Plant-pathogen Interaction and

Disease Development; Molecular

Mechanisms of Fungal and

Bacterial Infection in Plants;

Changes in Metabolism

Diseases ?• Plant diseases are the result of infection by any

living organisms that adversely affect the growth,

development, physiological functioning and

productivity of a plant, manifesting outwardly as

visible symptoms.

• Any Parasite organisms that cause disease are

called pathogens and pathogens on parasitoid is

known as host.

Plant – Pathogen Interaction

• Plants exist in a world filled with bacteria,

fungi, nematodes, and possibly parasitic

plants (stariga, casccuta, orobaincy etc).

• They may be inoculated with viruses during

feeding by insects or by other vectors

(insects/pest, water, air, humans etc).

• 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 activitiesmostly through secretions of chemicalsubstances that affect certain componentsor metabolic mechanisms of their hosts.

• Penetration and invasion, however, seem tobe aided by or in some cases be entirely theresult of, the mechanical force exerted bycertain pathogens on the cell walls of theplant.

Plant – Pathogen Interaction

Harold Henry Flor, 1900–1991

• Flor received a Ph.D. degree in 1929 from

the University of Minnesota. He worked

for the USDA for three years at

Washington State University and then for

the remainder of his career at North

Dakota State University.

In the 1940s, Flor developed the gene-for-gene

concept to explain the genetic interactions between

Melampsora lini and flax. His theories were put to

use in Flor’s own flax breeding program to

successfully develop rust-resistant flax. This concept

provided the underpinnings for research on the

genetics of host-pathogen interactions for the next

70 years.

Gene For Gene Hypothesis

• The concept of gene for gene hypothesis was first developed by Flor in

1956 based on his studies of host pathogen interaction in flax, for rust

caused by Malampsora lini.

• 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.

Biochemistry of Gene-for-Gene Hypothesis

• The Receptor-Elicitor Model of gene-for-gene interactions.

• The resistance allele of the plant encodes a receptor thatrecognizes an elicitor produced by the pathogen. Recognitionof the pathogen elicitor by the plant receptor initiates plantdefense 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

Mode of Infection Virus

Bacteria

Fungi

(A) Mechanical Forces Exerted

on Host Tissues by Pathogens

• Viruses are usually introduced directly through the plantcells by insects (BPH, WF, Aphids), Natural openings(stomata), wounding site, therefore they do not exertmechanical forces to inter in the host.

• Many fungi are known to apply mechanical forces on theplant they are about to attack. When fungal spores landson a plant surface, and contact is established, diameter ofthe tip of the hypha or radical in contact with the hostincreases and forms the flattened, bulb-like structure calledthe Appressorium.

• This increases the area of adherence between the twoorganisms and securely fastens the pathogen to the plant.From the appressorium, a fine growing point, called thePenetration Peg arises and advances into and through thecuticle and the cell wall.

(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).

• 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.

(B) CHEMICAL WEAPONS OF PATHOGENS

(I) ENZYMES

(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.

• Cutinases break down cutin molecules and release

monomers as well as oligomers of the component fatty

acid derivatives from the insoluble cutin polymer e.g.

Fusarium spp. and Botrytis cinerea.

(B) Pectinases:• Pectin substances constitute the main components of

the middle lamella i.e. the intercellular cement thatholds in place the cells of plant tissues. Severalenzymes degrade pectic substances and are known aspectinases or pectolylic enzymes.

• 1. Pectic enzymes is pectin methyl-esterases, whichremoves small branches off the pectin chains.

• 2. Pectic enzymes is a chain splitting pectinases calledpolygalacturonases. It split the pectic chain by addinga molecule of water and breaking the linkage betweentwo galacturonan molecules.

• 3. Pectin lyases is split the chain by removing amolecule of water from the linkage, there by breaking itand releasing products with an unsaturated doublebond. Examples of pathogens include Ralstoniasolanacearum, Didymella bryoniae.

(C) Cellulases:• Cellulose is also a polysaccharide, but it

consists of chains of glucose (1-4) β-D-glucan

molecules. Saprophytes 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, however, cellulolytic

enzymes secreted by pathogens play a role in

the softening and disintegration of cell wall

material.

