bottleneck, making it necessary to optimize the efficiency of current protocols. The strengths and weaknesses of using explants from young in vitro plantlets, and from mature plants, will be discussed. Other major issues addressed in this review are related to the requirement for marker-free systems and shortening the long juvenility phase. This review aims to summarize methods and approaches available in the literature that are suitable to citrus, focusing on the principles observed before the use of NPBTs.

Functional study of Csrbohs in defence response against Xanthomonas citri ssp. citri

NADPH oxidases, encoded by rbohs (respiratory burst oxidase homologues), transfer electrons from NADPH to molecular oxygen (O2) to generate superoxide anion (O2•-), which is the first step in the formation of hydrogen peroxide (H2O2) in the plant-pathogen interaction system. In the present work, six citrus rbohs (Csrbohs) genes were identified in citrus, and their possible involvement in resistance to Xanthomonas citri ssp. citri (Xcc) was examined. Inoculation with Xcc promoted the H2O2 production and induced expression of the Csrbohs, especially CsrbohD. Results showed that CsrbohD was markedly induced in the resistant genotype kumquat 'Luofu' [Fortunella margarita (Lour.) Swingle] compared with grapefruit 'Duncan' [Citrus paradisi (Linn.) Macf.]. Virus-induced gene silencing (VIGS) of CsrbohD resulted in reduced resistance to Xcc in grapefruit, but not in kumquat. Compared with non-silenced plants, canker-like symptoms were observed earlier, and they were more extensive in the CsrbohD-silenced grapefruit. Silencing of CsrbohD also suppressed the Xcc induced reactive oxygen species (ROS) burst, and resulted in accumulation of more Xcc bacterial colonies. Taken together, these data indicate that CsrbohD promotes resistance to Xcc, especially in grapefruit.

Development of Infectious cDNA Clones of Citrus Yellow Vein Clearing Virus Using a Novel and Rapid Strategy

Yellow vein clearing disease (YVCD) causes significant economic losses in lemon and other species of citrus. Usually, citrus yellow vein clearing virus (CYVCV) is considered to be the causal agent of YVCD. However, mixed infection of CYVCV and Indian citrus ringspot virus (ICRSV) or other pathogens is often detected in citrus plants with YVCD. In this study, we re-examined the causal agent of YVCD to fulfill Koch's postulates. First, the full-length genome of CYVCV isolate AY (CYVCV-AY) was amplified by long-distance RT-PCR from a Eureka lemon (Citrus limon) tree with typical YVCD symptoms. The genomic cDNAs were then cloned into a ternary Yeast-Escherichia coli-Agrobacterium tumefaciens shuttle vector, pCY, using transformation-associated recombination (TAR) strategy, and 15 full-length cDNA clones of CYVCV-AY were obtained. Subsequently, four of these clones were selected randomly and inoculated on Jincheng (C. sinensis) seedlings through Agrobacterium-mediated vacuum-infiltration, and it was found that 80 to 100% of inoculated plants were infected with CYVCV by RT-PCR at 20 to 40 days postinoculation (dpi) and by direct tissue blot immunoassay at 60 dpi. The progeny of CYVCV-AY from cDNA clones caused typical symptoms of YVCD such as yellow vein clearing, leaf distortion, and chlorosis, which were the same as that elicited by wild-type virus. Finally, the regeneration of CYVCV-AY genome was confirmed by long-distance RT-PCR in lemon trees inoculated with the infectious cDNA clone. These results proved that CYVCV was the primary causal agent of YVCD. This is the first report on the development of infectious cDNA clones of CYVCV, which lays the foundation for further studies on viral gene functions and virus-host interactions.

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.

