2.1. Effect of Deletions within the 3′ NTR of AlMV sgRNA4 on Chimeric Virus Replication and CP Accumulation
To investigate the effect of deletion of the AlMV sgRNA4 3′ NTR on replication of chimeric TMV (Av/A4), we generated Av/A4 mutants by deleting various length of sequences within CP binding site 2 (CPB2) and its downstream sequences, except from CPB1 which is needed for efficient RNA translation [
21]. The mutants were p0 (no CPB2 domain is included), p1 (containing only CPB2), p3 (containing an extended CPB2 sequence) and p8 (containing a further extended CPB2 sequence) (
Figure 2).
Suspensions of tobacco protoplasts were inoculated with equivalent amounts of
in vitro generated transcripts of each construct, and total RNAs were assessed by the Northern blot analysis at 24 h post inoculation. The results indicated that all the constructs produced genomic RNAs and CP sg mRNAs at comparable levels (
Figure 3a). The construct Av also produced a similar level of CP sg RNA, but of a smaller size than the other constructs, which was expected since Av lacks the sequences of AlMV sgRNA4. Uninfected healthy protoplasts were used as a negative control and showed no detectable signal with the mobility of the genomic RNA or sgRNA in the Northern blot analysis, thus suggesting that the deletions within the AlMV sgRNA4 3′ NTR sequence did not affect chimeric Av/A4 virus replication or mRNA accumulation in protoplasts.
The expression of CP from these constructs was also compared using the Western blot analysis. For that, total proteins were isolated from the transcript-inoculated protoplasts at 24 h post inoculation and probed with an anti-AlMV CP monoclonal antibody. The results demonstrated CP accumulation from all the mutants except Av. Among these, p3 and p8 showed a moderate to high level of CP accumulation compared to p0 and p1 (
Figure 3b). Interestingly, both p0, which has a complete deletion of CPB2, and p1, which includes CPB2, showed similar levels of CP (
Figure 3b). This suggests that inclusion or deletion of the CPB2 region (
i.e., p0 and p1 mutants) had no impact on CP accumulation, whereas inclusion of an extended (p3) and a further extended (p8) sequence downstream of CPB2 had a strong positive effect on CP accumulation (
Figure 3b).
Thus, the results indicate that, although AlMV mutants are as competent as Av/A4 in replication, they varied in CP accumulation in protoplasts.
Figure 2.
Construction of various deletions in CPB of Av/A4. Top: secondary structure of the 3′ non‑translated region (NTR) of AlMV RNA3 (or sgRNA4) in the CPB conformation. Hairpin loops in the 3′ NTR as shown by CPB1 and CPB2 are the CPB sites that are involved in replication and translation, respectively. Numbers indicate nt position in AlMV RNA3. Bottom: Av/A4 mutants p0, p1, p3 and p8 with respective deletion regions (shown by dotted lines within CPB regions). The lines upstream of MP represent sequences from TMV included in each of the Av/A4 mutants.
Figure 2.
Construction of various deletions in CPB of Av/A4. Top: secondary structure of the 3′ non‑translated region (NTR) of AlMV RNA3 (or sgRNA4) in the CPB conformation. Hairpin loops in the 3′ NTR as shown by CPB1 and CPB2 are the CPB sites that are involved in replication and translation, respectively. Numbers indicate nt position in AlMV RNA3. Bottom: Av/A4 mutants p0, p1, p3 and p8 with respective deletion regions (shown by dotted lines within CPB regions). The lines upstream of MP represent sequences from TMV included in each of the Av/A4 mutants.
Figure 3.
