Pathogenesis Level-3 Semester-2

1. Lecture Notes
For Level-3 Semester-2
Prepared by
Prof. Dr. Md. Rashidul Islam
Department of Plant Pathology
Bangladesh Agricultural University
Mymensingh

2.  The first contact of pathogens with their host plants occurs at a
plant surface. Aerial plant part surfaces consist primarily of cuticle
and/or cellulose, whereas root cell wall surfaces consist only of
cellulose.
 Cuticle consists primarily of cutin, more or less impregnated
with wax and frequently covered with a layer of wax. The lower
part of cutin is intermingled with pectin and cellulose lamellae and
lower yet there is a layer consisting predominantly of pectic
substances; below that there is a layer of cellulose.
 Polysaccharides of various types are often found in cell walls.
 Proteins of many different types, both structural, e.g., elastin,
which helps loosen the cell wall, and extensin, which helps add
rigidity to the cell wall, some enzymes, and some signal molecules
that help receive or transmit signals inward or outward, are normal
constituents of cell walls.
Finally, epidermal cell walls may also contain suberin and lignin.
 Enzymes in Pathogenesis

3.  The penetration of pathogens into parenchymatous tissues is
facilitated by the breakdown of the internal cell walls, which
consist of cellulose, pectins, hemicelluloses, and structural
proteins, and of the middle lamella, which consists primarily of
pectins.
 In addition, complete plant tissue disintegration involves the
breakdown of lignin.
 The degradation of each of the above mentioned substances
is brought about by the action of one or more sets of enzymes
secreted by the pathogen.

4. Cutinases
 Schematic representation of the structure and composition of the
cuticle and cell wall of foliar epidermal cells along with the enzymes
involve in degradation of these components

5.  Schematic diagram of morphology and arrangement of some cell wall
components along with the respective enzymes responsible for the
degradation of the cell wall components
Hemicellulase
Cellulase
Proteases
Pectate lyase
Proteases
Cellulase
Hemicellulase
Cellulase

6.  Cutinase
 Cutin is the main component of the cuticle. The upper part of the
cuticle is admixed with waxes, whereas its lower part, in the region
where it merges into the outer walls of epidermal cells, is admixed
with pectin and cellulose.
 Many fungi and a few bacteria have been shown to produce
cutinases and/or nonspecific esterases, i.e., enzymes that can
degrade cutin.
 Cutinases break cutin molecules and release monomers (single
molecules) as well as oligomers (small groups of molecules) of the
component fatty acid derivatives from the insoluble cutin polymer.

7.  The involvement of cutinase in the penetration of the host
cuticle by plant pathogenic fungi is shown by several facts. For
example, the enzyme reaches its highest concentration at the
penetrating point of the germ tube and at the infection peg of
appressoriumforming fungi.
 Pathogens that produce higher levels of cutinase seem to be
more virulent than others.
e. g. The germinating spores of a virulent isolate of the fungus
Fusarium produced much more cutinase than those of an
avirulent isolate of the same fungus.
The fungus Botrytis cinerea, the cause of numerous types of
diseases on many plants, produces a cutinase and a lipase, both
of which break down cutin.

8. Diagrammatic representation of cuticle penetration by a
germinating fungus spore. Constitutive cutinase releases a few
cutin monomers from the plant cuticle. These trigger expression
of the cutinase genes of the fungus, leading to the production of
more cutinase(s), which macerates the cuticle and allows
penetration by the fungus.

9.  Pectinases
 Pectic substances constitute the main components of the middle
lamella, i.e., the intercellular cement that holds in place the cells of
plant tissues.
 Pectic substances also make up a large portion of the primary cell
wall in which they form an amorphous gel filling the spaces
between the cellulose microfibrils.
 Several enzymes degrade pectic substances and are known as
pectinases or pectolytic enzymes. Some of them, e.g., the pectin
methyl esterases, remove small branches off the pectin chains.
 Pectin-degrading enzymes have been shown to be involved in the
production of many fungal and bacterial diseases, particularly
those characterized by the soft rotting of tissues. Various
pathogens produce different sets of pectinases and their isozymes.

