The methyltransferase domain of the 1a protein of cowpea chlorotic mottle virus controls local and systemic accumulation in cowpea

The type strain of cowpea chlorotic mottle virus (CCMV-T) induces a local and systemic infection in California Blackeye cowpea (Vigna unguiculata (L.) Walp. subs. unguiculata cv. California Blackeye), but accumulates to low levels in inoculated leaves and fails to accumulate systemically in the cowpea plant introduction (PI) 186465. CCMV-R, a mutant strain derived from CCMV-T, accumulates to higher levels than CCMV-T in inoculated leaves and systemically infects PI 186465 plants. The phenotypic determinant of CCMV-R was previously mapped to viral RNA1, but the location of the determinant within RNA1 was not identified. Pseudorecombinants generated from genomic cDNA clones of CCMV-T and CCMV-R indicated that the phenotypic differences on PI 186465 were independent of replication. Through the use of chimeric RNA1 cDNA clones containing portions of CCMV-T and CCMV-R and site-directed mutagenesis, two nucleotides, 299 (amino acid residue 77) and 951 (amino acid residue 294), were identified as being independently critical for the local and systemic accumulation patterns of CCMV-R in PI 186465 plants. A second independently derived CCMV-R-like mutant, identified nucleotide 216 (amino acid residue 49) as being critical for induction of the CCMV-R infection phenotype. Amino acid residues 49, 77, and 294 are within the methytransferase domain of the CCMV 1a protein, suggesting that the methytransferase domain has a role in cell-to-cell and systemic accumulation of the virus that is independent of replication.

Acquisition of Tomato spotted wilt virus by Adults of Two Thrips Species

ABSTRACT Only larval thrips that acquire Tomato spotted wilt virus (TSWV), or adults derived from such larvae, transmit the virus. Nonviruliferous adults can ingest virus particles while feeding on TSWV-infected plants, but such adult thrips have not been shown to transmit TSWV. Immunofluorescence microscopy was used to show that thrips 1, 5, 10, and 20 days after adult emergence (DAE) fed on TSWV-infected plants acquired TSWV with virus replication and accumulation occurring in both epithelial and muscle cells of Frankliniella fusca (tobacco thrips [TT]) and F. occidentalis (western flower thrips [WFT]), as indicated by immunodetection of the nonstructural (NSs) protein encoded by the small RNA and the nucleocapsid (N) protein, respectively. Adult WFT acquired TSWV more efficiently than TT. There was no significant effect of insect age on TSWV acquisition by TT. In contrast, acquisition by adult WFT at 1 and 5 DAE was higher than acquisition at 10 and 20 DAE. Subsequent transmission competence of adult cohorts was studied by vector transmission assays. All adult thrips tested that had an acquisition access period as an adult were unable to transmit the virus. These results indicate the susceptibility of adult TT and WFT to infection of midgut cells by TSWV and subsequent virus replication and confirm earlier studies that adult thrips that feed on virus-infected plants do not transmit the virus. The role of a tissue barrier in TSWV movement and infection from midgut muscle cells to the salivary glands is discussed.

Molecular diversity of RNA-2 genome segments in pecluviruses causing peanut clump disease in West Africa and India

The complete nucleotide sequence of RNA-2 genome segments of four isolates of Peanut clump virus (PCV) and two isolates of Indian peanut clump virus (IPCV) were determined. Comparisons among the complete RNA-2 sequences of six isolates from this study and two published earlier, revealed a high degree of variability in size (between 4290 and 4652 nucleotides) and nucleotide sequence identities (between 58 % and 79 %). Amino acid sequence alignments of the five open reading frames (ORF) showed that ORF 4, which encodes the second of the triple gene block proteins, is highly conserved (90 to 98 % identical) whereas the protein encoded by ORF 2, whose function is unknown, is less conserved (25 to 60 % identical). The coat protein of the eight isolates showed amino acid identities between 37 % and 89 % and contained several conserved residues. Phylogenetic comparisons, based on complete RNA-2 sequences, revealed that the eight isolates grouped into two distinct clusters with no geographical distinction between PCV and IPCV isolates. Phylogenetic tree topologies for individual ORFs showed an overall similarity with that obtained from entire RNA-2 sequences, although the relative positions of individual isolates vary within each cluster. The results indicate that there is substantial divergence among the RNA-2 genomes of pecluviruses and suggest that different proteins have evolved differently, possibly due to different selection pressures.

Dynamics of Tomato spotted wilt virus Replication in the Alimentary Canal of Two Thrips Species

ABSTRACT Transmission of Tomato spotted wilt virus (TSWV) is dependent on virus uptake in the midgut prior to virus movement to the salivary glands. Replication of TSWV in the alimentary canal of tobacco thrips (TT, Frankliniella fusca) and western flower thrips (WFT, F. occidentalis) was investigated by immunolocalization of the nonstructural protein (NSs) encoded by the small RNA of TSWV and fluorescence microscopy. Analysis of cohorts during development from larva to adults following virus acquisition by first instar larva indicated that virus replication followed a specific time-course pattern in the foregut, regions of the midgut, salivary glands, and ligaments between the midgut and salivary glands. Initial virus replication occurred only in epithelial cells of midgut-1 but, upon infection of muscle cells, the virus moved to the midgut-2, foregut, midgut-3, and salivary glands. The ligaments between the midgut and salivary glands appeared to be a route for virus to invade the salivary glands. No virus replication was observed in the hindgut, Malpighian tubules, or tubular salivary glands. The dynamics of TSWV replication, as measured by NSs accumulation, were similar in both TT and WFT.

