Table of Contents
Plant defense systems are not only important for the survival of plants in nature; they are also a valuable resource for humans. Many defense mechanisms or their components are used directly or indirectly in agriculture, medicine, industry, and food preservation. This chapter focuses on these economic applications, assuming you are already familiar with the basic principles of plant defense.
1. Plant Defense in Crop Protection and Breeding
1.1 Using Natural Resistance in Breeding
Crop plants are constantly threatened by pathogens (fungi, bacteria, viruses), herbivores, and abiotic stress. One of the most important economic applications of plant defense is the targeted use and improvement of natural resistance.
Key strategies:
- Conventional breeding for resistance
- Crossing a cultivated variety with a naturally resistant wild relative.
- Selection of offspring that combine high yield with effective defense traits (e.g., resistance to mildew, rust fungi, potato blight).
- Resistance can be:
- Qualitative (often based on single major genes; strong but sometimes easy for pathogens to overcome).
- Quantitative (many genes; partial but often more durable).
- Marker-assisted selection
- Molecular markers linked to resistance genes are used to select seedlings in the lab before they are grown in the field.
- Saves time and costs in breeding programs.
- Pyramiding resistance genes
- Combining several different resistance genes in one variety.
- Makes it harder for the pathogen to overcome resistance.
- Economically important in crops like wheat, rice, and maize.
Benefits:
- Lower pesticide use and costs.
- More stable yields.
- Better suitability for low-input or organic farming systems.
1.2 Induced Resistance and Plant Strengtheners
Plants can be “primed” so that their defense is activated faster and more strongly when a real attack happens.
Economic uses:
- Chemical elicitors
Substances that stimulate plant defenses without acting as direct pesticides, e.g.: - Analogs of plant hormones involved in defense (such as salicylic acid or jasmonic acid derivatives).
- Specific microbial cell wall fragments that signal “danger” to the plant.
- Biological plant strengtheners
- Beneficial microbes (bacteria, fungi) that colonize roots or leaves and enhance the plant’s immune readiness.
- Some products are marketed as “biostimulants” or “induced resistance agents”.
Advantages:
- Reduced dependence on synthetic pesticides.
- Often acceptable in organic agriculture.
- Can help manage pathogens that are hard to control otherwise.
Limitations:
- Effects may be variable depending on environment and crop.
- Often preventive, not curative; require good timing and management.
1.3 Biological Control Using Plant-Microbe Interactions
Plant-associated microorganisms that contribute to defense are used in biological control:
- Endophytes and rhizobacteria
- Live in or around plant roots.
- Compete with pathogens or produce their own antimicrobial substances.
- Some strains are commercialized to protect crops from root diseases or leaf pathogens.
- Mycorrhizal fungi
- Mainly known for improving nutrient uptake.
- Can also increase resistance to some soil-borne pathogens and stress.
- Used as inoculants in horticulture, forestry, and restoration projects.
Economically, such biological control agents:
- Open new markets in “bio-based crop protection”.
- Can reduce losses where chemical control is ineffective or restricted (e.g., in greenhouses, after pesticide bans).
2. Plant Defensive Compounds as Pesticides and Repellents
Plants produce a wide range of chemical defense substances that we use directly or adapt for pest control.
2.1 Botanical Insecticides and Repellents
Examples used in practice:
- Pyrethrins
- Natural insecticidal compounds from chrysanthemum flowers.
- Affect insect nervous systems.
- Basis for many synthetic pyrethroids widely used in agriculture and household insecticides.
- Neem (Azadirachtin and related compounds)
- Extracts from the neem tree seeds.
- Act as feeding deterrents and growth regulators for many insect species.
- Used in agriculture, forestry, stored-product protection, and household gardening.
- Often considered more environmentally friendly and used in integrated and organic systems.
- Rotenone, nicotine, and others (historical and limited use)
- Formerly important botanical insecticides.
- Some are now restricted or banned due to toxicity or environmental concerns.
- Essential oils and plant volatiles
- Oils from plants such as peppermint, eucalyptus, citronella, and clove.
- Used as repellents against mosquitoes, moths, and agricultural pests.
- Marketed in insect repellent sprays, candles, and natural household products.
Economic significance:
- Provide alternatives to synthetic pesticides.
- Open niche markets for “green” pest control products.
- Useful tools in integrated pest management (IPM).
2.2 Allelopathy and Natural Herbicides
Some plants release compounds that suppress neighbors (allelopathy). This can be exploited:
- Cover crops with allelopathic effects
- Rye, sorghum, and some mustards leave residues that inhibit weed germination.
- Used in conservation agriculture to reduce herbicide needs.
- Derivatives as herbicide models
- Natural allelopathic compounds can serve as templates for new herbicides.
- Aim: more selective, biodegradable weed control agents.
Constraints:
- Effects in the field can be inconsistent.
- Careful management is needed to avoid inhibiting the main crop.
