Study

1. Molecular Basis of Infection
Plant – Pathogen Interaction
• Plants exist in a world filled with bacteria, fungi, nematodes, and possibly
parasitic plants
• They may be inoculated with viruses during feeding by insects or by other
vectors
• Plant pathogens have made many adaptations to enable them to invade
plants, overcome plant defense mechanisms, and colonize plant tissues for
growth, survival, and reproduction.
• Pathogens accomplish these activities mostly through secretions of chemical
substances that affect certain components or metabolic mechanisms of their
hosts.
• Penetration and invasion, however, seem to be aided by or in some cases be
entirely the result of, the mechanical force exerted by certain pathogens on
the cell walls of the plant.
• H H Flor in 1946 developed the gene-for-gene concept to explain the
genetic interactions between Melampsora lini and flax (flax rust disease).
This concept provided the underpinnings for research on the genetics of
host-pathogen interactions for the next 70 years.

2. • The gene for gene hypothesis states that “for each gene
controlling resistance in the host, there is corresponding gene
controlling pathogenicity in the pathogen”. The resistance of
host is governed by dominant genes and virulence of pathogen
by recessive genes. The genotype of host and pathogen
determine the disease reaction.
• When genes in host and pathogen match for all loci, then only
the host will show susceptible reaction. If some gene loci
remain unmatched, the host will show resistant reaction. The
gene for gene hypothesis is also known as “Flor Hypothesis.”
• At molecular level, it is considered that gene for gene
resistance usually involves production of toxins, antibiotic
proteins by a resistance gene. The production of toxins is
related to gene dosage.
• The resistance controlled by domain gene is the most
desirable. Gene for gene relationship are rare or unknown for
disease caused by viruses, bacteria, Fusarium.

3. The Receptor-Elicitor Model of gene-for-gene interactions.
• The resistance allele of the plant encodes a receptor that recognizes an
elicitor produced by the pathogen. Recognition of the pathogen elicitor by the
plant receptor initiates plant defense responses that lead to plant resistance.
• If the pathogen produces the elicitor, it is avirulent.
• If the pathogen does not produce the elicitor, it is virulent

5. Mode of Infection
(A)Mechanical Forces Exerted on Host Tissues by Pathogens
• Viruses are usually introduced directly through the plant cells by insects
(BPH, WF, Aphids), Natural openings (stomata), wounding site, therefore
they do not exert mechanical forces to inter in the host.
• Many fungi are known to apply mechanical forces on the plant they are
about to attack. When fungal spores lands on a plant surface, and contact
is established, diameter of the tip of the hypha or radical in contact with the
host increases and forms the flattened, bulb-like structure called the
Appresorium.
• This increases the area of adherence between the two organisms and
securely fastens the pathogen to the plant.
• From the appresorium, a fine growing point, called the Penetration Peg
arises and advances into and through the cuticle and the cell wall.

6. (B) Chemical Weapons of Pathogens
• Some pathogens in plant are largely chemical in nature. Therefore, the
effects caused by pathogens on plants are almost entirely the result of
biochemical reactions taking place between substances (Hydrolytic
enzymes) secreted by the pathogen and those present in or produced by
the plant.
• The main groups of substances secreted by pathogens in plants that seem
to be involved in production of disease either directly or indirectly, are
 Enzymes
 Toxins
 Growth regulators and
 Polysaccharides (plugging substances).
• Enzymes are mainly involved in the degradation of plant cell wall and other
materials for gaining entry into the host tissues
• Toxins seem to act directly on protoplast components and interfere with the
permeability of its membrane and with its functions.
• Growth regulators exert a hormonal effect on the cells and their increase or
decrease their ability to divide and enlarge.
• Polysaccharides seem to play a role only in the vascular diseases, in which
they interfere passively with the translocation of water in the plants.

