Short Interfering RNA Accumulation Correlates with Host Recovery in DNA Virus-Infected Hosts, and Gene Silencing Targets Specific Viral Sequences

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Abstract
Viruses are both inducers and targets of posttranscriptional gene silencing (PTGS), a natural defense mechanism in plants. Here we report molecular evidence of the ability of single-stranded DNA (ssDNA) viruses to induce PTGS in infected plants irrespective of the severity of or recovery from the symptoms. Our results reveal that five distinct species of cassava-infecting geminiviruses were capable of triggering PTGS by producing two classes of virus-specific short interfering RNAs (siRNAs) of 21 to 26 nucleotides in two plant hosts, tobacco (Nicotiana benthamiana) and cassava (Manihot esculenta, Crantz). However, the efficacy of virus-induced PTGS varied depending on the intrinsic features of the virus and its interaction with the plant host. We found that symptom recovery over time in plants infected with the isolates of African cassava mosaic virus (ACMV-[CM]) or Sri Lankan cassava mosaic virus was associated with a much higher level of virus-derived siRNA accumulation compared to plants infected with viruses that do not show symptom recovery. Furthermore, we determined that the C terminus of AC1 that overlaps with the N terminus of AC2 early viral genes involved in virus replication were the primary targets for ACMV-[CM]-induced PTGS, whereas the C terminus of BC1 was targeted for the East African cassava mosaic Cameroon virus. In addition, our results reveal the possibility for double-stranded RNA formation during transcription in ssDNA viruses, which explains in part how these viruses can trigger PTGS in plants.