Figure 4

A schematic model illustrating the predicted mechanisms of virus resistance to TSWV in Sw-7 tomato plants or of symptom expression in the susceptible (S) plants. The Sw-7 resistance requires defense-related signaling molecules, including pathogenesis-related 1 (PR-1) protein, pathogenesis-related 5 (PR-5) (osmotic-like protein), glycine/proline rich protein (GRP), nodulin (PR-10), Pto-like R-gene (bacterial resistance), MADS box transcription factors (candidate Ty-2 gene), and subtilisin serine protease, all of which showed elevated expression in the Sw-7 line relative to S-line. The potential functional roles of the above stated genes, and signaling pathways including GRP-triggered PR proteins, are to actively communicate to neighboring cells, resulting in callose, lignin, and suberin deposition to the cell wall and leading to restricted cell-to-cell movement of TSWV. This in turn leads to the resistance phenotype in the Sw-7 plants. For the susceptible response in the S-line, we speculate that the virus-encoded molecular factors would suppress the host immune pathways, leading to TSWV replication, transcription and translation. Abundance of viral RNA accumulation in the cells would trigger the expression of RNA silencing pathway genes in the S-line, including Argonaute 1 (Ago1) and Dicer-like 2 (DCL 2), resulting in antiviral defense. In the meantime, TSWV-encoded silencing suppressor protein (NSs) would suppress (sequester) the host antiviral defense pathway, leading to over-accumulation of viral particles. This in turn results in the opening of the cell wall/plasmodesmata to virus cell-to-cell and systemic movement, producing the disease phenotype.