7. ENZYMES
Important enzymes produced by the plant
pathogens mainly fungi and few bacteria
are cutinases, pectinases and cellulases
a) Cutinases:
• 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 like Puccinia hordei,
Fusarium spp. and Botrytis cinerea
and few bacteria produce cutinases
and / or non-specific esterases which
are the enzymes capable of
degrading cultin
• Cutinases break down cutin
molecules into monomers and
oligomers of the component fatty acid
derivatives.

8. Structure of a Typical Plant Cell Wall

10. b) Cellulases:
• Cellulose is a polysaccharide, consisting of chains of glucose (1-4) β-D-
glucan molecules.
• In living plant tissues, cellulolytic enzymes secreted by pathogens play a role
in the softening and disintegration of cell wall material.
• To break cellulose into glucose monomers, a series of enzymatic reactions
are carried out by several cellulases and other enzymes produced by fungi
 Cellulase C1 attacks cellulose and breaks the cross linkages between
cellulose chains
 Cellulase C2 attacks native cellulose and breaks it into smaller chains
 These small chains are then attacked by a group of cellulases (Cx)
which degrade them into dimer cellobiose
 Finally, Cellobiase also called β-glucosidase degrades the cellobiose
into glucose
c) Hemicellulases:
• Many fungi produce different types of hemicellulases depending upon the
monomer released from the polymer on which they act eg. Xylanase,
Galactanase, Glucanase. Arabinase and Mannase etc.
• Some of the hemicelluloses are broken down by activated oxygen, hydroxyl
and other radicals produced by attacking fungi

11. d) Pectinases:
• Pectin substances constitute the main components of the middle lamella
i.e. the intercellular cement that holds in place the cells of plant tissues.
• These are the polysaccharides made up of chains of galacturonan
molecules interspersed with a much smaller number of rhamnose
molecules and small side chains of galacturonan, xylan and other C5
sugars
• Several enzymes degrade pectic substances and are known as
pectinases or pectolylic enzymes.
• Mainly three pectinases namely, pectin methyl esterases,
polygalacturonases and pectin lyases are produced by different fungi
• Pectic enzymes is pectin methyl-esterases, which removes small
branches off the pectin chains.
• The chain splitting pectinases called polygalacturonases. It splits the
pectic chain by adding a molecule of water and breaking the linkage
between two galacturonan molecules.
• Pectin lyases split the chain by removing a molecule of water from the
linkage, there by breaking it and releasing products with an unsaturated
double bond. Examples of pathogens include Ralstonia solanacearum,
Didymella bryoniae.

12. MICROBIAL TOXINS
• Toxins are metabolites that are produced by invading microorganisms and
act directly on living host protoplast, seriously damaging or killing the cells
of the plant.
• Some toxins act as a general protoplasmic poisons and affect many species
of plant representing different families.
• Others are toxic to only a few plant species or varieties and are completely
harmless to others.
• Many toxins exist in multiple forms that have different potency.
• Mainly, the microbial toxins are are of two types
 a) Non-host specific toxin
 c) Host specific toxin
a) Non-Host Specific Toxin Or Non Host-selective Toxins
• These are the toxins produced by any pathogen which act non specifically
against many host species. Important toxins included in this category are
Tab toxin, Phaseolo toxin, Ten toxin and Cercosporin
• 1) Tab-toxin:- It is produced by the bacterium Pseudomonas syringae pv
tabaci which causes the wildfire disease of tobacco. But it is also produced
by the strains of pv tabaci occurring on other hosts such as bean and
soybean and by other pathovars of P. syringae such as those occurring on
oats maize and coffee.

