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Chapter 21

Responses to
Plant Pathogens

21.1 Ways in which plant pathogens cause disease
21.2 Plant defense systems
21.3 Genetic basis of plant–pathogen interactions
21.4 R genes and R gene–mediated disease resistance
21.5 Biochemistry of plant defense reactions
21.6 Systemic plant defense responses
21.7 Control of plant pathogens by genetic engineering


Kim Hammond-Kosack
Jonathan D.G. Jones



Plants must continuously defend themselves against attack from bacteria, viruses, fungi, invertebrates, and even other plants. Because their immobility precludes escape, each plant cell possesses both a preformed and an inducible defense capacity. This is in striking contrast to the vertebrate immune system, in which specialized cells devoted to defense are rapidly mobilized to the infection site, where they either kill the invading organism or limit its spread. The noncirculatory defense strategy of the plant nevertheless minimizes infections. In wild plant populations, most plants are healthy most of the time; if disease does occur, it is usually restricted to only a few plants and affects only a small amount of tissue (Fig. 21.1). Disease, the outcome of a successful infection, rarely kills a plant. Natural selection probably acts to curtail fatal pathogen toxicity to plants; after all, diseases that keep a host alive longer may permit more reproduction of pathogen.
      Why study the interactions between plants and pathogenic organisms? There are three main reasons. First, a detailed study of plant–microbe interactions should provide sustainable practical solutions for the control of plant disease in agricultural crops. Growing monocultures of genetically uniform crop species over vast tracts of land is a practice that frequently leads to severe outbreaks of disease; such epidemics lower both crop yield and quality and can diminish the safety of the end product (Fig. 21.2). In addition, the use of agro-chemicals to control plant disease can cause serious pollution and increase the costs of production. Second, such studies should help elucidate the signaling mechanisms by which plant cells cope with a stress situation. For example, do plant responses provoked by the invasion of a pathogenic organism differ from those provoked by mechanical wounding or by the stresses of low temperature, high salinity, or desiccation? Third, study of plant–pathogen interactions can lead us to discover how organisms from different kingdoms communicate with one another. What type of messages do they exchange and how are appropriate responses evoked?
      In this chapter we examine the biochemical and molecular mechanisms by which plants defend themselves against attack from microbial pathogens and invertebrate pests. To begin, we will look at the numerous strategies utilized by pathogenic organisms for a successful invasion of plant tissue. A plant pathogen is defined as an organism that, to complete a part or all of its life cycle, grows inside the plant and in so doing has a detrimental effect on the plant.

Figure 21.1
The leaf mold fungus Cladosporium fulvum sporulating through the lower surface of tomato leaves, 11 days after infection. This biotrophic pathogen is restricted to attacking only a few plant species of the genus Lycopersicon.


Figure 21.2
Sugar beet–root nematode interaction. (A) The central rows of sugar beet show severe damage from the endoparasitic cyst nematode Heterodera schachtii. (B) Mature female nematode bodies filled with eggs, attached to the sugar beet roots at the end of the pathogen’s life cycle.

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