Symptom induction by Cowpea chlorotic mottle virus on Vigna unguiculata is determined by amino acid residue 151 in the coat protein

The type strain of Cowpea chlorotic mottle virus (CCMV-T) produces a bright chlorosis in cowpea (Vigna unguiculata cv. California Blackeye). The attenuated variant (CCMV-M) induces mild green mottle symptoms that were previously mapped to RNA 3. Restriction fragment exchanges between RNA 3 cDNA clones of CCMV-T and CCMV-M that generate infectious transcripts and site-directed mutagenesis indicated that the codon encoding amino acid residue 151 of the coat protein determines the symptom phenotypes of CCMV-T and CCMV-M. Amino acid 151 is within an alpha-helical structure required for calcium ion binding and virus particle stability. No differences in virion stability or accumulation were detected between CCMV-T and CCMV-M. Mutational analysis suggested that the amino acid at position 151 and not the nucleotide sequence induce the symptom phenotype. Thus, it is likely that subtle influences by amino acid residue 151 in coat protein-host interactions result in chlorotic and mild green mottle symptoms.

First Report of Frankliniella fusca as a Vector of Impatiens necrotic spot tospovirus

Of more than a dozen members of the genus Tospovirus, Tomato spotted wilt virus (TSWV) and Impatiens necrotic spot virus (INSV) are among the most damaging viruses found in North America (3). TSWV is a major problem in vegetable and field crops, whereas INSV is commonly encountered in the floriculture and nursery industries. TSWV is transmitted by several thrips species, of which the western flower thrips (WFT, Frankliniella occidentalis Pergande) is the most predominant vector. INSV has been reported to be transmitted only by WFT (1). To determine if tobacco thrips (TT, F. fusca Hinds) can transmit INSV, a virus-free culture of TT was reared on detached peanut cv. Florunner leaves in 0.5-liter polypropylene cups with closed lids at 25 ± 2°C with constant light. Fresh peanut leaves were exchanged every 2 to 3 days to maintain the thrips colony. For transmission studies, adult thrips were confined on peanut leaves for 24 h for oviposition and then the peanut leaves, sans adults thrips, were transferred to a new cup. Leaves were examined daily for larval emergence, and similarly aged first instar larvae (<12 h old) were given an acquisition access period of 24 to 48 h on INSV-infected detached leaves of Emilia sonchifolia. The larvae were subsequently transferred to healthy peanut leaves and reared until adult emergence. Groups of 10 adults per plant were given a 48-h inoculation access period on 10-day-old healthy E. sonchifolia seedlings. Thrips were subsequently killed, and the plants were maintained in a growth chamber at 28 ± 2°C, and with a 16/8 light/dark photoperiod. Transmission studies were repeated 10 times with different sources of infected plants and different batches of larvae following acquisition access periods. Seven to ten days after inoculation, plants developed symptoms consisting of chlorotic spots, mosaic, and mottling. The presence of INSV in these symptomatic plants was confirmed by ELISA using INSV ImmunoStrip Test (Agdia, Inc., Elkhart, IN) and by reverse transcription-polymerase chain reaction assay with primers specific to the INSV-NSs gene. Our results demonstrate that TT can serve as a vector of INSV. INSV has been reported in peanut in the southeastern United States (2). WFT and TT transmit TSWV in peanuts, with the latter being the predominant vector species in Georgia and other parts of the region. TT transmission of INSV is of concern because of the increased incidence in recent years of INSV in peanuts and the potential for synergistic or gene exchange between TSWV and INSV, since mixed infections with both viruses have been observed (4). References: (1) M. L. Daughtrey et al. Plant Dis. 81:1220, 1997. (2) S. S. Pappu et al. Plant Dis. 83:966, 1999. (3). J. L. Sherwood et al. Pages 1034-1040 in: Encyclopedia of Plant Pathology. C. Maloy and T. D. Murray, eds. John Wiley and Sons, Inc., New York, 2001. (4) L. Wells et al. Phytopathology (Abstr.) 94:S94, 2001.

Distribution and Characteristics of Groundnut Rosette Disease in Kenya

Groundnut rosette is a major virus disease of peanut in sub-Saharan Africa. The disease is caused by a complex of three agents: GRAV (groundnut rosette assistor luteovirus), GRV (groundnut rosette umbravirus), and the associated satellite RNA (Sat-RNA). During the 1997 to 1998 crop season, the incidence of rosette in farmers' fields was estimated at 24 to 40% in western Kenya and 30% in the Rift Valley. Sequence analysis of Kenyan isolates revealed that GRAV-CP sequences shared 97 to 100% and 95 to 98% sequence homology at nucleotide and amino acid levels, respectively, amongst themselves and with the Malawian and Nigerian isolates. The ORFs 3 and 4 of GRV were similar, with a homology of 99% at the nucleotide and amino acid levels among Kenyan isolates. The GRV sequences of Kenyan isolates were closer to the Malawian (95 to 96%) than to the Nigerian (87 to 88%) isolates. Sat-RNA shared 89 to 94% nucleotide identity with those from Malawi and Nigeria. A closer sequence relationship was observed between Kenyan and Malawian isolates in all regions compared. This is the first report on the distribution and molecular characterization of groundnut rosette disease complex in East Africa.

