Cauliflower mosaic virus aphid transmission factor protein is expressed in cells infected with some aphid nontransmissible isolates

Electron-lucent inclusion bodies are structures specialized for aphid transmission of cauliflower mosaic virus

Cauliflower mosaic virus (CaMV) is transmitted by aphids. For acquisition by the vector, a transmissible complex must form, composed of the virus particle, the viral coat-associated protein P3 and the helper protein P2. However, the components of the transmissible complex are largely separated in infected plant cells: most P3 virions are confined in electron-dense inclusion bodies, whereas P2 is sequestered in electron-lucent inclusion bodies (elIBs). This spatial separation controls virus acquisition by favouring the binding of virus-free P2 to the vector first, rendering the vector competent for later uptake of P3 virions. Consequently, sequential acquisition of virus from different cells or tissues is possible, with important implications for the biology of CaMV transmission. CaMV strains Campbell and CM1841 contain a single amino acid mutation (G94R) in the helper protein P2, rendering them non-transmissible from plant to plant. However, the mutant P2-94 protein supports aphid transmission when expressed heterologously and supplied to P3-CaMV complexes in vitro. The non-transmissibility of P2-94 was re-examined in vivo and it is shown here that the non-transmissibility of this P2 mutant is not due to low accumulation levels in infected plants, as suggested previously, but more specifically to the failure to form elIBs within infected plant cells. This demonstrates that elIBs are complex viral structures specialized for aphid transmission and suggests that viral inclusion bodies other than viral factories, most often considered as 'garbage cans', can in fact exhibit specific functions.

Requirement of cauliflower mosaic virus open reading frame VI product for viral gene expression and multiplication in turnip protoplasts

Cauliflower mosaic virus (CaMV) open reading frame (ORF) VI product (P6) has been shown to be the major constituent of viral inclusion body, to function as a post-transcriptional transactivator, and to be essential for infectivity on whole plants. Although these findings suggest that P6 has an important role in viral multiplication, it is unknown whether P6 is required for viral multiplication in a single cell. To address this question, we transfected turnip protoplasts with an ORF VI frame-shift (4 bp deletion) mutant (pCaFS6) of an infectious CaMV DNA clone (pCa122). The mutant was uninfectious. Co-transfection of plasmids expressing P6 complemented the mutant. Overexpression of P6 elevated the infection rate in co-transfection experiments with either pCa122 or pCaFS6. This would have been achieved by elevating the level of pregenomic 35S RNA, a putative polycistronic mRNA for ORFs I, II, III, IV and V, and by enhancing the accumulation of these five viral gene products. When CaMV ORFs I, II, III, IV and V were expressed from monocistronic constructs in which each of the ORFs was placed just downstream of the 35S promoter, the accumulation of ORF III, IV and V products depended on the co-expression of P6. The accumulation of ORF I and II products was not detected, even in the presence of P6. These results suggest that P6 is involved in the stabilization of other viral gene products as well as in the activation of viral gene expression, and thus, is a prerequisite for CaMV multiplication.

Accumulation kinetics of viral gene products in cauliflower mosaic virus-infected turnip protoplasts

The expression of cauliflower mosaic virus (CaMV) genes was studied in a turnip protoplast system. Six CaMV-encoded gene products were detected in infected turnip protoplasts by means of Western blotting. The infected turnip protoplasts showed different patterns of protein accumulation; e.g. an open reading frame (ORF) I-encoded movement protein, an ORF V-encoded reverse transcriptase and an ORF VI-encoded posttranscriptional transactivator representing the early accumulated proteins, an ORF II-encoded aphid transmission factor and an ORF IV-encoded coat protein the late accumulated proteins and an ORF III-encoded DNA binding protein the intermediate protein. The results suggest that the expression of CaMV genes is differentially regulated.

Helper-dependent vector transmission of plant viruses

A variety of noncirculatively transmitted viruses have evolved a vector transmission strategy that involves, in addition to virions, virus-encoded proteins that are not constituents of virions. These "helpers" and the genes encoding them have been characterized for viruses in the genera Potyvirus and Caulimovirus. Several lines of evidence support the hypothesis that these helpers act by mediating retention of virions in regions of the vector's alimentary tract from which they subsequently can be egested to initiate an infection. The possible advantage this convergently evolved strategy could confer to noncirculatively transmitted virus quasispecies is discussed.

