Biological and physicochemical properties of tobacco streak virus

Three isolates of tobacco streak virus (TSV) occurring in Australia (A-TSV, As-TSV and S-TSV) were physiochemically, serologically and biologically compared. All isolates had isometric particles of three different sizes (27, 31 and 35 nm in diameter), three major species of RNA of estimated Mr(x10^-6), 1.11-1.19 (RNA-1), 0.92-0.95 (RNA-2), and 0.64-0.70 (RNA-3) and a subgenomic (RNA-4) of Mr, 0.30-0.31x10⁶ sometimes detected in A-TSV and As-TSV isolates, and one major component of coat protein of Mr about 30x10³. The three isolates cross reacted with two antisera-produced from As-TSV and A-TSV but they were not serologically identical as indicated by spur formation in gel diffusion tests. Symptoms caused by the As-TSV isolate more or less conform to those caused by the Brazilian TSV whereas A-TSV and S-TSV isolates produced symptoms rather similar to those described for the TSVs occurring in North America and Europe. In cross-protection tests, As-TSV and A-TSV completely protected against S-TSV, whereas the S-TSV isolate did not protect against either of the other two isolates. Predarkening of Bountiful bean before inoculation with A-TSV resulted in increased lesion size.

In transmission tests, the As-TSV isolate was seed transmitted in mechanically inoculated tomato (Lycopersicon esculentum cv. Grosse Lisse) and weeds (Nicandra physalodes and Solanum niqrum) at rates of 10-88%. With infected tomato seed, the virus was detected by ELISA more frequently in the endosperm (40-90%) than embryo (10-50%) and little or no virus was present in the seed coat. As-TSV also affected the viability of seed as the germination rate of the infected seed was reduced by 6-24% in weeds and 66-85% in tomato, compared to healthy seeds. In tomato, although the virus was transmitted from pollen to seed via fertilization, none of the plants which were pollinated with infective pollen became infected.      

In insect transmission tests, As-TSV was regularly transmitted by adults and nymphs of Thrips tabaci from virus-carrying pollen to the test plants by either mixing thrips with the virus-carrying pollen and then placing them on test plants, or placing thrips on pollen-dusted test plants. However, no transmission occurred when the thrips were fed on TSV-infected leaves and then transferred to test plants in the absence of pollen. Pollen-borne virus from one host species could be transmitted by thrips to leaves of other species, and the minimum time for thrips to transmit pollen-borne TSV was 1 h. This method of virus transmission has not previously been reported.

In studies of relationships with cells and tissues, TSV was detected by electron microscopy and immunogoldlabelling in anther wall cells and pollen grains. Further, ELISA and immunogoldlabelling indicated that TSV was both internally and externally associated with the pollen.        

In ecological studies, TSV was shown to infect 4 species of crop plants and 13 weed species. Cyphomandra betacea (tamarillo) and 8 of the weed species were new records for Queensland. Gel diffusion tests indicated that all Queensland isolates except the isolate from strawberry (S-TSV) were closely related serologically but were more distant to the Victorian Ajuga isolate (A-TSV). Some naturally infected weeds produced flowers and seeds containing TSV but none of the seedlings derived from those seeds became infected.