(II) 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.

• (A) Non-host specific toxin

• (B) Host specific toxin

• (1) Tab-toxin:-

its produced by the bacterium Pseudomonas syringae pv tabaci

which causes the wildfire disease of tobacco, by strain 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.

• (2) Phaseolo-toxin:-

its produced by the bacterium Pseudomonas syringae pv

phaseolicola, the cause of halo blight of bean and some other

legumes.

• (3) Ten-toxin:-

its produced by the fungus Alternaria alternata which causes

spots and chlorosis in plants by many species.

• (4) Cercosporin-

its produced by the fungus Cercospora and by several other fungi. 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.

(A) Non-Host Specific Toxin Or

Non Host-selective Toxins

(B) Host Specific Or Host-selective Toxins

1. Victorin or HV toxin – Its produced by the fungus

Cochliobolus 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- is produced by race T of Cochliobolus

heterostrophus, the cause of southern corn leaf

blight. Race T is indistinguishable from other all

other C. heterostrophus races except for its ability to

produce the T toxin.

3. HC-toxin- Its produced by Race 1 of C. carbonum

causing northern leaf spot and ear rot disease in

maize.

(iii) Growth Regulators

(a)Auxins-• It occurs naturally in plants as indole-3-acetic acid

(IAA). Increased IAA levels occur in many plantsinfected by fungi, bacteria, viruses, nematodes andmollicutes, although some pathogens seem to lowerthe auxin level of the host e.g Exobasidium azaleacausing azalea and flower gall, Ustilago maydiscausative organism of corn smut.

(b) Cytokinins-Cytokinin activity increases in clubroot galls, in smutand rust infected bean leaves. it is partly responsiblefor several bacterial galls of leafy gall disease of sweetpea caused by bacterium Rhodococus fasciens.

(c) Gibberellins-• High level of gibberellins due to foolish seedling

diseases of rice, in which rice seedlings infectedwith the fungus Gibberella fujikuroi grow rapidly andbecome much taller than healthy plants isapparently the result, to a considerable extent atleast, of the gibberellins secreted by the pathogen.

(d) Ethylene –• In the fruit of banana infected with Ralstoniasolanacearum, the ethylene content increasesproportionately with the (premature) yellowing of thefruits, whereas no ethylene can be detected in thehealthy fruits.

(iii) Growth Regulators

(IV) 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.

Disease Development

• The amount of disease that

develops in a plant community

is dependent on properties of

• The Host,

• The Pathogen and

• The Environment

• The environment can affect

both the susceptibility of the

host (e.g. by creating stress in

the plant) and the activity of

the pathogen (e.g. providing

moisture for spore

germination).

Disease Development

Factors of Disease Development

Pathogen Host Environment

Presence of pathogen

Pathogenicity

Adaptability

Dispersal efficiency

Survival efficiency

Reproductive fitness

Susceptibility

Growth stage & form

Population density &

structure

General health

Temperature

Rainfall / Dew

Leaf wetness period

Soil properties

Wind

Fire history

Air pollution

Herbicide damage

Factors of Disease Development

Molecular Mechanisms of Fungal Pathogenicity to Plants

1. Entry

• Fungi rarely cause disease in healthy immunocompetent hosts. Disease resultswhen fungi accidentally penetrate host barriers or when immunologic defects orother debilitating conditions exist that favor fungal entry and growth.

2. Adaptation and Propagation

• Fungi often develop both virulence mechanisms (e.g., capsule and ability to growat 37oC) and morphologic forms (e.g., yeasts, hyphae, spherules, and scleroticbodies) that facilitate their multiplication within the host.

3. Dissemination

• Dissemination of fungi in the body indicates a breach or deficiency of hostdefenses (e.g., endocrinopathies and immune disorders).

4. Host Factors

• Healthy, immunologically-competent individuals have a high degree of innateresistance to fungi. Resistance to fungi is based primarily upon cutaneous andmucosal physical barriers. Severity of disease depends on factors such asinoculum, magnitude of tissue destruction, ability of fungus to multiply in thetissue, and the immune status of the host.

