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Key Words plant defense, salicylic acid, SAR, NPR1, TGA factor
Abstract Systemic acquired resistance (SAR) is a mechanism of induced defense that confers long-lasting protection against a broad spectrum of microorganisms. SAR requires the signal molecule salicylic acid (SA) and is associated with accumulation of pathogenesis-related proteins, which are thought to contribute to resistance. Much progress has been made recently in elucidating the mechanism of S AR. Using the model plant Arabidopsis, it was discovered that the isochorismate pathway is the major source of SA during SAR. In response to SA, the positive regulator protein NPR1 moves to the nucleus where it interacts with TGA transcription factors to induce defense gene expression, thus activating SAR. Exciting new data suggest that the mobile signal for SAR might be a lipid molecule. We discuss the molecular and genetic data that have contributed to our understanding of SAR and present a model describing the sequence of events leading from initial infection to the induction of defense genes.
INTRODUCTION
Plants have evolved a number of inducible defense mechanisms against pathogen attack. Recognition of a pathogen often triggers a localized resistance reaction, known as the hypersensitive response (HR), which is characterized by rapid cell death at the site of infection (40). In the 1960s, Ross showed that tobacco plants challenged with tobacco mosaic virus (TMV) subsequently developed increased resistance to secondary infection in distal tissues (86). This spread of resistance throughout the plant's tissues was termed systemic acquired resistance (SAR). We now know that SAR can be activated in many plant species by pathogens that cause necrosis, either as part of the HR or as a symptom of disease. The resistance conferred is long-lasting, sometimes for the lifetime of the plant, and effective against a broad-spectrum of pathogens including viruses, bacteria, fungi, andoomycetes (91, 102).
Molecularly, SAR is characterized by the increased expression of a large number of pathogenesis-related genes (PR genes), in both local and systemic tissues. PR proteins were first described in the 197Os by Van Loon, who observed accumulation of various novel proteins after infection of tobacco with TMV (108,109). Although many PR proteins have antimicrobial properties in vitro (109), the function of each in the defense response has not been clearly defined. It is...