Antiviral strategies in plants based on RNA silencing ☆ Carmen Simón-Mateo ⁎, Juan Antonio García ⁎⁎ Centro Nacional de Biotecnología (CNB-CSIC), Campus Universidad Autónoma de Madrid, 28049 Madrid, Spain abstract article info Article history: Received 25 March 2011 Received in revised form 17 May 2011 Accepted 18 May 2011 Available online 30 May 2011 Keywords: Virus resistance RNA interference Pathogen-derived resistance One of the challenges being faced in the twenty-first century is the biological control of plant viral infections. Among the different strategies to combat virus infections, those based on pathogen-derived resistance (PDR) are probably the most powerful approaches to confer virus resistance in plants. The application of the PDR concept not only revealed the existence of a previously unknown sequence-specific RNA-degradation mechanism in plants, but has also helped to design antiviral strategies to engineer viral resistant plants in the last 25 years. In this article, we review the different platforms related to RNA silencing that have been developed during this time to obtain plants resistant to viruses and illustrate examples of current applications of RNA silencing to protect crop plants against viral diseases of agronomic relevance. This article is part of a Special Issue entitled: MicroRNAs in viral gene regulation. Published by Elsevier B.V. 1. Introduction Plant viruses represent important threats to modern agriculture. Although accurate figures for crop losses due to viruses are not available, it is generally accepted that among the different plant pathogens, the economic relevance of viruses comes second to fungi. Until the emergence of genetic engineering technologies, plant viruses have been partially controlled using conventional cultivation techniques such as crop rotation, early detection and eradication of the diseased plants, cross protection, breeding for resistance, or chemical control of their vectors [1]. In the 1980s, the successful transfer of foreign DNA into the nuclear genome using Agrobacterium as a vector prompted the introduction of genetic engineering for crop improvement and the development of virus-resistant plants [2,3]. Today, different antiviral strategies are being undertaken, either by exploiting natural plant defense mechanisms, or designing new tools, which in most cases are ultimately also based on natural defense mechanisms. Most of the achievements obtained in plant biotechnology in the area of plant virus resistance are based on the principle of pathogen- derived resistance (PDR) [4]. The concept of PDR was proposed by Sanford and Johnston [5] twenty-five years ago using the bacteriophage Qß as a model, and considers that expression of pathogen genetic elements outside the context of infection may lead to resistance. This approach opened an interesting possibility for the practical control of diseases. For plant viruses, the concept of PDR was first validated with its use in tobacco plants transformed with the tobamovirus Tobacco mosaic virus (TMV) coat protein (CP) gene [6]. Soon this observation was validated using other viral CPs and other viral sequences that code for proteins such as replicases, proteinases and movement proteins (for reviews, see [7–11]). CP is the most successful and widely applied viral protein for PDR. However, the protection conferred by CP-mediated resistance varies significantly from strong interference with virus multiplication to delay or attenuation of symptoms. The PDR based on the expression of viral proteins, with either the wild type or the mutated one, in transgenic plants has several general characteristics: i) it is not very specific, and protects against a broad range of viral strains; ii) it shows a positive correlation between the levels of accumulation of the viral product and the effectiveness in resistance; iii) it is usually overcome by high doses of inoculum. Despite extensive studies, the molecular mechanisms underlying protein-mediated resistance are not fully understood. What appears to be certain is that they are diverse, that they probably affect several steps of the infection process, and that each virus/transgenic plant combination has specific features. Moreover, it soon became apparent that many virus resistances initially envisaged as protein-mediated PDR did not rely on the expression of the correspond- ing viral proteins and that a majority of PDR phenomena seemed to work through RNA-mediated mechanisms [12]. 2. RNA silencing and virus resistance In the early nineties, two independent research groups found that the expression of a transgene mRNA with a high sequence similarity to an endogenous mRNA, led to specific degradation of both mRNAs through post-transcriptional gene silencing (PTGS), also known as “cosuppression” [13,14]. Later, the W. Dougherty research group suggested that a similar mechanism might be involved in the resistance phenomena observed in transgenic plants transformed with viral genes. Some of the transgenic lines showed anomalous phenotypes; unexpectedly and unpredictably the highest level of Biochimica et Biophysica Acta 1809 (2011) 722–731 ☆ This article is part of a Special Issue entitled: MicroRNAs in viral gene regulation. ⁎ Corresponding author. Tel.: + 34 915855397; fax: + 34 915854506. ⁎⁎ Corresponding author. Tel.: + 34 915854535; fax: + 34 915854506. E-mail addresses: csimon@cnb.csic.es (C. Simón-Mateo), jagarcia@cnb.csic.es (J.A. García). 1874-9399/$ – see front matter. Published by Elsevier B.V. doi:10.1016/j.bbagrm.2011.05.011 Contents lists available at ScienceDirect Biochimica et Biophysica Acta journal homepage: www.elsevier.com/locate/bbagrm