Endoplasmic reticulum as a hub organizing microRNAs and a subset of endogenous siRNAs in Arabidopsis [Ribo-Seq and RNA-Seq]

Endoplasmic reticulum (ER) was implicated as the site of microRNA (miRNA)-mediated translational repression in plants. Here, we examined the ER- and rough ER- associated small RNAome, transcriptome and translatome. We found that nearly all cellular transcripts were present on membrane-bound polysomes (MBPs), and miRNAs and a small set of endogenous siRNAs were particularly enriched on MBPs. The MBP- enriched miRNAs and siRNAs associated with, and were recruited to membranes by, their effector protein ARGONAUTE1 (AGO1). AGO1 associated with ER in a partly RNA-independent manner. Reduced membrane association of 22-nt miRNAs, which trigger the biogenesis of phased, secondary siRNAs (phasiRNAs) from their target transcripts, was accompanied by decreased production or loss of phasing of phasiRNAs. The phasiRNA precursor transcripts, previously thought to be noncoding, were associated with MBPs in a manner that supported phasiRNA production. These findings point to the ER as a hub that hosts and organizes endogenous small RNAs in plants. Ribo-Seq and mRNA-Seq libraries from RNA extracted from membrane bound polysome fractions (One biological replicate)