Examination of Av/A4 and deletion mutants in protoplasts and plants. (a) Northern blot analysis of total RNA isolated from tobacco protoplasts inoculated with in vitro transcripts of Av, p0, p1, p3, p8 or Av/A4 (as shown on blot) at 24 h post inoculation. A digoxigenin-labeled TMV 3′ NTR plus-strand–specific riboprobe was used for detection. Lane H indicates healthy protoplasts and rRNA is ribosomal RNA. (b) Western blot analysis of total protein from protoplasts inoculated with in vitro transcripts of each construct. Lanes are the same as in (a); Std indicates wild-type purified AlMV CP, 200 ng. (c) Western blot analysis of total protein from plants inoculated with in vitro transcripts of each construct. Tissue samples were obtained from inoculated (Lanes 1, 3, 5, 7 and 9) and upper non-inoculated leaves (Lanes 2, 4, 6, 8 and 10) at 7 and 20 days post inoculation (dpi). CP was detected by an anti-AlMV CP monoclonal antibody followed by a secondary antibody labeled with horseradish peroxidase.
Figure 3.
Examination of Av/A4 and deletion mutants in protoplasts and plants. (a) Northern blot analysis of total RNA isolated from tobacco protoplasts inoculated with in vitro transcripts of Av, p0, p1, p3, p8 or Av/A4 (as shown on blot) at 24 h post inoculation. A digoxigenin-labeled TMV 3′ NTR plus-strand–specific riboprobe was used for detection. Lane H indicates healthy protoplasts and rRNA is ribosomal RNA. (b) Western blot analysis of total protein from protoplasts inoculated with in vitro transcripts of each construct. Lanes are the same as in (a); Std indicates wild-type purified AlMV CP, 200 ng. (c) Western blot analysis of total protein from plants inoculated with in vitro transcripts of each construct. Tissue samples were obtained from inoculated (Lanes 1, 3, 5, 7 and 9) and upper non-inoculated leaves (Lanes 2, 4, 6, 8 and 10) at 7 and 20 days post inoculation (dpi). CP was detected by an anti-AlMV CP monoclonal antibody followed by a secondary antibody labeled with horseradish peroxidase.

2.2. Symptom Expression, Encapsidation and Long-Distance Movement by AlMV sgRNA4 Mutants
Although the deletions within the AlMV sgRNA4 3′ NTR had no effect on chimeric virus replication in inoculated tobacco protoplasts, it was unknown whether the symptom expression in plants inoculated with these mutants would be similar. To address this question, we inoculated N. benthamiana plants with in vitro generated RNA transcripts of the mutants and monitored for symptom development. Plants infected with p0 or p1 did not develop any observable symptoms until 15–20 days post inoculation (dpi), whereas plants inoculated with p3 or p8, as well as with intact Av/A4, demonstrated visible curling and yellowing symptoms in upper non-inoculated leaves within 7 to 10 dpi. Moreover, these plants developed significant stunting.
To examine whether symptomatic or asymptomatic plants contained AlMV CP, total proteins were extracted from upper systemic leaves of the infected plants at 7 and 20 dpi and analyzed by Western blotting (
Figure 3c). The results demonstrated that at 7 dpi p3, p8 and Av/A4, but not p0 and p1, showed strong CP signals detected using an anti-AlMV CP monoclonal antibody (
Figure 3c; compare lanes 6, 8 and 10 with lanes 2 and 4). Furthermore, CP signals were visible in leaves inoculated with any of these constructs (
Figure 3c; lanes 1, 3, 5, 7 and 9), except from Av (data not shown). This suggests that at 7 dpi encapsidated virus particles of p3, p8 and Av/A4 moved efficiently into upper systemic leaves, while p0 and p1 mutants did not. However, at 20 dpi all the mutants, including p0 and p1, showed more or less equivalent CP signals in both inoculated as well as upper systemic leaves (
Figure 3c), These results demonstrate that compared to p3, p8 and Av/A4, the mutants p0 and p1 were less efficiently encapsidated by CP, which affected their long-distance movement as evidenced by symptom expression and CP accumulation in upper systemic leaves of the inoculated plants at 7 dpi.
Reduced encapsidation of p0 and p1 was further confirmed by passaging infectivity from the p0- and p1-infected plant sap to the local lesion host, N. tabacum cv. Xanthi NN plants, and counting the number of lesions on each inoculated leaf at 2 to 3 dpi. We have found that the mutants p3 and p8 passaged infectivity at a high efficiency (60 to 65%, similar to intact Av/A4), whereas p0 and p1 did it at a very low efficiency (less than 20%). This marked reduction in the efficiency of passaging with p0 and p1 suggests low efficiency in encapsidation for these chimeric constructs compared to p3, p8 and Av/A4.
