Role of Growth Regulators in Disease Development

Plant growth regulators (hormones) such as auxins, gibberellins, cytokinins, and ethylene play critical roles in regulating growth, development, and stress responses in plants. During pathogen infection, these regulatory mechanisms are often disrupted, leading to various physiological and structural changes in the plant that facilitate disease progression. Pathogens manipulate hormone levels and signaling pathways to establish infection, reproduce, and spread, contributing to symptom development and disease severity.

Key Growth Regulators and Their Role in Disease Development

1. Auxins

• Normal Function: Auxins regulate cell elongation, differentiation, and growth.
• Effect During Pathogen Infection:
  • Increased auxin levels due to:
    • Pathogen-induced synthesis of auxins.
    • Inhibition of indole acetic acid oxidase (enzyme responsible for auxin degradation).
  • Overproduction leads to abnormal cell growth and tissue deformation.
• Examples of Symptoms:
  • Gall formation (e.g., crown gall disease by Agrobacterium tumefaciens).
  • Hyperplasia (abnormal cell division) and hypertrophy (abnormal cell enlargement).

2. Gibberellins

• Normal Function: Gibberellins control stem elongation, seed germination, and flowering.
• Effect During Pathogen Infection:
  • Altered gibberellin levels lead to:
    • Hyper-elongation of tissues in some diseases (e.g., Bakane disease of rice caused by Fusarium moniliforme).
    • Dwarfing or stunting in others due to reduced gibberellin activity.
• Examples of Symptoms:
  • Excessive elongation (e.g., Bakane disease).
  • Stunted growth and reduced plant height.

3. Cytokinins

• Normal Function: Cytokinins promote cell division, delay senescence, and regulate nutrient mobilization.
• Effect During Pathogen Infection:
  • Pathogens interfere with cytokinin metabolism, leading to:
    • Gall and tumor formation.
    • Nutrient redirection towards infected tissues.
    • Delayed senescence in localized areas (green island effect).
  • Disruption in cytokinin and auxin balance exacerbates abnormal growth.
• Examples of Symptoms:
  • Gall formation (e.g., crown gall by Agrobacterium tumefaciens).
  • Green islands on leaves, as seen in leaf spot diseases.

4. Ethylene

• Normal Function: Ethylene regulates fruit ripening, leaf abscission, and senescence.
• Effect During Pathogen Infection:
  • Elevated ethylene levels due to pathogen activity accelerate tissue degradation and symptom expression.
  • Ethylene contributes to:
    • Premature leaf drop (abscission).
    • Epinasty (downward curling of leaves).
    • Yellowing and senescence of tissues.
• Examples of Symptoms:
  • Premature leaf fall (e.g., black spot of rose by Diplocarpon rosae).
  • Senescence and rotting of fruits and tissues.

5. Other Hormones

• Abscisic Acid (ABA):
  • Pathogens may manipulate ABA to suppress plant immune responses.
  • ABA is involved in stomatal closure, which some pathogens exploit to limit plant defense.
• Salicylic Acid and Jasmonic Acid:
  • Although not growth regulators in the traditional sense, these hormones are critical in plant defense and are often suppressed by pathogens.

Mechanisms of Hormonal Disruption by Pathogens

1. Hormone Synthesis: Pathogens produce or induce the host to produce excessive hormones.
2. Enzyme Inhibition: Enzymes responsible for hormone degradation are suppressed.
3. Signal Manipulation: Pathogens interfere with hormone signaling pathways to alter host responses.

Impact on Disease Development

1. Abnormal Growth: Disrupted hormone levels cause galls, tumors, hyper-elongation, or stunting.
2. Symptom Amplification: Hormones like ethylene promote tissue senescence, yellowing, and abscission.
3. Host Manipulation: Cytokinins and auxins redirect nutrients to infected tissues, aiding pathogen reproduction.
4. Defense Suppression: ABA and other hormone disruptions weaken plant defense mechanisms, allowing pathogen invasion.