10. e.g.
 In some Colletotrichum-caused anthracnoses, the fungus
produces one pectin lyase that is a key virulence factor in disease
development.
 In some diseases, e.g., the bacterial wilt of solanaceous crops
caused by Ralstonia solanacearum, pectinolytic enzymes
collectively are absolutely essential for disease to develop,
although some of them individually seem to not be required for
disease but rather for accelerated colonization and enhanced
aggressiveness by bacteria.
 Pectin-degrading enzymes are produced and play a role in the
ability of nematodes, such as the root knot nematode, Meloidogyne
javanica, for the penetration of root tissues, movement between
plant cells along the middle lamella, and possibly in the formation
of tee multinucleate giant cells on which the nematode feeds
throughout the rest of its life.

11.  Cellulases
 Cellulose is also a polysaccharide, but it consists of chains of
glucose (1-4) β-d-glucan molecules. The glucose chains are held to
one another by a large number of hydrogen bonds. Cellulose
occurs in all higher plants as the skeletal substance of cell walls in
the form of microfibrils.
 The enzymatic breakdown of cellulose results in the final
production of glucose molecules. The glucose is produced by a
series of enzymatic reactions carried out by several cellulases and
other enzymes.
 Cellulose-degrading enzymes (cellulases) have been shown to be
produced by several phytopathogenic fungi, bacteria, and
nematodes and are undoubtedly produced by parasitic higher
plants.

12.  Saprophytic fungi, mainly certain groups of basidiomycetes,
and, to a lesser degree, saprophytic bacteria cause the
breakdown of most of the cellulose decomposed in nature.
• In living plant tissues, cellulolytic enzymes secreted by
pathogens play a role in the softening and disintegration of cell
wall material. They facilitate the penetration and spread of the
pathogen in the host and cause the collapse and disintegration
of the cellular structure, thereby aiding the pathogen in the
production of disease.
• Cellulolytic enzymes may further participate indirectly in
disease development by releasing, from cellulose chains,
soluble sugars that serve as food for the pathogen and, in the
vascular diseases, by liberating into the transpiration stream
large molecules from cellulose, which interfere with the normal
movement of water.
• In the bacterial wilt of tomato, production of an endocellulase
by the bacterium was required for the latter to be pathogenic
and induce the disease.

13.  Hemicellulases
 Cross-linking glycans, known earlier as hemicelluloses, are
complex mixtures of polysaccharide polymers that can
hydrogen-bond to and may cover and link cellulose microfibrils
together.
 Their composition and frequency seem to vary among plant
tissues, plant species, and with the developmental stage of the
plant.
 Cross-linking glycans are a major constituent of the primary
cell wall and may also make up a varying proportion of the
middle lamella and secondary cell wall.
 Hemicellulosic polymers include primarily xyloglucans and
glucuronoarabinoxylans, but also glucomannans,
galactomannans, arabinogalactans, and others.

14.  The enzymatic breakdown of hemicelluloses appears to
require the activity of many enzymes. Several hemicellulases
seem to be produced by many plant pathogenic fungi. e. g. Rice
blast fungus, Magnaporthe oryae.
 Depending on the monomer released from the polymer on
which they act, the particular enzymes are called xylanase,
galactanase, glucanase, arabinase, mannase, and so on. e.g.
Xylanases secreted by Botrytis cinerea.

15.  Lignin degrading enymes
 Lignin is found in the middle lamella, as well as in the secondary
cell wall of xylem vessels and the fibers that strengthen plants. It is
also found in epidermal and occasionally hypodermal cell walls of
some plants. The lignin content of mature woody plants varies
from 15 to 38% and is second only to cellulose in abundance.
 It is generally accepted, however, that only a small group of
microorganisms is capable of degrading lignin.

16.  Actually, only about 500 species of fungi, almost all of them
basidiomycetes, have been reported so far as being capable of
decomposing wood. About one-fourth of these fungi (the brown
rot fungi) seem to cause some degradation of lignin but cannot
utilize it.
e. g.
 Most of the lignin in the world is degraded and utilized by a
group of basidiomycetes called white rot fungi. It appears that
white rot fungi secrete one or more enzymes (ligninases), which
enable them to utilize lignin.
 In addition to wood-rotting basidiomycetes, several other
pathogens, primarily several ascomycetes and imperfect fungi
and even some bacteria, apparently produce small amounts of
lignin-degrading enzymes and cause soft rot cavities in wood
they colonize.

