L.19-G.Biology
Mycology
D.Ebtihal Muiz
Metabolic Defense Induced by the Attacking Pathogen
Biochemical Inhibitors Produced in Plants in Response to
Injury by the Pathogen :
Plant cells and tissues respond to injury , whether caused by a
pathogen or a mechanical or chemical agent , through a series of
biochemical reactions that seem to be aimed at isolating the irritant
and at healing the wound .
Defense through the Hypersensitive Reaction
The hypersensitive reaction is one of the most important
defense mechanisms in plants . It occurs only in incompatible
combinations of host plant and fungi , bacteria , viruses , and
nematodes . In such combinations , no difference is observable in the
manner of penetration of the epidermis in susceptible and resistant
plants . After infection , however , infected cells in resistant varieties
rapidly lose turgor , turn brown , and die , while infected cells of
susceptible varieties survive considerably longer . In resistant
varieties a number of physiological changes occur in the
infected cells and in the cells surrounding them , while in
susceptible varieties such changes either do not occur or
they occur at a much slower rate . Such changes in
hypersensitive reactions include loss of permeability of cell
membranes , increased respiration , accumulation and
oxidation of phenolic compounds , and production of
phytoalexins . The end result of the intense mobilization of
internal reactions in these cells is always death and collapse of the
infected and a few surrounding cells . Fungal and bacterial pathogens
within the area of the hypersensitive reaction are isolated by necrotic
tissue and quickly die . In virus diseases , the hypersensitive reaction
always results in formation of the so-called local lesions in which the
virus may survive for considerable time but is , generally , found in
low concentrations , and its spread beyond the lesion is as a rule
checked .
Defense through Increased Levels of Phenolic Compounds
Some of the phenolics implicated in disease resistance occur
widely in plants and are found in healthy as well as diseased plants ,
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but their synthesis or accumulation seems to be accelerated after
infection . Such compounds may be called " common " phenolic
compounds . Certain other phenolics, however , are not present in
healthy plants but are produced upon stimulation-by a pathogen or
by a mechanical or chemical injury . Such compounds are known as
phytoalexins .
" Common " phenolic :
It has often been observed that certain " common " phenolic
compounds that are toxic to pathogens are produced and accumulate
at a faster rate after infection in a resistant variety of plant than in a
susceptible variety . Chlorogenic acid , caffeic acid , and scopoletin
are examples of such phenolic compounds ( Figure 5 - 8 ) . Although
some of the common phenolics may each reach concentrations that
could be toxic to the pathogen , it should be noted that several of
them appear concurrently in the same diseased tissue , and it is
possible that the combined toxic effect of all fungitoxic phenolics
present , rather than that of each one separately , is responsible for
the inhibition of infection in resistant varieties .
Phytoalexins :
Phytoalexins are toxic substances produced in appreciable
amounts in plants only after stimulation by the various types of
phytopathogenic microorganisms or by chemical and mechanical
injury . Phytoalexins are produced by healthy cells adjacent to
localized damaged and necrotic cells in response to materials
diffusing from the damaged cells . Phytoalexins are not produced
during biotophic infections . Phytoalexins accumulate around both
resistant and susceptible necrotic tissues . Resistance occurs when
one or more phytoalexins reach a concentration sufficient to restrict
pathogen development .
The Role of Induced Synthesis of Enzymes
Pathogen attacks on plants appear to induce alterations in
enzyme synthesis in the plant that can lead to the development of
various degrees of resistance around infection sites . An enzyme that
generally exhibits increased activity or greater new synthesis in
diseased tissues is phenylalanine ammonia lyase ( PAL ) . PAL is a
key enzyme in the production of the basic molecule used for the
biosynthesis of most phenolics , including phytoalexins and lignin .
The resistance of plants to the pathogen may depend on the speed
and extent of synthesis of one or more enzymes induced in the host by
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the pathogen or closely related nonpathogens . This type of defense
seems to be related to that afforded by increased amounts of common
phenolics , phytoalexins , and other compounds , the additional
enzymes being those required for synthesis of phytoalexins , although
it is possible that the two mechanisms operate separately .