13. • (2) Phaseolo-toxin:- It is produced by the bacterium Pseudomonas
syringae pv phaseolicola, the cause of halo blight of bean and some other
legumes.
• (3) Ten-toxin:- It is produced by the fungus Alternaria alternata which
causes spots and chlorosis in plants of many species.
• (4) Cercosporin- It is produced by the fungus Cercospora. It causes
damaging leaf spot and blight diseases of many crop plants such as
Cercospora leaf spot of Zinnia and gray leaf spot of corn.
(B) Host Specific Or Host-selective Toxins
• These are the toxins which are produced by the pathogens in their specific
host and are effective against the same hosts. Important toxins under this
category are Victorin or HV toxin, T-toxin and HC toxin
• (1). Victorin or HV toxin – It is produced by the fungus Cochliobolus
(Helminthosporium) victoriae. This fungus infects the basal portions of
susceptible oat plants and produces a toxin that is carried to the leaves,
causes a leaf blight and destroys the entire plant.
• (2). T-toxin- It is produced by race T of Cochliobolus heterostrophus
(helminthosporium maydis), the cause of southern corn leaf blight. Race T
is indistinguishable from other all C. heterostrophus races except for its
ability to produce the T toxin.
• (3). HC-toxin- It is produced by Race 1 of Cochliobolus (Helminthosporium)
carbonum causing northern leaf spot and ear rot disease in maize.

14. GROWTH REGULATORS
• Some of the pathogens especially fungi and bacteria produce some growth
regulators or increase or decrease their levels in the host plants
(a)Auxins-
• It occurs naturally in plants as indole-3-acetic acid (IAA).
• Increased IAA levels occur in many plants infected by fungi, bacteria,
viruses, nematodes and mollicutes, although some pathogens seem to lower
the auxin level of the host e.g Exobasidium azalea causing flower gall of
Azalea and Ustilago maydis causative organism of corn smut.
(b) Cytokinins-
• Cytokinin activity increases in clubroot galls, in smut and rust infected bean
leaves.
• It is partly responsible for several bacterial galls of leafy gall disease of
sweet pea caused by bacterium Rhodococus fasciens.
• (c) Gibberellins-
• High level of gibberellins are detected due to foolish seedling diseases of
rice, in which rice seedlings infected with the fungus Gibberella fujikuroi
grow rapidly and become much taller than healthy plants, which is
apparently the result of the gibberellins secreted by the pathogen.
• (d) Ethylene –
• In the fruit of banana infected with Ralstonia solanacearum, the ethylene
content increases proportionately with the (premature) yellowing of the fruits.

15. POLYSACCHARIDES
• Fungi, bacteria, nematodes and possibly other pathogens constantly release
varying amounts of mucilaginous substances that coat their bodies and
provide the interface between the outer surface of the microorganism and its
environment.
• The role of the slimy polysaccharides in plant disease appears to be
particularly important in wilt diseases caused by pathogens that invade the
vascular system of the plant.
• Large polysaccharide molecules released by the pathogen in the xylem may
be sufficient to cause a mechanical lockage of vascular bundles and thus
initiate wilting.
DEFENSE MECHANISM IN PLANTS
• Analysis of most of the host parasite relationships reveals that on the pattern
of pathogenesis, the plants on their part, do exhibit defence mechanisms
(structural and chemical) as soon as challenged by the pathogen.
• The moment pathogen propagules come in contact with host surface, the
plants due to hereditary characters have several naturally occuring physical
and chemical barriers (pre-existing) resisting penetration, and if at all the
penetration occurs, the host reacts by different means resulting in formation
of physical and chemical barriers.

16. • There are three main types of defense systems in plants:
 Pre-existing or passive defense
 Defense through lack of essential factors
 Post –infectional or active or induced defense
1. Pre-existing or Passive Defense
A. Pre-existing Structural Defenses
• The first line of defence in plants is present in its surface.
• Several characters of the plants surface function as barriers to penetration
which pathogen must breach to enter the host.
• The pathogens enter the plant host by penetrating the epidermis and cuticle
• Cuticular wax and number of natural openings existing before the onset of
the pathogenesis can obstruct penetration.
• If the pathogen succeeds in penetration; it encounters pre-existing internal
structural barriers.
• The external and internal structural barriers existing before pathogen attack
are also called Pre-existing defence structures or passive/static or anit-
infection structures. These include:
 Cuticular wax
 Thickness of cuticle
 Structure of epidermal cell wall
 Structure of natural openings

