Formation of large viroplasms and virulence of Cauliflower mosaic virus in turnip plants depend on the N-terminal EKI sequence of viral protein TAV

Cauliflower mosaic virus: Virus-host interactions and its uses in biotechnology and medicine

Cauliflower mosaic virus (CaMV) was the first discovered plant virus with genomic DNA that uses reverse transcriptase for replication. The CaMV 35S promoter is a constitutive promoter and thus, an attractive driver of gene expression in plant biotechnology. It is used in most transgenic crops to activate foreign genes which have been artificially inserted into the host plant. In the last century, producing food for the world's population while preserving the environment and human health is the main topic of agriculture. The damage caused by viral diseases has a significant negative economic impact on agriculture, and disease control is based on two strategies: immunization and prevention to contain virus spread, so correct identification of plant viruses is important for disease management. Here, we discuss CaMV from different aspects: taxonomy, structure and genome, host plants and symptoms, transmission and pathogenicity, prevention, control and application in biotechnology as well as in medicine. Also, we calculated the CAI index for three ORFs IV, V, and VI of the CaMV virus in host plants, the results of which can be used in the discussion of gene transfer or antibody production to identify the CaMV.

The N-terminus of the cauliflower mosaic virus aphid transmission protein P2 is involved in transmission body formation and microtubule interaction

Cauliflower mosaic virus (CaMV) is transmitted by aphids using the non-circulative transmission mode: when the insects feed on infected leaves, virus particles from infected cells attach rapidly to their stylets and are transmitted to a new host when the aphids change plants. Mandatory for CaMV transmission, the viral helper protein P2 mediates as a molecular linker binding of the virus particles to the aphid stylets. P2 is available in infected plant cells in a viral inclusion that is specialized for transmission and named the transmission body (TB). When puncturing an infected leaf cell, the aphid triggers an ultra-rapid viral response, necessary for virus acquisition and called transmission activation: The TB disrupts and P2 is redistributed onto cortical microtubules, together with virus particles that are simultaneously set free from virus factories and join P2 on the microtubules to form the so-called mixed networks (MNs). The MNs are the predominant structure from which CaMV is acquired by aphids. However, the P2 domains involved in microtubule interaction are not known. To identify P2 regions involved in its functions, we generated a set of P2 mutants by alanine scanning and analyzed them in the viral context for their capacity to form a TB, to interact with microtubules and to transmit CaMV. Our results show that contrary to the previously characterized P2-P2 and P2-virion binding sites in its C-terminus, the microtubule binding site is contained in the N-terminal half of P2. Further, this region is important for TB formation since some P2 mutant proteins did not accumulate in TBs but were retained in the viral factories where P2 is translated. Taken together, the N-terminus of P2 is not only involved in vector interaction as previously reported, but also in interaction with microtubules and in formation of TBs.

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.

Small hydrophobic viral proteins involved in intercellular movement of diverse plant virus genomes

Most plant viruses code for movement proteins (MPs) targeting plasmodesmata to enable cell-to-cell and systemic spread in infected plants. Small membrane-embedded MPs have been first identified in two viral transport gene modules, triple gene block (TGB) coding for an RNA-binding helicase TGB1 and two small hydrophobic proteins TGB2 and TGB3 and double gene block (DGB) encoding two small polypeptides representing an RNA-binding protein and a membrane protein. These findings indicated that movement gene modules composed of two or more cistrons may encode the nucleic acid-binding protein and at least one membrane-bound movement protein. The same rule was revealed for small DNA-containing plant viruses, namely, viruses belonging to genus Mastrevirus (family Geminiviridae) and the family Nanoviridae. In multi-component transport modules the nucleic acid-binding MP can be viral capsid protein(s), as in RNA-containing viruses of the families Closteroviridae and Potyviridae. However, membrane proteins are always found among MPs of these multicomponent viral transport systems. Moreover, it was found that small membrane MPs encoded by many viruses can be involved in coupling viral replication and cell-to-cell movement. Currently, the studies of evolutionary origin and functioning of small membrane MPs is regarded as an important pre-requisite for understanding of the evolution of the existing plant virus transport systems. This paper represents the first comprehensive review which describes the whole diversity of small membrane MPs and presents the current views on their role in plant virus movement.

Plant virus interaction mechanism and associated pathways in mosaic disease of small cardamom (Elettaria cardamomum Maton) by RNA-Seq approach

Small cardamom (Elettaria cardamomum), grown in limited coastal tropical countries is one of the costliest and widely exported agri-produce having global turnover of >10 billion USD. Mosaic/marble disease is one of the major impediments that requires understanding of disease at molecular level. Neither whole genome sequence nor any genomic resources are available, thus RNA seq approach can be a rapid and economical alternative. De novo transcriptome assembly was done with Illumina Hiseq data. A total of 5317 DEGs, 2267 TFs, 114 pathways and 175,952 genic region putative markers were obtained. Gene regulatory network analysis deciphered molecular events involved in marble disease. This is the first transcriptomic report revealing disease mechanism mediated by perturbation in auxin homeostasis and ethylene signalling leading to senescence. The web-genomic resource (SCMVTDb) catalogues putative molecular markers, candidate genes and transcript information. SCMVTDb can be used in germplasm improvement against mosaic disease in endeavour of small cardamom productivity. Availability of genomic resource, SCMVTDb: http://webtom.cabgrid.res.in/scmvtdb/.

