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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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