Phylogenetic studies of tospoviruses (family: Bunyaviridae) based on intergenic region sequences of small and medium genomic RNAs

Analysis of the intergenic region (IGR) of S and M RNAs of tospoviruses (Family Bunyaviridae) indicated their heterogeneity both in length and sequence. In general, IGRs of M RNA were shorter in length compared to the IGRs of their respective S RNA species. Percent identity among the S RNA IGR sequences of distinct tospovirus species varied from 42 to 57%, whereas it was 79 to 99% among isolates of the same species. Similarly, when IGRs of M RNAs were compared, there was higher sequence identity among isolates of the same tospovirus species (84 to 98%) than among distinct tospovirus species (46 to 59%). Percent nucleotide identities and maximum likelihood trees of IGR sequences of S and M RNAs indicated that their sequence divergence is similar to that of nucleocapsid gene at inter and intra-species levels. This is the first detailed sequence analysis of IGRs of S and M RNAs of known tospoviruses.

Sequence diversity within the three agents of groundnut rosette disease

ABSTRACT Sequence diversity was examined in the coat protein (CP) gene of Groundnut rosette assistor virus (GRAV), the overlapping open reading frames (ORFs) 3 and 4 of Groundnut rosette virus (GRV), and the satellite RNA (sat-RNA) of GRV obtained from field isolates from Malawi and Nigeria. These three agents cause groundnut rosette disease, a major disease of groundnut in sub-Saharan Africa (SSA). Sequence analysis showed that the GRAV CP gene was highly conserved (97 to 99%) independent of its geographic source. The nucleotide sequence of the overlapping ORFs 3 and 4 of GRV was highly conserved (98 to 100%) from isolates within a geographic region but less conserved (88 to 89%) between isolates from the two distinct geographic regions. Phylogenetic analysis of the overlapping ORFs 3 and 4 show that the GRV isolates cluster according to the geographic region from which they were isolated, indicating that Malawian GRV isolates are distinct from Nigerian GRV isolates. Similarity within the sat-RNA sequences analyzed ranged from 88 to 99%. Phylogenetic analysis also showed clustering within the sat-RNA isolates according to country of origin, as well as within isolates from two distinct regions of Malawi. Because the GRAV CP sequence is highly conserved, independent of the geographic source of the GRAV isolates, the GRAV CP sequence represents the most likely candidate to use for pathogen-derived resistance in groundnut and may provide effective protection against groundnut rosette disease throughout SSA.

Spatiotemporal separation of groundnut rosette disease agents

ABSTRACT Analysis by triple-antibody sandwich enzyme-linked immunosorbent assay of groundnut samples from fields in two seasons from different regions of Malawi showed the absence of groundnut rosette assistor virus (GRAV) from some plants showing groundnut rosette disease symptoms and the presence of GRAV in some symptomless plants. Viruliferous Aphis craccivora collected from fields transmitted either GRAV alone, groundnut rosette virus (GRV) with its satellite RNA (sat RNA), or all three agents together, in different proportions. More plants became infected with all three agents when increasing numbers of potentially viruliferous aphids were used per plant, suggesting a dosage response. Electrical pentration graph studies of aphid stylet activities indicated successful transmission of GRV and its sat RNA during both the "stylet pathway phase" and salivation into sieve elements, whereas GRAV was transmitted only during the latter phase. Aphids transmitted all three agents together only during the salivation phase. Reverse-transcriptase polymerase chain reaction testing of viruliferous aphids and of inoculated plants revealed no correlation between the presence of all three agents in prospective aphid vectors and their simultaneous transmission to groundnut plants. These results show that separation of the groundnut rosette disease agents occurs over time and space.

First Report of the Green Rosette Variant of Groundnut Rosette Disease in Kenya

Groundnut (Arachis hypogaea L.) is an important food crop in sub-Saharan Africa. One of the major production constraints is groundnut rosette disease, which is caused by a complex of two viruses, groundnut rosette assistor luteovirus (GRAV) and groundnut rosette umbravirus (GRV) together with the associated satellite RNA (satRNA) (1). Two main forms of the disease have been described: chlorotic and the green rosette. Variants of the satRNA have been shown to be largely responsible for the different forms of the disease (1). Chlorotic rosette has been the predominant form in all of sub-Saharan Africa while green rosette has been reported in the western and southern regions of Africa (2). During the 1997-1998 crop season, disease surveys conducted in Kenya showed the incidence of the rosette disease in farmers' fields to be 24 to 40% in a total of 23 fields surveyed in the western regions of the country (Homabay, Kendubay, Kisumu) and 30% in 8 fields sampled in the Rift Valley (Cheplamus, Marigat) regions. Representative peanut plants showing rosette symptoms were analyzed for the presence of GRV by reverse transcription polymerase chain reaction (RT-PCR). With primers specific to a portion of ORF4 of GRV RNA (3), RT-PCR gave a product of expected size (approximately 300 bp). The PCR product was cloned in pGEM-T vector and sequenced. The sequenced region showed 89% nucleotide sequence identity with published GRV sequences. Green rosette was observed on groundnut cultivars Nyaela Red and Homabay Local in the Kendu Bay region. The incidence of the green rosette was 5.3% of the plants with rosette symptoms. References: (1) A. F. Murant and I. K. Kumar. Ann. Appl. Biol. 117:85, 1990. (2) R. A. Naidu et al. Ann. Appl. Biol. 132:525, 1998. (3). M. E. Taliansky et al. J. Gen. Virol. 77:2335, 1996.

