A New Natural Host of Lisianthus necrosis virus in Taiwan

Lisianthus necrosis virus (LNV) was first identified as a fungus-borne virus that induced systemic necrosis in lisianthus (Eustoma russellianum) in Japan (2). In Taiwan, LNV causes systemic bright yellow chlorosis followed by necrosis in lisianthus (1). The disease was able to spread through the infested soil. Isolation of a fungus vector was attempted but was not successful (1). Calla lilies (Zantedeschia spp.) showing symptoms of systemic necrosis were observed in the fields of central Taiwan. A virus culture was established through single-lesion isolation from a local lesion host, Chenopodium quinoa, and maintained in Nicotiana benthamiana. Mechanical inoculation of the virus resulted in systemic infection in E. russellianum and Datura stramonium and local infection in Celosia argentea, Gomphrena globosa, Chenopodium amaranticolor, Zinnia elegans, Cucumis melo, Cucumis sativus, Cucurbita pepo, Vigna angularis, and Petunia hybrida. Electron microscopic examination of ultrathin sections of infected plant tissues revealed the presence of spherical viral particles approximately 33 nm in diameter. Scattered and aggregated virion particles were frequently observed in the cytoplasm of infected cells. Results of enzyme-linked immunosorbent assay, western blotting, and immunoelectron microscopy indicate that the virus is serologically related to LNV (1). Reverse transcription-polymerase chain reaction (RT-PCR) with degenerate primers (forward primer 5'-ATGGAAATCGTTAGG and reverse primer 5'-CTATAGCAATGTTGC) for LNV coat protein gene produced a cDNA of approximately 1.1 kb. The RT-PCR product was cloned into pGEM-T vector (Promega, Madison, WI) and sequenced. Sequence analysis showed that the cloned fragment (GenBank Accession No. DQ523229) was 1,167 bp long and shared 99% identity at the nucleotide and deduced amino acid levels with that of the LNV isolated from lisianthus (GenBank Accession No. DQ011234). To our knowledge, this is the first report of the natural occurrence of LNV infection in calla lily. References: (1) C. C. Chen et al. Plant Dis. 84:506, 2000. (2) M. Iwaki et al. Phytopathology 77:867, 1987.

A minimum length of N gene sequence in transgenic plants is required for RNA-mediated tospovirus resistance

We showed previously that transgenic plants with the green fluorescent protein (GFP) gene fused to segments of the nucleocapsid (N) gene of tomato spotted wilt virus (TSWV) displayed post-transcriptional gene silencing of the GFP and N gene segments and resistance to TSWV. These results suggested that a chimeric transgene composed of viral gene segments might confer multiple virus resistance in transgenic plants. To test this hypothesis and to determine the minimum length of the N gene that could trans-inactivate the challenging TSWV, transgenic plants were developed that contained GFP fused with N gene segments of 24-453 bp. Progeny from these plants were challenged with: (i) a chimeric tobacco mosaic virus containing the GFP gene, (ii) a chimeric tobacco mosaic virus with GFP plus the N gene of TSWV and (iii) TSWV. A number of transgenic plants expressing the transgene with GFP fused to N gene segments from 110 to 453 bp in size were resistant to these viruses. Resistant plants exhibited post-transcriptional gene silencing. In contrast, all transgenic lines with transgenes consisting of GFP fused to N gene segments of 24 or 59 bp were susceptible to TSWV, even though the transgene was post-transcriptionally silenced. Thus, virus resistance and post-transcriptional gene silencing were uncoupled when the N gene segment was 59 bp or less. These results provide evidence that multiple virus resistance is possible through the simple strategy of linking viral gene segments to a silencer DNA such as GFP.

Turnip mosaic potyvirus resistance in Nicotiana benthamiana derived by post-transcriptional gene silencing

The coat protein (CP) gene of turnip mosaic potyvirus isolate ESC8 (TuMV-ESC8) was cloned and sequenced. Comparisons of the 867-nucleotide (nt) CP region with those of 11 TuMV isolates showed 86.7-89.3% nucleotide identity and 92.4-95.5% amino acid identity. The CP gene was cloned into a plant expression vector and transformed into Nicotiana benthamiana plants via Agrobacterium tumefaciens-mediated leaf disk transformation. Progeny from R0 lines was screened for resistance to TuMV-ESC8. Five of 29 tested lines showed TuMV protection in more than 50% of their progeny. Interestingly, some of the resistant plants transformed with the CP gene of TuMV displayed mild mosaicism in the new growing leaves at the later stages of evaluation; but these mosaic symptoms disappeared when the leaves were fully expanded. Collective data from steady-state RNA analysis and nuclear run-on assay of a line showed that the resistance was RNA-mediated through the post-transcriptional gene silencing mechanism.

