First field isolation of wound tumor virus from a plant host: minimal sequence divergence from the type strain isolated from an insect vector

Genetic and molecular variability of a Turnip mosaic virus population from horseradish (Cochlearia armoracia L.)

Variability and genetic structure of a novel Turnip mosaic virus (TuMV) population from horseradish (Cochlearia armoracia L.) were examined. Over 60 horseradish plants were tested to identify a total of 28 TuMV isolates, constituting the Cochlearia ARmoracia (CAR) TuMV population. Two subgroups of the CAR TuMV isolates could be distinguished: subgroup N did not infect oilseed rape (Brassica napus var. oleifera) cv. Westar plants, while subgroup A infected these plants systemically. Two types of infection of oilseed rape plants were induced by inoculation with the CAR TuMV isolates: systemic mosaic infection and systemic necrotic lesions. The complete sequences of isolates CAR37 (subgroup N) and CAR37A (subgroup A) were determined and compared. The sequences of HC-Pro and CP genes of CAR37 and CAR37A and other isolates of TuMV from other countries were compared to provide some insight into their relatedness. CAR37A, initially regarded as a variant, proved to be very different from CAR37. Re-sequencing after repeated passages confirmed the genetic stability of both isolates.

Molecular analysis of six segments of tobacco leaf enation virus, a novel phytoreovirus from tobacco

Tobacco leaf enation virus (TLEV) is a putative member of the genus Phytoreovirus within the family Reoviridae. Previous western blot analysis of structural viral proteins (apparent molecular weights of 93 kDa; 58 kDa; 48 kDa; 39 kDa and 36 kDa) associated with TLEV, isolated from infected tobacco in South Africa, suggested that these proteins may correspond to structural Wound tumor virus (WTV) proteins. To further establish the nature of this novel virus disease phenotype in tobacco, molecular characterization of six dsRNA components was undertaken. Full-length cDNA clones were obtained by an optimized modified single-primer amplification sequence-independent dsRNA cloning method. Results of this study revealed the conserved terminal sequence: 5'GG(U/C)...UGAU 3' of segments S6-S12, while adjacent to these conserved terminal sequences are imperfect inverted repeats (7-15 bp in length), both features being common to reoviruses. The complete nucleotide sequences of segments S5 (2,610 bp), S7 (1,740 bp), S8 (1,439 bp), S10 (1,252 bp), S11 (1,187 bp) and S12 (836 bp) were determined. Comparison of full-length nucleotide sequences with corresponding segments of other phytoreoviruses, Rice gall dwarf virus (RGDV), Rice dwarf virus (RDV) and WTV has shown nucleotide and predicted amino acid identities within the range of 30-60%. TLEV consistently shows a higher identity to WTV than to other phytoreovirus species where sequence data is available. Each segment had a single predicted open reading frame encoding proteins with calculated molecular weights of S5 (90.6 kDa); S7 (58.1 kDa); S8 (47.7 kDa); S10 (39.8 kDa); S11 (35 kDa) and S12 (19.5 kDa). The relatively low nucleotide and amino acid identity to other members of the genus demonstrates that TLEV is a novel phytoreovirus, distinct from the only other reported dicotyledenous-infecting WTV and is the first phytoreovirus reported to emerge in Africa.

Variability and genetic structure of plant virus populations

Populations of plant viruses, like all other living beings, are genetically heterogeneous, a property long recognized in plant virology. Only recently have the processes resulting in genetic variation and diversity in virus populations and genetic structure been analyzed quantitatively. The subject of this review is the analysis of genetic variation, its quantification in plant virus populations, and what factors and processes determine the genetic structure of these populations and its temporal change. The high potential for genetic variation in plant viruses, through either mutation or genetic exchange by recombination or reassortment of genomic segments, need not necessarily result in high diversity of virus populations. Selection by factors such as the interaction of the virus with host plants and vectors and random genetic drift may in fact reduce genetic diversity in populations. There is evidence that negative selection results in virus-encoded proteins being not more variable than those of their hosts and vectors. Evidence suggests that small population diversity, and genetic stability, is the rule. Populations of plant viruses often consist of a few genetic variants and many infrequent variants. Their distribution may provide evidence of a population that is undifferentiated, differentiated by factors such as location, host plant, or time, or that fluctuates randomly in composition, depending on the virus.

