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A serologically distinct member of the tomato spotted wilt virus (TSWV) group was isolated from the hybrid flower crop New Guinea impatiens (Impatiens sp.) and termed TSWV-I. TSVW-I type isolates have frequently been detected in a wide variety of flower crops throughout the United States. TSWV-I~-~i-eg~-many characteristics with TSWV, such as symptomatology and possession of three ssRNA species (L, M and S of 8-3 kb, 5.2 kb and 3-4 kb, respectively) and three structural proteins (G1, G2 and N of 78K, 52K and 28K respectively). The TSWV-I G1 and G2 glycoproteins were serologically related to the respective proteins of TSWV, but the TSWV-I nucleocapsid or N protein was serologically unrelated to that of TSWV. Hybridization analysis under high stringency conditions revealed no hybridization between clones of TSWV-I S and M and the S and M RNAs of TSWV, respectively and in addition, a TSWV S clone hybridized only with TSWV S RNA. The cytopathology of TSWV-I also differed from that of TSWV. TSWV-Iinfected tissue primarily contained filamentous structures arranged in paracrystalline arrays, which were also observed by immunosorbent electron microscopy of tissue extracts. The filamentous structures were only trapped by TSWV-I antibodies. The conserved serological relatedness between TSWV types for G1 and G2, but not N, is consistent with serological analyses of the nairovirus and phlebovirus genera of the Bunyaviridae, the virus family that TSWV most closely resembles.

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Tobacco lines expressing transgenes that encode tobacco etch virus (TEV) coat protein (CP) mRNA with or without nonsense codons give rise to TEV-resistant tissues that have reduced levels of TEV CP mRNA while maintaining high levels of transgene transcriptional activity. Two phenotypes for virus resistance in the lines containing the transgene have been described: immune (no virus infection) and recovery (initial systemic symptoms followed by gradual recovery over several weeks). Here, we show that at early times in development, immune lines are susceptible to TEV infection and accumulate full-length CP mRNA. Therefore, immune lines also exhibit meiotic resetting, as is seen in the recovery lines, providing molecular evidence for a common mechanism of gene silencing and virus resistance in both cases. We also investigated the characteristics of two sets of low molecular weight RNAs that appear only in silenced tissue. One set has nearly intact 5[prime] ends, lacks poly(A) tails, and is associated with polyribosomes; the second set contains the 3[prime] end of the mRNA. Treating silenced leaf tissue with cycloheximide resulted in decreased levels of full-length mRNA and an increase in the levels of the low molecular weight RNAs, supporting a cytoplasmic decay mechanism that does not require ongoing translation. Surprisingly, mRNA from the transgene containing nonsense codons was associated with more ribosomes than expected, possibly resulting from translation from a start codon downstream of the introduced translational stop codons. We present a hypothesis for transgene/viral RNA degradation in which RNA degradation occurs in the cytoplasm while in association with polyribosomes.

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