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. 2019 Mar 27;10:360. doi: 10.3389/fpls.2019.00360

FIGURE 1.

FIGURE 1

Regulation of miRNA biogenesis, RISC loading, and action in plants. MIRNA genes are transcribed by RNA Polymerase II (Pol II) and fold into stem-loop structures called pri-miRNAs. pri-miRNAs are mainly processed by DCL1 from either “base-to-loop” or “loop-to-base” direction. Nascent miRNA/miRNA∗ duplexes are methylated by the small RNA methyltransferase HEN1. RISC loading occurs in the nucleus, but may also occur in the cytoplasm. miRNAs mediate gene silencing through either target cleavage or translation inhibition. Some miRNAs can trigger the production of secondary siRNAs through sequential actions of RDR6 and DCL2/4, generating 21–22 nt secondary siRNAs called phasiRNAs and easiRNAs, which in turn repress gene expression via PTGS (for phasiRNAs) or TGS (for easiRNAs) (D). It is important to note that although the steps in the model are separate, they could be closely coupled. Factors involved in the regulation of MIR transcription, pri-miRNA processing, and RISC assembly are shown in parts (A–C), respectively.