miRNAs in the Plant Genome: All Things Great and Small

Miniature Inverted-Repeat Transposable Elements: Small DNA Transposons That Have Contributed to Plant MICRORNA Gene Evolution

Angiosperms form the largest phylum within the Plantae kingdom and show remarkable genetic variation due to the considerable difference in the nuclear genome size of each species. Transposable elements (TEs), mobile DNA sequences that can amplify and change their chromosome position, account for much of the difference in nuclear genome size between individual angiosperm species. Considering the dramatic consequences of TE movement, including the complete loss of gene function, it is unsurprising that the angiosperms have developed elegant molecular strategies to control TE amplification and movement. Specifically, the RNA-directed DNA methylation (RdDM) pathway, directed by the repeat-associated small-interfering RNA (rasiRNA) class of small regulatory RNA, forms the primary line of defense to control TE activity in the angiosperms. However, the miniature inverted-repeat transposable element (MITE) species of TE has at times avoided the repressive effects imposed by the rasiRNA-directed RdDM pathway. MITE proliferation in angiosperm nuclear genomes is due to their preference to transpose within gene-rich regions, a pattern of transposition that has enabled MITEs to gain further transcriptional activity. The sequence-based properties of a MITE results in the synthesis of a noncoding RNA (ncRNA), which, after transcription, folds to form a structure that closely resembles those of the precursor transcripts of the microRNA (miRNA) class of small regulatory RNA. This shared folding structure results in a MITE-derived miRNA being processed from the MITE-transcribed ncRNA, and post-maturation, the MITE-derived miRNA can be used by the core protein machinery of the miRNA pathway to regulate the expression of protein-coding genes that harbor homologous MITE insertions. Here, we outline the considerable contribution that the MITE species of TE have made to expanding the miRNA repertoire of the angiosperms.

The Plant microRNA Pathway: The Production and Action Stages

Plant microRNAs are an endogenous class of small regulatory RNA central to the posttranscriptional regulation of gene expression in plant development and environmental stress adaptation or in response to pathogen challenge. The plant microRNA pathway is readily separated into two distinct stages: (1) the production stage, which is localized to the plant cell nucleus and where the microRNA small RNA is processed from a double-stranded RNA precursor transcript, and (2) the action stage, which is localized to the plant cell cytoplasm and where the mature microRNA small RNA is loaded into an effector complex and is used by the complex as a sequence specificity guide to direct expression repression of target genes harboring highly complementary microRNA target sequences. Historical research indicated that the plant microRNA pathway was a highly structured, almost linear pathway requiring a small set of core machinery proteins. However, contemporary research has demonstrated that the plant microRNA pathway is highly dynamic, and to allow for this flexibility, a large and highly functionally diverse set of machinery proteins is now known to be required. For example, recent research has shown 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 of RNA silencing: a mode of RNA silencing originally assigned to all plant microRNAs. Using Arabidopsis thaliana as our model system, here we report on both the core and auxiliary sets of machinery proteins now known to be required for both microRNA production and microRNA action in plants.

MicroRNA expression profiles during cotton (Gossypium hirsutum L) fiber early development

The role of microRNAs (miRNAs) during cotton fiber development remains unclear. Here, a total of 54 miRNAs belonging to 39 families were selected to characterize miRNA regulatory mechanism in eight different fiber development stages in upland cotton cv BM-1. Among 54 miRNAs, 18 miRNAs were involved in cotton fiber initiation and eight miRNAs were related to fiber elongation and secondary wall biosynthesis. Additionally, 3,576 protein-coding genes were candidate target genes of these miRNAs, which are potentially involved in cotton fiber development. We also investigated the regulatory network of miRNAs and corresponding targets in fiber initiation and elongation, and secondary wall formation. Our Gene Ontology-based term classification and KEGG-based pathway enrichment analyses showed that the miRNA targets covered 220 biological processes, 67 molecular functions, 45 cellular components, and 10 KEGG pathways. Three of ten KEGG pathways were involved in lignan synthesis, cell elongation, and fatty acid biosynthesis, all of which have important roles in fiber development. Overall, our study shows the potential regulatory roles of miRNAs in cotton fiber development and the importance of miRNAs in regulating different cell types. This is helpful to design miRNA-based biotechnology for improving fiber quality and yield.

