TAS3 Genes for small ta-siARF RNAs in plants belonging to subtribe Senecioninae: occurrence of prematurely terminated RNA precursors.

The various classes of plant 21 - to 24-nt siRNAs derive from long dsRNA precursors that are processed by the ribonuclease Dicer-like (DCL). The species of ta-siRNA were originally discovered in Arabidopsis thaliana. Four gene families have been identified in Arabidopsis that each produces a number of ta-siRNAs: TAS1, TAS2, TAS3 and TAS4. The TAS3 genes encode tasiR-ARF species which target the mRNA of three Auxin Response Factor (ARF) genes (ARF2, ARF3/ETT and ARF4) for subsequent degradation. The function of TAS3 precursor RNA is controlled by two miR390 target sites flanking tandem of ta-siARF sequences. In this paper, we have studied the presence ofta-siARF RNA genes in the representatives of subtribe Senecioninae. Senecioneae is the largest tribe of Asteraceae, comprised of ca. 150 genera and 3,000 species which include many common succulents of greenhouses. Approximately one-third of species are placed in genus Senecio, making it one of the largest genera of flowering plants. However, there was no information on the structure of TAS genes in these plants. We revealed that the TAS3 species (TAS3-Sen1) in Senecio representatives was actively transcribed, and its homologues are distributed among many Asteracea plants and found to be similar to Arabidopsis AtTAS3a gene. We revealed several prematurely terminated transcripts of TAS3-Sen1. Finding the alternative shortened transcripts of TAS3-Sen1 lacking the 3'-terminal site cleaved by miR390 and retaining the 5'-terminal miR390 non-cleaved site suggested their using as decoys for the modulation of miR390 activity to regulate synthesis of ta-siARF RNAs in different Senecioninae species.