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© 2005 Nature Publishing Group Department of Molecular, Cellular, and Developmental Biology, Yale University, New Haven, CT 06520-8103, USA. *These authors contributed equally to this work. Correspondence to S.P.D.-K. e-mail: savithramma. dinesh-kumar@yale.edu doi:10.1038/nrmicro1239 Published online 10 August 2005 INNATE IMMUNITY The suite of host responses to pathogens that result in rapid defence without requiring prior stimulation. Plants and viruses enter into various relationships that do not necessarily result in damage to the host BOX 1. If a pathogenic virus succeeds in infecting a plant, a selection of INNATE IMMUNITY mechanisms might defeat the virus. Should the virus circumvent these defence mechanisms, disease outbreaks and epidemics occur. For example, during the 1990s, cassava production in Uganda was devastated by cassava mosaic gemini- viruses, resulting in famine-related deaths 1 . It is esti- mated that on the African continent in 2003 more than 19 million tons of cassava, valued at more than US$1.9 billion, was lost 1 . Clearly, understanding plant defence is required to develop approaches to protect the world’s food supply. Perhaps the best-characterized mechanism of plant antiviral defence is mediated by resistance (R) genes. R genes confer resistance to organisms including viruses, bacteria, fungi and even nematodes 2 . R proteins and their signal-transduction molecules are strikingly similar to the components of the animal innate immune system. How these conserved signalling modules function in plants is the subject of intense research. Of increasing interest in plant antiviral strategies is the role of RNA silencing, an ancient cellular mechanism of defence against foreign nucleic acids that also func- tions in gene regulation. The RNA-silencing pathway is found in organisms that are separated by millions of years of evolution. How do R proteins and RNA silencing interact to limit viral pathogenesis? We review recent advances in the field of plant defence against viruses. R genes Plant R genes confer resistance to many pathogens, including viruses. Here, we describe the responses mediated by these genes and how R proteins probably function. R-gene mediated responses. Each R gene confers resist- ance to a specific pathogen BOX 1. The first pheno- type of defence in most R -gene-mediated resistance responses is the hypersensitive response (HR). The HR includes programmed cell death (PCD), which occurs in cells at the site of infection and manifests as discrete necrotic lesions in otherwise phenotypically normal tissue (FIG. 1a). The virus is usually confined to the lesion and to the cells immediately surround- ing it and fails to spread from lesions into adjacent MECHANISMS OF PLANT RESISTANCE TO VIRUSES Jennifer L. M. Soosaar*, Tessa M. Burch-Smith* and Savithramma P. Dinesh-Kumar Abstract | Plants have evolved in an environment rich with microorganisms that are eager to capitalize on the plants’ biosynthetic and energy-producing capabilities. There are approximately 450 species of plant-pathogenic viruses, which cause a range of diseases.
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