Figure 3.

Proposed flow diagram of an airborne virus infection in grafted and non-grafted tomato plants. By the time of virus infection, a general graft wound-response is already active only in grafted plants (pink oval in the background). After cell-to-cell movement in epidermal cells, the virus enters the abaxial phloem, bypasses graft union (circled in red), and accumulates in roots following the source-to-sink phloem sap movement. In rootstock tissues, heavy loads of replicating viral RNA trigger RNAi, probably induces changes in defense gene expression, and signals molecules via interactions with specific transcription factors. Reduced virus loads move from roots into the scion bypassing the graft union and old leaves and spreads into young leaves where it may induce symptoms. Virus load is still sufficient to trigger RNAi and changes in defense gene expression. Between 14-21 days post infection (dpi), virus replication and load are very low, a tolerant state is activated and plant recovers from disease symptoms. In roots of non-grafted plants, the virus replicates actively and accumulates in root tissues by suppressing RNAi via the expression of its VSRs. Then, the virus spreads upward to infect systemically tomato leaves. Active virus replication and expression of its VSRs induce disease symptoms and hinders recovery.