Analysis of the intergenic region of tomato spotted wilt Tospovirus medium RNA segment

The intergenic region (IGR) of the medium (M) RNA of tomato spotted wilt Tospovirus (TSWV) isolates naturally infecting peanut (groundnut), pepper, potato, stokesia, tobacco and watermelon in Georgia (GA) and a peanut isolate from Florida (FL) was cloned and sequenced. The IGR sequences were compared with one another and with respective M RNA IGRs of TSWV isolates from Brazil and Japan and other tospoviruses. The length of M IGR of GA and FL isolates varied from 271 to 277 nucleotides. The M IGRs of TSWV from potato and stokesia, and tobacco and watermelon were identical with each other in their length and sequence. IGR sequences were more conserved (95-100%) among the populations of TSWV from GA and FL, than when compared with those of TSWV isolates from other countries (83-94%). The conserved motif (CAAACTTTGG) present in the IGRs of both M and small (S) RNAs of a Brazilian isolate of TSWV was also conserved in the isolates studied. Cluster analysis of the IGR sequences showed that all GA and FL isolates are closely clustered and are distinct from the TSWV isolates from other countries as well as from other tospoviruses.

Defective Movement of Viruses in the Family Bromoviridae Is Differentially Complemented in Nicotiana benthamiana Expressing Tobamovirus or Dianthovirus Movement Proteins

ABSTRACT Taxonomically distinct tobacco mosaic tobamovirus (TMV), red clover necrotic mosaic dianthovirus (RCNMV), cucumber mosaic cucumovirus (CMV), brome mosaic bromovirus (BMV), and cowpea chlorotic mottle bromovirus (CCMV) exhibit differences in their host range. Each of these viruses encodes a functionally similar nonstructural movement protein (MP) that is essential for cell-to-cell movement of a progeny virus. Despite the lack of significant amino acid identity among the MPs of CMV, TMV, and RCNMV, movement-defective CMV (CMVFnyDeltaMP-DeltaKPN) was able to move locally and systemically in transgenic Nicotiana benthamiana expressing either TMV MP (NB-TMV-MP(+)) or RCNMV MP (NB-RCNMV-MP(+)). These observations contrast with those of previous studies in which transgenic N. tabacum cv. Xanthi plants expressing TMV MP supported only the cell-to-cell movement of CMVFnyDeltaMP-DeltaKPN. To verify whether similar complementation could be observed for movement-defective bromoviruses, NB-TMV-MP(+) and NB-RCNMV-MP(+) plants were inoculated independently with movement-defective variants of BMV (B3DeltaMP) and CCMV (CC3DeltaMP). Neither NB-TMV-MP(+) nor NB-RCNMV-MP(+) was able to rescue the defective cell-to-cell and long-distance movement of B3DeltaMP. In contrast, NB-RCNMV-MP(+) complemented the cell-to-cell, but not the long-distance, movement of CC3DeltaMP. Taken together, these studies suggest that virus movement is a complex process and that, in some cases, the host species plays a major role in determining the long-distance movement function of a virus.

Second-site reversion of a dysfunctional mutation in a conserved region of the tobacco mosaic tobamovirus movement protein

The N-terminal two-thirds of tobamovirus movement proteins (MPs) contain two well conserved regions. Within region I (amino acids 56-96) is an area predicted by computer analysis to have loop secondary structure (amino acids 76-87). A single or two double amino acid mutations were introduced into the loop in region I of the TMV MP to destabilize the structure. The three mutant MPs were defective in movement function. The single amino acid mutation resulted in a Pro81-->Ser substitution. The mutant virus, TP81S, containing the Pro81-->Ser substitution, was propagated on a transgenic line of Nicotiana tabacum that expresses the sunn-hemp mosaic tobamovirus MP. Inoculation of virus progeny from the transgenic plants onto hypersensitive N. tabacum indicated the presence of infectious virus at a low frequency. Necrotic lesions were detected at 4 days postinoculation, 2 days later than those induced by wild-type TMV. Inoculation of virus extracted from necrotic lesions onto N. tabacum resulted in a delayed and attenuated systemic infection relative to that induced by TMV, indicating that a second-site mutation restored movement function rather than a reversion of the original mutation. Sequence analysis revealed that the revertant MP gene had two additional amino acid substitutions, a Thr104-->Ile and a Arg167-->Lys. Introduction of the amino acid substitutions individually or in combination into the MP of TP81S indicated that both substitutions were required for the revertant phenotype. The data indicate that structure within region I is important in maintaining an active conformation for functional MP, that changes outside region I can compensate for alterations within the region, and suggest that region I may interact with a distal portion of the protein.

