Characterization of tomato yellow vein streak virus, a begomovirus from Brazil.

Tomato yellow vein streak virus (ToYVSV) is a tentative begomovirus (Family Geminiviridae) species that seriously affects tomato and potato production in Brazil. Here, we have determined the genomic and biological characteristics of a ToYVSV isolate (Ba3) from a potato plant sampled in Rio Grande do Sul State, Brazil. The DNA-A nucleotide sequence of Ba3 and another previously reported ToYVSV isolate share 89.7% sequence identity. These ToYVSV isolates should be classified as a new species in that they are most closely related to Soybean blistering mosaic virus with which they share only approximately 80% identity. Cloned constructs containing 1.5 mer copies of the ToYVSV genomic components were found, by biolistic bombardment, to be infectious in at least 11 plant species in 2 families (Solanaceae and Malvaceae). Symptoms on tomato and potato plants were identical to those originally observed on field-infected plants. ToYVSV was also sap-transmissible from Nicotiana benthamiana to N. benthamiana and tomato, but not to potato plants.

Widespread Occurrence of Tomato Geminiviruses in Brazil, Associated with the New Biotype of the Whitefly Vector.

Although tomato golden mosaic virus (TGMV) was reported in Brazil more than 20 years ago (3), tomato-infecting geminiviruses have not been of economic significance in the country until recently. However, a sharp increase in the incidence of geminivirus-like symptoms in tomatoes has been reported in several areas of Brazil since 1994. This has coincided with the appearance of the B biotype of Bemisia tabaci, which, as opposed to the A biotype, readily colonizes solanaceous plants (2). We have isolated geminiviruses from symptomatic tomato plants in the Federal District, in two different areas of the state of Minas Gerais, and in the state of Pernambuco. Tomato plants in these areas showed a variety of symptoms, including yellow mosaic, severe leaf distortion, down-cupping, and epinasty. Whitefly infestation was high in all fields sampled, and in some fields, particularly in Pernambuco, incidence of virus-like symptoms was close to 100%, and no tomatoes of commercial value were harvested (1). Using primer pairs PAL1v1978/PAR1c496 and PCRc1/PBL1v2040 (4), DNA-A and -B fragments were polymerase chain reaction (PCR)-amplified from total DNA extracted from diseased plants, cloned, and sequenced. Sequence comparisons of the PCR fragments indicated the existence of at least six different geminiviruses. The nucleotide sequence homologies for DNA-A fragments ranged from 67 to 80% for the 5' end of the cp gene, and from 44 to 80% for the 5' end of the rep gene. Data base comparisons indicated the viruses are most closely related to TGMV, bean golden mosaic virus from Brazil (BGMV-Br), and tomato yellow vein streak virus (ToYVSV), although homologies were less than 80% for the fragments compared. A similar lack of a close relationship with each other and other geminiviruses was obtained with two DNA-B component PCR products compared, corresponding to the 5' end of the BC1 open reading frame. Infectious, full-length genomic clones from the tomato viruses are being generated for biological and molecular characterization. References: (1) I. C. Bezerra et al. Fitopatol. Bras. 22:331, 1997. (2) F. H. França et al., Ann. Soc. Entomol. Bras. 25:369, 1996. (3) J. C. Matyis et al. Summa Phytopathol. 1:267, 1975. (4) M. R. Rojas et al. Plant Dis. 77:340, 1993.

Immuno-electron microscopical detection of tomato spotted wilt virus and its nucleocapsids in crude plant extracts.

Tomato spotted wilt virus (TSWV) particles were identified in thin sections and in crude extracts of leaves from plants infected with different TSWV isolates, using gold labelled protein A and antibodies prepared against purified virus particles or against nucleocapsid preparations. In addition, both in thin sections and in dip preparations aggregates were detected using either gold-labelled antiserum. These aggregates were not detected using antisera against envelope proteins or against the non-structural protein NSs. They were the only detectably labelled material in preparations made from plants infected by morphologically defective isolates of TSWV, i.e. isolates that did not produce enveloped particles. The aggregates were discerned in dip preparations as cloudy amorphous structures having a cottonwool boll-like appearance. These 'cotton bolls' were interpreted as being the in vitro structures of the nucleocapsid aggregates occurring in plant cells infected by isolates maintained by mechanical inoculation and are the only structures in cells infected by morphologically defective isolates (Ie, 1982; Kitajima et al., 1992). The detection of the 'cotton boll-like' structures in crude extracts provides an adequate and rapid method to establish the presence of nucleocapsid aggregates in cells of infected plants, irrespective of whether they are infected by morphologically defective isolates or not.