Cauliflower mosaic virus: Pathways of infection

Cauliflower mosaic virus P6 inclusion body formation: A dynamic and intricate process

During an infection, Cauliflower mosaic virus (CaMV) forms inclusion bodies (IBs) mainly composed of viral protein P6, where viral activities occur. Because viral processes occur in IBs, understanding the mechanisms by which they are formed is crucial. FL-P6 expressed in N. benthamiana leaves formed IBs of a variety of shapes and sizes. Small IBs were dynamic, undergoing fusion/dissociation events. Co-expression of actin-binding polypeptides with FL-P6 altered IB size distribution and inhibited movement. This suggests that IB movement is required for fusion and growth. A P6 deletion mutant was discovered that formed a single large IB per cell, which suggests it exhibited altered fusion/dissociation dynamics. Myosin-inhibiting drugs did not affect small IB movement, while those inhibiting actin polymerization did. Large IBs colocalized with components of the aggresome pathway, while small ones generally did not. This suggests a possible involvement of the aggresome pathway in large IB formation.

Setting Up Shop: The Formation and Function of the Viral Factories of Cauliflower mosaic virus

Similar to cells, viruses often compartmentalize specific functions such as genome replication or particle assembly. Viral compartments may contain host organelle membranes or they may be mainly composed of viral proteins. These compartments are often termed: inclusion bodies (IBs), viroplasms or viral factories. The same virus may form more than one type of IB, each with different functions, as illustrated by the plant pararetrovirus, Cauliflower mosaic virus (CaMV). CaMV forms two distinct types of IBs in infected plant cells, those composed mainly of the viral proteins P2 (which are responsible for transmission of CaMV by insect vectors) and P6 (required for viral intra-and inter-cellular infection), respectively. P6 IBs are the major focus of this review. Much of our understanding of the formation and function of P6 IBs comes from the analyses of their major protein component, P6. Over time, the interactions and functions of P6 have been gradually elucidated. Coupled with new technologies, such as fluorescence microscopy with fluorophore-tagged viral proteins, these data complement earlier work and provide a clearer picture of P6 IB formation. As the activities and interactions of the viral proteins have gradually been determined, the functions of P6 IBs have become clearer. This review integrates the current state of knowledge on the formation and function of P6 IBs to produce a coherent model for the activities mediated by these sophisticated virus-manufacturing machines.

Protein encoded by ORF I of cauliflower mosaic virus is part of the viral inclusion body

Coding sequences of ORF I from cauliflower mosaic virus were cloned in an Escherichia coli expression vector. A protein derived from this ORF was used to raise antibodies in rabbits. Immunoblots revealed that in infected plants the ORF I protein with an apparent molecular weight of 41 kDa is part of the viral inclusion bodies and is absent from purified virus particles. Amino acid sequence homologies of the ORF I protein with other proteins are discussed.

Nuclear import and export of plant virus proteins and genomes

SUMMARY Nuclear import and export are crucial processes for any eukaryotic cell, as they govern substrate exchange between the nucleus and the cytoplasm. Proteins involved in the nuclear transport network are generally conserved among eukaryotes, from yeast and fungi to animals and plants. Various pathogens, including some plant viruses, need to enter the host nucleus to gain access to its replication machinery or to integrate their DNA into the host genome; the newly replicated viral genomes then need to exit the nucleus to spread between host cells. To gain the ability to enter and exit the nucleus, these pathogens encode proteins that recognize cellular nuclear transport receptors and utilize the host's nuclear import and export pathways. Here, we review and discuss our current knowledge about the molecular mechanisms by which plant viruses find their way into and out of the host cell nucleus.

Multiple domains within the Cauliflower mosaic virus gene VI product interact with the full-length protein

The Cauliflower mosaic virus (CaMV) gene VI product (P6) is a multifunctional protein essential for viral propagation. It is likely that at least some of these functions require P6 self-association. The work described here was performed to confirm that P6 self-associates and to identify domains involved in this interaction. Yeast two-hybrid analyses indicated that full-length P6 self-associates and that this interaction is specific. Additional analyses indicated that at least four independent domains bind to full-length P6. When a central domain (termed domain D3) was removed, these interactions were abolished. However, this deleted P6 was able to bind to the full-length wild-type protein and to isolated domain D3. Viruses lacking domain D3 were incapable of producing a systemic infection. Isolated domain D3 was capable of binding to at least two of the other domains but was unable to self-associate. This suggests that domain D3 facilitates P6 self-association by binding to the other domains but not itself. The presence of multiple domains involved in P6 self-association may help explain the ability of this protein to form the intracellular inclusions characteristic of caulimoviruses.

