Replication of tobacco ringsport virus. I. Detection of a low molecular weight double-stranded RNA from infectd plants

Silicon delays Tobacco ringspot virus systemic symptoms in Nicotiana tabacum

Soluble silicon (Si) provides protection to plants against a variety of abiotic and biotic stress. However, the effects of Si on viral infections are largely unknown. To investigate the role of Si in viral infections, hydroponic studies were conducted in Nicotiana tabacum with two pathogens: Tobacco ringspot virus (TRSV) and Tobacco mosaic virus (TMV). Plants grown in elevated Si showed a delay in TRSV systemic symptom formation and a reduction in symptomatic leaf area, compared to the non-supplemented controls. TRSV-infected plants showed significantly higher levels of foliar Si compared to mock-inoculated plants. However, the Si effect appeared to be virus-specific, since the element did not alter TMV symptoms nor did infection by this virus alter foliar Si levels. Hence, increased foliar Si levels appear to correlate with Si-modulated protection against viral infection. This is all the more intriguing since N. tabacum is classified as a low Si accumulator.

Studies on encapsidated viroid-like RNA I. Characterization of velvet tobacco mottle virus

Velvet tobacco mottle virus (VTMoV) isolated from Nicotiana velutina growing wild in arid Central Australia was transmitted by inoculation to a limited number of plant species of which N. clevelandii was the most convenient experimental host. The virus was also transmitted from field-grown plants toN. velutina and N. clevelandii by the mirid Cyropeltis nicotianae. VTMoV preparations purified by clarification with organic solvents and differential centrifugation contained polyhedral particles about 30 nm in diameter sedimenting as a single component at about 115 S. The particles were shown to be located in the nucleus, cytoplasm, and vacuoles of infected plant cells. Virus dissociated in the presence of mercaptoethanol and sodium dodecyl sulfate (SDS) separated into one major and two minor polypeptides with estimated molecular weights of 33,000, 36,000 and 31,000, respectively. Single-stranded RNA isolated from VTMoV by extraction with phenol was separated into five components with apparent molecular weights of 1.5 x 10(6), 0.63 x 10(6), 0.25 x 10(6), 0.16 x 10(6), and 0.12 x 10(6) referred to as RNAs 1, 1a, 1b, 2, and 3, respectively. It appears that RNAs 1a and 1b are breakdown products of RNA 1, as shown elsewhere, and electron microscopic examination of the other species showed that whereas RNAs 1 and 3 are linear molecules, RNA 2 is circular. The similarity of RNAs 2 and 3 to the RNA of viroids is discussed. VTMoV has been compared with several RNA plant viruses with small polyhedral particles. Only solanum nodiflorum mottle virus appears to share some of its unique features and the two have been shown to be antigenically related.

The structure of tobacco ringspot virus: a link in the evolution of icosahedral capsids in the picornavirus superfamily

BACKGROUND

Tobacco ringspot virus (TRSV) is a member of the nepovirus genus of icosahedral RNA plant viruses that cause disease in fruit crops. Nepoviruses, comoviruses and picornaviruses are classified in the picornavirus superfamily. Crystal structures of comoviruses and picornaviruses and the molecular mass of the TRSV subunit (sufficient to accommodate three beta-barrel domains) suggested that nepoviruses may represent a link in the evolution of the picornavirus capsids from a T = 3 icosahedral virus. This evolutionary process is thought to involve triplication of the capsid protein gene, to encode a three-domain polyprotein, followed by development of cleavage sites in the interdomain linking regions. Structural studies on TRSV were initiated to determine if the TRSV subunit corresponds to the proposed uncleaved three-domain polyprotein.

RESULTS

The 3.5 A resolution structure of TRSV shows that the capsid protein consists of three beta-barrel domains covalently linked by extended polypeptides. The order of connectivity of the domains in TRSV confirms the proposed connectivity for the precleaved comovirus and picornavirus capsid polyprotein. Structural differences between equivalent domains in TRSV and comoviruses are confined to the external surface loops, interdomain connecting polypeptides and N termini. The three different domains within TRSV and comoviruses are more closely related at the structural level than the three individual domains within picornaviruses.