Detection of cauliflower mosaic virus (CaMV) in single aphids by the polymerase chain reaction (PCR)

The detection for the first time of a plant virus in a single aphid by the high sensitivity polymerase chain reaction (PCR) technology is reported. The application of PCR for the detection of viruses in their vectors will aid the understanding of the complex virus-vector relationship and therefore allow the development of new approaches for control of spread of plant virus diseases.

Cauliflower mosaic virus gene II product forms distinct inclusion bodies in infected plant cells

Turnip leaves infected with the aphid transmissible isolate of cauliflower mosaic virus (CaMV Cabb B-JI) showed two types of virus-containing inclusion bodies (IBs), which differed morphologically and in their protein composition when analyzed by immunogold labeling of ultrathin sections. Vacuolated IBs, typical of CaMV infections, contained P62 (the generally accepted IB protein) but lacked P18 (the aphid transmission factor), while electron-lucent IBs did not contain P62 but were the only detectable sites of P18 accumulation within the infected leaf cells. Both types of inclusions were detected in cells of the epidermis, vascular bundles, mesophyll, and spongy parenchyma. Electron-lucent IBs were not found in the aphid nontransmissible isolates of CaMV, Campbell and CM4-184.

Comparative sequence of the helper component (HC) region of potato virus Y and a HC-defective strain, potato virus C

Potato virus C (PVC), a non-aphid transmissible strain of potato virus Y (PVY), was found to code for a protein (PVC-HC) which is similar in molecular weight and immunological reactivity to the helper component protein of PVY (PVY-HC). PVC-HC, however, was inactive with respect to its ability to effect aphid transmission of either PVC or PVY. The 5'-terminal 2.7-kb regions of PVC and PVY were sequenced. Within the HC region there was 92% nucleotide homology between the two strains; comparison of the derived amino acid sequences revealed 24 amino acid differences. Comparison of the PVC-HC sequence with that of five potyviruses revealed 2 amino acid changes which were specific to PVC-HC. These amino acids are prime targets for mutational analysis of HC activity.

The pattern of accumulation of cauliflower mosaic virus-specific products in infected turnips

The concentrations of cauliflower mosaic virus (CaMV) DNA and protein products in the developing leaves of a host, turnip, have been measured and the results have been correlated with symptom production. Virus-specific products were limited to the symptomatic leaves. CaMV DNA was detected in the youngest foliar tissues showing full systemic symptoms and continued to accumulate as the leaf expanded, indicating that virus multiplication was not restricted to meristematic tissues of the host plant and that virus concentration was not a primary determinant for symptom production. Using specific antisera for Western blot analysis, the distribution of CaMV-specific proteins (P1-P6) in a range of subcellular fractions of infected tissue was determined. The protein products (P2-P6) of genes II-VI were all detected in fractions enriched for virus inclusion bodies, although P5 was present only at low levels. A high-speed pellet fraction enriched for virus replication complexes revealed P5 in higher concentrations, and also contained P4 and small amounts of P6 in proportions which indicated that replication complexes had been released from inclusion bodies. In the different leaves of the host, P2, 3, 4, 5, and 6 all increased in concentration in parallel with viral DNA, although there appeared to be a bias toward protein rather than DNA synthesis in the very young leaves. P1 showed a different pattern of accumulation; it was most concentrated in the very young and the oldest infected tissues, and showed a different spectrum of products between leaves. The experiments described provide a more complete picture of the relationship between CaMV multiplication and expression, and leaf development, and an increased understanding of how the disease syndrome is established.

Expression of CaMV ORF IV in Escherichia coli

A CaMV DNA fragment corresponding to nucleotides 2200-3992 and including the coding sequence (2200-3670) of open reading frame IV was inserted in the pTG908 prokaryotic expression vector. In the recombinant pTG-IV plasmid, ORF IV, which codes for the coat protein precursor, was fused to the N-terminal coding sequence of the lambda CII gene, which is under transcriptional control of the lambda PL promoter. The expected fusion protein CII-ORF IV had a calculated molecular weight of 58.4 Kd. Nevertheless, temperature induction of the PL promoter resulted in synthesis of a major 76-Kd fusion protein: the coat protein precursor migrated abnormally on SDS polyacrylamide gel.