GSE107070
Degradation of unmethylated miRNA/miRNA*s by a DEDDy-type 3' to 5' exoribonuclease ATRIMMER2 in Arabidopsis
3' end methylation catalyzed by HUA ENHANCER1 (HEN1) is a crucial step of small RNA stabilization in plants, yet how unmethylated small RNAs undergo degradation remains largely unknown. Using a reverse genetic approach, we here show that ATRIMMER2 (ATRM2), a DEDDy-type 3' to 5' exoribonuclease, acts in the degradation of unmethylated miRNAs and miRNA*s in Arabidopsis. A loss-of-function mutations in ATRM2 partially suppress the morphological defects caused by HEN1 malfunction, with restored levels of a subset of miRNAs and receded expression of corresponding miRNA targets. Loss of ATRM2 has negligible effect on miRNA trimming, and further increase the fertility of hen1 heso1 urt1, a mutant with an almost complete abolishment of miRNA uridylation, indicating that ATRM2 may neither be involved in 3' to 5' trimming nor be the enzyme that specifically degrades uridylated miRNAs. Notably, the fold changes of miRNAs and their corresponding miRNA*s were significantly co-regulated in hen1 atrm2 as compared with those in hen1. Unexpectedly, we also observed a markedly increase of 3' to 5' trimming of several miRNA*s but not miRNAs in ATRM2 compromised backgrounds. These data suggest an action of ATRM2 on miRNA/miRNA* duplexes, and the existence of an unknown exoribonuclease for specific trimming of miRNA*. This asymmetric effect on miRNA/miRNA* is likely related to ARGONAUTE (AGO) proteins, which can distinguish miRNAs from miRNA*s. Finally, we show that ATRM2 colocalizes and physically interacts with ARGONAUTE1 (AGO1). Taken together, our results suggest that ATRM2 may be involved in the surveillance of unmethylated miRNA/miRNA* duplexes during the initiation step of RISC assembly. Overall design: Examination of total and  AGO1-associated small RNAs in different genotypes