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Plant Signaling & Behavior - Systemic Acquired Resistance

Plant Signaling and Behavior (Journal)


Systemic Acquired Resistance




Upon infection with necrotizing pathogens many plants develop an enhanced resistance to further pathogen attack also in the uninoculated organs. This type of enhanced resistance is referred to as systemic acquired resistance (SAR). In the SAR state, plants are primed (sensitized) to more quickly and more effectively activate defense responses the second time they encounter pathogen attack. Since SAR depends on the ability to access past experience, acquired disease resistance is a paradigm for the existence of a form of “plant memory”. Although the phenomenon has been known since the beginning of the 20th century, major progress in the understanding of SAR was made over the past sixteen years. This review covers the current knowledge of molecular, biochemical and physiological mechanisms that are associated with SAR.
Key Words: Arabidopsis, benzothiadiazole, defense response potentiation, 2,6-dichloroisonicotinic acid, elicitor, MAP kinase, parsley cell culture, priming, salicylic acid, sensitization





In 1901, Beauverie and Ray independently realized that plants previously infected by a pathogen could better resist further infection.1,2 Over the 30 years that followed these reports, many studies suggested the existence of various induced disease resistance phenomena in plants. These have been summarized by Chester in 1933.3 One prominent induced resistance phenomenon is nowadays known as systemic acquired resistance (SAR).4–6 SAR is induced by most pathogens that cause tissue necrosis, either as a part of a hypersensitive response (HR)7 or as a symptom of disease.8 One characteristic of SAR is the development of the enhanced resistance in the distal, uninoculated plant organs.4–8 Another hallmark of SAR is its activity against a broad and distinctive spectrum of pathogens which includes viruses, bacteria, oomycetes, and fungi.4,9 In addition, SAR confers a long-lasting protection that can last for weeks to month, and sometimes throughout an entire season.9 Thus, for SAR to be realized, a plant requires the ability to recall past experience. Therefore, in addition to serving as a paradigm for signal transduction and having practical value, acquired resistance is a prime example for the existence of a form of “plant memory.”


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SAR is a widely observed phenomenon in plants and priming for potentiated activation of defense responses has emerged as an important part of it (Fig. 1). In the primed state, plants are able to more quickly and more effectively activate defense responses when exposed to biotic or abiotic stress. A thorough elucidation of the primed state on the molecular, biochemical and physiological levels will not only contribute to a better understanding of signal transduction in plants. It will also enable utilization of the broad-spectrum, natural defense capacity of plants in the field. Finally, in-depth analysis of priming and SAR will provide access toward understanding this form of “plant memory”.





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