CONCLUSIONS

The structural results confirm the notion of divergent evolution of the capsid polyproteins of nepoviruses, comoviruses and picornaviruses from a common ancestor. A number of residues were found to be conserved among various nepoviruses, some of which stabilize the quaternary structure of the three domains in the TRSV capsid protein subunit. Two conserved regions were identified on the external surface of TRSV, however, mutational studies will be needed to understand their functional significance. Nepoviruses transmitted by the same nematode species do not share regions with similar amino acid composition on the viral surface.

Effect of actinomycin D on replication of satellite tobacco ringspot virus RNA in plant protoplasts

We have developed a three-component system of host, tobacco ringspot virus (TobRV), and satellite tobacco ringspot virus RNA (sTobRV RNA) for investigating the specific contributions of host components or TobRV gene products to the propagative cycle of satellite RNA. Cowpea (Vigna unguiculata) protoplasts were inoculated with sTobRV and TobRV genomic RNAs by electroporation. An increase in sTobRV RNA was detected both by blot hybridization and by incorporation of [14C]uridine into material with the electrophoretic mobility of sTobRV RNA. DNA-dependent RNA synthesis in uninoculated protoplasts was effectively inhibited by 50 micrograms/ml actinomycin D (Act D) in the medium. Addition of Act D to protoplasts 24 or 48 hr after coinoculation with sTobRV RNA and TobRV genomic RNAs had little effect on accumulation of sTobRV RNA, whereas addition at 24 hr prior to coinoculation prevented any detected accumulation of sTobRV RNA of either polarity. Our results and previous findings of RNA complementary to encapsidated satellite RNA in extracts of infected tissue suggest that an RNA-dependent RNA polymerase is responsible for the synthesis of sTobRV RNA. The strongly inhibitory effect of Act D when added early implies a role for a host factor in the early phase of sTobRV RNA replication.

Kinetics of velvet tobacco mottle virus satellite RNA synthesis and encapsidation

Synthesis of circular (RNA 2) and linear (RNA 3) molecules of velvet tobacco mottle virus (VTMoV) satellite RNA (sat RNA) has been studied by incubating strips of tissues excised from systemically infected Nicotiana clevelandii in solutions of [14C]uridine. After a short lag, RNA and virus synthesis proceeded at a constant rate for at least 24 hr, during which time most of the synthesis was directed to the production of RNAs 2 and 3. The kinetics of [14C]uridine incorporation into the sat RNA molecules after increasing times of incubation and during pulses of [14C]uridine followed by chase incubation with excess [12C]uridine suggest that RNA 3 is a percursor of RNA 2. However, not all the RNA 3 synthesized was shown to end up as RNA 2, even after 72 hr of incubation. Several lines of evidence are presented supporting the conclusion that VTMoV-infected cells contain large pools of unencapsidated sat RNA. It is suggested that the sat RNA may have a greater affinity for the VTMoV replicase than the helper viral RNA which results in copious production of the sat RNA.

Immunochemical detection of ds-RNA in healthy and virus-infected plants and specific detection of viral ds-RNA by hybridization to labelled complementary DNA

Double-stranded (ds) RNA was detected in nucleic acid preparations from virus-free leaf tissues of several plant species by radial double-diffusion tests with antiserum raised against polyinosinic : polycytidylic acid [poly(I) : poly(C)]. No significant differences in the concentrations of ds-RNA were detected by such tests in virus-free French bean leaves and those infected with tobacco mosaic virus or tobacco ringspot virus. However, virus-specific ds-RNAs as well as genomic RNAs of both viruses were readily detected and estimated quantitatively by hybridization to 3H-labelled complementary DNA probes. It is concluded that it is not feasible to screen plant material infected with single-stranded RNA viruses by immunochemical tests for virus-associated ds-RNAs.