Completion of the nucleotide sequence of sunn-hemp mosaic virus: a tobamovirus pathogenic to legumes

Sunn-hemp mosaic virus (SHMV) is a member of the tobamovirus group of plant viruses. The nucleotide sequence of the 5'-untranslated region, the 129 kD protein gene, and a portion of the 186 kD protein gene of SHMV was determined. The 4,683 nucleotides (nts) reported here completes the sequence of the SHMV genome and complements previous work (Meshi, Ohno, and Okada, Nucleic Acids Res. 10, 6111-6117 [1982]; Mol. Gen. Genet. 184, 20-25 [1981]) to provide the first complete nucleotide sequence for a tobamovirus that is pathogenic to leguminous plants.

Tobamovirus and dianthovirus movement proteins are functionally homologous

The movement proteins (MPs) of tobacco mosaic tobamovirus (TMV) and red clover necrotic mosaic dianthovirus (RCNMV) enlarge plasmodesmata size exclusion limits, transport RNA from cell to cell, and bind nucleic acids in vitro. Despite these functional similarities, they have no sequence homology. However, they do appear to have similar secondary structures. We have used transgenic plants expressing either the TMV MP or the RCNMV MP, and a chimeric TMV that encodes the RCNMV MP as its only functional MP gene, to demonstrate that the MPs of TMV and RCNMV are functionally homologous. Further, both TMV and RCNMV can act as helper viruses to allow the cell-to-cell movement of the heterologous movement-defective viruses. These data support the conclusion that, despite other differences, such as particle morphology, host range, and sequence, TMV and RCNMV share a common mechanism for cell-to-cell movement.

Studies of coat protein-mediated resistance to tobacco mosaic virus (TMV). II. Challenge by a mutant with altered virion surface does not overcome resistance conferred by TMV coat protein

Transgenic tobacco plants expressing the coat protein (CP) gene of the U1 strain of tobacco mosaic virus (TMV) exhibit CP-mediated resistance (CP-MR) against some, but not all, tobamoviruses. To investigate the role of the amino acid sequences on the surface of the challenge virus in CP-MR, mutant strains of U1 TMV were constructed to contain the amino or carboxy termini of the CP of Sunn hemp mosaic tobamovirus (SHMV). The modified virus was unable to overcome CP-MR in transgenic plants that contained the TMV CP. In contrast, TMV in which the CP was replaced by the SHMV CP overcame CP-MR to the same extent as did SHMV. We conclude that CP-MR conferred by TMV CP involves interactions between amino acid sequences of the challenge viruses and the transgene protein other than those on the surface of the challenge virus.

Influence of heterologous tobamovirus movement protein and chimeric-movement protein genes on cell-to-cell and long-distance movement

Sunn-hemp mosaic tobamovirus (SHMV) moves slowly from cell to cell in Nicotiana tabacum cv. Xanthi, but fails to move long distance. To determine the role of the SHMV movement protein (MP) in cell-to-cell and long-distance movement in tobacco, the SHMV MP gene was inserted into a TMV-cDNA clone that had approximately the 5'-half of the endogenous MP gene deleted. RNA transcripts inoculated onto tobacco induced systemic infections by 8 days postinoculation. Sequence analysis of the MP genes from purified virus isolated from systemically infected leaf tissue indicated that chimeric SHMV/TMV MP genes had been generated through RNA-RNA recombination within the 3'-termini of the MP gene sequences. When exchanged for the MP gene of TMV, three of four chimeric MP genes analyzed provided long-distance movement function for the hybrid viruses in tobacco. Two of the three hybrid viruses that moved long distance showed enhanced cell-to-cell movement relative to a recombinant TMV that expressed the intact SHMV MP gene. These observations suggest that the C-terminus of the TMV MP contains a determinant that can influence cell-to-cell movement in tobacco. A recombinant virus, TLSM, that expressed the intact SHMV MP gene exhibited cell-to-cell movement that was intermediate to SHMV and TMV, but failed to produce coat protein and was defective in long-distance movement. To further examine the role of the SHMV MP gene in long-distance movement, transgenic N. tabacum cv. Xanthi that expressed the wild-type SHMV MP gene were generated and found to facilitate rapid and efficient long-distance movement of a TMV mutant that contained a dysfunctional MP gene. Therefore, the inability of SHMV to systemically infect tobacco is a function of virus components and sequences other than those encoded by the SHMV MP gene.