Defense through Formtion of Substrates Resisting the
Enzymes of the Pathogen
Plant resistance to some pathogens is apparently a result of the
presence or appearance of compounds that are not easily degraded
by the enzymes of pathogens attempting to invade the plant . These
compounds are usually complexes between pectins , pr0teins , and
polyvalent cations such as calcium or magnesium . The availability or
accumulation of either cation near the infection results in formation
of pectin salts or other complexes that resits degradation by the
pathogen enzymes . Thus they inhibit tissue maceration and confine
the pathogen to lesions of limited size . The effectiveness of
impregnation of cell walls and papillae with ligninlike substances
against invading pathogens was mentioned previously .
Defense through Inactivation of Pathogen Enzymes
Several phenolic compounds or their oxidation products seem
to induce resistance to disease though their inhibitory action on the
pectolytic and other enzymes of the pathogen rather than on the
pathogen itself . In some diseases the more resistant the varieties or
the tissues , the higher is their content in polyphenols , and although
these phenols do not inhibit the growth of the pathogen , they do
inhibit the activity of its pectionlytic enzymes and apparently
contribute to the resistance of the plant . The resistance of many
young immature fruit to fungal infection , for example m apple to
Monilinia and grape to Botrytis m seems to be the result of their high
content content of such enzyme-inhibiting phenolic compounds The
polygalacturonase of the fungus Diplodia , causing post-harvest
decay of oranges , is inhibited by a protein produced by cells in the
rind , but the role of this protein in disease resistance has not been
established .
The Role of Induced Synthesis of Enzymes
Pathogen attacks on plants appear to induce alterations in
enzyme synthesis in the plant that can lead to the development of
various degrees of resistance around infection sites . An enzyme that
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generally exhibits increased activity or greater new synthesis in
diseased tissues is phenylalanine ammonia lyase ( PAL ) . PAL is a
key enzyme in the production of the basic molecule used for the
biosynthesis of most phenolics , including phytoalexins and lignin .
The resistance of plants to the pathogen may depend on the speed
and extent of synthesis of one or more enzymes induced in the host by
the pathogen or closely related nonpathogens . This type of defense
seems to be related to that afforded by increased amounts of common
phenolics , phytoalexins , and other compounds , the additional
enzymes being those required for synthesis of phytoalexins , although
it is possible that the two mechanisms operate separately .
Defense through Formtion of Substrates Resisting the
Enzymes of the Pathogen
Plant resistance to some pathogens is apparently a result of the
presence or appearance of compounds that are not easily degraded
by the enzymes of pathogens attempting to invade the plant . These
compounds are usually complexes between pectins , pr0teins , and
polyvalent cations such as calcium or magnesium . The availability or
accumulation of either cation near the infection results in formation
of pectin salts or other complexes that resits degradation by the
pathogen enzymes . Thus they inhibit tissue maceration and confine
the pathogen to lesions of limited size . The effectiveness of
impregnation of cell walls and papillae with ligninlike substances
against invading pathogens was mentioned previously .
Defense through Inactivation of Pathogen Enzymes
Several phenolic compounds or their oxidation products seem
to induce resistance to disease though their inhibitory action on the
pectolytic and other enzymes of the pathogen rather than on the
pathogen itself . In some diseases the more resistant the varieties or
the tissues , the higher is their content in polyphenols , and although
these phenols do not inhibit the growth of the pathogen , they do
inhibit the activity of its pectionlytic enzymes and apparently
contribute to the resistance of the plant . The resistance of many
young immature fruit to fungal infection , for example m apple to
Monilinia and grape to Botrytis m seems to be the result of their high
content content of such enzyme-inhibiting phenolic compounds The
polygalacturonase of the fungus Diplodia , causing post-harvest
decay of oranges , is inhibited by a protein produced by cells in the
rind , but the role of this protein in disease resistance has not been
established .
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