TasiRNA

TasiRNA, short for trans-acting small interfering RNA, is a class of plant small interfering RNAs derived from transcripts of noncoding TAS genes. TasiRNA production is initiated by a microRNA (miRNA) trigger that binds to a TAS transcript, with TAS3 being the best characterized example in Arabidopsis. In TAS3, miR390 guides an AGO protein to two miR390 target sites; cleavage occurs at one site by the AGO-miRNA complex, while the other site remains non-cleaved. The resulting fragment is converted into double-stranded RNA by RNA-dependent RNA polymerase 6 (RDR6) in a SGS3-dependent manner. Dicer-like enzymes, primarily DCL4, process the dsRNA into 21-nt tasiRNAs in a defined phasing relative to the initial miRNA-guided cleavage site.

The mature tasiRNAs are incorporated into Argonaute-containing silencing complexes and act in trans to regulate other transcripts. The best-characterized tasiRNAs are TAS3-derived tasiRNAs, which target members of the AUXIN RESPONSE FACTOR (ARF) family, notably ARF2, ARF3 (ETT), and ARF4. Through regulating ARF expression, tasiRNAs influence auxin signaling and developmental processes such as leaf polarity, organ formation, and overall plant architecture.

TAS gene families also include TAS1, TAS2, and TAS4, which generate additional tasiRNAs with distinct targets across plant species. TasiRNA biogenesis is a hallmark of the broader phasiRNA ( phased secondary siRNA) pathway, linking miRNA triggers to downstream gene regulation. Disruption of tasiRNA pathways often results in developmental defects, underscoring their role in plant growth and patterning.

---