Sequences of Citrus tristeza virus separated in time and space are essentially identical

The first Citrus tristeza virus (CTV) genomes completely sequenced (19.3-kb positive-sense RNA), from four biologically distinct isolates, are unexpectedly divergent in nucleotide sequence (up to 60% divergence). Understanding of whether these large sequence differences resulted from recent evolution is important for the design of disease management strategies, particularly the use of genetically engineered mild (essentially symptomless)-strain cross protection and RNA-mediated transgenic resistance. The complete sequence of a mild isolate (T30) which has been endemic in Florida for about a century was found to be nearly identical to the genomic sequence of a mild isolate (T385) from Spain. Moreover, samples of sequences of other isolates from distinct geographic locations, maintained in different citrus hosts and also separated in time (B252 from Taiwan, B272 from Colombia, and B354 from California), were nearly identical to the T30 sequence. The sequence differences between these isolates were within or near the range of variability of the T30 population. A possible explanation for these results is that the parents of isolates T30, T385, B252, B272, and B354 have a common origin, probably Asia, and have changed little since they were dispersed throughout the world by the movement of citrus. Considering that the nucleotide divergence among the other known CTV genomes is much greater than those expected for strains of the same virus, the remarkable similarity of these five isolates indicates a high degree of evolutionary stasis in some CTV populations.

Evidence for a phytoreovirus associated with tobacco exhibiting leaf curl symptoms in South Africa

ABSTRACT Three forms of tobacco leaf curl (termed classes I, II, and III, based on symptomatology) recently have been described in southern Africa. Numerous attempts to isolate virus particles responsible for a nongeminivirus-induced leaf curl disease (class I) of tobacco in South Africa have been unsuccessful. Recently, 12 dsRNA segments were isolated from tobacco exhibiting class I leaf curl symptoms, suggesting a possible reovirus genome. The objective of our study was to confirm whether the dsRNA segments are associated with a reovirus. Isolation of icosahedral particles with an outer core 60 to 65 nm in diameter and an inner core 40 to 45 nm in diameter was achieved. Twelve distinct nonpolyadenylated dsRNAs were isolated from purified virions, and the total molecular masses of the dsRNAs ranged from 17.86 to 18.40 x 10(6) Da in polyacrylamide and agarose gels, respectively. Using hybridization analysis, dsRNAs were identified as non-homologous distinct segments. Comparisons with other known reoviruses revealed a unique banding pattern that was most similar to the wound tumor virus (WTV), the type species of the genus Phytoreovirus. Hybridizations of WTV cloned DNA probes (segments S4 and S6 to S9) and dsRNAs from infected tobacco indicated no significant sequence similarity, whereas indirect enzyme-linked immunosorbent assay with a polyclonal antiserum to WTV showed strong positive cross-reactivity to tobacco virions. Our results indicate a virus with features consistent with those of phytoreoviruses. This is the first report of a plant reovirus in tobacco, the first record in Africa, and the second example of a field-isolated dicot phytoreovirus.

Molecular analysis of rice ragged stunt oryzavirus segment 9 and sequence conservation among isolates from Thailand and India

Nucleotide sequences of rice ragged stunt virus (RRSV) genome segment 9 (S9) from a Thai and an Indian isolate were determined. Both sequences are 1132 bp long, contain a single large open reading frame (ORF) spanning nucleotide residues 14 to 1027 and are capable of encoding a protein of 38.5K. The two isolates are 94.6% and 99.4% identical at the nucleotide and amino acid level, respectively. The authenticity of these coding sequences was confirmed by identifying a approximately 38K protein in the RRSV particle with an N-terminal amino acid sequence identical to that inferred from the S9 ORF. Furthermore, cDNA of S9 from each isolate incorporated into the bacterial expression vector pGEX3-X produced a fusion protein that reacted with antibodies raised against purified RRSV particles. Cleaving these fusion proteins with protease factor X liberated a approximately 38K polypeptide.