Construction of Full-length Infectious cDNA Clones of Apple chlorotic leaf spot virus and Their Agroinoculation to Woody Plants by a Novel Method of Vacuum Infiltration

Construction and agroinoculation of full-length infectious cDNA clones of plant RNA viruses have been used in plant virology to prove Koch's postulates and for development of viruses as vectors for expressing foreign genes in plants. Four full-length cDNA clones (pIF3-12, pIF3-14, pIF3-15, and pIF3-19) of Apple chlorotic leaf spot virus (ACLSV) isolate 38/85 were produced. Two of the four full-length cDNA clones (pIF3-15 and pIF3-19) proved to be infectious on Nicotiana occidentalis 37B test plants by agroinoculation and were then mechanically transmissible to healthy N. occidentalis 37B. The genomic cDNAs of ACLSV pIF3-15 and pIF3-19 shared nucleotide identity of 77.5%, demonstrating mixed infections of multiple strains of ACLSV in the source tree of isolate 38/85. The two full-length cDNA clones were agroinoculated to apple seedlings by a newly developed vacuum infiltration method. The success rate of agroinoculation was greater than 78%, defined as the number of PCR positive seedlings to the number of apple seedlings that survived. ACLSV was transmissible from agroinoculated seedlings by cleft grafting. The results of this study will be useful for construction of infectious cDNA clones of plant viruses from full-length PCR fragments and agroinoculating woody host plants using the vacuum infiltration method outlined here.

Precocious flowering of juvenile citrus induced by a viral vector based on Citrus leaf blotch virus: a new tool for genetics and breeding

The long juvenile period of citrus trees (often more than 6 years) has hindered genetic improvement by traditional breeding methods and genetic studies. In this work, we have developed a biotechnology tool to promote transition from the vegetative to the reproductive phase in juvenile citrus plants by expression of the Arabidopsis thaliana or citrus FLOWERING LOCUS T (FT) genes using a Citrus leaf blotch virus-based vector (clbvINpr-AtFT and clbvINpr-CiFT, respectively). Citrus plants of different genotypes graft inoculated with either of these vectors started flowering within 4-6 months, with no alteration of the plant architecture, leaf, flower or fruit morphology in comparison with noninoculated adult plants. The vector did not integrate in or recombine with the plant genome nor was it pollen or vector transmissible, albeit seed transmission at low rate was detected. The clbvINpr-AtFT is very stable, and flowering was observed over a period of at least 5 years. Precocious flowering of juvenile citrus plants after vector infection provides a helpful and safe tool to dramatically speed up genetic studies and breeding programmes.

A simplified strategy for studying the etiology of viral diseases: Apple stem grooving virus as a case study

A simple method to amplify infective, complete genomes of single stranded RNA viruses by long distance PCR (LD PCR) from woody plant tissues is described in detail. The present protocol eliminates partial purification of viral particles and the amplification is achieved in three steps: (i) easy preparation of template RNA by incorporating a pre processing step before loading onto the column (ii) reverse transcription by AMV or Superscript reverse transcriptase and (iii) amplification of cDNA by LD PCR using LA or Protoscript Taq DNA polymerase. Incorporation of a preprocessing step helped to isolate consistent quality RNA from recalcitrant woody tissues such as apple, which was critical for efficient amplification of the complete genomes of Apple stem pitting virus (ASPV), Apple stem grooving virus (ASGV) and Apple chlorotic leaf spot virus (ACLSV). Complete genome of ASGV was cloned under T7 RNA polymerase promoter and was confirmed to be infectious through transcript inoculation producing symptoms similar to the wild type virus. This is the first report for the largest RNA virus genome amplified by PCR from total nucleic acid extracts of woody plant tissues.

Effectiveness of gene silencing induced by viral vectors based on Citrus leaf blotch virus is different in Nicotiana benthamiana and citrus plants

Virus induced gene silencing (VIGS) is an effective technology for gene function analysis in plants. We assessed the VIGS effectiveness in Nicotiana benthamiana and citrus plants of different Citrus leaf blotch virus (CLBV)-based vectors, using insets of the phytoene desaturase (pds) gene. While in N. benthamiana the silencing phenotype was induced only by the construct carrying a 58-nt pds hairpin, in citrus plants all the constructs induced the silencing phenotype. Differences in the generation of secondary small interfering RNAs in both species are believed to be responsible for differential host-species effects. The ability of CLBV-based vectors to silence different endogenous citrus genes was further confirmed. Since CLBV-based vectors are known to be stable and induce VIGS in successive flushes for several months, these vectors provide an important genomic tool and it is expected that they will be useful to analyze gene function by reverse genetics in the long-lived citrus plants.