Nontarget DNA sequences reduce the transgene length necessary for RNA-mediated tospovirus resistance in transgenic plants

RNA-mediated virus resistance has recently been shown to be the result of post-transcriptional transgene silencing in transgenic plants. This study was undertaken to characterize the effect of transgene length and nontarget DNA sequences on RNA-mediated tospovirus resistance in transgenic plants. Transgenic Nicotiana benthamiana plants were generated to express different regions of the nucleocapsid (N) protein of tomato spotted wilt (TSWV) tospovirus. Transgenic plants expressing half-gene segments (387-453 bp) of the N gene displayed resistance through post-transcriptional gene silencing. Although smaller N gene segments (92-235 bp) were ineffective in conferring resistance when expressed alone in transgenic plants, these segments conferred resistance when fused to the nontarget green fluorescent protein gene DNA. These results demonstrate that (i) a critical length of N transgene (236-387 bp) is required for a high level of transgene expression and consequent gene silencing, and (ii) the post-transcriptional gene silencing mechanism can trans-inactivate the incoming tospovirus genome with homologous transgene segments that are as short as 110 bp. Therefore, the activation of post-transcriptional transgene silencing requires a significantly larger transgene than is required for the trans-inactivation of the incoming viral genome. These results raise the possibility of developing a simple new strategy for engineering multiple virus resistance in transgenic plants.

Complete nucleotide sequence and genetic organization of papaya ringspot virus RNA

The complete nucleotide sequence of the RNA genome of papaya ringspot virus (PRSV) was determined from four overlapping cDNA clones and by direct sequencing of viral RNA. The genomic RNA is 10326 nucleotides in length, excluding the poly(A) tract, and contains one large open reading frame that starts at nucleotide positions 86 to 88 and ends at positions 10118 to 10120, encoding a polyprotein of 3344 amino acids. The highly conserved sequence AAAUAAAANANCUCAACACAACAUA at the 5' end of the RNA of PRSV and those of the other five reported potyviruses shows 80% similarity, suggesting that this region may play a common important role for potyvirus replication. Two cleavage sites of the polyprotein were determined by amino acid sequencing of the N termini of helper component (HC-Pro, amorphous inclusion) and cylindrical inclusion (CI) proteins. Other cleavage sites were predicted by analogy with the other potyviruses. The genetic organization of PRSV is similar to that of the other potyviruses except that the first protein processed from the N terminus of the polyprotein (NT protein) has an M(r) of 63K, 18K to 34K larger than those of the other potyviruses. The cleavage site for liberating the N terminus of the HC-Pro protein was found at the same location down-stream from the consensus sequence FI(V)VRG as that reported for tobacco vein mottling virus. The NT protein of potyviruses is the most variable and may be considered important for identification of individual potyviruses. The most conserved protein of potyviruses appears to be the NIb protein, the putative polymerase for the replication of the potyviral RNA. The genetic organization of PRSV RNA is tentatively proposed to be VPg-5' leader-63K NT-52K HC-Pro-46K-72K CI-6K-48K NIa-59K NIb-35K coat protein-3' non-coding region-poly(A) tract.

Physicochemical surface properties of nonencapsulated and encapsulated coagulase-negative staphylococci

Cell surfaces of three nonencapsulated and three encapsulated coagulase-negative staphylococci were characterized by their surface free energies, zeta potentials, and elemental and molecular compositions. Surface free energies were calculated from contact angle measurements with various liquids. All six strains showed a high surface free energy (103 to 126 mJ.m-2), estimated from the concept of polar and dispersion components. However, the hydrogen-donating surface free energy parameter was zero for all nonencapsulated strains. The zeta potential profile measured as a function of pH in phosphate-buffered saline for the nonencapsulated strains was completely different from that of the encapsulated strains. X-ray photoelectron spectroscopy was used to determine the elements (O, C, N, P, and K) in the outer 2 to 5 nm of the freeze-dried cell surface and showed that the hydrophilic character of the staphylococci was related to oxygen (O/C ratio, approximately 0.52)- and phosphorus (P/C ratio, approximately 0.03)-containing groups. Both the elemental and molecular characterizations (done by infrared spectroscopy) pointed to the presence of polysaccharides and polypeptides on the cell surface of the nonencapsulated and encapsulated strains.