Third position codon composition suggests two classes of genes within the Cauliflower mosaic virus genome

The translation of viral mRNAs by host ribosomes is essential for infection. Hence, codon usage of virus genes may influence efficiency of infection. In addition, composition of nucleotides in the third position within codons of genes can reflect evolutionary relationships. In this study, third position codon composition was examined for the seven genes of eight Cauliflower mosaic virus isolates. Genes IV-VII had similar codon composition values and were termed Class 1 genes. Genes I-III possessed corresponding codon composition values and were termed Class 2 genes. The codon composition values of Class 1 and genes differed significantly. Neither Class 1 nor Class 2 genes had codon composition values identical to that of the host plant, Arabidopsis thaliana. However, Class 1 genes possessed codon composition values closer to those of the host than Class 2 genes. Examination of the genomes of three Rous sarcoma virus isolates indicated that codon composition values were similar for the gag, pol, and env genes but these genes differed significantly from the src genes. Since codon composition values for Rous sarcoma virus distinguished a "foreign" gene from the rest of the viral genome, it is possible that the Cauliflower mosaic virus genome is composed of genes from two different sources. Others have suggested that Cauliflower mosaic virus evolved in this manner and our data provide support for this hypothesis.

The 5' Third of Cauliflower mosaic virus Gene VI Conditions Resistance Breakage in Arabidopsis Ecotype Tsu-0

ABSTRACT Arabidopsis thaliana ecotypes vary in their responses to viruses. In this study, we analyzed the variation in response of A. thaliana ecotype Tsu-0 to Cauliflower mosaic virus (CaMV). This ecotype was previously reported to be resistant to two CaMV isolates (CM1841 and CM4-184), but susceptible to W260. In this study, we show that Tsu-0 is resistant to four additional CaMV isolates. CaMV propagated within the rosette leaves of Tsu-0 plants, but did not appear to spread systemically into the inflorescence. However, virus viability in rosette leaves of Tsu-0 plants apparently was not compromised because infectious CaMV could be recovered from these organs. W260 overcomes Tsu-0 resistance by a passive mechanism (i.e., this virus avoids activating plant defenses). The portion of the viral genome responsible for W260 resistance breakage was mapped to the 5' third of gene VI, which we have termed RBR-1. This region is also responsible for controlling the ability of CaMV to infect different types of solanaceous plants. Hence, the pathways by which plants of different families interact with CaMV may be conserved through evolution.

Nucleic acid transport in plant-microbe interactions: the molecules that walk through the walls

Many microbes "genetically invade" plants by introducing DNA or RNA molecules into the host cells. For example, plant viruses transport their genomes between host cells, whereas Agrobacterium spp. transfer T-DNA to the cell nucleus and integrate it into the plant DNA. During these events, the transported nucleic acids must negotiate several barriers, such as plant cell walls, plasma membranes, and nuclear envelopes. This review describes the microbial and host proteins that participate in cell-to-cell transport and nuclear import of nucleic acids during infection by plant viruses and Agrobacterium spp. Possible molecular mechanisms by which these transport processes occur are discussed.

Interaction between the tobacco mosaic virus movement protein and host cell pectin methylesterases is required for viral cell-to-cell movement

Virus-encoded movement protein (MP) mediates cell-to-cell spread of tobacco mosaic virus (TMV) through plant intercellular connections, the plasmodesmata. The molecular pathway by which TMV MP interacts with the host cell is largely unknown. To understand this process better, a cell wall-associated protein that specifically binds the viral MP was purified from tobacco leaf cell walls and identified as pectin methylesterase (PME). In addition to TMV MP, PME is recognized by MPs of turnip vein clearing virus (TVCV) and cauliflower mosaic virus (CaMV). The use of amino acid deletion mutants of TMV MP showed that its domain was necessary and sufficient for association with PME. Deletion of the PME-binding region resulted in inactivation of TMV cell-to-cell movement.

