Direct molecular evidence for an ancient, conserved developmental toolkit controlling post-transcriptional gene regulation in land plants

Abstract

RNA interference (RNAi) plays important roles in organism development through post-transcriptional regulation of specific target mRNAs. Target specificity is largely controlled by base-pair complementarity between micro-RNA (miRNA) regulatory elements and short regions of the target mRNA. The pattern of miRNA production in a cell interacts with the cell's mRNA transcriptome to generate a specific network of post-transcriptional regulation that can play critical roles in cellular metabolism, differentiation, tissue/organ development and developmental timing. In plants, miRNA production is orchestrated in the nucleus by a suite of proteins that control transcription of the pri-miRNA gene, post-transcriptional processing and nuclear export of the mature miRNA. In the model plant, Arabidopsis thaliana, post-transcriptional processing of miRNAs is controlled by a pair of physically-interacting proteins, HYL1 and DCL1. However, the evolutionary history of the HYL1-DCL1 interaction is unknown, as is its structural basis. Here we use ancestral sequence reconstruction and functional characterization of ancestral HYL1 in vitro and in vivo to better understand the origin and evolution of the HYL1-DCL1 interaction and its impact on miRNA production and plant development. We found the ancestral plant HYL1 evolved high affinity for both double-stranded RNA (dsRNA) and its DCL1 partner very early in plant evolutionary history, before the divergence of mosses from seed plants (~500 Ma), and these high-affinity interactions remained largely conserved throughout plant evolutionary history. Structural modeling and molecular binding experiments suggest that the second of two double-stranded RNA-binding motifs (DSRMs) in HYL1 may interact tightly with the first of two C-terminal DCL1 DSRMs to mediate the HYL1-DCL1 physical interaction necessary for efficient miRNA production. Transgenic expression of the nearly 200 Ma-old ancestral flowering-plant HYL1 in A. thaliana was sufficient to rescue many key aspects of plant development disrupted by HYL1- knockout and restored near-native miRNA production, suggesting that the functional partnership of HYL1-DCL1 originated very early in and was strongly conserved throughout the evolutionary history of terrestrial plants. Overall, our results are consistent with a model in which miRNA-based gene regulation evolved as part of a conserved plant 'developmental toolkit'; its role in generating developmental novelty is probably related to the relatively rapid evolution of miRNA genes.