Acts as a suppressor of RNA-mediated gene silencing, also known as post-transcriptional gene silencing (PTGS), a mechanism of plant viral defense that limits the accumulation of viral RNAs. Binds to short interfering RNAs (siRNAs) with high affinity. Acts as a molecular caliper to specifically select siRNAs based on the length of the duplex region of the RNA. Homodimer. Belongs to the tombusvirus protein p19 family.