Identification of a new enamovirus associated with citrus vein enation disease by deep sequencing of small RNAs

To identify the causal agent of citrus vein enation disease, we examined by deep sequencing (Solexa-Illumina) the small RNA (sRNA) fraction from infected and healthy Etrog citron plants. Our results showed that virus-derived sRNAs (vsRNAs): (i) represent about 14.21% of the total sRNA population, (ii) are predominantly of 21 and 24 nucleotides with a biased distribution of their 5' nucleotide and with a clear prevalence of those of (+) polarity, and (iii) derive from all the viral genome, although a prominent hotspot is present at a 5'-proximal region. Contigs assembled from vsRNAs showed similarity with luteovirus sequences, particularly with Pea enation mosaic virus, the type member of the genus Enamovirus. The genomic RNA (gRNA) sequence of a new virus, provisionally named Citrus vein enation virus (CVEV), was completed and characterized. The CVEV gRNA was found to be single-stranded, positive-sense, with a size of 5,983 nucleotides and five open reading frames. Phylogenetic comparisons based on amino acid signatures of the RNA polymerase and the coat protein clearly classifies CVEV within the genus Enamovirus. Dot-blot hybridization and reverse transcription-polymerase chain reaction tests were developed to detect CVEV in plants affected by vein enation disease. CVEV detection by these methods has already been adopted for use in the Spanish citrus quarantine, sanitation, and certification programs.

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.

A genetic system for Citrus Tristeza Virus using the non-natural host Nicotiana benthamiana: an update

In nature Citrus tristeza virus (CTV), genus Closterovirus, infects only the phloem cells of species of Citrus and related genera. Finding that the CTV T36 strain replicated in Nicotiana benthamiana (NB) protoplasts and produced normal virions allowed development of the first genetic system based on protoplast transfection with RNA transcribed from a full-genome cDNA clone, a laborious and uncertain system requiring several months for each experiment. We developed a more efficient system based on agroinfiltration of NB leaves with CTV-T36-based binary plasmids, which caused systemic infection in this non-natural host within a few weeks yielding in the upper leaves enough CTV virions to readily infect citrus by slash inoculation. Stem agroinoculation of citrus and NB plants with oncogenic strains of Agrobacterium tumefaciens carrying a CTV-T36 binary vector with a GUS marker, induced GUS positive galls in both species. However, while most NB tumors were CTV positive and many plants became systemically infected, no coat protein or viral RNA was detected in citrus tumors, even though CTV cDNA was readily detected by PCR in the same galls. This finding suggests (1) strong silencing or CTV RNA processing in transformed cells impairing infection progress, and (2) the need for using NB as an intermediate host in the genetic system. To maintain CTV-T36 in NB or assay other CTV genotypes in this host, we also tried to graft-transmit the virus from infected to healthy NB, or to mechanically inoculate NB leaves with virion extracts. While these trials were mostly unsuccessful on non-treated NB plants, agroinfiltration with silencing suppressors enabled for the first time infecting NB plants by side-grafting and by mechanical inoculation with virions, indicating that previous failure to infect NB was likely due to virus silencing in early infection steps. Using NB as a CTV host provides new possibilities to study virus-host interactions with a simple and reliable system.

Characterization of the complete genome of a novel citrivirus infecting Actinidia chinensis

A ssRNA virus from kiwifruit (Actinidia spp.) was identified as a member of the family Betaflexiviridae. It was mechanically transmitted to the herbaceous indicators Nicotiana benthamiana, N. clevelandii, N. glutinosa and N. occidentalis. The complete genome was comprised of three ORFs and a 3'poly (A) tail. Phylogenetic analysis of the entire genome indicated it was a novel member of the genus Citrivirus (family Betaflexiviridae). The complete nucleotide sequence differed from that of citrus leaf blotch virus (CLBV) by ~ 26 %. The movement protein (ORF2) and coat protein (ORF3) shared 95-96 % and 90-92 % amino acid sequence identity, respectively, with CLBV. The replicase polyprotein (ORF1) was distinctly different from published CLBV sequences, with 78-79 % amino acid sequence identity, while the 5' UTR and 3' UTR differed from CLBV by 28 % and 29 %, respectively. The sequence differences indicate that the citrivirus from Actinidia is either a divergent strain of CLBV or a member of a new citrivirus species.

