The Use of Arabidopsis thaliana to Characterize the Production and Action Stages of the Plant MicroRNA Pathway.

Plant microRNAs are an endogenous class of small regulatory RNA central to the posttranscriptional control of gene expression as part of normal development to adapt to environmental stress and respond to pathogen challenges. The plant microRNA pathway is separated into two distinct stages: (1) production stage, which is localized to the nucleus of the cell and, in this cell compartment, the microRNA silencing signal is processed from its double-stranded RNA precursor transcript, and (2) action stage, which is localized to the cytoplasm of the cell. It is in this cellular compartment where the now mature microRNA functions as a regulatory RNA molecule to control target gene expression via its loading into the protein effector complex termed microRNA-induced silencing complex. Historical research indicated that the plant microRNA pathway was a highly structured, almost linear pathway that only required the functional activity of a small set of core, highly conserved pieces of protein machinery. However, contemporary research continues to illustrate that the plant microRNA pathway is highly dynamic, with such flexibility provided by an extremely large and functionally diverse set of auxiliary protein machinery that perform highly specific roles as part of either the production or action stage of the pathway. For example, recent research has elegantly demonstrated that plant microRNAs can regulate target gene expression via a translational repression mechanism of RNA silencing in addition to the standard messenger RNA cleavage-based mechanism: a mode of RNA silencing originally assigned to all plant microRNAs. Using Arabidopsis thaliana as our model system, we report on both the core and auxiliary sets of protein machinery now demonstrated functionally essential for the plant microRNA pathway.