Taken together, our findings indicate that deletions of the extended part of the CPB2 region (from nt position 1924 to 1970;
Figure 2) of the 3′ NTR of sgRNA4, as in the p0 and p1 mutants, impaired the ability of viral RNA to be encapsidated efficiently and thus resulted in poor host infection and long-distance movement of the encapsidated virus particles. In contrast, the deletions in the p3 and p8 mutants did not have substantial effect.
2.3. Characterization of Purified Mutant Virus Particles
In order to characterize virus particles formed by the deletion mutants of Av/A4, we have purified them from upper systemic leaves 3 weeks post inoculation, following a previously described procedure for purification of AlMV virions [
22]. The presence of the virus in the purified preparations was demonstrated by SDS-PAGE followed by Coomassie staining (
Figure 4a) and the Western blot analysis (
Figure 4b). We also assessed the virus yield from plants infected with the AlMV sgRNA4 3′ NTR deletion mutants. The comparative analysis of total protein in crude plant extracts and purified virus preparations demonstrated that the amount of purified particles from plants infected with p3, p8 or Av/A4 was quantitatively high compared with p0- and p1-infected plants (
Figure 4a,b), whereas the yield of p3 and p8 was ~2-fold lower compared with Av/A4-infected plants. In contrast, the yield of p0- and p1-produced particles was negligible (~1% of the yield of Av/A4), although AlMV CP was detected in both inoculated and upper systemic leaves at 20 dpi (
Figure 3c).
Figure 4.
Analysis of virus preparations and the crude extracts by SDS-PAGE followed by (a) Coomassie staining and (b) Western blot analysis using an anti-AlMV CP antibody. N. benthamiana plants were inoculated with equivalent amounts of in vitro transcripts of p0, p1, p3, p8 or Av/A4. Virus was purified from upper non-inoculated leaves of the plants at 3 to 4 weeks post inoculation from equivalent amounts of tissue. Total protein present in the crude extracts from equivalent amounts of tissue infected with these constructs was also examined. CE: crude extract, v: purified virus preparation.
Figure 4.
Analysis of virus preparations and the crude extracts by SDS-PAGE followed by (a) Coomassie staining and (b) Western blot analysis using an anti-AlMV CP antibody. N. benthamiana plants were inoculated with equivalent amounts of in vitro transcripts of p0, p1, p3, p8 or Av/A4. Virus was purified from upper non-inoculated leaves of the plants at 3 to 4 weeks post inoculation from equivalent amounts of tissue. Total protein present in the crude extracts from equivalent amounts of tissue infected with these constructs was also examined. CE: crude extract, v: purified virus preparation.
We characterized the purified mutant virus preparations by transmission electron microscopy (TEM). The wild-type AlMV virus preparation typically contains bacilliform-shaped particles of four major sizes—56, 43, 35 or 30 nm long and 18 nm wide (
Figure 5), whereas TMV assembles into rigid rods that are 300 nm long and 18 nm wide (
Figure 5). Preparations of the Av/A4 hybrid virus contain only AlMV-like bacilliform-shaped particles, but no TMV-like rigid rods (
Figure 5). These preparations are very heterogeneous and contain particles of at least six different sizes—95, 70, 62, 44–48, 30–35 or 16–24 nm long, and ~18 nm wide (
Figure 5,
Table 1). Approximately 10% of the particles were longer than 62 nm, and ~90% were in the 16–48 nm length range. The preparation of the p8 mutant contained virus particles similar to the virions of intact Av/A4, whereas in the preparation of the mutant p3 we could detect only 62 nm or shorter particles and none longer than 70 and 95 nm (
Figure 5,
Table 1). Furthermore, in the preparations of the p0 and p1 mutants we observed even shorter particles, 16–48 nm long.
