Heterologous movement protein strongly modifies the infection phenotype of cucumber mosaic virus

Peanut stunt virus movement protein is the limiting factor in Capsicum annuum infection

Cucumber mosaic virus (CMV) is one of the most devastating plant viruses, with more than 1,200 species of host plants. The host range and economic importance of peanut stunt virus (PSV) are mostly limited to legumes, despite the similar taxonomy and genome structure with CMV. Since no data are available on the background of the limited host range of PSV, RNA 3 recombinant and reassortant viruses were generated (C12P3, P12C3, C12CP3, C12PC3, C12PΔC3) to study their infection phenotype on a common host (Nicotiana benthamiana) and on a selective host (Capsicum annuum cv. Brody). The PSV movement protein (MP) was not able to function with the coat protein (CP) of CMV unless the C-terminal 42 amino acids were deleted from the PSV MP. As a result of the inoculation experiments, MP was considered the protein influencing symptom phenotypes on N. benthamiana and responsible for the host range difference on the pepper. Since plasmodesmata (PD) localization of viral MPs is essential for cell-to-cell movement, subcellular localization of GFP-tagged MPs (CMV-MP-eGFP, PSV-MP-eGFP) was observed. In the case of CMV-MP-eGFP, clear colocalization with PD was detected in both hosts, but PSV-MP-eGFP was not tightly connected to the PD in N. benthamiana and barely localized to the PD in C. annuum epidermal cells. Measuring Pearson correlation coefficients (PCCs) also supported the visual observation.

Alanine scanning of cucumber mosaic virus (CMV) 2b protein identifies different positions for cell-to-cell movement and gene silencing suppressor activity

The multifunctional 2b protein of CMV has a role in the long distance and local movement of the virus, in symptom formation, in evasion of defense mediated by salicylic acid as well as in suppression of RNA silencing. The role of conserved amino acid sequence domains were analyzed previously in the protein function, but comprehensive analysis of this protein was not carried out until recently. We have analyzed all over the 2b protein by alanine scanning mutagenesis changing three consecutive amino acids (aa) to alanine. We have identified eight aa triplets as key determinants of the 2b protein function in virus infection. Four of them (KKQ/22-24/AAA, QNR/31-33/AAA, RER/34-36/AAA, SPS/40-42/AAA) overlap with previously determined regions indispensable in gene silencing suppressor function. We have identified two additional triplets necessary for the suppressor function of the 2b protein (LPF/55-57/AAA, NVE/10-12/AAA), and two other positions were required for cell-to-cell movement of the virus (MEL/1-3/AAA, RHV/70-72/AAA), which are not essential for suppressor activity.

Transcomplementation and synergism in plants: implications for viral transgenes?

In plants, viral synergisms occur when one virus enhances infection by a distinct or unrelated virus. Such synergisms may be unidirectional or mutualistic but, in either case, synergism implies that protein(s) from one virus can enhance infection by another. A mechanistically related phenomenon is transcomplementation, in which a viral protein, usually expressed from a transgene, enhances or supports the infection of a virus from a distinct species. To gain an insight into the characteristics and limitations of these helper functions of individual viral genes, and to assess their effects on the plant-pathogen relationship, reports of successful synergism and transcomplementation were compiled from the peer-reviewed literature and combined with data from successful viral gene exchange experiments. Results from these experiments were tabulated to highlight the phylogenetic relationship between the helper and dependent viruses and, where possible, to identify the protein responsible for the altered infection process. The analysis of more than 150 publications, each containing one or more reports of successful exchanges, transcomplementation or synergism, revealed the following: (i) diverse viral traits can be enhanced by synergism and transcomplementation; these include the expansion of host range, acquisition of mechanical transmission, enhanced specific infectivity, enhanced cell-to-cell and long-distance movement, elevated or novel vector transmission, elevated viral titre and enhanced seed transmission; (ii) transcomplementation and synergism are mediated by many viral proteins, including inhibitors of gene silencing, replicases, coat proteins and movement proteins; (iii) although more frequent between closely related viruses, transcomplementation and synergism can occur between viruses that are phylogenetically highly divergent. As indicators of the interoperability of viral genes, these results are of general interest, but they can also be applied to the risk assessment of transgenic crops expressing viral proteins. In particular, they can contribute to the identification of potential hazards, and can be used to identify data gaps and limitations in predicting the likelihood of transgene-mediated transcomplementation.

