Systemic Acquired Resistance (SAR) in Plants

Systemic Acquired Resistance (SAR)

• Plants infected with necrotizing pathogens develop enhanced resistance to subsequent infections by various fungal, bacterial, and viral pathogens, known as Systemic Acquired Resistance (SAR).
• SAR confers broad-spectrum resistance, effective against many but not all pathogens, including Phytophthora parasitica, Cercospora nicotianae, Peronospora tabacina, tobacco mosaic virus, and Pseudomonas syringae pv.tabaci.

Characteristics of SAR

• SAR is characterized by a heightened state of resistance in distal tissues of the plant after pathogen recognition and HR.
• It is a long-lasting heightened resistance state, lasting for a few weeks to sometimes the entire life of the plant.
• SAR is effective against a broad-spectrum of pathogens, including viruses, bacteria, fungi, and oomycetes.
• The time needed for the establishment of SAR depends on both the plant and type of inducing factor.

Historical Perspective

• SAR was first recognized as a significant phenomenon in 1933 by Chester.
• Infection of plants with necrotizing pathogens often results in enhanced resistance to subsequent infections by various fungal, bacterial, and viral pathogens.
• SAR was associated with the coordinated induction of a set of SAR genes encoding proteins known as Pathogenesis-related (PR) proteins.

Signal Molecule and PR Proteins

• Salicylic acid (SA) is the endogenous signal molecule required for the induction of SAR.
• SAR is characterized by the increased expression of a large number of pathogenesis-related genes (PR genes) in both local and systemic tissues.
• SA application on tobacco leaves mimics pathogen-induced expression of PR genes and pathogen resistance in treated tissues.

Key Discoveries

• White (1979) found that the application of aspirin, salicylic acid, and benzoic acid resulted in enhanced resistance to TMV.
• Malamy et al. (1990) showed that the endogenous SA concentration rises in both local and systemic tissues after infection of tobacco with TMV, correlating with PR gene induction.
• Ryals and co-workers (1994) demonstrated that tobacco plants defective in SA accumulation were also defective in their ability to develop SAR and accumulate PR proteins.