Mycology BB 451

Mycology - BB 451

Structural characteristics act as physical barriers to inhibit pathogen entry and spread.
- Physical barriers include:
- Waxes and hairs on the plant surface.
- Thick cuticle covering epidermal cells.
- Structure of epidermal cell walls.
- Size, location, and shape of stomata and lenticels.
- Thick-walled tissue that hinders pathogen advance.
Biochemical reactions within plant cells lead to production of:
- Toxic substances for pathogen inhibition.
- Conditions that inhibit pathogen growth.

Plant defense is ultimately controlled by the genetic material of both the host plant and the pathogen.
- Genetic interaction dictates resistance or susceptibility.

Nonhost Resistance
- Plants exhibit immune response to pathogens for which they are not hosts, e.g., apple trees are immune to tomato and citrus pathogens.
Horizontal Resistance (Partial, Polygenic, Quantitative)
- Many genes involved provide a broader, less specific resistance.
- Collected genetic contributions enhance overall defense against a variety of pathogens.
Vertical Resistance
- Defined as race-specific or monogenic resistance.
- Involves one or few resistance (R) genes and matching avirulence (Avr) genes from the pathogen.

Effective resistance occurs when plants confront pathogens they are not genetically compatible with.

Genes involved:
- Code for compounds that are toxic to or neutralize toxins from pathogens.
- Produce structures that impede pathogen growth.

First line of defense includes structural defenses present before pathogen contact.

Phytoanticipins:
- Antimicrobial compounds present prior to infection, including phenolic compounds (tannins and dienes).
- Hydrolytic enzyme inhibitors affect pathogens' ability to break down plant tissues.
- Phytocystatins inhibit cysteine proteinases from nematodes and fungi.
Lectins:
- Bind sugars and induce lysis and growth inhibition of fungi.
Hydrolytic enzymes:
- Glucanases and chitinases can degrade pathogen cell wall components.

Pathogens release substances acting as elicitors, leading to host recognition:
- Nonspecific elicitors include toxins, glycoproteins, carbohydrates, and enzymes (proteases, pectic enzymes).
- Specific elicitors include avr gene products and type III secreted proteins (hrp gene products).

Resistance occurs through matching pairs of R and Avr gene interactions:
- The avirulence gene from the pathogen triggers the corresponding R gene in the host, initiating defense responses.

Interaction of avirulence (Avr) gene and resistance (R) gene:
- Triggers defense responses that may lead to programmed cell death surrounding the infection site.
- This response is categorized as hypersensitive response (HR).

Upon elicitor recognition, alarm signals activate:
- Host proteins and nuclear genes for defensive action.
Transmission Types:
- Intracellular Signal Transduction:
- Key molecules include protein kinases, calcium ions, phospholipases, hydrogen peroxide, and ethylene.
- Systemic Signal Transduction leading to systemic acquired resistance (SAR).

Localized defense mechanism at the site of infection:
- Triggers a series of defense responses to release toxic compounds, killing pathogens and surrounding cells.
Acts through:
- Burst of reactive oxygen species (ROS).
- Increased ion movement across membranes (e.g., K+ and H+).
- Strengthening of cell walls through phenolic cross-linking.
- Activation of various protein kinases and production of antimicrobial substances (phytoalexins, PR proteins).

Functions include:
- Punching holes in plant membranes.
- Interfering with plant gene regulation.
- Inhibiting plant proteases.
- Degrading plant toxins.

Cytoplasmic NBS-LRR Class - 75% of R genes.
Extracellular LRR-TM Class (+ kinase) - 20% of R genes.
These classes form a universal surveillance system in plants against varied pathogens.

Resistance proteins have specialized structures:
- Leucine-rich repeats (LRRs) for signal perception.
- Leucine Zipper (LZ) and Toll/Interleukin Receptor-like domain (TIR) for signal transduction.

Suggest the use of diverse R protein mixtures for broad-spectrum resistance against pathogens.

Plants like tomatoes, rice, and Arabidopsis show varying resistance levels against specific pathogens (e.g., pseudomonas, magnaporthe, peronospora).

Understanding plant defense mechanisms through genetic interactions and biochemical pathways is essential for improving disease resistance in crops.