Cauliflower Mosaic Virus Isolate NY8153 Breaks Resistance in Arabidopsis Ecotype En-2

ABSTRACT Arabidopsis thaliana ecotype En-2 was previously shown to be resistant to cauliflower mosaic caulimovirus (CaMV) isolate CM4-184. In this study, En-2 plants were screened with eight other isolates of CaMV to identify viruses capable of overcoming resistance and to determine if the mechanism of resistance was the same for each virus. En-2 resistance to most CaMV isolates was mediated by the same mechanism, i.e., preventing virus long-distance movement. One CaMV isolate, NY8153, was found that produced a severe systemic infection on En-2 plants. In addition, the CM1841 isolate was able to spread systemically through En-2 plants, to a limited extent, without producing visible symptoms. These data indicate that the resistance shown by En-2 plants is not an all-or-none phenomenon. En-2 plants were susceptible to turnip mosaic potyvirus, suggesting that resistance is specific to CaMV.

Three regions of cauliflower mosaic virus strain W260 are involved in systemic infection of solanaceous hosts

We have identified regions of CaMV strain W260 involved in systemic infection of Nicotiana bigelovii and Datura stramonium by constructing chimeric viruses between W260 and CM1841, a strain that is unable to systemically infect any solanaceous host. All of the chimeric viruses systemically infected turnips, demonstrating the viability of the chimeric viruses in a host that is susceptible to both CM1841 and W260. Three regions of W260, containing primarily genes I, IV, and VI, influenced the ability of that virus to induce systemic symptoms in the solanaceous hosts. The involvement of the regions containing gene I, and to a lesser extent gene IV, were affected by environmental conditions. When infected plants were grown under conditions of low light, low temperatures (18 degrees), and short days (9.5-hr day), the source of genes I and IV no longer influenced whether a chimeric virus moved systemically. As light intensity and day length were increased, the genetic requirements became more stringent and genes I and IV, as well as gene VI, had to be derived from W260.

Biophysical and biochemical properties of baculovirus-expressed CaMV P1 protein

Cauliflower mosaic virus (CaMV) gene I encodes a protein (P1) that has been implicated in the control of virus movement in infected plants. To assist in the characterization of the mechanism of action of P1, gene I has been expressed efficiently in Spodoptera frugiperda (Sf) cells using recombinant baculovirus. Control of the expression of CaMV gene I by the polyhedrin late promoter in the baculovirus Autographa californica nuclear polyhedrosis virus (AcNPV) resulted in very high levels of P1 accumulation late in the infection cycle. This was predominantly as insoluble inclusion bodies within the cytoplasm of infected Sf cells, and not extracellularly. Evidence from anomalous gel migration and sequence homology with an analogous viral protein (tobacco mosaic virus 30K) indicated that P1 may be post-translationally processed. However, neither phosphorylation nor glycosylation of P1 occurred in this system, suggesting a functional distinction between P1 and TMV 30K. P1 from insect cells and native P1 from infected plants were immunologically related, allowing the expressed product to be used in the preparation of anti-P1 serum for detecting P1 in plant extracts. The full-size (46 kD) P1 product from insect cells, from plants, and from in vitro translations of in vitro gene I transcripts all showed similar behavior on two-dimensional protein gels, with a major pI of 7.0. Using a combination of 4 M urea, 1 M NaCl, and high temperature, P1 was solubilized. Approximately 5% of the starting material remained in solution after dialysis and remained stable to freeze/thawing. This preparation should enable us to identify the biochemical function of P1 and to resolve its role in controlling virus spread.

How do plant virus nucleic acids move through intercellular connections?