5. Fungal Factors

• Enzymes such as keratinase, the presence of capsule in Cryptococcusneoformans, the ability to grow at 37°C, dimorphism, and other as yet undefinedfactors contribute to fungal pathogenesis which involves a complex interplay ofmany fungal and host factors.

• The hyphae composed of protein polypepetides,

polysaccharide carbohydrates, fibrinogen or fibrin

and polynucleotides that contain RNA and DNA

material. This structure is bound together with

lignands that have stickiness properties.

• The cell wall of candida is composed of fibrin and theplant fiber chitin or cellulose, mannoprotein-1, glucanand mannoprotein-2, which are plantpolysaccharrides. Then comes a plasma membraneand a protein nucleus. These components arebasically what candida uses to build the biofilm aswell.

• Candidase is a very strong enzyme for candida yeastthat eats the cellulose layers of the biofilm and cellwall and the protein of the nucleus. But there are noenzymes to eat the fibrin or plant polysaccharides.

• Candizyme is more complete than the above threecandida eating enzyme formulas. It contains enzymesto eat the chitin/cellulose and the plantpolysaccharrides in both the biofilm and cell wall. Italso has protease enzymes to eat the protein of thenucleus. However, like the rest it does not have anenzyme that eats the fibrin in the biofilm.

First, bacterial nodulation (nod) genes are activated in response to

plant-secreted signal molecules such as flavonoids, resulting in

biosynthesis and secretion of lipo-chito-oligosaccharides (LCOs) by

rhizobium bacteria.

In the second step, LCOs elicit nodule formation on the host plant

roots and trigger the infection process. LCOs, which induce the

formation of the root nodules on the host plants, are termed Nod

factors.

Molecular Mechanisms of Bacterial Pathogenicity to Plants

Changes in Plant Metabolism After Infection

Programmed Cell Death

• Programmed cell

death is a

genetically

regulated process of

cell suicide that is

central to the

development,

homeostasis and

integrity of

multicellular

organisms

NECROSIS VS APOPTOSIS

Purpose of Cell Death

• Cells that are produced in excess

• Cell that have no function

• Cell that develop improperly

• Cell that have stop further infection

• Cells that are harmful

Hypersensitive Response

• Rapid, localized plant cell death upon contact with

avirulent pathogens. HR is considered to be a key

component of multifaceted plant defense responses to

restrict attempted infection by avirulent pathogens

• Rapid - within 24 h

• Not always needed for resistance

• HR also contributes to the establishment of the long-

lasting systemic acquired resistance against

subsequent attack by a broad range of normally

virulent pathogens

• HR Includes:

• Oxidative bust (production of

reactive oxygen species)

• Disruption of cell membranes

• opening of ion channels

• Cross linking of phenolics

with cell wall component

• Production of anti-microbial

phytoalexins and PR protein

• apoptosis (programmed cell

death)

• A type III secretion pathway, broadly conserved among gram-negative

pathogens of plants and animals

• Macromolecular structure, Hrp pilus, acts as conduit for traffic (called

needle complex in animal pathogens)

• Encoded by clustered hrp genes

• Required for hypersensitive reaction and pathogenicity

• Expression induced in plant and in defined minimal media

• Capable of delivering proteins into host cells

• Secretes and delivers “effector proteins”

a) virulence factors

b) avirulence factors

The Hrp pathway

• Pectic enzymes- Soft rot Erwinia spp.

- Multiple isozymes, some plant regulated

• Toxins- e.g., coronatine acts as JA mimic to down regulate plant defense

• Extracellular polysaccharides- Important in many diseases, specially vascular diseases

- Postulated roles in protection from plant-derived antimicrobials, osmotic stress,

dessication; evading recognition; causing wilt, etc.

• Quorum sensing (cell-cell signaling) and global regulation

of virulence- Soft rot Erwinia spp. regulation of virulence associated genes, including pectic

enzymes

- Cell wall degradation products elicit plant defense, so benefit to repressing pectolytic

activity until high numbers of bacteria accumulate.

• The Type III secretion (Hrp) pathway- Essential for necrogenic Gram negative pathogens

Major changes after Infection of Erwinia spp.

K K

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