Plants are subject to attach and infection by a
remarkable variety of symbiotically species and have evolved a diverse array of
mechanisms designed to frustrate the potential colonists. These can be divided
into preformed or passive defense mechanisms and inducible or active systems.
Passive plant defense comprises physical and chemical barriers that prevent
entry of pathogens(病原体), such as bacteria, or render tissues unpalatable or
toxic to the invader. The external surfaces of plants, in addition to being
covered by an epidermis(表皮层) and a waxy cuticle, often carry spiky hairs known
as trichomes(毛状物), which either prevent feeding by insects or may even puncture
and kill insect larvae. Other trichomes are sticky and glandular and effectively
trap and immobilize insects. If the physical barriers of the
plant are breached, then preformed chemicals may inhibit or kill the intrude,
and plant tissues contain a diverse array of toxic or potentially toxic
substances, such as resins, tannins, glycosides, and alkaloids, many of which
are highly effective deterrents to insects that feed on plants. The success of
the Colorado beetle in infesting potatoes, for example, seems to be correlated
with its high tolerance to alkaloids that normally repel potential pests. Other
possible chemical defenses, while not directly toxic to the parasite, may
inhibit some essential step in the establishment of a parasitic relationship.
For example, glycoproteins(糖蛋白) in plant cell walls may inactivate enzymes that
degrade cell walls. These enzymes are often produced by bacteria and
fungi. Active plant defense mechanisms are comparable to the
immune system of vertebrate animals, although the cellular and molecular bases
are fundamentally different. Both, however, are triggered in reaction to
intrusion, implying that the host has some means of recognizing the presence of
a foreign organism. The most dramatic example of an inducible plant defense
reaction is the hypersensitive response. In the hypersensitive response, cells
undergo rapid necrosis—that is, they become diseased and die—after being
penetrated by a parasite; the parasite itself subsequently ceases to grow and is
therefore restricted to one or a few cells around the entry site. Several
theories have been put forward to explain the basis of hypersensitive
resistance. Why does the author mention "glycoproteins" in Line 16
A. To compare plant defense mechanisms to the immune system of
animals.
B. To introduce the discussion of active defense mechanisms in plants.
C. To illustrate how chemicals function in plant defense.
D. To show the advantages of plant defense mechanisms.