Figure 7. Scenarios to explain the cleavage of ChlI mRNA by Y-sat in the RNA silencing pathway.

The most probable scenario is that AGO1 associated with the primary Y-sat siRNAs cleaves the ChlI mRNA at the SYR-YR portion, then a host RNA-dependent RNA polymerase (RDR) may access the cleaved ChlI mRNA fragments to produce dsRNAs, which are subsequently processed into secondary siRNAs. In addition, Y-sat itself is cleaved by AGO1 loaded with siRNAs derived from Y-sat, inducing the production of a large amount of siRNAs containing the SYR sequence.