Ribosome biogenesis is a fundamental and tightly regulated cellular process, including synthesis, processing, and assembly of rRNAs with ribosomal proteins. Protein arginine methyltransferases (PRMTs) have been implicated in many important biological processes, such as ribosome biogenesis. Two alternative precursor rRNA (prerRNA) processing pathways coexist in yeast and mammals; however, how PRMT affects ribosome biogenesis remains largely unknown. Here we show that Arabidopsis PRMT3 (AtPRMT3) is required for ribosome biogenesis by affecting pre-rRNA processing. Disruption of AtPRMT3 results in pleiotropic developmental defects, imbalanced polyribosome profiles, and aberrant pre-rRNA processing. We further identify an alternative pre-rRNA processing pathway in Arabidopsis and demonstrate that AtPRMT3 is required for the balance of these two pathways to promote normal growth and development. Our work uncovers a previously unidentified function of PRMT in posttranscriptional regulation of rRNA, revealing an extra layer of complexity in the regulation of ribosome biogenesis.arginine methylation | protein arginine methyltransferase | AtPRMT3 | ribosome biogenesis | rRNA processing T he fundamental, complicated, and highly cooperative process of ribosome biogenesis involves ribosomal DNA (rDNA) transcription, precursor rRNA (pre-rRNA) processing, and assembly with ribosomal proteins and related assembly factors (1, 2). As a multistep, error prone, and energy-consuming process, ribosome biogenesis is also highly regulated (3, 4). In eukaryotic cells, mutations in ribosomal proteins or ribosome assembly factors usually lead to aberrant pre-rRNA processing (5-7) and activation of the polyadenylation-mediated RNA quality control system (4, 8, 9), resulting in various genetic diseases in humans (10, 11).Work in budding yeast, Saccharomyces cerevisiae, has deciphered the mechanisms of ribosome biogenesis (1, 2, 12). After transcription by RNA polymerase I and site-specific modification by small nucleolar ribonucleoproteins, the nascent 35S rRNA, the common precursor of 18S, 5.8S, and 25S rRNAs, is quickly assembled with many assembly factors and ribosomal proteins into small subunit processome/90S preribosomal particles (13-15). Then it mainly undergoes the "U3-dependent cleavage occurs first" pathway, which first removes the 5′ external transcribed sequence (5′ ETS) of 35S rRNA, to generate the 32S rRNA (16, 17). Next, after cleavage at the A2 site of intergenic transcribed sequence 1 (ITS1) between 18S rRNA and 5.8S rRNA, the 90S preribosomal particle splits into two independent complexes of pre-40S and pre-60S ribosomal particles. Finally, ribosomal subunits are further matured and assembled into 80S ribosomes for translation in the cytoplasm (12). However, in contrast to budding yeast, pre-rRNA processing in Xenopus laevis oocytes, mouse cells, and human cells preferentially cleave in ITS1 before the complete removal of the 5′ ETS (18-21), which may represent a common pathway in metazoans.In plants, a pre-rRNA processi...

Epigenetics refers to the study of heritable changes in gene expression or cellular phenotype without changes in DNA sequence. Epigenetic regulation of gene expression is accomplished by DNA methylation, histone modifications, histone variants, chromatin remodeling, and may involve small RNAs. DNA methylation at cytosine is carried out by enzymes called DNA Methyltransferases and is involved in many cellular processes, such as silencing of transposable elements and pericentromeric repeats, X-chromosome inactivation and genomic imprinting, etc. Histone modifications refer to posttranslational covalent attachment of chemical groups onto histones such as phosphorylation, acetylation, and methylation, etc. Histone variants, the non-canonical histones with amino acid sequences divergent from canonical histones, can have different epigenetic impacts on the genome from canonical histones. Higher-order chromatin structures maintained or modified by chromatin remodeling proteins also play important roles in regulating gene expression. Small non-coding RNAs play various roles in the regulation of gene expression at pre- as well as posttranscriptional levels. A special issue of Molecular Plant on ‘Epigenetics and Plant Development’ (Volume 4, Number 2, 2009) published a variety of articles covering many aspects of epigenetic regulation of plant development. We have tried here to present a bird's-eye view of these credible efforts towards understanding the mysterious world of epigenetics. The majority of the articles are about the chromatin modifying proteins, including histone modifiers, histone variants, and chromatin remodeling proteins that regulate various developmental processes, such as flowering time, vernalization, stem cell maintenance, and response to hormonal and environmental stresses, etc. Regulation of expression of seed transcriptome, involvement of direct tandem repeat elements in the PHE1 imprinting in addition to PcG proteins activity, paramutation, and epigenetic barriers in species hybridization are described well. The last two papers are about the Pol V-mediated heterochromatin formation independent of the 24nt-siRNA and the effect of genome position and tissue type on epigenetic regulation of gene expression. These findings not only further our current understanding of epigenetic mechanisms involved in many biological phenomena, but also pave the path for the future work, by raising many new questions that are discussed in the following lines.