Unit5 - Subjective Questions

In plant pathology, pathogens are classified based on their mode of nutrition and parasitism:

1. Biotrophs (Obligate Parasites):

   • These organisms can obtain nutrients only from living host cells.
   • They do not kill the host cells rapidly; rather, they establish a long-term feeding relationship.
   • Examples: Rusts (Puccinia), Powdery mildews (Erysiphe), and Viruses.

2. Necrotrophs:

   • These pathogens kill the host cells first (often by secreting toxins or enzymes) and then derive nutrients from the dead matter.
   • They are essentially saprophytes that can parasitize.
   • Examples: Alternaria, Botrytis, and Soft rot bacteria (Pectobacterium).

3. Hemibiotrophs:

   • These organisms start their infection cycle as biotrophs (living on live tissue) and later switch to a necrotrophic phase (killing the tissue).
   • Examples: Phytophthora infestans (Late blight of potato) and Colletotrichum species (Anthracnose).

This classification is based on the organism's ability to switch between parasitic and saprophytic modes of life:

1. Facultative Saprophytes:

• Primary Nature: These are essentially parasites (usually attacking living tissue).
• Capability: They have the ability or faculty to live as saprophytes (on dead organic matter) if the living host is not available.
• Significance: This ability allows them to survive in soil or debris between crop seasons.
• Example: Phytophthora infestans, Venturia inaequalis (Apple scab pathogen).

2. Facultative Parasites:

• Primary Nature: These are essentially saprophytes (living on dead organic matter).
• Capability: Under certain favorable conditions, they can act as parasites and attack living plants.
• Significance: They are generally opportunistic pathogens affecting weakened plants.
• Example: Many soil-borne fungi like Pythium, Rhizoctonia, and Fusarium species.

Variability in fungi allows them to overcome host resistance. Two key asexual mechanisms are:

1. Heterokaryosis:

• Definition: The condition in which a fungal cell (hypha) contains two or more genetically different nuclei within a common cytoplasm.
• Mechanism: It arises through the anastomosis (fusion) of hyphae from different strains or by mutation of nuclei within a hypha.
• Significance: It allows the fungus to carry different genetic traits (e.g., virulence factors) without sexual reproduction. If the nuclei are and , the phenotype may exhibit traits of both parents.

2. Parasexualism:

• Definition: A cycle that mimics sexual reproduction but occurs in the somatic (vegetative) phase, discovered by Pontecorvo (1956).
• Steps:
  1. Heterokaryosis: Formation of heterokaryotic mycelium.
  2. Karyogamy: Fusion of two genetically different nuclei to form a diploid nucleus ().
  3. Mitotic Crossing Over: Exchange of genetic material during mitosis (rare event).
  4. Haploidization: The diploid nucleus loses chromosomes (aneuploidy) until it returns to a haploid state ().
• Result: New genetic recombinants are formed, leading to new physiological races.

Definition:
Cutinases are esterase enzymes that break down cutin, the main structural component of the plant cuticle.

Role in Pathogenesis:

1. Penetration Barrier: The plant cuticle is the first physical barrier a pathogen encounters. It is composed of hydroxy fatty acids impregnated with wax.
2. Enzymatic Action: Pathogens secrete cutinases to hydrolyze the ester bonds in cutin polymers, disrupting the cuticle structure.
3. Induction: Cutin monomers produced by small amounts of constitutive cutinase on the spore surface often act as inducers, signaling the fungus to produce more enzymes.
4. Significance:
   • Many fungi, such as Fusarium solani f.sp. pisi, require cutinase to penetrate the host directly.
   • Pathogens lacking cutinase often require wounds or natural openings (stomata) to enter.
5. Proof of Role: Inhibitors of cutinase (e.g., organophosphates) often prevent infection by fungi that rely on direct penetration.

Pectin is the major component of the middle lamella, which acts as the cement holding plant cells together.

Types of Pectolytic Enzymes:

1. Pectin Methyl Esterases (PME): Remove methyl groups from pectin chains, making them susceptible to chain-splitting enzymes.
2. Polygalacturonases (PG): Hydrolyze the glycosidic bonds between galacturonic acid units (adding water).
3. Pectin Lyases (PL): Split the chain by trans-elimination (removing a proton and forming a double bond).

Role in Pathogenesis (Tissue Maceration):

• Middle Lamella Dissolution: Pathogens secrete these enzymes to degrade the middle lamella.
• Cell Separation: As pectin degrades, individual cells separate from one another, a process known as maceration.
• Soft Rot: The loss of structural integrity results in a mushy, watery consistency typical of diseases like Soft Rot of Vegetables caused by bacteria like Pectobacterium carotovorum.
• Cell Death: The separation often alters osmotic balances and cell wall permeability, leading to cell death.

