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Summary BOT 2601 LEARNING UNIT 7
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- University Of South Africa (Unisa)
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BOT 2601 LEARNING UNIT 7DEFENCE MECHANISMS AND STRUCTURAL RESPONSES OF PLANTS TO DISEASES, PESTS AND MECHANICAL INJURY
Textbook reference: chapter 10 pg. 357-371
Diseases are any disturbances that cause the breaking down of plant metabolism.
Causes of disease can be parasitic or non-parasitic
An organism that lives in or on another individual for part or all of its life, securing its food directly from the
tissues of the host, but not benefitting the host in any way is a parasite
THE NATURE OF PLANT DEFENCES:
o Plant defences can be passive, in the sense that they are pre-existing barriers to infection
o Examples of passive, preinfectional structural defences are the cell walls and outer surface covering of the plant
o The multi-layered cuticle provides an effective barrier to invasion by pathogenic microbes. Some fungi produce
extracellular enzymes capable of hydrolysing cutin. The enzymes are called cutinases
o Resin is secreted as a defence barrier
o Trichomes and thorns minimise invasion of some pathogens
o Bark of woody plants provides a barrier to infection
o Active mechanisms of resistance, referred to as a postinfectional host response, are changes induced in cells and
tissues in direct response to injury
RESPONSES OF PLANTS TO WOUNDING AND INVASION BY MICROORGANISMS:
o The principal anatomical response of plants to the formation of wounds and infection is to wall off the injured or
diseased region in an effort to contain the damage
o This restricts the movement of pathogens to surrounding tissues
o The walling off process is called compartmentalization and occurs in the primary tissues of stems and leaves
o The compartmentalisation of infection is a continuous process and is followed by regeneration of new tissues
o Plants survive only as long as they are able to successfully wall off the spread of infection
o Tissues are modified to form structural and chemical barriers. One of these modifications is the ability to alter
cell wall structure.
o The activation of genes coding for cell wall proteins following wounding or pathogenic infection results in rapid
polymerization of proline-rich proteins that form a tightly bound wall to render the wall indigestible by invading
pathogens
o Lignification and suberization forms a resistance barrier
o The addition of suberin to walls of wounded tissues seals of exposed surfaces and helps replace the damaged
original tissue layers
o The carbohydrate callose is formed in cells in response to injury and plugs the site of a wound.
o Protective barriers may also form as a result of meristematic activity. Meristematic cells are initiated from
parenchymatous cells and form a mass of protective callus by cell division and enlargement. The walls of callus
cells typically become filled with suberin or lignin
o A periderm wound stimulates more layers to be created, pushing peripheral tissue outwards and moving the
wound outwards
Abscission:
During normal growth, plants shed organs and form exposed wounds on the stem
These wounds are liable to infection immediately following abscission if they are not sealed or isolated
Abscission results in the separation of cells along a narrow band at a predetermined location at the base of the
organ to be shed.
The hydrolytic enzymes involved in cell wall softening and breakdown within the site of organ detachment are
known as cellulases
The plant growth regulator ethylene promotes abscission, whereas the hormone auxin retards the separation
process
A narrow transverse plane of cells forms at the base of the petiole, known as the abscission zone
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