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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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