Cellulose is the primary framework of the plant cell wall.

1. Cellulases:

• These enzymes degrade cellulose into glucose.
• Mechanism: It involves a complex of enzymes (, , and -glucosidase).
• Role in Wilt: In vascular wilts (e.g., Fusarium wilt), the breakdown of vessel walls releases large molecular weight compounds that clog the xylem, obstructing water flow.
• Role in Wood Decay: In diseases like Brown Rot, fungi degrade cellulose, leaving behind lignin, causing the wood to crumble.

2. Hemicellulases:

• Hemicellulose (e.g., Xyloglucan, Xylan) links cellulose microfibrils.
• Role: Pathogens secrete xylanases and other hemicellulases to loosen the cell wall matrix, allowing easier access for cellulases and pectinases to completely destroy the cell wall integrity.

Summary: While pectinases separate cells, cellulases and hemicellulases break down the primary and secondary cell walls, facilitating deep invasion and nutrient extraction.

Host Specific Toxins (HST):
These are metabolic products produced by a pathogen that are toxic only to the susceptible host of that pathogen. They determine the host range and pathogenicity. If the pathogen loses the ability to produce the HST, it becomes non-pathogenic.

Victorin (HV-toxin):

• Pathogen: Produced by Cochliobolus victoriae (Helminthosporium victoriae).
• Host: Victoria variety of Oats.
• Mode of Action:
  1. Receptor Binding: Victorin binds to a specific protein receptor (Vb gene product) on the plasma membrane of susceptible oat cells.
  2. Permeability Change: It causes a rapid alteration in the structure and permeability of the cell membrane.
  3. Electrolyte Leakage: This leads to significant loss of electrolytes and cell contents.
  4. Metabolic Disruption: It interferes with oxidative phosphorylation, leading to cell death and the characteristic blight symptoms.

Definition:
Non-Host Specific Toxins are toxic substances produced by pathogens that can affect a wide range of plants, not just the specific host of the pathogen. They act as virulence factors—increasing the severity of the disease—but are not strictly required for the pathogen to cause disease.

Examples:

1. Tabtoxin (Wildfire Toxin):

• Pathogen: Pseudomonas syringae pv. tabaci (causes Wildfire of Tobacco).
• Mechanism:
  • In the plant cell, Tabtoxin is hydrolyzed to release Tabtoxinine--lactam.
  • This active form inhibits the enzyme Glutamine Synthetase.
  • Inhibition leads to the accumulation of toxic levels of ammonia (), which destroys thylakoid membranes.
• Symptom: Chlorotic halos surrounding necrotic spots.

2. Tentoxin:

• Pathogen: Alternaria alternata (specifically tenuis).
• Mechanism:
  • It binds to the chloroplast-coupling factor protein involved in energy transfer.
  • It inhibits photophosphorylation (ATP production in chloroplasts).
• Symptom: Chlorosis (interference with chlorophyll synthesis and function).

Indole-3-Acetic Acid (IAA) is the primary naturally occurring auxin in plants, regulating cell elongation and differentiation.

Hyperauxiny in Pathogenesis:
Many pathogens (bacteria like Agrobacterium, fungi like Ustilago) induce increased levels of IAA in host tissues (Hyperauxiny). This occurs via:

1. Direct synthesis of IAA by the pathogen.
2. Pathogen inducing the host to synthesize more IAA.
3. Inhibition of IAA degradation (inhibiting IAA oxidase).

Effects on Host:

• Hypertrophy: Abnormal enlargement of cells.
• Hyperplasia: Excessive cell division.
• Symptoms:
  • Crown Gall: Agrobacterium tumefaciens transfers genes that overproduce auxin, causing tumor-like growths.
  • Corn Smut: Ustilago maydis causes large galls on corn ears due to high auxin levels.
  • Root Formation: Increased rooting or adventitious roots.

Pathogen: Gibberella fujikuroi (conidial stage Fusarium moniliforme).
Disease: Bakanae or "Foolish Seedling" disease of rice.

Mechanism:

1. Production: The fungus produces excessive amounts of Gibberellins (specifically Gibberellic Acid or GA).
2. Normal Function: Normally, gibberellins regulate internodal elongation.
3. Pathogenic Effect: The massive influx of fungal gibberellins causes uncontrollable cell elongation in the internodes of the rice plant.