Effects of terminal deletion mutations on function of the movement protein of tobacco mosaic virus

A series of carboxy- and amino-terminal deletion mutations in the movement protein (MP) gene of tobacco mosaic virus (TMV) were ligated into a cloned TMV cDNA deleted for the endogenous MP gene. RNA transcripts were produced in vitro from clones carrying the various mutated MP genes. The effect of the deletion mutations on local and systemic movements of the infection was evaluated. Deletion of 9 or 33 amino acids from the carboxy terminus of the movement protein did not effect cell-to-cell movement as reflected by local lesion formation on Nicotiana tabacum cv. Xanthi NN plants. Deletion of 55 amino acids resulted in impaired MP that supported the formation of local lesions of 1 mm in diameter compared to lesions of 3-5 mm caused by the wild-type MP. Deletion of 74 amino acids (or more) from the carboxy terminus resulted in a protein that could not support virus movement. Modified viruses that contained repeated sequences in the 3' region of the MP gene lost the repeated sequences during replication and reverted to the wild type. This was evidenced by the size of the MP produced and by sequence analysis of reverse-transcribed PCR-amplified products, following infection by the modified virus. MP deleted for as few as 3 amino acids at the amino terminus could not support virus movement thus indicating that the amino-terminal domain is critical for MP activity.

Plasmodesmatal function is probed using transgenic tobacco plants that express a virus movement protein

A gene encoding a temperature-sensitive mutant (MPP154A) of the 30-kilodalton movement protein (MP) of tobacco mosaic virus (TMV) was transformed into Nicotiana tabacum cv Xanthi. Transgenic plants expressing the MPP154A gene complemented local and systemic movement of an MP-defective mutant of TMV (U3/12MPfs) at the permissive temperature of 24 degrees C but not at 32 degrees C, the nonpermissive temperature. A microinjection procedure was used to investigate the effects of the modified TMV MP on plasmodesmatal size-exclusion limits. Movement of fluorescein isothiocyanate-labeled dextran (F-dextran), with an average molecular mass of 9.4 kilodaltons, was detected between leaf mesophyll cells of the transgenic plants at 24 degrees C; however, no movement of either 3.9-kilodalton or 9.4-kilodalton F-dextrans was detected when the transgenic plants were held for 6 hours (or longer) at 32 degrees C. When these plants were shifted back to 24 degrees C for 6 hours, cell-to-cell movement of the F-dextrans was again observed. Accumulation of MPP154A was not affected by the temperature regime, nor was the subcellular distribution of the MP altered. These results are consistent with a change in the protein conformation of MPP154A at the nonpermissive temperature, which gives rise to a protein that fails to modify the molecular size-exclusion limits of plasmodesmata to the same extent as wild-type MP. Surprisingly, at 32 degrees C, movement of the F-dextrans was inhibited in transgenic plants expressing the wild-type MP gene; however, the inhibition was transient and was no longer detected after 48 hours at this elevated temperature. This transient inhibition of plasmodesmatal function was alleviated with Sirofluor, an inhibitor of callose ([1----3]-beta-D-glucan) synthesis. This result provides experimental evidence that callose deposition is involved in regulating the molecular size-exclusion limit of plasmodesmata in plants. Sirofluor had no effect on the inhibition of F-dextran movement at 32 degrees C in plants expressing the MPP154A gene, indicating that callose formation was not responsible for the failure of the temperature-sensitive mutant protein to alter the size-exclusion limit of plasmodesmata.

Altered function of the tobacco mosaic virus movement protein in a hypersensitive host

The N gene in Nicotiana sp. confers hypersensitive resistance to all strains of tobacco mosaic virus (TMV) and limits the rate of virus spread in infected leaves. To examine the role of the movement protein (MP) of TMV in the hypersensitive reaction (HR), transgenic Nicotiana tabacum cv. Xanthi-nc (genotype NN) plants that express the MP gene were produced and the molecular size exclusion limit of plasmodesmata in leaf mesophyll cells was monitored. At the HR-permissive temperature (24 degrees) movement from cell to cell of fluorescein isothiocyanate-labeled dextran of molecular mass 3.9 kDa was detected while 9.4-kDa molecules failed to move. At the HR-nonpermissive temperature (33 degrees) the 9.4-kDa probe moved readily from cell to cell. In contrast, in transgenic Xanthi (genotype nn) which express the MP gene the 9.4-kDa probe moved from cell to cell at 24 and 33 degrees. These results suggest that the N gene may modify the ability of the MP to alter plasmodesmata molecular exclusion limits, although expression of the TMV-MP gene alone did not induce the HR. Furthermore, when MP(+) Xanthi-nc tobacco lines were inoculated with a TMV that lacked a MP gene the HR was induced, and the concentration of MP in the transgenic lines was correlated with the degree of the HR.

Molecular characterization and biological function of the movement protein of tobacco mosaic virus in transgenic plants

We previously demonstrated, in transgenic tobacco plants, that the role of the movement protein (MP) of tobacco mosaic virus is to facilitate the cell-to-cell spread of viral progeny during infection. An analysis of different tissues of these transgenic plants indicated that the MP accumulated in leaf, stem, and root tissue. The highest levels were detected in older leaves. The relative levels of MP in leaf tissue from transgenic plants were equivalent to, or higher than, the levels of MP in tobacco mosaic virus-infected leaf tissue. Results of subcellular fractionation of homogenates of transgenic leaf tissue showed that the MP was most abundant in the cell wall fraction of older leaves and that the protein remained at high levels in the cell wall fraction as the leaves continued to age. Significant levels of the MP were detected in a crude membrane/organelle fraction and a soluble fraction in younger leaves but decreased to low levels in older leaves. These results suggest that the MP accumulates and is stable in cell walls. We have previously shown that the MP modifies the molecular exclusion limit of plasmodesmata, which is consistent with the hypothesis that plant viruses move from cell to cell through altered plasmodesmata. We show here that the ability of the tobacco mosaic virus MP to modify the molecular exclusion limit of plasmodesmata in tobacco depends on the developmental stage of the leaf. The implications of these findings on understanding virus movement and how plasmodesmata function are discussed.

