3_RNAs

The Arabidopsis genome encodes two major classes of 2024-nucleotide riboregulators: microRNAs and small interfering RNAs. These small RNAs act as sequence-specific repressors of target gene expression, either at the transcriptional level through DNA and/or histone methylation or at the post-transcriptional level through transcript cleavage or translational inhibition. Small RNAs are processed from precursor RNAs by one or more of the four DICER-LIKE RNase III enzymes, modified by HUA ENHANCER 1, a small RNA methyltransferase, and loaded onto an argonaute protein-containing RNA-induced silencing complex. Here, we review the biogenesis of small RNAs, and we discuss the major outstanding questions in small RNA metabolism and function.

http://arjournals.annualreviews.org/doi/abs/10.1146/annurev.arplant.043008.092111

Abstract

Annual Review of Plant Biology

Vol. 60: 485-510 (Volume publication date June 2009)

(doi:10.1146/annurev.arplant.043008.092111)

Roles of Plant Small RNAs in Biotic Stress Responses

Virginia Ruiz-Ferrer and Olivier Voinnet

Institut de Biologie Moléculaire des Plantes du CNRS, UPR2357, 67084 Strasbourg Cedex, France; email: olivier.voinnet@ibmp-ulp.u-strasbg.fr

A multitude of small RNAs (sRNAs, 18–25 nt in length) accumulate in plant tissues. Although heterogeneous in size, sequence, genomic distribution, biogenesis, and action, most of these molecules mediate repressive gene regulation through RNA silencing. Besides their roles in developmental patterning and maintenance of genome integrity, sRNAs are also integral components of plant responses to adverse environmental conditions, including biotic stress. Until recently, antiviral RNA silencing was considered a paradigm of the interactions linking RNA silencing to pathogens: Virus-derived sRNAs silence viral gene expression and, accordingly, viruses produce suppressor proteins that target the silencing mechanism. However, increasing evidence shows that endogenous, rather than pathogen-derived, sRNAs also have broad functions in regulating plant responses to various microbes. In turn, microbes have evolved ways to inhibit, avoid, or usurp cellular silencing pathways, thereby prompting the deployment of counter-counterdefensive measures by plants, a compelling illustration of the neverending molecular arms race between hosts and parasites.

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