Figure 8

Effect of oligonucleotides on the metabolism of the homologous mRNA or the modulation of homologous DNA. A short (18-25 nucleotides), single stranded DNA fragment when introduced into a plant cell may bind to a homologues sequence in a transcript and therefore activate RNA-dependent RNA polymerase (RdRP). RdRP synthesizes a second strand creating double-stranded RNA (dsRNA), whose presence brings to action RISC and DICER complexes, and Argonaute (AGO) proteins. The dsRNA fragment could down-regulate the gene by an RNA interference (RNAi) mechanism and/or may also activate the target gene by an RNA activatory (RNAa) mechanism. Furthermore, the presence of dsRNA can also provoke RNA dependent methylation of the homologous DNA fragment. Involved in this process are AGO proteins, DNA methyltransferases, demethylases, and, present only in plants, polymerase IV and V.