ABSTRACT A model for the maturation of tomato spotted wilt virus (TSWV) particles is proposed, mainly based on results with a protoplast infection system, in which the chronology of different maturation events could be determined. By using specific monoclonal and polyclonal antisera in immunofluorescence and electron microscopy, the site of TSWV particle morphogenesis was determined to be the Golgi system. The viral glycoproteins G1 and G2 accumulate in the Golgi prior to a process of wrapping, by which the viral nucleocapsids obtain a double membrane. In a later stage of the maturation, these doubly enveloped particles fuse to each other and to the endoplasmic reticulum to form singly enveloped particles clustered in membranes. Similarities and differences between the maturation of animal-infecting (bunya)viruses and plant-infecting tospoviruses are discussed.

A plant protoplast system for studying tomato spotted wilt tospovirus (TSWV) infection was established and tested. Using polyethylene glycol-mediated inoculation with highly infectious TSWV particles, generally 50% or more of Nicotiana rustica protoplasts were infected. In these cells viral RNA and viral protein synthesis became detectable at 16 h post-inoculation (p.i.) and continued at least until 90 h p.i. Both the structural viral proteins [nucleoprotein (N) and the envelope glycoproteins G1 and G2] and the nonstructural viral proteins NSs and NSm accumulated to amounts sufficient for detection and immunocytological analysis. Local lesion tests on petunia leaves and electron microscopical analysis confirmed the production of mature, infectious virus particles, underlining the conclusion that a full infection cycle was completed in this system. Upon inoculation of Vigna unguiculata (cowpea) protoplasts with TSWV particles, comparable proportions of infected cells and amounts of NSs, NSm and N protein were obtained, but much lower amounts of viral glycoproteins were detected than in N. rustica protoplasts, and progeny virus particles were less abundant. With the N. rustica-based protoplast system, a powerful synchronized single-cell infection system has now become available for more precise in vivo studies of the processes occurring during tospovirus infection.

Sequence analysis of the complete, tripartite RNA genomes of two tospoviruses, tomato spotted wilt virus (TSWV) and impatiens necrotic spot virus (INSV), demonstrated that they possess five genes that specify six functional proteins. The negative-stranded L RNA encodes the putative polymerase (TSWV 331.5 kDa, INSV 330.3 kDa), the ambisense M RNA encodes a common precursor to the two glycoproteins (GI and G2) and a nonstructural protein (NSm), and the likewise ambisense S RNA encodes the nucleocapsid (N) protein and a second nonstructural protein (NSs). These viral proteins are expressed from mRNAs that contain 12-20 nontemplated nucleotides at the 5' ends. This indicates that ''cap-snatching'' is the mechanism used by the viral polymerase to initiate transcription. Sequence analysis revealed that there was no strict base preference at the endonucleolytic site of the cellular leaders. Whereas the function of NSs, the least conserved tospoviral protein, has remained enigmatic, evidence is accumulating that NSm represents the viral movement protein that is involved in a tubule-guided cell-to-cell movement of nonenveloped nucleocapsids. Protoplasts infected with TSWV or transfected with the NSm gene solely, develop long, NSm-containing tubules that extend from the plasma membrane into the culture medium and are similar to tubules found in plasmodesmata of infected plant tissues. Experiments with transgenic plants confirm that NSm is a plasmodesma-associated protein that can modify intercellular communication in plants.

The 33.6-kDa nonstructural (NSM) protein gene, located on the ambisense M RNA segment of tomato spotted wilt virus (TSWV), was cloned and expressed using the Escherichia coli pET-11t expression system. The protein thus produced was purified and used for the production of a polyclonal antiserum. Western immunoblot analyses of TSWV-infected Nicotiana rustica plants showed NSM synthesis only during a short period early in systemic infection. Although NSM was found associated with cytoplasmic nucleocapsid preparations, it was absent from purified virus particles. Analyses of subcellular fractions from young, systemically infected leaves showed the presence of NSM in fractions enriched for cell walls and cytoplasmic membranes, respectively. Furthermore, immunogold labeling of tissue sections of TSWV-infected N. rustica plants showed that this protein was found associated with nucleocapsid aggregates in the cytoplasm and in close association with plasmodesmata. The data obtained provide evidence that NSM represents the viral movement protein of TSWV, involved in cell-to-cell movement of nonenveloped ribonucleocapsid structures.

In cowpea plant cells infected with cowpea mosaic virus, tubular structures containing virus particles are formed in the plasmodesmata between adjacent cells; these structures are supposedly involved in cell-to-cell spread of the virus. Here we show that similar tubular structures are also formed in cowpea protoplasts, from which the cell wall and plasmodesmata are absent. Between 12 and 21 h post-inoculation, tubule formation starts in the periphery of the protoplast at the level of the plasma membrane. Upon assembly, the virus-containing tubule is enveloped by the plasma membrane and extends into the culture medium. This suggests that the tubule has functional polarity and makes it likely that a tubule 'grows' into a neighbouring cell in vivo. On average, 75% of infected protoplasts were shown to possess tubular structures extending from their surface. The tubule wall was 3 to 4 nm thick and they were up to 20 microns in length, as shown by fluorescent light microscopy and negative staining electron microscopy. By analogy to infected plant cells, both the viral 58K/48K movement and capsid proteins were located in these tubules, as determined by immunofluorescent staining and immunogold labelling using specific antisera against these proteins. These results demonstrate that the formation of tubules is not necessarily dependent on the presence of plasmodesmata or the cell wall, and that they are composed, at least in part, of virus-encoded components.