10.2 Biopesticides and biofertilizers

Biopesticides and biofertilizers are game-changers in sustainable agriculture. They use natural microbes and plant-derived substances to control pests and boost crop growth, reducing the need for harsh chemicals and synthetic fertilizers.

These eco-friendly alternatives work with nature, not against it. From Bt toxins that target specific pests to nitrogen-fixing bacteria that feed plants, they offer smart solutions for farmers looking to protect crops and improve yields sustainably.

Biopesticides

Microbial Pest Control Agents

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• Microbial pesticides are naturally occurring or genetically modified microorganisms (bacteria, fungi, viruses, or protozoa) that suppress pests and plant diseases
• Bacillus thuringiensis (Bt) is a common bacterial biopesticide that produces crystal proteins toxic to certain insect larvae (lepidopteran, coleopteran, and dipteran pests)
  • Bt crystal proteins bind to specific receptors in the insect gut, causing pores to form and leading to cell lysis and death
  • Bt toxins are highly specific to target pests and have minimal impact on non-target organisms (beneficial insects, mammals, birds)
• Entomopathogenic fungi infect and kill insects by penetrating their cuticle, growing inside their body, and producing toxins
  • Examples include Beauveria bassiana and Metarhizium anisopliae, which target a wide range of insect pests (aphids, thrips, whiteflies, and beetles)
  • These fungi can be applied as spores or formulated into biopesticide products for spray applications

Genetically Engineered Biopesticides

• Plant-incorporated protectants (PIPs) are pesticidal substances produced by plants through genetic engineering
• Common PIPs include Bt crops (corn, cotton, soybeans) that express Bt toxins in their tissues, providing built-in protection against target pests
  • Bt crops have significantly reduced the need for synthetic insecticide applications in many cropping systems
• Other PIPs may include transgenic plants expressing antifungal proteins, viral coat proteins for virus resistance, or enzymes that degrade fungal cell walls

Biofertilizers and Plant Growth Promoters

Microbial Inoculants for Nutrient Acquisition

• Nitrogen-fixing bacteria form symbiotic relationships with leguminous plants (soybeans, alfalfa, clover) and convert atmospheric nitrogen (N2) into plant-available forms (ammonia)
  • Rhizobium and Bradyrhizobium are common genera of nitrogen-fixing bacteria used as biofertilizers for legume crops
  • Inoculating legume seeds or soil with these bacteria enhances nitrogen fixation and reduces the need for synthetic nitrogen fertilizers
• Mycorrhizal fungi form mutualistic associations with plant roots, extending the root system and improving nutrient and water uptake
  • Arbuscular mycorrhizal fungi (AMF) colonize the roots of most land plants and exchange nutrients (phosphorus, nitrogen, micronutrients) for plant-derived carbon
  • Ectomycorrhizal fungi (EMF) associate with many tree species (pines, oaks, eucalyptus) and enhance nutrient acquisition in forest ecosystems

Plant Growth-Promoting Rhizobacteria (PGPR)

• PGPR colonize plant roots and stimulate growth through various mechanisms, including nutrient solubilization, phytohormone production, and pathogen suppression
• Examples of PGPR include Pseudomonas, Bacillus, and Azospirillum species, which can be applied as seed treatments or soil inoculants
  • Pseudomonas fluorescens produces siderophores that chelate iron and make it more available to plants, promoting growth in iron-deficient soils
  • Bacillus subtilis produces antibiotics and enzymes that suppress soil-borne pathogens (Fusarium, Pythium, Rhizoctonia) and induce systemic resistance in plants

Environmental Applications

Bioremediation Using Microorganisms

• Bioremediation involves the use of microorganisms to degrade or detoxify environmental pollutants (petroleum hydrocarbons, pesticides, heavy metals)
• Bacteria and fungi possess diverse metabolic capabilities that allow them to break down complex organic compounds and transform them into less toxic or non-toxic substances
  • Pseudomonas putida is a common bacterial species used in bioremediation of oil spills and hydrocarbon-contaminated soils
  • White-rot fungi (Phanerochaete chrysosporium, Trametes versicolor) produce lignin-degrading enzymes that can also degrade persistent organic pollutants (PCBs, PAHs, dioxins)
• Bioremediation strategies include biostimulation (adding nutrients to stimulate indigenous microbial populations) and bioaugmentation (introducing specific microorganisms with desired degradative capabilities)
  • Biostimulation of oil-contaminated beaches with fertilizers containing nitrogen and phosphorus accelerates the breakdown of petroleum hydrocarbons by native bacteria
  • Bioaugmentation of pesticide-contaminated soils with Sphingomonas species that can degrade organophosphate insecticides (chlorpyrifos, parathion) enhances the rate of pesticide removal