17. B. Pre-existing biochemical defence
• Plants liberate different chemicals, which interfere with activities of the
pathogen and pathogenesis, thereby preventing or reducing infection.
• These chemicals and the biochemical conditions that develop may act either
directly through toxic or lytic effect on the invader or indirectly through
stimulating antagonistic plant surface microflora.
• The compounds pre-existing in plants as constitutive antibiotics and those,
which are formed in response to wounds as wounds antibiotics.
• Important pre- existing biochemical defense include:
 Release of antimicrobial compounds in plants: Plants while growing
and developing release gases as well as organic substances, from
leaves and roots (leaf and root exudates), containing sugars, amino
acid, organic acids, enzymes, glycoside etc. Although these substances
are ideal nutrients for microbes and help in germination and growth of
several saprophytes and parasites, but a number of inhibitory
substances are also present in these exudates. These inhibitory
substances directly affect the microorganism, or encourage certain
groups to dominate the environment and function as antagonists of the
pathogen.
 Inhibitors like phenolics, tannins and some fatty acid like
compounds such as dienes present in the plant cells also play an
important role in defense of plants

18.  Lack of nutrients essential for the plant pathogens
 Absence of Common Antigen in Host plant: It is now clear that the
presence of a common protein (antigen) in both the pathogen and host
determines disease development in the host. But if the antigen is
present in the host and absent in the pathogen or vice-versa, it makes
the host resistant to the pathogen.
2. Defense Through Lack of Essential Factors
A. Recognition factors
• The first step in infection process is the cell-to-cell communication between
host and pathogens.
• Plants of species or varieties may not be infected by pathogen if their
surface cells lack specific recognition factors.
• If the pathogen does not recognize the plant as one of its hosts it may not
adhere to the host surface or it may not produce infection substances such
as enzymes, or structures (appresoria, haustoria).
• These recognition molecules are of various types of oligosaccharides and
polysaccharides and glycoproteins..
B. Host receptors and sites for toxins
• In many host parasite interactions the pathogen produces host specific
toxins, which are responsible for symptoms and disease development.

19. • The molecules of toxin are supposed to attach to specific sensitive sites or
receptors in the cell. Only the plants that have such sensitive sites become
diseased
C. Essential nutrients and growth factors
• The fact that many facultative saprophytes and most of the obligate
parasites are host specific and sometimes are so specialized that they can
grow and reproduce only on certain varieties of those species
• it suggests that for these pathogens the essential nutrients and growth
factors are available only in these hosts.
• Absence of these nutrients and stimulus make the other varieties and
species unsuitable hosts.
3. Post Infectional or Active or Induced Defense
• Plants have to face the wide variety of pathogens (enemies) standing at a
place.
• Thus a strategically designed pre-existing (structural and biochemical)
defense mechanism in plants exists.
• It appears that these preexisting defense mechanisms help plants in warding-
off most of microbes as non-pathogens.
• But it does not seems to be sufficient.

20. • The defense activated after the entry or attack of the pathogen in the plant is
termed as active or induced defence.
• The induced/active defense mechanism in plants may operate at different
levels
 Biochemical defense
 Defense at cellular level
 Defenses at tissue level
A. Defense Structures Developed after the Attack of the
Pathogen / Induced Structural Defense
• After the pathogen has successfully managed to overcome the preexisting
defense mechanisms of the host, it invades the cells and tissues of the host.
• In order to check the further invasion by the pathogen, the host plants
develop some structures/mechanisms which may be
 Defense reactions in the cytoplasm, which ultimately kill the pathogen
 Cell wall defense structures,
 Defense structures developed by the tissues eg. Gum deposition,
abscission layers, tyloses, lignification and suberization etc.
 The death of the invaded cell i.e. necrosis or hypersensitivity.

