Degradation of unmethylated miRNA/miRNA*s by a DEDDy-type 3' to 5' exoribonuclease Atrimmer 2 in Arabidopsis

The 3' end methylation catalyzed by HUA Enhancer 1 (HEN1) is a crucial step of small RNA stabilization in plants, yet how unmethy-lated small RNAs undergo degradation remains largely unknown. Using a reverse genetic approach, we here show that Atrimmer 2 (ATRM2), a DEDDy-type 3' to 5' exoribonuclease, acts in the degradation of unmethylated miRNAs and miRNA*s in Arabidopsis. 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. Dysfunction 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 correlated in hen1 atrm2 versus hen1. Unexpectedly, we observed a marked 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 Argonaute 1 (AGO1). Taken together, our results suggest that ATRM2 may be involved in the surveillance of unmethylated miRNA/miRNA* duplexes during the initiation step of RNA-induced silencing complex assembly.