Biological and molecular characterization of a U.S. isolate of Hosta virus X

Hosta virus X (HVX) is rapidly becoming a serious pathogen of commercially important hosta plants worldwide. We report here biological and molecular characterization of a U.S. isolate of HVX, HVX-37. HVX-37 infectivity was tested in 21 hosta cultivars over three growth seasons, and three types of responses were defined based upon the ability of the virus to cause local and/or systemic infections. Four cultivars resistant to systemic HVX infection were identified. The full-length sequence of the HVX-37 genome was determined, the first complete sequence of a U.S. HVX isolate. Comparison with the previously sequenced HVX-Korea (Kr) genome revealed a high level of sequence similarity, as well as some differences. Notably, a 105-nucleotide long, near-perfect direct repeat in the Kr isolate is absent in HVX-37. The accuracy of the HVX-37 genome sequence was confirmed by infectivity of in vitro transcripts synthesized from a full-length HVX-37 cDNA on Nicotiana benthamiana and hosta plants.

Development of viral vectors based on Citrus leaf blotch virus to express foreign proteins or analyze gene function in citrus plants

Viral vectors have been used to express foreign proteins in plants or to silence endogenous genes. This methodology could be appropriate for citrus plants that have long juvenile periods and adult plants that are difficult to transform. We developed viral vectors based on Citrus leaf blotch virus (CLBV) by duplicating a minimum promoter (92 bp) either at the 3' untranslated region (clbv3'pr vector) or at the intergenic region between the movement and coat protein (CP) genes (clbvINpr vector). The duplicated fragment (-42/+50) around the transcription start site of the CP subgenomic RNA (sgRNA) had the full promoter activity and induced synthesis of a new sgRNA in infected plants. Agroinoculation with these vectors resulted in systemic infection of Nicotiana benthamiana and the resulting virions systemically infected citrus plants. A clbvINpr vector carrying the green fluorescent protein (GFP) gene expressed GFP in citrus plants and triggered gfp silencing in gfp-transgenic citrus plants, and vectors carrying fragments of the phytoene desaturase or the magnesium chelatase genes incited a silencing phenotype in citrus plants. These silenced phenotypes persisted in successive flushes. Because CLBV infections are symptomless in most citrus species, the effective silencing induced by CLBV-derived vectors will be helpful to analyze citrus gene function.

The Citrus leaf blotch virus movement protein acts as silencing suppressor

To counteract plant antiviral defense based on RNA silencing, many viruses express proteins that inhibit this mechanism at different levels. The genome of Citrus leaf blotch virus (CLBV) encodes a 227-kDa protein involved in replication, a 40-kDa movement protein (MP), and a 41-kDa coat protein (CP). To determine if any of these proteins might have RNA silencing suppressor activities, we have used Agrobacterium-mediated transient assays in the green fluorescent protein (GFP)-expressing Nicotiana benthamiana line 16c. Only CLBV MP was able to suppress intracellular GFP silencing induced by expression of either single- or double-stranded (ds) GFP RNA, but not cell-to-cell or long distance spread of the silencing signal. The MP suppressor activity was weak compared to other characterized viral suppressor proteins. Overall our data indicate that MP acts as a suppressor of local silencing probably by interfering in the silencing pathway downstream of the steps of dsRNA and small RNAs generation.

Strategies to facilitate the development of uncloned or cloned infectious full-length viral cDNAs: Apple chlorotic leaf spot virus as a case study

BACKGROUND

Approaches to simplify and streamline the construction of full-length infectious cDNA clones (FL-cDNAs) are needed. Among desirable improvements are the ability to use total nucleic acids (TNA) extracts from infected hosts (to bypass viral purification limitations) for the direct one-step amplification of large FL-cDNAs, the possibility to inoculate plants with uncloned FL-cDNAs and the simplified cloning of these large molecules.

RESULTS

Using the 7.55 kb genome of Apple chlorotic leaf spot trichovirus (ACLSV) approaches allowing the rapid generation from TNA extracts of FL-cDNAs under the control of the T7 promoter and the successful inoculation of plants using in vitro transcripts obtained from these uncloned amplification products have been developed. We also show that the yeast homologous recombination system permits efficient cloning of FL-cDNAs and the simultaneous one-step tailoring of a ternary Yeast-Escherichia coli-Agrobacterium tumefaciens shuttle vector allowing efficient inoculation of both herbaceous and woody host plants by agroinfiltration.