17.  Proteases/proteinases/peptidases
 Plant cells contain innumerable different proteins, which play
diverse roles as catalysts of cellular reactions (enzymes) or as
structural material (in membranes and cell walls). Proteins are
formed by the joining together of numerous molecules of about 20
different kinds of amino acids.
 All pathogens seem to be capable of degrading many kinds of
protein molecules. The plant pathogenic enzymes involved in
protein degradation are similar to those present in higher plants
and animals and are called proteases or proteinases or,
occasionally, peptidases.
 Considering the paramount importance of proteins as enzymes,
constituents of cell membranes, and structural components of
plant cell walls, the degradation of host proteins by proteolytic
enzymes secreted by pathogens can profoundly affect the
organization and function of the host cells.

18.  Amylases
 Starch is the main reserve polysaccharide found in plant
cells. Starch is synthesized in the chloroplasts and, in
nonphotosynthetic organs, in the amyloplasts.
 Starch is a glucose polymer and exists in two forms:
amylose, an essentially linear molecule, and amylopectin, a
highly branched molecule of various chain lengths.
 Most pathogens utilize starch, and other reserve
polysaccharides, in their metabolic activities. The degradation
of starch is brought about by the action of enzymes called
amylases. The end product of starch breakdown is glucose and
it is used by the pathogens directly.

19.  Lipases & phospholipases
 Various types of lipids occur in all plant cells, with the most
important being phospholipids and glycolipids, both of which,
along with protein, are the main constituents of all plant cell
membranes.
 Oils and fats are found in many cells, especially in seeds where
they function as energy storage compounds; wax lipids are found
on most aerial epidermal cells. The common characteristic of all
lipids is that they contain fatty acids, which may be saturated or
unsaturated.

20.  Several fungi, bacteria, and nematodes are known to be
capable of degrading lipids. Lipolytic enzymes, called lipases,
phospholipases, and so on, hydrolyze liberation of the fatty
acids from the lipid molecule. The fatty acids are presumably
utilized by the pathogen directly.
 Some of them, before or after hyperoxidation by plant
lipoxygenases or active oxygen species, provide signal
molecules for the development of plant defenses and also act
as antimicrobial compounds that inhibit the pathogen directly.

21.  Pathogenic effects on physiological functions of plants
1. Effect of pathogens on photosynthesis
2. Effect of pathogens on translocations of water and nutrients
in the host plant
a. Interference with upward translocation of water and
inorganic nutrients
b. Effect on absorption of water by roots
c. Effect on translocation of water through the xylem
d. Effect on Transpiration
e. Interference with translocation of organic nutrients
through the phloem
3. Effect of pathogens on host plant respiration
4. Effect of pathogens on permeability of cell membranes
5. Effect of pathogens on transcriptions and translations
6. Effect of pathogens on plant growth
7. Effect of pathogens on plant reproduction

22. 1. Effect of pathogens on photosynthesis
 In leaf spot, blight, and other kinds of diseases in which there is
destruction of leaf tissue, e.g., in cereal rusts and fungal leaf
spots, bacterial leaf spots, viral mosaics and yellowing and
stunting diseases or in defoliations, photosynthesis is reduced
because the photosynthetic surface of the plant is lessened.
 In some fungal and bacterial diseases, photosynthesis is
reduced because the toxins, such as tentoxin and tabtoxin,
produced by these pathogens inhibit some of the enzymes that
are involved directly or indirectly in photosynthesis.
 In plants infected by many vascular pathogens, stomata remain
partially closed, chlorophyll is reduced, and photosynthesis
stops even before the plant eventually wilts.
 Most virus, mollicute, and nematode diseases also induce
varying degrees of chlorosis and stunting. In the majority of
such diseases, the photosynthesis of infected plants is reduced
greatly.