Symptoms:

• Infected plants grow much taller and thinner than healthy plants.
• They appear chlorotic (pale) and spindly.
• The stems are weak and often lodge (fall over).
• In severe cases, the plants may die or produce empty panicles (sterility).

This disease was actually the source of the discovery of gibberellins as plant hormones.

Cytokinins: Plant hormones (e.g., Zeatin, Kinetin) essential for cell division and the prevention of senescence (aging).

The Green Island Effect:

• Observation: In diseases caused by obligate parasites like Rusts and Powdery Mildews, the tissue immediately surrounding the fungal infection site remains green and photosynthetically active, while the surrounding tissue yellows (senesces).
• Mechanism:
  1. The pathogen secretes cytokinins or stimulates the host to accumulate cytokinins at the infection site.
  2. Cytokinins create a strong physiologic sink, drawing nutrients (amino acids, sugars) from the surrounding leaves toward the infection site.
  3. They inhibit the breakdown of chlorophyll and protein in that specific zone.
• Significance: This ensures a steady food supply for the obligate parasite, keeping the host cells alive at the site of feeding even as the rest of the leaf dies.

Ethylene: A gaseous plant hormone associated with fruit ripening, senescence, and stress responses.

Role in Pathogenesis:
Ethylene production often increases significantly in infected tissues (e.g., in diseases caused by Ralstonia solanacearum or various fruit rots).

Effects:

1. Epinasty: Downward curling of leaves (a common symptom in vascular wilts like Fusarium wilt of tomato). The toxin fusaric acid may stimulate ethylene production.
2. Premature Defoliation: Promotes the formation of the abscission layer, causing leaves or fruits to drop early (e.g., Bacterial spot of peach).
3. Chlorosis: Accelerates the degradation of chlorophyll, causing yellowing.
4. Premature Ripening: In fruit rots, pathogens stimulate ethylene to soften the fruit tissue (ripening), making it easier for enzymes to degrade the cell walls.
5. Defense Signaling: Conversely, ethylene acts as a signal to trigger host defense genes (PR proteins), though pathogens often exploit it for tissue senescence.

1. Constitutive Enzymes:

• Definition: Enzymes that are always present in the pathogen's cells or secretions, regardless of the presence of a substrate.
• Role: They enable the pathogen to initiate the attack.
• Example: A spore landing on a leaf may continuously secrete low levels of Cutinase or Pectinase. These trace enzymes release small breakdown products (monomers) from the plant surface.

2. Inducible (Adaptive) Enzymes:

• Definition: Enzymes produced only when the specific substrate (inducer) is present in the environment.
• Role: They allow the pathogen to conserve energy by synthesizing massive amounts of enzymes only when needed.
• Mechanism: The monomers released by constitutive enzymes enter the fungal cell and trigger the genetic expression of inducible enzymes.
• Example: Once pectin degradation products (galacturonic acid) are detected, the fungus ramps up production of Polygalacturonases to rapidly macerate the tissue.

Pathogen: Cochliobolus heterostrophus (Race T) (Asexual: Bipolaris maydis).
Disease: Southern Corn Leaf Blight (Epidemic of 1970 in USA).

The Mechanism:

1. Host Susceptibility: The epidemic affected corn hybrids containing Texas Male Sterile Cytoplasm (T-cms). This cytoplasm was widely used to avoid manual detasseling.
2. T-Toxin: Race T of the fungus produces a specific Host Specific Toxin (HST) called T-toxin (a linear polyketide).
3. Target Site: The toxin specifically attacks the mitochondria of maize varieties with T-cms.
4. Action: It binds to a specific protein (URF13) in the inner mitochondrial membrane, causing pore formation.
5. Result: Loss of mitochondrial respiration and massive leakage of molecules, leading to rapid necrosis and death of the plant.
6. Specificity: Corn with Normal (N) cytoplasm is resistant to the toxin.

Lignin:
A complex, amorphous polymer of phenylpropanoid units. It provides rigidity and acts as a barrier against enzymatic degradation of cellulose.

Ligninases:
Enzymes capable of degrading lignin. The primary enzymes involved are oxidative rather than hydrolytic:

1. Lignin Peroxidase (LiP)
2. Manganese Peroxidase (MnP)
3. Laccases

Role in Wood Rot:

• White Rot Fungi: (e.g., Phanerochaete chrysosporium) produce ligninases. They degrade lignin completely, exposing the white cellulose fibers (hence "White Rot"). This allows them to access the energy-rich cellulose and hemicellulose protected inside.
• Mechanism: These enzymes generate free radicals that break the stubborn chemical bonds in lignin.
• Importance: Without ligninases, pathogens cannot effectively colonize woody tissues, as lignin physically protects cell wall carbohydrates and contains antimicrobial phenolic compounds.