Identification of microRNA genes and their mRNA targets in Festuca arundinacea

MicroRNAs (miRNAs) have emerged as a novel class of endogenous, small, non-coding RNAs of 22 nucleotides (nts) in length, which plays important roles in post-transcriptional degradation of target mRNA or inhibition of protein synthesis through binding the specific sites of target mRNA. Growing evidences have shown that miRNAs play an important role in various biological processes, including growth and development, signal transduction, apoptosis, proliferation, stress responses, maintenance of genome stability, and so on. In our study, we used bioinformatic tools to predict miRNA and the corresponding target genes of Festuca arundinacea. We used known miRNAs of other plants from miRBase to search against expressed sequence tags (EST) databases and genome survey sequences (GSS) of F. arundinacea. A total of 8 potential miRNAs were predicted. Phylogenetic analysis of the predicted miRNAs revealed that miRNA398c of F. arundinacea species was evolutionary highly conserved with Populus trichocarpa. The 8 potential miRNAs corresponding to 20 target genes were found. Most of the miRNA target genes were predicted to encode transcription factors that regulate cell growth and development, signaling, metabolism, and other biology processes. By bioinformatics methods, we can effectively predict novel miRNAs and its target genes and add information to F. arundinacea miRNA database. Moreover, it shows a path for the prediction and analysis of miRNAs to those species whose genomes are not available through bioinformatics tools.

A historical overview of RNAi in plants

RNA interference, or RNAi, is arguably one of the most significant discoveries in biology in the last several decades. First recognized in plants (where it was called post-transcriptional gene silencing, PTGS) RNAi is a gene down-regulation mechanism since demonstrated to exist in all eukaryotes. In RNAi, small RNAs (of about 21-24 nucleotides) function to guide specific effector proteins (members of the Argonaute protein family) to a target nucleotide sequence by complementary base pairing. The effector protein complex then down-regulates the expression of the targeted RNA or DNA. Small RNA-directed gene regulation systems were independently discovered (and named) in plants, fungi, worms, flies, and mammalian cells. Collectively, PTGS, RNA silencing, and co-suppression (in plants); quelling (in fungi and algae); and RNAi (in Caenorhabditis elegans, Drosophila, and mammalian cells) are all examples of small RNA-based gene regulation systems. From the very beginning, plant research has had a major impact on our understanding of RNAi. The purpose of this chapter is to provide an historical perspective and overview on the discovery, characterization, and applications of RNAi in plants.

Hairpin transcription does not necessarily lead to efficient triggering of the RNAi pathway

Previously, we had shown that stable expression of a hairpin RNA sharing homology with the coat protein (CP) of the Cucumber mosaic virus (CMV) (hpRNA(CMV)) produced CMV resistant Nicotiana tabacum plants. However, only 17% of the hpRNA(CMV)-expressing plants generated substantial amounts of siRNAs that mediated CMV resistance (siRNAs(CMV)). Here, we demonstrate that the transcription of a hpRNA(CMV) per se is not sufficient to trigger cytoplasmic and nuclear RNAi. A multiple-transgene copy line showed a strong resistance phenotype. Segregation of individual copies revealed that in one locus, the transgene-produced hpRNA(CMV) transcript was processed into 21-nt and 24-nt siRNAs(CMV) and lines containing this locus were resistant. At a second locus, where the transgene was shown to be transcribed, no siRNAs(CMV) were produced and lines harbouring only this locus were susceptible. In addition, the second locus failed to trigger de novo RNA-directed DNA methylation (RdDM) in cis, of its cognate sequence. However, after being induced in trans, methylation in the transcribed region of the transgene was maintained in both CG and CHG residues. Sequence-specific maintenance of methylation in transcribed regions, as well as diverse RNA degradation pathways in plants are discussed in view of our observations.

Conserved miRNA analysis in Gossypium hirsutum through small RNA sequencing

Several miRNA family and their targets in cotton had been identified by computational methods based on the conserved characterization of miRNAs. So far, there are no experiments to validate the existence of miRNAs in cotton. In this study, to analyze the miRNAs in cotton, a small RNA library of sequences from 18 to 26 nt of Gossypium hirsutum seedling has been built by high-throughput sequencing. In this library, 34 conserved miRNA families were identified by homology search and the miRNA sequences of them were also found in the library. Furthermore, potential targets of these conserved miRNA families were predicted in cotton TC library. However, based on the mature miRNAs and their miR sequences, only 8 conserved miRNA encoding loci (miR156, miR157a, miR157b, miR162, miR164, miR393, miR399, miR827) were identified from cotton EST sequences. Multiple encoding loci of some miRNAs were identified by comparing the cloned miRNA and miR sequences.