TasiRNAs

TasiRNAs, or trans-acting small interfering RNAs, are a class of plant small RNAs derived from noncoding TAS transcripts. They are generated in a phased manner and act in trans to silence target mRNAs, often those that are not the source of the tasiRNAs themselves.

Biogenesis of tasiRNAs begins when the microRNA miR390 binds to conserved sites on TAS transcripts, guiding loading into AGO7. This interaction recruits RNA-dependent RNA polymerase 6 (RDR6) and the SGS3 protein to convert the TAS transcript into double-stranded RNA. The Dicer-like enzyme DCL4 processes the dsRNA into 21-nucleotide, phased siRNAs. The mature tasiRNAs are then loaded into AGO proteins to guide sequence-specific cleavage or repression of target mRNAs, typically at sites distant from the TAS gene.

The most studied tasiRNAs are those derived from TAS3 transcripts, which generate tasiRNAs targeting Auxin Response Factor genes ARF2, ARF3, and ARF4. Through repression of these ARFs, TAS3-derived tasiRNAs modulate auxin signaling and various developmental processes. Other TAS-family loci, such as TAS1, TAS2, and TAS4, produce tasiRNAs with different targets and regulatory roles in different plant species.

Functionally, tasiRNAs contribute to plant development by coordinating organ formation, leaf morphology, meristem activity, and responses to environmental cues via regulation of transcription factors and signaling components. The tasiRNA pathway is conserved among land plants, with variability in TAS genes and their targets across species. Key components in the biogenesis pathway include miR390-AGO7, RDR6, SGS3, and DCL4.