23. 2. Effect of pathogens on translocations of water and nutrients in
the host plant
a. Interference with upward translocation of water and inorganic
nutrients
Many plant pathogens interfere in one or more ways with the
translocation of water and inorganic nutrients through plants.:
 Some pathogens affect the integrity or function of the roots,
causing them to absorb less water;
 Other pathogens, by growing in the xylem vessels or by other
means, interfere with the translocation of water through the stem;
and,
 In some diseases, pathogens interfere with the water economy
of the plant by causing excessive transpiration through their
effects on leaves and stomata.

24. b. Effect on absorption of water by roots
Root injury affects the amount of functioning roots directly and
decreases proportionately the amount of water absorbed by the
roots in many ways such as:
 Many pathogens, such as damping-off fungi, root-rotting fungi
and bacteria, most nematodes, and some viruses, cause an
extensive destruction of the roots before any symptoms appear on
the aboveground parts of the plant and some bacteria and
nematodes cause root galls or root knots, which interfere with the
normal absorption of water and nutrients by the roots.
 Some vascular parasites, along with their other effects, seem to
inhibit root hair production, which reduces water absorption.
 These and other pathogens also alter the permeability of root
cells, an effect that further interferes with the normal absorption of
water by roots.

25. c. Effect on Translocation of Water through the Xylem
Fungal and bacterial pathogens that cause damping off, stem
rots and cankers may reach the xylem vessels in the area of the
infection and inhibits the translocation of water through xylem
vessels.
The most typical and complete dysfunction of xylem in
translocating water, however, is observed in the vascular wilts
caused by the fungi Ceratocystis, Ophiostoma, Fusarium, and
Verticillium and bacteria such as Pseudomonas, Ralstonia, and
Erwinia. In vascular wilts, affected vessels may be filled with the
bodies of the pathogen and with substances secreted by the
pathogen or by the host in response to the pathogen and may
become clogged.
Certain gall forming pathogens such as Agrobacterium
tumefaciens, Plasmodiophora brassicae), and Meloidogyne sp.,
induce gall formation in the stem, roots, or both. The enlarged
and proliferating cells near or around the xylem exert pressure
on the xylem vessels, which may be crushed and dislocated,

26. d. Effect on Transpiration
In plant diseases in which the pathogen infects the leaves,
transpiration is usually increased. This is the result of destruction
of at least part of the protection afforded the leaf by the cuticle, an
increase in the permeability of leaf cells, and the dysfunction of
stomata.
If water absorption and translocation cannot keep up with the
excessive loss of water, loss of turgor and wilting of leaves follow.
The suction forces of excessively transpiring leaves are increased
abnormally and may lead to collapse or dysfunction of underlying
vessels through the production of tyloses and gums. In diseases
such as rusts, in which numerous pustules form and break up the
epidermis, in most leaf spots, in which the cuticle, epidermis, and all
the other tissues, including xylem, may be destroyed in the infected
areas, in the powdery mildews, in which a large proportion of the
epidermal cells are invaded by the fungus, and in apple scab, in
which the fungus grows between the cuticle and the epidermis-in all
these examples, the destruction of a considerable portion of the
cuticle and epidermis results in an uncontrolled loss of water from
the affected areas.

27. e. Interference with translocation of organic nutrients through the
phloem
In stem diseases of woody plants in which cankers develop, the
pathogen attacks and remains confined to the bark for a considerable
time. During that time the pathogen attacks and may destroy the
phloem elements in that area, thereby interfering with the downward
translocation of nutrients.
In diseases caused by phytoplasmas, as well as in diseases caused
by phloem-limited fastidious bacteria, bacteria exist and reproduce in
the phloem sieve tubes, thereby interfering with the downward
translocation of nutrients.
In some virus diseases, particularly the leaf-curling type and some
yellows diseases, starch accumulation in the leaves is mainly the
result of degeneration (necrosis) of the phloem of infected plants,
which is one of the first symptoms. It is also possible, however, at
least in some virus diseases, that the interference with translocation
of starch stems from inhibition by the virus of the enzymes that break
down starch into smaller, translocatable molecules