Compensatory capsid protein mutations in cucumber mosaic virus confer systemic infectivity in squash (Cucurbita pepo)

Cucumber mosaic virus (CMV) systemically infects both tobacco and zucchini squash. CMV capsid protein loop mutants with single-amino-acid substitutions are unable to systemically infect squash, but they revert to a wild-type phenotype in the presence of an additional, specific single-site substitution. The D118A, T120A, D192A, and D197A loop mutants reverted to a wild-type phenotype but did so in combination with P56S, P77L, A162V, and I53F or T124I mutations, respectively. The possible effect of these compensatory mutations on other, nonsystemically infecting loop mutants was tested with the F117A mutant and found to be neutral, thus indicating a specificity to the observed changes.

Compatibility of the movement protein and the coat protein of cucumoviruses is required for cell-to-cell movement

For the cell-to-cell movement of cucumoviruses both the movement protein (MP) and the coat protein (CP) are required. These are not reversibly exchangeable between Cucumber mosaic virus (CMV) and Tomato aspermy virus (TAV). The MP of CMV is able to function with the TAV CP (chimera RT), but TAV MP is unable to promote the cell-to-cell movement in the presence of CMV CP (chimera TR). To gain further insight into the non-infectious nature of the TR recombinant, RNA 3 chimeras were constructed with recombinant MPs and CPs. The chimeric MP and one of the CP recombinants were infectious. The other recombinant CP enabled virus movement only after the introduction of two point mutations (Glu-->Lys and Lys-->Arg at aa 62 and 65, respectively). The mutations served to correct the CP surface electrostatic potential that was altered by the recombination. The infectivity of the TR virus on different test plants was restored by replacing the sequence encoding the C-terminal 29 aa of the MP with the corresponding sequence of the CMV MP gene or by exchanging the sequence encoding the C-terminal 15 aa of the CP with the same region of TAV. The analysis of the recombinant clones suggests a requirement for compatibility between the C-terminal 29 aa of the MP and the C-terminal two-thirds of the CP for cell-to-cell movement of cucumoviruses.

Cucumoviruses

Research on the molecular biology of cucumoviruses and their plant-virus interactions has been very extensive in the last decade. Cucumovirus genome structures have been analyzed, giving new insights into their genetic variability, evolution, and taxonomy. A new viral gene has been discovered, and its role in promoting virus infection has been delineated. The localization and various functions of each viral-encoded gene product have been established. The particle structures of Cucumber mosaic virus (CMV) and Tomato aspermy virus have been determined. Pathogenicity domains have been mapped, and barriers to virus infection have been localized. The movement pathways of the viruses in some hosts have been discerned, and viral mutants affecting the movement processes have been identified. Host responses to viral infection have been characterized, both temporally and spatially. Progress has been made in determining the mechanisms of replication, gene expression, and transmission of CMV. The pathogenicity determinants of various satellite RNAs have been characterized, and the importance of secondary structure in satellite RNA-mediated interactions has been recognized. Novel plant genes specifying resistance to infection by CMV have been identified. In some cases, these genes have been mapped, and one resistance gene to CMV has been isolated and characterized. Pathogen-derived resistance has been demonstrated against CMV using various segments of the CMV genome, and the mechanisms of some of these forms of resistances have been analyzed. Finally, the nature of synergistic interactions between CMV and other viruses has been characterized. This review highlights these various achievements in the context of the previous work on the biology of cucumoviruses and their interactions with plants.

Macromolecular Transport and Signaling Through Plasmodesmata

Plasmodesmata (Pd) are channels in the plant cell wall that in conjunction with associated phloem form an intercellular communication network that supports the cell-to-cell and long-distance trafficking of a wide spectrum of endogenous proteins and ribonucleoprotein complexes. The trafficking of such macromolecules is of importance in the orchestration of non-cell autonomous developmental and physiological processes. Plant viruses encode movement proteins (MPs) that subvert this communication network to facilitate the spread of infection. These viral proteins thus represent excellent experimental keys for exploring the mechanisms involved in intercellular trafficking and communication via Pd.