The Movement Protein of Cucumber Mosaic Virus Traffics into Sieve Elements in Minor Veins of Nicotiana clevelandii

Abstract

The location of the 3a movement protein (MP) of cucumber mosaic virus (CMV) was studied by quantitative immunogold labeling of the wild-type 3a MP in leaves of Nicotiana clevelandii infected by CMV as well as by using a 3a-green fluorescent protein (GFP) fusion expressed from a potato virus X (PVX) vector. Whether expressed from CMV or PVX, the 3a MP targeted plasmodesmata and accumulated in the central cavity of the pore. Within minor veins, the most extensively labeled plasmodesmata were those connecting sieve elements and companion cells. In addition to targeting plasmodesmata, the 3a MP accumulated in the parietal layer of mature sieve elements. Confocal imaging of cells expressing the 3a-GFP fusion protein showed that the 3a MP assembled into elaborate fibrillar formations in the sieve element parietal layer. The ability of 3a-GFP, expressed from PVX rather than CMV, to enter sieve elements demonstrates that neither the CMV RNA nor the CMV coat protein is required for trafficking of the 3a MP into sieve elements. CMV virions were not detected in plasmodesmata from CMV-infected tissue, although large CMV aggregates were often found in the parietal layer of sieve elements and were usually surrounded by 3a MP. These data suggest that CMV traffics into minor vein sieve elements as a ribonucleoprotein complex that contains the viral RNA, coat protein, and 3a MP, with subsequent viral assembly occurring in the sieve element parietal layer.

Constraints to Genetic Exchange Support Gene Coadaptation in a Tripartite RNA Virus

“…This would not be the case for the combination of RNA1 and RNA3. There was a sharp increase in relative frequency of allele A at locus i in RNA1 associated to systemic movement, suggesting that the homologous combination of the three RNAs performs the function of systemic colonization better, which occurs in the form of assembled viral particles and may depend on interactions between the CMV capsid and host factors [39,40]. Alternatively, a higher fitness for homologous allele combinations in RNAs 1 and 2, related to the interaction of their protein products in the viral replicase [41], could lead to a delayed increase of allele A at locus i as it increases at locus j, and explain the association between loci i and j observed in the systemic host.…”

THE GREAT ESCAPE: Phloem Transport and Unloading of Macromolecules

“…However, larger vein classes could also potentially be entry points for systemic macromolecules, and there is no a priori case as to why macromolecules should enter the phloem at the same sites as solutes (58). Minor veins of dicotyledons usually have a characteristic architecture, comprising two or three mature sieve elements, associated companion cells, and phloem parenchyma elements (4,19,26,46,58,69,75,76,92) (Figure 1a). Prior to the sink-source transition these minor vein complexes are immature (69,89) and symplastically coupled to the mesophyll.…”

“…For many viruses, encapsidated virions appear to represent the functional longdistance movement complex (4,27,57,58,81,91) (Figure 1c). However, the preceding cell-cell movement steps through mesophyll cells may occur as a ribonucleoprotein complex (4,18,27,58,71) or as an intact virion (58,76), depending on the virus. In CMV, virions have not been detected in mesophyll plasmodesmata nor in the PPUs that connect the SE and CC (4).…”

“…Research on phloem exudates has also demonstrated the presence of numerous proteins (25,28,29,32,34,76,79,103) and endogenous mRNAs (70,78,101) in sieve tube exudate. Together with a large body of evidence that viral genomes, either as nucleoprotein complexes or intact virions, may be translocated in the phloem (4,10,27,50,58,65,69,72,75,77,81,91), the picture that emerges is one in which the transport phloem functions as an information superhighway (41,70), translocating a wide range of macromolecular traffic throughout the plant. What is the role of the many diverse macromolecules in the phloem and what is their final destination?…”

Morphological Characterization of the Self‐Assembly of Virus Movement Proteins into Nanotubes in the Absence of Virus Particles

“…These VTs may be able to grow in cell walls containing plasmodesmata and outward into neighboring cells to deliver viruses . This group of viruses includes alfalfa mosaic virus (AMV, genus Alfamovirus ), cowpea mosaic virus (CPMV, genus Comovirus ), brome mosaic virus (BMV, genus Bromovirus ), cucumber mosaic virus (CMV, genus Cucumovirus ), and prunus necrotic ringspot virus (PNRSV, genus Ilarvirus ) . In virus‐infected plants, such VTs reach an outer diameter of ≈100 nm and a length in the micrometer range with a diameter of ≈50 nm for the inner lumen, as shown by fluorescent light microscopy and negative staining electron microscopy .…”