Impeded thrips transmission of defective Tomato spotted wilt virus isolates

Two defective RNA-containing isolates (Pe-1 and 16-2) and an envelope-defic ient (env(-)) isolate of Tomato spotted wilt virus (TSWV) were tested for t heir transmissibility by Frankliniella occidentalis. The Pe-1 isolate conta ined a truncated L RNA segment that barely interfered with symptom expressi on and replication of the wild-type (wt) L RNA segment. This isolate was tr ansmitted with an efficiency of 51%, a value comparable to that found for w t TSWV (54%). Isolate 16-2, which contained a genuine defective interfering L RNA as concluded from its ability to suppress wt L RNA synthesis and att enuation of symptom expression, was not transmitted at all. The midguts of all larvae that ingested Pe-l became infected, whereas limited midgut infec tions were found in 24% of the larvae that ingested 16-2. This difference i n infection could be explained by the presence of a low number of infectiou s units in the inoculum ingested from plants as demonstrated in infection e xperiments and verified by northern blot analysis. The env(-) isolate faile d to infect the midgut after ingestion and could not be transmitted by any thrips stage. This isolate also cannot infect primary thrips cell cultures. Taken together, these results suggest that the envelope of TSWV contains t he determinants required for binding and subsequent infection of thrips cel ls.