Cauliflower mosaic virus transactivator protein (TAV) can suppress nonsense-mediated decay by targeting VARICOSE, a scaffold protein of the decapping complex

During pathogenesis, viruses hijack the host cellular machinery to access molecules and sub-cellular structures needed for infection. We have evidence that the multifunctional viral translation transactivator/viroplasmin (TAV) protein from Cauliflower mosaic virus (CaMV) can function as a suppressor of nonsense-mediated mRNA decay (NMD). TAV interacts specifically with a scaffold protein of the decapping complex VARICOSE (VCS) in the yeast two-hybrid system, and co-localizes with components of the decapping complex in planta. Notably, plants transgenic for TAV accumulate endogenous NMD-elicited mRNAs, while decay of AU-rich instability element (ARE)-signal containing mRNAs are not affected. Using an agroinfiltration-based transient assay we confirmed that TAV specifically stabilizes mRNA containing a premature termination codon (PTC) in a VCS-dependent manner. We have identified a TAV motif consisting of 12 of the 520 amino acids in the full-length sequence that is critical for both VCS binding and the NMD suppression effect. Our data suggest that TAV can intercept NMD by targeting the decapping machinery through the scaffold protein VARICOSE, indicating that 5'-3' mRNA decapping is a late step in NMD-related mRNA degradation in plants.

Split green fluorescent protein as a tool to study infection with a plant pathogen, Cauliflower mosaic virus

The split GFP technique is based on the auto-assembly of GFP when two polypeptides–GFP1-10 (residues 1–214; the detector) and GFP11 (residues 215–230; the tag)–both non-fluorescing on their own, associate spontaneously to form a fluorescent molecule. We evaluated this technique for its efficacy in contributing to the characterization of Cauliflower mosaic virus (CaMV) infection. A recombinant CaMV with GFP11 fused to the viral protein P6 (a key player in CaMV infection and major constituent of viral factory inclusions that arise during infection) was constructed and used to inoculate transgenic Arabidopsis thaliana expressing GFP1-10. The mutant virus (CaMV_11P6) was infectious, aphid-transmissible and the insertion was stable over many passages. Symptoms on infected plants were delayed and milder. Viral protein accumulation, especially of recombinant 11P6, was greatly decreased, impeding its detection early in infection. Nonetheless, spread of infection from the inoculated leaf to other leaves was followed by whole plant imaging. Infected cells displayed in real time confocal laser scanning microscopy fluorescence in wild type-looking virus factories. Thus, it allowed for the first time to track a CaMV protein in vivo in the context of an authentic infection. 11P6 was immunoprecipitated with anti-GFP nanobodies, presenting a new application for the split GFP system in protein-protein interaction assays and proteomics. Taken together, split GFP can be an attractive alternative to using the entire GFP for protein tagging.

Ribosome Shunting, Polycistronic Translation, and Evasion of Antiviral Defenses in Plant Pararetroviruses and Beyond

Viruses have compact genomes and usually translate more than one protein from polycistronic RNAs using leaky scanning, frameshifting, stop codon suppression or reinitiation mechanisms. Viral (pre-)genomic RNAs often contain long 5′-leader sequences with short upstream open reading frames (uORFs) and secondary structure elements, which control both translation initiation and replication. In plants, viral RNA and DNA are targeted by RNA interference (RNAi) generating small RNAs that silence viral gene expression, while viral proteins are recognized by innate immunity and autophagy that restrict viral infection. In this review we focus on plant pararetroviruses of the family Caulimoviridae and describe the mechanisms of uORF- and secondary structure-driven ribosome shunting, leaky scanning and reinitiation after translation of short and long uORFs. We discuss conservation of these mechanisms in different genera of Caulimoviridae, including host genome-integrated endogenous viral elements, as well as in other viral families, and highlight a multipurpose use of the highly-structured leader sequence of plant pararetroviruses in regulation of translation, splicing, packaging, and reverse transcription of pregenomic RNA (pgRNA), and in evasion of RNAi. Furthermore, we illustrate how targeting of several host factors by a pararetroviral effector protein can lead to transactivation of viral polycistronic translation and concomitant suppression of antiviral defenses. Thus, activation of the plant protein kinase target of rapamycin (TOR) by the Cauliflower mosaic virus transactivator/viroplasmin (TAV) promotes reinitiation of translation after long ORFs on viral pgRNA and blocks antiviral autophagy and innate immunity responses, while interaction of TAV with the plant RNAi machinery interferes with antiviral silencing.