CONCLUSIONS

The fast and efficient strategies described here should have broad applications, in particular for the study of "difficult" plant viruses, such as those infecting woody hosts, and potentially for other, non plant-infecting viral agents.

Agroinoculation of Citrus tristeza virus causes systemic infection and symptoms in the presumed nonhost Nicotiana benthamiana

Citrus tristeza virus (CTV) naturally infects only some citrus species and relatives and within these it only invades phloem tissues. Failure to agroinfect citrus plants and the lack of an experimental herbaceous host hindered development of a workable genetic system. A full-genome cDNA of CTV isolate T36 was cloned in binary plasmids and was used to agroinfiltrate Nicotiana benthamiana leaves, with or without coinfiltration with plasmids expressing different silencing-suppressor proteins. A time course analysis in agroinfiltrated leaves indicated that CTV accumulates and moves cell-to-cell for at least three weeks postinoculation (wpi), and then, it moves systemically and infects the upper leaves with symptom expression. Silencing suppressors expedited systemic infection and often increased infectivity. In systemically infected Nicotiana benthamiana plants, CTV invaded first the phloem, but after 7 wpi, it was also found in other tissues and reached a high viral titer in upper leaves, thus allowing efficient transmission to citrus by stem-slash inoculation. Infected citrus plants showed the symptoms, virion morphology, and phloem restriction characteristic of the wild T36 isolate. Therefore, agroinfiltration of Nicotiana benthamiana provided the first experimental herbaceous host for CTV and an easy and efficient genetic system for this closterovirus.

Mapping the subgenomic RNA promoter of the Citrus leaf blotch virus coat protein gene by Agrobacterium-mediated inoculation

Citrus leaf blotch virus has a single-stranded positive-sense genomic RNA (gRNA) of 8747 nt organized in three open reading frames (ORFs). The ORF1, encoding a polyprotein involved in replication, is translated directly from the gRNA, whereas ORFs encoding the movement (MP) and coat (CP) proteins are expressed via 3' coterminal subgenomic RNAs (sgRNAs). We characterized the minimal promoter region critical for the CP-sgRNA expression in infected cells by deletion analyses using Agrobacterium-mediated infection of Nicotiana benthamiana plants. The minimal CP-sgRNA promoter was mapped between nucleotides -67 and +50 nt around the transcription start site. Surprisingly, larger deletions in the region between the CP-sgRNA transcription start site and the CP translation initiation codon resulted in increased CP-sgRNA accumulation, suggesting that this sequence could modulate the CP-sgRNA transcription. Site-specific mutational analysis of the transcription start site revealed that the +1 guanylate and the +2 adenylate are important for CP-sgRNA synthesis.

Nucleotide sequence and genome organization of Dweet mottle virus and its relationship to members of the family Betaflexiviridae

The nucleotide sequence of Dweet mottle virus (DMV) was determined and compared to sequences of members of the families Alphaflexiviridae and Betaflexiviridae. The DMV genome has 8,747 nucleotides (nt) excluding the 3′ poly-(A) tail. DMV genomic RNA contains three putative open reading frames (ORFs) and untranslated regions of 73 nt at the 5′ and 541 nt at 3′ termini. ORF1 potentially encoding a 227.48-kDa polyprotein, which has methyltransferase, oxygenase, endopeptidase, helicase, and RNA-dependent RNA polymerase (RdRP) domains. ORF2 encodes a movement protein of 40.25 kDa, while ORF3 encodes a coat protein of 40.69 kDa. Protein database searches showed 98–99% matches of DMV ORFs with citrus leaf blotch virus (CLBV) sequences. Phylogenetic analysis based on the RdRP core domain revealed that DMV is closely related to CLBV as a member of the genus Citrivirus. DMV did not satisfy the molecular criteria for demarcation of an independent species within the genus Citrivirus, family Betaflexiviridae, and hence, DMV can be considered a CLBV isolate.