miR390

miR390 is a plant microRNA belonging to the MIR390 family. It is one of the conserved microRNAs that regulate plant development by guiding the generation of trans-acting small interfering RNAs (ta-siRNAs). In Arabidopsis thaliana and other land plants, MIR390 genes produce a 21-nucleotide miRNA that is processed from a stem-loop precursor.

Biogenesis and mechanism: miR390 is loaded into an Argonaute protein, AGO7 (also called ZIP), and targets the non-coding TAS3 transcripts at two conserved sites. The 5' miR390 target site typically contains mismatches that prevent slicing, while the 3' site is cleaved by AGO7. This cleavage triggers RNA-dependent RNA polymerase RDR6–SGS3–DCL4–mediated production of phased ta-siRNAs from TAS3.

The TAS3-derived siRNAs regulate members of the AUXIN RESPONSE FACTOR (ARF) family, notably ARF2, ARF3, and ARF4, reducing their mRNA levels and thereby modulating auxin responses and developmental programs.

Genomic and evolutionary context: In Arabidopsis, two MIR390 loci (MIR390a and MIR390b) encode functional miR390. The miR390–TAS3 ta-siRNA pathway is highly conserved in land plants and serves as an important example of tasiRNA biogenesis.

Functional significance: The pathway influences leaf polarity, shoot architecture, and lateral organ formation by fine-tuning ARF transcription factor activity. It is also used as a model for understanding non-canonical miRNA–ta-siRNA interactions and tasiRNA biogenesis.