Taken together, our purification results and TEM observations indicated that the longer the sequence deleted from the 3′ NTR of AlMV sgRNA4, the lower the yield of the mutant virus particles and the shorter their length.
Figure 5.
Transmission electron microscopy (TEM) analysis of virus particles purified from plants inoculated with in vitro transcripts of AlMV, TMV, Av/A4, p8, p3, p1 or p0.
Figure 5.
Transmission electron microscopy (TEM) analysis of virus particles purified from plants inoculated with in vitro transcripts of AlMV, TMV, Av/A4, p8, p3, p1 or p0.
Table 1.
Distribution of virus particles in purified virus preparations of Av/A4, p8, p3, p1 and p0.
Table 1.
Distribution of virus particles in purified virus preparations of Av/A4, p8, p3, p1 and p0.
Construct | Particle size (nm) a |
---|
95 nm | 70 | 62 | 44–48 | 30–35 | 16–24 |
---|
Av/A4 | + | + | + | + | + | + |
P8 | + | + | − | + | + | + |
P3 | − | − | + | + | + | + |
P1 | − | − | − | + | + | + |
P0 | − | − | − | + | + | + |
The results of the presented study, using tobacco protoplasts and
N. benthamiana plants inoculated with the deletion mutants of Av/A4 chimeric TMV virus, have demonstrated that the 3′ NTR of AlMV sgRNA4 is not essential for replication of Av/A4 but is required for efficient RNA encapsidation, long-distance movement and virus accumulation. Our long-term goal is to identify the AlMV OAS which appears to be within the 3′ NTR of sgRNA4 based on the current findings. However, additional research with more compelling evidence is needed to demonstrate this. In this study, we used the Av/A4 chimeric virus as a model, because it allows for studying functions of CP and 3′ NTR sequences of AlMV sgRNA4 independently. This has bypassed the need for creating viable CP mutants using AlMV RNAs 1, 2 and 3. Since the OAS of many viruses typically consists of stem-loop structures [
1,
5,
23] and the 3′ NTR of AlMV sgRNA4 (and RNA3) is highly structured, we examined the role of the 3′ NTR in encapsidation of Av/A4. For this purpose, we have generated mutants of Av/A4 that differed in the length of the sequences deleted from the 3′ NTR and explored their ability to replicate in protoplasts, encapsidate RNA and infect plants.
Because the sequences required for Av/A4 encapsidation were found to localize within the AlMV 3′ NTR, we hypothesized that Av/A4 would form at least three types of major-size virus particles. These particles were expected to encapsidate genomic RNA (~7,350 nts), MP sgRNA (~2,500 nts) or CP sgRNA (~1,650 nts). The TEM analysis of purified Av/A4 demonstrated the presence of bacilliform-shaped particles with rounded ends typical for AlMV, rather than the rigid rods typical for TMV. Moreover, the Av/A4 virus preparation was more heterogeneous than expected, showing at least six types of particles of different sizes (
Table 1). When similar-size particles were grouped together (shorter particles, 16–48 nm; longer particles, 62–95 nm), the shorter particles predominated (~90%), suggesting that encapsidation of the longer particles in the Av/A4 virus preparation was relatively inefficient (10%), which could be due to the large size of the Av/A4 genome (~7,350 nts, twice as the size of AlMV RNA1 [3,644 nts; the particle size, 56 nm]).
Deletion of certain elements of the 3′ NTR has been also found to have an effect on the efficiency of encapsidation and size of Av/A4 mutant s and their ability to infect plants. Although the shorter particles (16–48 nm) were observed even in the purified preparations of the mutants p0 and p1, these preparations may have infected plants very inefficiently, indicating poor encapsidation of these mutants. Although we did not examine the encapsidated RNA, it is also possible that p0 or p1 could form empty particles, and this may be the reason why we observed CP expression on a Western blot. Studies with a TMV-BMV hybrid [
3] in which
Brome mosaic virus CP replaced native CP revealed that in the hybrid virus only three sgRNAs, but not genomic RNA, were encapsidated. CP or sequences and/or structural features within the chimeric viral genome were thought to play a role in promoting virion assembly [
3].