In addition to their function in transport of water, ions, small metabolites, and growth factors in normal plant tissue, the plasmodesmata presumably serve as routes for cell-to-cell movement of plant viruses in infected tissue. Virus cell-to-cell spread through plasmodesmata is an active process mediated by specialized virus encoded movement proteins; however, the mechanism by which these proteins operate is not clear. We incorporate recent information on the biochemical properties of plant virus movement proteins and their interaction with plasmodesmata in a model for transport of nucleic acids through plasmodesmatal channels. We propose that only single stranded (ss) nucleic acids can be transported efficiently through plasmodesmata, and that movement proteins function as molecular chaperones for ss nucleic acids to form unfolded movement protein-ss nucleic acid complexes. These complexes are targeted to plasmodesmata. Plasmodesmatal permeability is then increased following interaction with movement protein and the entire movement complex or its nucleic acid component is translocated across the plasmodesmatal channel.

Segregation of cauliflower mosaic virus symptom genetic determinants

We have created a series of hybrid cauliflower mosaic virus (CaMV) genomes between a severe virus strain (Cabb BJI) and a mild strain (Bari 1) to map the virus genetic loci responsible for specific systemic symptom characters produced in infected turnip plants. Recombinants were generated in vivo by recombinational rescue and in vitro by restriction enzyme fragment exchange. On infection, hybrids induced either parental (wild-type) symptoms or segregated parental characters. Some of the engineered hybrid genomes produced novel symptomatic effects not observed in either of the parental strains whilst others reverted to express parental symptom characters following passaging. Determinants defining differences between the two CaMV strains in respect of four specific symptom characters were delimited to separate genome regions. A locus involved in determining the rate of spread of systemic vein clearing symptoms mapped to a region containing part of gene VII and gene I (nts 109-780). This phenomenon is consistent with the putative involvement of the CaMV gene I product in mediating virus movement within infected plants. Determinants influencing the degree of leaf chlorosis were located in a separate genome domain encompassing part of gene VI together with the large intergenic region and part of gene VII (nts 6103-90). Determinants controlling timing of initial systemic symptom appearance were mapped to a region between nts 2150 and 4438 containing part of gene III, gene IV, and part of gene V. Plant stunting was influenced by loci in at least two separate regions, one containing parts of gene I and II, and a second within the reverse transcriptase gene (V). We conclude that symptoms produced by CaMV infection can be subdivided into individual characters, the genetic determinants of which segregate to different virus genetic loci and are not restricted to a single gene product.

Plant viruses: a tool-box for genetic engineering and crop protection

Traditionally, plant viruses are viewed as harmful, undesirable pathogens. However, their genomes can provide several useful 'designer functions' or 'sequence modules' with which to tailor future gene vectors for plant or general biotechnology. The majority (77%) of known plant viruses have single-stranded RNA of the messenger (protein coding) sense as their genetic material. Over the past 4 years, improved in vitro transcription systems and the construction of partial or full-length DNA copies of several plant RNA viruses have enhanced our ability to manipulate and study their genomes, particularly in the context of their pathogenic interactions with host plants. Recently, two forms of genetically engineered protection against plant virus infections have been reported. In both, a virus-related 'interfering' molecule was stably introduced into plants via the DNA-transfer mechanism of Agrobacterium tumefaciens. To date, the choice of 'interfering' molecule has been guided by empirical field-observations and each is effective against only a narrow range of closely-related viruses. As yet, we do not fully understand the molecular mechanism(s) responsible for the observed protection. The ability to manipulate the plant-pathogen relationship is a powerful tool to increase our knowledge and improve future strategies for unconventional cropprotection by genetic engineering techniques.

The pattern of accumulation of cauliflower mosaic virus-specific products in infected turnips

The concentrations of cauliflower mosaic virus (CaMV) DNA and protein products in the developing leaves of a host, turnip, have been measured and the results have been correlated with symptom production. Virus-specific products were limited to the symptomatic leaves. CaMV DNA was detected in the youngest foliar tissues showing full systemic symptoms and continued to accumulate as the leaf expanded, indicating that virus multiplication was not restricted to meristematic tissues of the host plant and that virus concentration was not a primary determinant for symptom production. Using specific antisera for Western blot analysis, the distribution of CaMV-specific proteins (P1-P6) in a range of subcellular fractions of infected tissue was determined. The protein products (P2-P6) of genes II-VI were all detected in fractions enriched for virus inclusion bodies, although P5 was present only at low levels. A high-speed pellet fraction enriched for virus replication complexes revealed P5 in higher concentrations, and also contained P4 and small amounts of P6 in proportions which indicated that replication complexes had been released from inclusion bodies. In the different leaves of the host, P2, 3, 4, 5, and 6 all increased in concentration in parallel with viral DNA, although there appeared to be a bias toward protein rather than DNA synthesis in the very young leaves. P1 showed a different pattern of accumulation; it was most concentrated in the very young and the oldest infected tissues, and showed a different spectrum of products between leaves. The experiments described provide a more complete picture of the relationship between CaMV multiplication and expression, and leaf development, and an increased understanding of how the disease syndrome is established.

