Dennis J. Lewandowski

Conundrum of the Lack of Defective RNAs (dRNAs) Associated with Tobamovirus Infections: dRNAs That Can Move Are Not Replicated by the Wild-Type Virus; dRNAs That Are Replicated by the Wild-Type Virus Do Not Move

ABSTRACT Two classes of artificially constructed defective RNAs (dRNAs) ofTobacco mosaic virus (TMV) were examined in planta with helper viruses that expressed one (183 kDa) or both (126 and 183 kDa) of the replicase-associated proteins. The first class of artificially constructed dRNAs had the helicase and polymerase (POL) domains deleted; the second had an intact 126-kDa protein open reading frame (ORF). Despite extremely high levels of replication in protoplasts, the first class of dRNAs did not accumulate in plants. The dRNAs with an intact 126-kDa protein ORF were replicated at moderate levels in protoplasts and in planta when supported by a TMV mutant that expressed the 183-kDa protein but not the 126-kDa protein (183F). These dRNAs were not supported by helper viruses expressing both replicase-associated proteins. De novo dRNAs were generated in plants infected by 183F but not in plants infected with virus with the wild-type replicase. These novel dRNAs each contained a new stop codon near the location of the wild-type stop codon for the 126-kDa protein and had most of the POL domain deleted. The fact that only dRNAs that contained a complete 126-kDa protein ORF moved systemically suggests that expression of a functional 126-kDa protein or the presence of certain sequences and/or structures within this ORF is required for movement of dRNAs. At least two factors may contribute to the lack of naturally occurring dRNAs in association with wild-type TMV infections: an inability of TMV to support dRNAs that can move in plants and the inability of dRNAs that can be replicated by TMV to move in plants.

Biological and molecular characterization of a novel tobamovirus with a unique host range

Tobamoviruses are among the best characterized and most studied plant viruses. Three subgroups of tobamoviruses correspond to viral genome sequence and host range to include those viruses infecting (i) solanaceous plants, (ii) brassicas, or (iii) cucurbits or legumes. We isolated a virus from Florida landscape plantings of the malvaceous plant hibiscus (Hibiscus rosasinensis) that appears to be a tobamovirus based upon its virion morphology, genome organization, and coat protein sequence. The experimental host range of this virus included five malvaceous species but excluded all tested brassica, cucurbit, and legume species and 12 of the 19 solanaceous species tested. The unique host range and comparison of coat protein gene and protein sequences with those of recognized tobamoviruses indicate that this is a novel to-bamovirus. A limited survey revealed that this virus is widespread in hibiscus and related species in the Florida landscape.

Identification and Characterization of a Novel Tobamovirus from Tropical Soda Apple in Florida

Foliar symptoms suggestive of virus infection were recently observed on the noxious weed tropical soda apple (Solanum viarum) in Florida. An agent was mechanically transmitted to Nicotiana benthamiana, and virions were isolated from systemically infected leaves. Rod-shaped particles ~300 nm in length were observed in the partially purified preparations by electron microscopy. The host range determined by mechanical inoculation with purified virions included all tested plants in the Solanaceae (16 species including the important vegetable crops, pepper and tomato) and Chenopodiaceae (2 species) but excluded all tested plants in the Ama-ranthaceae, Apocynaceae, Brassicaceae, Caryophyllaceae, Cucurbitaceae, Fabaceae, Lamiaceae, Malvaceae, and Tropaeolaceae, including several common virus indicator hosts. Comparisons of the coat and movement protein nucleotide and deduced amino acid sequences of this putative tobamovirus with recognized members of this genus, indicate that it is a novel tobamovirus that shares the highest level of sequence identity with Pepper mild mottle virus followed by other members of the Solanaceae-infecting subgroup of tobamoviruses. The virus, for which the name Tropical soda apple mosaic virus (TSAMV) is proposed, was found to be widespread in tropical soda apple in peninsular Florida during an initial survey. TSAMV contamination of seed from infected tropical soda apple plants was found, suggesting that seed transmission may be important for TSAMV dissemination and epidemiology.

Tobacco mosaic virus, not just a single component virus anymore

Summary Taxonomy: Tobacco mosaic virus (TMV) is the type species of the Tobamovirus genus and a member of the alphavirus-like supergroup. Historically, many tobamoviruses are incorrectly called strains of TMV, although they can differ considerably in sequence similarities and host range from each other and from TMV. Physical properties: TMV virions are 300 x 18 nm rods with a central hollow cavity (Fig. 1) and are composed of 95% capsid protein (CP), and 5% RNA. Each CP subunit interacts with 3-nts in a helical arrangement around the RNA. Virions are stable for decades; infectivity in sap survives heating to 90 degrees C. Hosts: The natural host range of TMV is limited; however, a broad range of weed and crop species, mostly Solanaceae that includes tobacco, pepper and tomato can be infected experimentally [Holmes, F.O. (1946) A comparison of the experimental host ranges of tobacco etch and tobacco mosaic viruses. Phytopathology, 36, 643-657]. TMV distribution is worldwide. No biological vectors are known. Useful website: http://www.ncbi.nlm.nih.gov/ICTVdb/ICTVdB/71010001.htm.

Biological and Molecular Characterization of a U.S. Isolate of Hosta virus X

Hosta virus X (HVX) is rapidly becoming a serious pathogen of commercially important hosta plants worldwide. We report here biological and molecular characterization of a U.S. isolate of HVX, HVX-37. HVX-37 infectivity was tested in 21 hosta cultivars over three growth seasons, and three types of responses were defined based upon the ability of the virus to cause local and/or systemic infections. Four cultivars resistant to systemic HVX infection were identified. The full-length sequence of the HVX-37 genome was determined, the first complete sequence of a U.S. HVX isolate. Comparison with the previously sequenced HVX-Korea (Kr) genome revealed a high level of sequence similarity, as well as some differences. Notably, a 105-nucleotide long, near-perfect direct repeat in the Kr isolate is absent in HVX-37. The accuracy of the HVX-37 genome sequence was confirmed by infectivity of in vitro transcripts synthesized from a full-length HVX-37 cDNA on Nicotiana benthamiana and hosta plants.