What is Trans-acting siRNA?

Trans-acting siRNA are a class of small interfering RNA that repress gene expression through post-transcriptional gene silencing in land plants. Precursor transcripts from TAS loci are polyadenylated and converted to double-stranded RNA, and are then processed into 21-nucleotide-long RNA duplexes with overhangs. These segments are incorporated into a RNA-induced silencing complex and direct the sequence-specific cleavage of target mRNA. Ta-siRNAs are classified as siRNA because they arise from double-stranded RNA.