21. B. Induced cellular defense
The cellular defense structures, ie. Changes in cell walls, have only a limited
role in defense. Following types are commonly observed.
1. Carhohydrate apposition (synthesis of secondary wall and papillae formation)
2. Callose deposition (hyphal sheathing just outside plasma lemma around the
haustorium which delays contact of pathogen (Phytophythora infestans) with
host cells.
3. Structural proteins
4. Induced cytoplasmic defense that present last line of host defense and may
effective against slow growing pathogens, weak parasites or some symbiotic
relationship.
C. Post-Infection-Biochemical Defense Mechanism:
• In order to sight infections caused by pathogens or injuries caused by any
other means, the plant cells and tissues produce by synthesis many
substances (chemicals) which inhibit the growth of causal organism.
• These include:
 Production of phenolic compounds
 Production of phytoalexins
 Substances Produced in Host to Resist Enzymes Produced by
Pathogen
 Detoxification of Pathogen Toxins and Enzymes
 Production of pathogenesis related proteins (PR Proteins)

22. PHYTOALEXINS
• Phytoalexins are toxic antimicrobial substances synthesized ‘de novo’ in the
plants in response to injury, infectious agents or their products and
physiological stimuli.
• The term phytoalexin was first used by the two phytopathologists Muller and
Borger (1940) for fungi static compounds produced by plants in response to
mechanical or chemical injury or infection.
• All phytoalexins are lipophilic compounds and were first detected after a
study of late blight of potato caused by Phytophthora infestans.
• Although the exact mechanism of production of phytoalexin has not been
properly understood, it is considered that a metabolite of the host plant
interacts with specific receptor on the pathogen’s membrane resulting in the
secretion of “phytoalexin elicitor” which enters the host plant cells and
stimulates the phytoalexin synthesis.
• Phytoalexins are considered to stop the growth of pathogens by altering the
plasma membrane and inhibiting the oxidative phosphorylation.
• Phytoalexins have been identified in a wide variety of species of plants such
as Soyabean, Potato, sweet potato, barley, carrot, cotton etc. and are being
investigated.
• Some common phytoalexins are Ipomeamarone, Orchinol, Pistatin,
Phaseolin, Medicarpin, Rishitin, Isocoumarin, ‘Gossypol’ Cicerin, Glyceolin,
Capisidiol etc.

23. Pathogenesis-related (PR) proteins
• These are the proteins produced in plant body in the vent of pathogen attack
• They are induced as part of systemic acquired resistance.
• Infections activate genes that produce PR proteins.
• Some of these proteins are antimicrobial, attacking molecules in the cell wall
of a bacterium or fungus.
• Others may function as signals that spread “news” of the infection to nearby
cells.
• Infections also stimulate the cross-linking of molecules in the cell wall and
the deposition of lignin i.e. the responses that set up a local barricade that
slows spread of the pathogen to other parts of the plant
• Salicylic acid plays a role in the resistance to pathogens by inducing the
production of pathogenesis-related proteins
• Many proteins found in wine are grape pathogen-related proteins Those
include thaumatin -like proteins and chitinases
Functions of PR Proteins
• An important common function of most PRs is their antifungal effects
• Some PRs also exhibit antibacterial, insecticidal or antiviral action.
• Function as signals that spread “news” of the infection to nearby cells.
• Infections also stimulate the cross-linking of molecules in the cell wall and
the deposition of lignin, responses that set up a local barricade that slows
spread of the pathogen to other parts of the plant

24. • Some of the PR proteins exhibit antimicrobial enzyme activities like
chitinase, peroxidase, ribonuclease and lysozyme activity
• They have hydrolytic, proteinase-inhibitory and membrane-permeabilizing
ability.
• They inactivate the proteins secreted by the parasites in the invaded plant
tissues