Unit 4: Biofertilizers and Biocontrol Agents

Biofertilizers

What are Biofertilizers?

Definition: Biofertilizers

Biofertilizers are substances that contain living microorganisms which, when applied to seeds, plants, or soil, colonize the rhizosphere and promote growth by increasing the supply or availability of primary nutrients to the host plant.

They are a sustainable alternative to chemical fertilizers, which can cause water pollution (eutrophication) and soil degradation.

Nitrogen Fixers

These microbes perform nitrogen fixation—the conversion of atmospheric nitrogen gas (N2), which plants cannot use, into ammonia (NH3), which they can use.

• 1. Symbiotic Nitrogen Fixers:
  • Organism: Rhizobium (bacteria).
  • Mechanism: They form a mutualistic relationship with legume plants (e.g., peas, beans, clover). The bacteria live in root nodules, where they are protected and fed by the plant. In return, they fix nitrogen directly for the plant.
  • Application: Seeds are coated with a Rhizobium inoculum before planting.
• 2. Free-Living (Asymbiotic) Nitrogen Fixers:
  • Organisms: Azotobacter (aerobic bacteria), Clostridium (anaerobic bacteria), Anabaena (cyanobacteria, also called blue-green algae).
  • Mechanism: They live freely in the soil and fix nitrogen, which becomes available to plants only after the microbes die and decompose.
  • Application: Anabaena is particularly important in rice paddies.

Mycorrhiza

• Definition: A symbiotic (mutualistic) association between a fungus and the roots of a plant ("myco" = fungus, "rhiza" = root).
• Mechanism: The fungus forms a vast network of fine threads (hyphae) that extend far into the soil, acting as an extension of the plant's root system.
• Benefits to Plant:
  • Nutrient Uptake: Greatly enhances the absorption of phosphorus and other immobile nutrients (like zinc and copper).
  • Water Uptake: Improves drought resistance.
  • Pathogen Protection: Can protect the root from soil-borne pathogens.
• Benefits to Fungus: The plant provides the fungus with sugars (carbohydrates) from photosynthesis.

VAM (Vesicular-Arbuscular Mycorrhiza)

• Definition: VAM is the most common type of endomycorrhiza (where the fungal hyphae penetrate *into* the root cells).
• Organisms: Fungi from the phylum Glomeromycota.
• Mechanism: They form two unique structures inside the plant's root cortex cells:
  • Arbuscules: Highly branched, tree-like structures. This is the primary site of nutrient exchange (fungus gives P, plant gives C).
  • Vesicles: Swollen, balloon-like structures used for energy (lipid) storage.
• Importance: VAM is found in >80% of all land plant species, including most major crops (corn, wheat, cotton). They are critical for phosphorus nutrition in low-phosphate soils.

Biocontrol Agents

What is Biological Control?

Definition: Biological Control (Biocontrol)

The use of living organisms (called "biocontrol agents" or "natural enemies") to suppress the population of a pest organism (e.g., an insect, weed, or pathogen) to a level below economic injury.

This is a key component of Integrated Pest Management (IPM) and an alternative to synthetic chemical pesticides.

Biological Control of Pests and Diseases

• 1. Microbial Biocontrol Agents:
  • Against Insect Pests: The bacterium Bacillus thuringiensis (Bt). It produces a crystal protein (Cry protein) that is toxic to specific insects (like caterpillars) but harmless to humans and other animals. When the insect eats the Bt spores, the toxin dissolves in its alkaline gut and paralyzes it.
  • Against Fungal Diseases: The fungus Trichoderma. It is a "hyperparasite" that attacks and feeds on pathogenic fungi (like *Fusarium* and *Pythium*) that cause root rot. It also outcompetes them for space and nutrients.
• 2. Insect Biocontrol Agents (Predators/Parasitoids):
  • Example 1: Ladybugs (Predator). Both adults and larvae are voracious predators of aphids, mealybugs, and other soft-bodied pests.
  • Example 2: Parasitic Wasps (Parasitoid). They lay their eggs inside the body of a pest (like a caterpillar). The wasp larvae hatch and eat the pest from the inside out.

Biopesticides

Definition: Biopesticides

Pesticides that are derived from natural materials, such as animals, plants, bacteria, and certain minerals. They are distinct from, but related to, biocontrol agents.

Biopesticides fall into three main categories:

1. Microbial Pesticides:
   • Consist of a microorganism (bacterium, fungus, virus) as the active ingredient.
   • Example: A commercial spray containing Bacillus thuringiensis (Bt) spores and its crystal toxin. This is the most widely used biopesticide in the world.
2. Plant-Incorporated-Protectants (PIPs):
   • Genetically modified (GM) plants that have had the gene for a pesticide added to them.
   • Example: Bt Cotton or Bt Corn. These plants have been given the gene from *Bacillus thuringiensis* so they produce the Bt Cry toxin in their own tissues. When a caterpillar bites the plant, it dies.
3. Biochemical Pesticides:
   • Naturally occurring substances that control pests by non-toxic mechanisms.
   • Examples: Pheromones (used to trap insects), neem oil (a plant extract that repels insects and disrupts their growth), and baking soda (used as a fungicide).
Biocontrol vs. Biopesticide:
• Biocontrol: The *strategy* of using one living thing to control another (e.g., releasing ladybugs).
• Biopesticide: The *product* you apply, derived from a natural source (e.g., a spray of Bt toxin).