Comparative analysis of alternating purine-pyrimidine tracts and potential Z-DNA sequences in DNA plant viruses

The DNAs of several plant viruses were analyzed for the presence of alternating purine-pyrimidine sequences that can potentially undergo B to Z transition. The DNA of the caulimoviruses (plant retroviruses) was compared with that of the geminiviruses, with the cDNA of an RNA plant virus, and with several computer-generated random sequences. Our analysis indicates that potential Z-DNA sites tend to be restricted in the DNA of the caulimoviruses, whereas the same does not occur significantly in the other viral DNAs examined. This result is discussed in relation to the mode of replication of the caulimoviral DNA and offers additional evidence of the existence of selection processes regulating the frequency and distribution of the Z-DNA sites in the different genomes.

Cauliflower mosaic virus 35 S RNA leader region inhibits translation of downstream genes

The cauliflower mosaic virus (CaMV) 35 S RNA is a full-length transcript of the viral genome. It encodes the genes VII and I-V, arranged in tandem along the RNA, preceded by a long leader region (600 bases) containing many short open reading frames. We have examined the effects of the leader and the first gene (gene VII) on downstream gene I translation in vitro and in an in vivo transient expression system (carrot protoplasts). RNAs from constructs containing the intact leader, and from various deletion constructs, were translated in a rabbit reticulocyte system. Gene I was translated efficiently only when the long leader region and the upstream gene VII were deleted. Translational fusions of gene VII or I to the firefly luciferase reporter gene were also constructed, and a similar series of leader sequence deletion mutants were examined in vivo and in vitro. The 600-base leader region was found to repress translation of gene VII 8- to 30-fold as compared to the truncated gene lacking the leader region. Gene I expression as compared to that of gene VII was reduced an additional 7- to 20-fold by the presence of the upstream leader region including gene VII. This represented an overall reduction in gene I expression of greater than 100-fold as compared to expression in the absence of any leader sequence. The reduced translation of gene I in the context of the 35 S RNA leader region was not due to the action of the gene VII protein product but may result from efficient blocking of scanning 40 S ribosomes by translation of upstream open reading frames.

Cauliflower mosaic virus gene I product detected in a cell-wall-enriched fraction

Gene I product of cauliflower mosaic virus was immunodetected in a cell-wall-enriched fraction from infected turnip leaves in addition to its detection in viroplasms and replication complexes. The immunoreaction was carried out with an antiserum raised against a 15 amino acid long synthetic peptide corresponding to the carboxy-terminus of potential gene I protein (P1). The presence of P1 in different subcellular fractions was investigated as a function of time during viral multiplication. At late infection times, P1 was found only in the cell-wall-enriched fraction.

Detection of CaMV gene I and gene VI protein products in vivo using antisera raised to COOH-terminal β-galactosidase fusion proteins

Specific antisera were prepared to the inclusion body protein (gene VI product) and the gene I product of cauliflower mosaic virus (CaMV). Translational fusions between the lacZ gene and gene VI or gene I were constructed by cloning the relevant DNA fragments into the expression vectors pUR290, pUR291 or pUR292. Large amounts of fusion protein were synthesized when the inserted DNA fragment was in frame with the lacZ gene of the expression vector. These fusion proteins were used to raise specific antisera to gene VI and gene I proteins of CaMV. Antiserum to the gene VI product detected a range of proteins in crude extracts and in a subcellular fraction enriched for virus inclusion bodies. This range of proteins was further shown to be related to gene VI by Staphylococcus aureus V8 partial proteolysis. Antiserum to the gene I product detected viral specific proteins of 46, 42 and 38 K in preparations of CaMV replication complexes from infected plants but not in any other subcellular fraction.