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Key Words Peronospora, Pseudomonas, Erysiphe, Allemand, Botrytis
Abstract It has been suggested that effective defense against biotrophic pathogens is largely due to programmed cell death in the host, and to associated activation of defense responses regulated by the salicylic acid-dependent pathway. In contrast, necrotrophic pathogens benefit from host cell death, so they are not limited by cell death and salicylic acid-dependent defenses, but rather by a different set of defense responses activated by jasmonic acid and ethylene signaling. This review summarizes results from Arabidopsis-pathogen systems regarding the contributions of various defense responses to resistance to several biotrophic and necrotrophic pathogens. While the model above seems generally correct, there are exceptions and additional complexities.
INTRODUCTION
Plants are subject to attack by a wide variety of microbial pathogens and insect herbivores. In response, they express numerous defense mechanisms, many of which are induced by pathogen attack. Appropriate regulation of defense responses is important for plant fitness, as activation of defense responses has deleterious effects on plant growth.
The molecular mechanisms underlying activation of plant defense responses are exceedingly complex. Responses often begin with gene-for-gene recognition of the pathogen. Production of certain virulence effectors by the pathogen leads to their recognition by plants that carry corresponding Resistance, or R genes. Recognition results in rapid activation of defense responses and consequent limitation of pathogen growth. R gene-mediated resistance is usually accompanied by an oxidative burst, that is, rapid production of reactive oxygen species (ROS). ROS production is required for another component of the response, hypersensitive cell death (HR), a type of programmed cell death thought to limit the access of the pathogen to water and nutrients. R gene-mediated resistance is also associated with activation of a salicylic acid (SA)-dependent signaling pathway that leads to expression of certain pathogenesis-related (PR) proteins thought to contribute to resistance. Some other plant defense responses are controlled by mechanisms dependent on ethylene (ET) and/or jasmonates (JA). These responses show considerable overlap with responses to wounding, which are also under ethylene and/or jasmonate control. SA, JA, and ET signaling pathways interact extensively. SA and JA are mutually inhibitory for the expression of many genes. Induced expression of some genes requires both ET and JA, whereas expression of other genes requires only one of...