Abstract
To keep mRNA homeostasis, the RNA degradation, quality control and silencing systems should act in balance in plants. Degradation of normal mRNA starts with deadenylation, then deadenylated transcripts are degraded by the SKI-exosome 3'-5' and/or XRN4 5'-3' exonucleases. RNA quality control systems identify and decay different aberrant transcripts. RNA silencing degrades double-stranded transcripts and homologous mRNAs. It also targets aberrant and silencing prone transcripts. The SKI-exosome is essential for mRNA homeostasis, it functions in normal mRNA degradation and different RNA quality control systems, and in its absence silencing targets normal transcripts. It is highly conserved in eukaryotes, thus recent reports that the plant SKI-exosome is associated with RST1 and RIPR proteins and that, they are required for SKI-exosome-mediated decay of silencing prone transcripts were unexpected. To clarify whether RST1 and RIPR are essential for all SKI-exosome functions or only for the elimination of silencing prone transcripts, degradation of different reporter transcripts was studied in RST1 and RIPR inactivated Nicotiana benthamiana plants. As RST1 and RIPR, like the SKI-exosome, were essential for Non-stop and No-go decay quality control systems, and for RNA silencing- and minimum ORF-mediated decay, we propose that RST1 and RIPR are essential components of plant SKI-exosome supercomplex.