microRNA biogenesis, degradation and activity in plants

Abstract

microRNAs (miRNAs) are important regulators of gene expression. After excised from primary miRNA transcript by dicer-like1 (DCL1, an RNAse III enzyme), miRNAs bind and guide their effector protein named argonaute 1 (AGO1) to silence the expression of target RNAs containing their complementary sequences in plants. miRNA levels and activities are tightly controlled to ensure their functions in various biological processes such as development, metabolism and responses to abiotic and biotic stresses. Studies have identified many factors that involve in miRNA accumulation and activities. Characterization of these factors in turn greatly improves our understanding of the processes related to miRNAs. Here, we review recent progress of mechanisms underlying miRNA expression and functions in plants.

Fig. 1

The framework of miRNA biogenesis and function. The transcription of pri-miRNAs is regulated by many transcription factors. Then, many protein factors are recruited to pri-miRNAs to form the processor complex of miRNAs through protein–protein and protein-RNA interactions. CDC5 and NOT2 do not interact with HYL1. Thus, whether CDC5 and NOT2 are in the D-body is unknown. After generation in nucleus, miRNA/miRNA* is methylated by HEN1 and exported into cytoplasm. miRNAs are loaded into AGO1 to direct target RNA cleavage or translational inhibition. It is not clear where the AGO1-miRNA assembly and miRNA methylation happens. Evidences suggest that translational inhibition by miRNA may occur at specific site of endoplasmic reticulum

Fig. 2

Model of uridylation-triggered miRNA degradation. Under normal condition, miRNAs are methylated by HEN1, which protects miRNAs from uridylation at the AGO1 complex. Methylated miRNAs maybe subject to SDN degradation, resulting in 3′ truncated miRNAs. HESO1 may uridylate unmethylated miRNAs and 3′ truncated miRNAs to trigger their degradation. Unmethylated miRNA can also be degraded through unknown 3′-to-5′ trimming activity