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Chapter 21

Responses to
Plant Pathogens

 

Plants are resistant to most plant pathogens. Every plant cell can defend itself from attacking pathogenic microorganisms and invertebrates. Some defenses, such as antimicrobial secondary metabolites, are constitutive, being located in specific cellular compartments and ready to be released on cell damage. Other defense responses, such as those induced by pathogen invasion, require detection of the pathogen by the plant. Defense activation is correlated with rapid activation of defense-related genes and often culminates in the HR, localized cell death, to impair pathogen spread. Plant resistance to pathogens can be mediated by dominant resistance (R) genes in plants that are complementary to avirulence (Avr) genes in pathogens. Avr proteins exhibit extensive sequence diversity, and their function in the pathogen is poorly understood. In contrast, plant R proteins are strikingly similar in structure, sharing such motifs as LRRs, a central NBS, and a serine/ threonine protein kinase domain, among others. R proteins both detect pathogens and initiate signal transduction to activate defense mechanisms. In addition, R loci/genes can evolve new R gene specificities to keep pace with the evolution of virulence in pathogen populations. Plant defense reactions involve complex biochemical pathways and multiple signal molecules, including ROS, NO, SA, JA, and ethylene, to provoke the induction of antifungal proteins, secondary metabolites, and cell wall fortification reactions—both at the infection site and systemically throughout the attacked plant. Specialist defenses against plant viruses include PTGS; those against insects involve PI proteins. Many aspects of induced plant defense appear to be conserved in other eukaryotes, perhaps indicating the existence of an ancient defense strategy against microbial attack. The genetic engineering of plants has started to achieve broad-spectrum and durable pest and pathogen control in crops. Still needed are a better understanding of the factors/processes involved and better ways to manipulate resistance mechanisms so as to reduce crop losses.


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