1. Physiological Race:

• Definition: A subdivision of a species (or special form/forma specialis) that is morphologically similar but differs in its physiological ability to infect specific varieties (cultivars) of a host plant.
• Identification: Races are identified using a set of "Differential Hosts" (cultivars with specific resistance genes).
• Example: In Puccinia graminis f.sp. tritici (Stem rust of wheat), Race 15B might attack wheat variety 'A' but not 'B', while Race 56 might attack 'B' but not 'A'.

2. Biotype:

• Definition: A population of individuals within a race that are genetically identical. It is a subdivision of a physiologic race.
• Differentiation: They may show subtle differences not distinguished by the standard differential hosts but perhaps by a new, highly specific resistance gene or environmental adaptation.
• Hierarchy: Species Forma Specialis Physiological Race Biotype.

While enzymes and toxins degrade tissue, polysaccharides produced by pathogens often act mechanically.

Mechanism:

1. Production: Bacteria (e.g., Ralstonia solanacearum, Xanthomonas) and some fungi secrete copious amounts of extracellular polysaccharides (EPS) or slimy mucilage.
2. Location: These substances are released into the vascular system (xylem vessels) of the host plant.
3. Blockage: The high molecular weight slime increases the viscosity of the xylem fluid and physically clogs the vessel elements and tracheids.
4. Water Stress: This mechanical blockage prevents water translocation from roots to shoots.
5. Result: The plant suffers from irreversible wilting, even if there is ample water in the soil.

Note: This is often a contributing factor alongside cellulolytic enzymes that break down vessel walls.

Pathogens use a chemical arsenal to breach host defenses and obtain nutrients:

1. Enzymes (The Breakers):

• Function: Disintegrate structural components of the host cell.
• Cutinases: Break cuticular barriers (Entry).
• Pectinases: Dissolve middle lamella (Maceration/Soft Rot).
• Cellulases/Hemicellulases: Break down cell walls (Wilt/Collapse).
• Ligninases: Degrade wood (Rot).

2. Toxins (The Killers):

• Function: Poison the host protoplasm, damaging membranes and organelles.
• Host Specific Toxins (HST): Selective toxicity. E.g., Victorin (Oats), T-Toxin (Corn). Essential for pathogenicity.
• Non-Host Specific Toxins: General toxicity. E.g., Tabtoxin (Tobacco wildfire), Fusaric Acid. Enhance virulence.

3. Growth Regulators (The Disruptors):

• Function: Alter host metabolism and growth patterns.
• Auxins: Cause tumors/galls (Agrobacterium).
• Gibberellins: Cause elongation (Bakanae disease).
• Cytokinins: Create nutrient sinks (Green islands).
• Ethylene: Cause premature ripening and epinasty.

Proposed by H.H. Flor (1940s) while working with Flax Rust (Melampsora lini).

Concept:

• For every resistance gene (R-gene) in the host plant, there is a corresponding avirulence gene (Avr-gene) in the pathogen.
• Incompatible Reaction (Resistance): If the host has a dominant R-gene and the pathogen has the corresponding dominant Avr-gene, the plant recognizes the pathogen (often via hypersensitive response) and disease does not occur.
• Compatible Reaction (Disease): If the pathogen lacks the Avr-gene (mutation to virulence) or the host lacks the R-gene, recognition fails, and disease develops.

Relation to Variability:

• Pathogens constantly mutate their Avr-genes to evade detection by Host R-genes.
• This evolutionary pressure leads to the development of new Physiological Races capable of attacking previously resistant varieties.

This classification is based on the criteria proposed by Wheeler and Luke:

1. Pathotoxin (e.g., Victorin):

• A toxin that plays a causal role in disease production.
• It produces all the typical symptoms of the disease in a susceptible host when applied in pure form.
• The pathogen produces it in the infected plant.
• It is generally synonymous with Host Specific Toxins.

2. Vivotoxin (e.g., Fusaric Acid):

• Defined as a substance produced in the infected host by the pathogen and/or its host.
• It functions in the production of disease, but is not necessarily the initial inciting agent.
• It can be isolated from the diseased plant.
• When introduced to a healthy plant, it reproduces some but not necessarily all symptoms.
• Example: Fusaric acid causes wilting but might not cause the vascular browning alone.