Movement protein of tobacco mosaic virus modifies plasmodesmatal size exclusion limit

The function of the 30-kilodalton movement protein (MP) of tobacco mosaic virus is to facilitate cell-to-cell movement of viral progeny in an infected plant. A novel method for delivering non-plasmalemma-permeable fluorescent probes to the cytosol of spongy mesophyll cells of tobacco leaves was used to study plasmodesmatal size exclusion limits in transgenic plants that express the MP gene. Movement of fluorescein isothiocyanate-labeled dextran (F-dextran) with an average molecular mass of 9400 daltons and an approximate Stokes radius of 2.4 nanometers was detected between cells of the transgenic plants, whereas the size exclusion limit for the control plants was 700 to 800 daltons. No evidence of F-dextran metabolism in the leaves of the transgenic plants was found. Thus, the tobacco mosaic virus movement protein has a direct effect on a plasmodesmatal function.

Selective recovery of foreign gene transcripts as virus-like particles in TMV-infected transgenic tobaccos

A short origin-of-assembly sequence (OAS) located in the 30kDa movement protein gene, about 1.0kb from the 3'-end of the common strain of tobacco mosaic virus (TMV) RNA, nucleates encapsidation of the 6395-nucleotide-long genome by TMV coat protein in vitro, and presumably also in vivo. Single-stranded RNAs containing a foreign reporter gene sequence and the TMV OAS at their 5' - and 3' -ends, respectively, can be synthesized in vitro from recombinant SP6-transcription plasmids and will assemble spontaneously in vitro to form TMV-like 'pseudovirus' particles. In this paper, we show that foreign gene transcripts derived from the nuclear DNA of plants transformed by Agrobacterium tumefaciens, and which contain the TMV OAS, can be assembled into stable 'pseudovirus' particles in vivo during a systemic infection by TMV (helper). This is the first report of structural complementation between a heritable function bestowed on a transgenic plant and an infecting virus. As a route to protect, accumulate and recover a specific mRNA in vivo, in transgenic plant cells, this novel approach may find wider applications in developmental plant molecular biology.

The 30-Kilodalton Gene Product of Tobacco Mosaic Virus Potentiates Virus Movement

The proposed role of the 30-kilodalton(kD) protein of tobacco mosaic virus is to facilitate cell-to-cell spread of the virus-during infection. To directly define the function of the protein, a chimeric gene containing a cloned complementary DNA of the 30-kD protein gene was introduced into tobacco cells via a Ti plasmid-mediated transformation system of Agrobacterium tumefaciens. Transgenic plants regenerated from transformed tobacco cells expressed the 30-kD protein messenger RNA and accumulated 30-kD protein. Seedlings expressing the 30-kD protein gene complemented the Lsl mutant of TMV, a mutant that is temperature-sensitive in cell-to-cell movement. In addition, enhanced movement of the Lsl virus at the permissive temperature was detected in seedlings that express the 30-kD protein gene. These results conclusively demonstrate that the 30-kD protein of tobacco mosaic virus potentiates the movement of the virus from cell to cell.

In vitro transcription and translation of cloned cDNAs encoding the 30-kDa protein gene of TMV

A cDNA clone encoding the nonstructural, 30-kDa protein of the common (U1) strain of tobacco mosaic virus (TMV) was isolated and characterized. cDNA clones representing the intact gene as well as deletions from the 5' end of the gene were subcloned into SP6 vectors. Capped RNAs produced by in vitro transcription reactions were translated in a wheat germ cell-free system. The resultant proteins were compared to proteins obtained from the in vitro translation of intermediate length (I2) rods of TMV. Transcripts of the cDNA clones encoded polypeptides of 30, 28, or 18 kDa that were immunoprecipitated by antibody prepared against a synthetic peptide representing the carboxy terminus of the 30-kDa protein. cDNA clones containing the intact 30-kDa sequence coded for 30-kDa polypeptides while clones lacking the 30-kDa initiation codon produced 28-kDa polypeptides. Surprisingly, translation of a transcript from a cDNA clone containing the 30-kDa gene plus 390 nucleotides 5' of the initiator AUG yielded a polypeptide with an approximate molecular mass of 18 kDa. The results indicate that an intact and functional 30-kDa protein gene has been cloned. The significance of these results, with respect to determining the function of the 30-kDa protein, is discussed.