28. 3. Effect of pathogens on host plant respiration
When plants are infected by pathogens, the rate of respiration
generally increases. This means that affected tissues use up their
reserve carbohydrates faster than healthy tissues would. The
increased rate of respiration appears shortly after infection-
certainly by the time of appearance of visible symptoms and
continues to pathogen. After that, respiration declines to normal
levels or to levels even lower than those of healthy plants.
Several changes in the metabolism of the diseased plant
accompany the increase in respiration after infection. Thus, the
activity or concentration of several enzymes of the respiratory
pathways seems to be increased.
Increased respiration in diseased plants is also accompanied by
an increased activation of the pentose pathway, which is the main
source of phenolic compounds.
Increased respiration is sometimes accompanied by considerably
more fermentation than that observed in healthy plants, probably
as a result of an accelerated need for energy in the diseased plant
under conditions in which normal aerobic respiration cannot
provide sufficient energy.

29. 4. Effect of pathogens on permeability of cell membranes
Membranes function as permeability barriers that allow passage
into a cell only of substances the cell needs and inhibit passage
out of the cell of substances needed by the cell.
Changes in cell membrane permeability are often the first
detectable responses of cells to infection by pathogens, to most
host-specific and several nonspecific toxins, to certain pathogen
enzymes, and to certain toxic chemicals, such as air pollutants.
The most commonly observed effect of changes in cell membrane
permeability is the loss of electrolytes,.
Disruption or disturbance of the cell membrane by chemical or
physical factors alters (usually increases) the permeability of the
membrane with a subsequent uncontrollable loss of useful
substances, as well as the inability to inhibit the inflow of
undesirable substances or excessive amounts of any substances.

30. 5. Effect of pathogens on transcriptions and translations
a. Effect on Transcription
Several pathogens, particularly viruses and fungal obligate
parasites, such as rusts and powdery mildews, affect the
transcription process in infected cells.
In some cases, pathogens affect transcription by changing the
composition, structure, or function of the chromatin associated
with the cell DNA and in some diseases, especially those caused
by viruses, the pathogen, through its own enzyme or by modifying
the host enzyme (RNA polymerase) that makes RNA.
b. Effect on Translation
Infected plant tissues often have increased activity in several
enzymes and affect the :
Increases in protein synthesis in infected tissues have been
observed primarily in hosts resistant to the pathogen and reach
their highest levels in the early stages of infection
Much of the increased protein synthesis in plants attacked by
pathogens reflects the increased production of enzymes and other
proteins involved in the defense reactions of plants.

31. 6. Effect of pathogens on plant growth
Pathogens that destroy part of the photosynthetic area of plants
and cause significantly reduced photosynthetic output often
result in smaller growth of these plants and smaller yields.
Pathogens that destroy part of the roots of a plant or clog their
xylem or phloem elements, thereby severely interfering with the
translocation of water and of inorganic or organic nutrients in
these plants, often cause a reduction in size and yields by these
plants and, sometimes, their death.
In many plant diseases, however, infected tissues or entire plants
increase or reduce abnormally in size.
Some characteristic effects on plant growth are caused by the
phloem inhabiting phytoplasmas producing short and bushy
appearance (witches’ brooms). The most frequent and unusual
effects on plant growth are those caused by viruses (and viroids)
e. g. Stunting or dwarfing of infected plants, whereas others
cause rolling or curling of leaves, abnormally shaped fruit, etc.

32. 7. Effect of pathogens on plant reproduction
 Pathogens reduce the plants size, reduce flowers and fruit
setting and seeds which are of inferior vigor and vitality.
 Many pathogens have a direct adverse effect on plant
reproduction because they attack and kill the flowers, fruit, or
seed directly, or interfere and inhibit their production,
 Pathogens interfere with the reproduction of their host
-by infecting and killing the flowers of the host.
-by killing the embryo, that would have produced the seed, and
replacing the contents of the seed with its own fruiting
structure or its own spores.
 Finally, in some diseases caused by viruses, phytoplasmas,
or phloem limited bacteria, no flowers are produced or those
produced are sterile, and therefore few or no fruit and seed are
produced.