Host cell-mediated selection of a mutant influenza A virus that has lost a complex oligosaccharide from the tip of the hemagglutinin

During serial passage in Madin-Darby bovine kidney (MDBK) cells, a substrain of influenza virus A/WSN is lost from the population and is replaced by a mutant virus with altered host cell binding properties. This selection does not occur during growth in chicken embryo fibroblasts (CEF). It occurs during growth in MDBK cells because the parental virus produced by these cells has a dramatically reduced affinity for cellular receptors [Crecelius, D.M., Deom, C. M. & Schulze, I. T. (1984) Virology 139, 164-177]. We have now compared the hemagglutinin (HA) subunits, HA1 and HA2, of the parent and mutant viruses by NaDodSO4/PAGE and have found that when the viruses are grown in either host cell the HA1 subunit of the mutant is smaller than that of the parent virus. The nonglycosylated HAs, made in the presence of tunicamycin, have the same apparent molecular weight, indicating that the HA1 subunit of the mutant virus contains less carbohydrate than that of the parent. This reduction in carbohydrate content was observed with 11 independently derived mutants that had been selected by growth in MDBK cells. The nucleotide sequence of the HA gene of the parent and mutant viruses indicates that there are five potential glycosylation sites on the parent HA1 subunit and four on the mutant and that the mutation responsible for this difference is a single base change that eliminates the glycosylation site at amino acid 125 of the parent HA1 subunit. Treatment of the parent and mutant HAs from both cell sources with endo-beta-N-acetylglucosaminidases F and H showed that the HA1 of the parent virus has four complex and one high-mannose oligosaccharides, whereas that of the mutant virus has three complex and one high-mannose oligosaccharides. Thus, all of the potential sites on both HA1 subunits are glycosylated. We conclude that the oligosaccharide attached to amino acid 125 of the parent HA by MDBK cells can reduce the affinity of the virus for cellular receptors and that the mutant virus has a higher affinity than the parent because the mutant HA is not glycosylated at that site. Since amino acid 125 of the parent HA is glycosylated by both CEF and MDBK cells, we further conclude that the host-determined structure of the oligosaccharide at that site affects the affinity of the parent virus for cellular receptors and, thereby, determines whether the mutant virus will have a growth advantage.

Oligosaccharide composition of an influenza virus hemagglutinin with host-determined binding properties

We have previously reported that the binding properties of the hemagglutinin (HA) of the WSN-F strain of influenza A are affected by the cells in which the virus is grown (Crecelius, D. M., Deom, C. M., and Schulze, I.T. (1984) Virology 139, 164-177); at 37 degrees C chick embryo fibroblast-grown F virus has a greater affinity for host cells than does the same virus grown in Madin-Darby bovine kidney (MDBK) cells. In an attempt to explain this host-determined property, we have characterized the carbohydrate put onto the viral HA by these two cells. Experiments using tunicamycin indicate that the HA made by MDBK cells contains about 4000 daltons of carbohydrate in excess of that on the HA from chick embryo fibroblast. Serial lectin affinity chromatography of the asparagine-linked oligosaccharides on the HA subunits, HA1 and HA2, detected a number of host-dependent differences in the complex oligosaccharides. Both HA1 and HA2 from MDBK cells contained more highly branched (i.e. tri- and tetraantennary) complex oligosaccharides than did the subunits from chick embryo fibroblasts. In addition, the HA subunits from the two sources differed in the amount of galactose-containing "bisected" complex oligosaccharides and in the presence of certain fucosylated triantennary oligosaccharides. Profiles of the asparagine-linked oligosaccharides from the host cells did not show these differences, indicating that the HA subunit profiles were not necessarily representative of the structures found on the cellular glycoproteins. The data support the conclusion that bulky oligosaccharides on the MDBK-HA subunits of WSN-F reduce the affinity of the virus for cellular receptors.

Biological properties of a hemagglutinin mutant of influenza virus selected by host cells

Chick embryo fibroblast (CEF)-grown stocks of the WSN strain of influenza A(HINI) contain two variants which were designated F and C for fuzzy and clear plaque morphology on Madin-Darby bovine kidney (MDBK) cells. During growth in MDBK cells plaque-isolated F virus was completely replaced by C virus (L. Noronha-Blob and I.T. Schulze (1976), Virology 69, 314-322). The parental (F) and the mutant (C) viruses contain hemagglutinins which differ in their ability to bind to host cells. In addition, the host cells from which the purified viruses are obtained affect their binding properties. Thus, as compared to MDBK-grown F virus (FBK), MDBK-grown C virus (CBK) produced high amounts of mRNA and high virus yields in MDBK cells. CBK had greater affinity for SA alpha 2,3Gal and SA alpha 2,6Gal linkages on derivatized human erythrocytes than did FBK, independent of whether neuraminidase was present on the virions. CBK was also resistant to components of calf serum which inhibited FBK hemagglutination at 37 degrees. As compared to FBK, CBK had increased ability to bind to both MDBK cells and CEF at 37 degrees in the presence or absence of an inhibitor of neuraminidase. In addition, when cells with virus bound at 0 degrees were transferred to 37 degrees, CBK remained cell associated whereas about 80% of FBK dissociated from both cells. Thus, mutation from F to C increased the ability of the virus to associate with MDBK cell receptors. Studies carried out with F and C viruses from both cells indicated that the expression of the mutation depended in part on the host cells in which the virus was grown and in part on the cells used to measure the binding properties. A model relating these observations to selection of HA variants in nature is presented.