2.3 Plant Antimicrobials in Crop and Food Protection
- Plant-derived fungicides
- Extracts rich in phenols, saponins, or other antimicrobials can suppress fungal growth on seeds or harvest products.
- Used in seed treatment, post-harvest storage, and some organic production systems.
- Smoke, spices, and herbs
- Traditional preservation practices (smoking, spicing, drying) often harness plant secondary metabolites that inhibit microbes and insects.
- Today, some of these are standardized and used in food industry as bio-preservatives.
3. Medical and Pharmaceutical Applications
Many plant defense substances have strong effects on animal and human cells, which can be used as medicines.
3.1 Antimicrobial and Antiviral Substances
Plant defense compounds that inhibit microbes are sources for:
- Antimicrobial drugs
- Essential oils (e.g., thymol from thyme, eugenol from clove) with antiseptic effects in mouthwashes and topical preparations.
- Extracts from garlic, tea tree, and others in complementary medicine and some pharmaceutical products.
- Antiviral agents
- Some flavonoids, tannins, and other secondary metabolites show antiviral activity in experimental studies.
- Serve as leads for drug development, although relatively few are in routine clinical use yet.
3.2 Analgesic, Anti-inflammatory, and Anti-cancer Agents
Plant defense chemicals often interact with animal signaling pathways:
- Anti-inflammatory compounds
- Salicylates from willow bark inspired the development of acetylsalicylic acid (aspirin).
- Various phenolic compounds from plants show anti-inflammatory effects and are investigated as drugs or dietary supplements.
- Anti-cancer agents
- Complex plant toxins originally evolved as defense against herbivores or pathogens.
- Some have been adapted for chemotherapy (e.g., paclitaxel from yew, vinca alkaloids from periwinkle).
- These toxins interfere with cell division, which is useful against rapidly dividing cancer cells.
Economic relevance:
- Large pharmaceutical markets based on molecules first identified as plant defense substances.
- Ongoing screening of plant diversity for new bioactive compounds (“bioprospecting”).
3.3 Nutraceuticals and Functional Foods
Many plant defense compounds act as antioxidants or signaling modulators in humans at low doses.
Examples:
- Polyphenols (in berries, tea, grapes) and carotenoids (in carrots, tomatoes).
- Marketed for potential benefits in cardiovascular health, aging, and disease prevention.
- Used in functional foods, dietary supplements, and cosmetics.
These applications connect plant defense chemistry with nutrition science and consumer health markets.
4. Industrial Uses of Plant Defensive Metabolites
Plant defense molecules also find roles outside medicine and pesticide use.
4.1 Natural Dyes and UV Protection
Some pigments originally serve in plants as protection against UV radiation or herbivores:
- Anthocyanins, flavonoids, carotenoids
- Used as natural food colorants.
- Interest in cosmetic products for UV protection and antioxidant claims.
- Niche uses in textiles and natural dyeing.
Economic impact:
- Growing demand for natural coloring agents as alternatives to synthetic dyes.
4.2 Biobased Materials and Additives
- Tannins
- Originally defense against herbivores and pathogens.
- Used in leather tanning and as wood preservatives, adhesives, and corrosion inhibitors.
- Resins and latex
- Protect plants from injury and pests.
- Used to produce varnishes, adhesives, rubbers, and sealants.
- Natural antioxidants
- Plant phenolics added to fats and oils to slow rancidity.
- Extend shelf life of food and non-food products.
5. Biotechnological Exploitation of Plant Defense
Modern biotechnology allows targeted modification and transfer of defense-related traits.
5.1 Transgenic and Genome-Edited Crops with Enhanced Defense
Economic motivations:
- Insect-resistant crops
- Introduction or upregulation of genes that produce insecticidal proteins or defense compounds.
- Reduces insecticide applications, lowers production costs, and can increase yields.
- Disease-resistant crops
- Overexpression of pattern-recognition receptors or key signaling components of plant immunity.
- Introduction of resistance genes from wild relatives more quickly than by conventional breeding.
- Use of genome editing (e.g., CRISPR) to modify susceptibility genes, making crops less vulnerable without adding foreign DNA.
Benefits:
- More efficient and predictable resistance traits.
- Potential reduction of yield losses and agrochemical inputs.
Challenges:
- Regulatory requirements and public acceptance.
- Risk of resistance evolution in pests and pathogens.
- Need for careful ecological risk assessment.
5.2 Production Platforms for Defense Compounds
Plants or plant cell cultures can be used to produce valuable defensive metabolites:
- Plant cell fermentation
- Growing plant cells in bioreactors to produce specific secondary metabolites independently of climate and season.
- Economically relevant for rare or slow-growing medicinal plants.
- Metabolic engineering
- Modifying biosynthetic pathways to increase the yield of desired defense compounds.
- Transferring pathways into more easily cultivated organisms (yeast, bacteria) to mass-produce plant-derived drugs and pesticides.
These technologies turn plant defense chemistry into controlled, scalable industrial production.
6. Socioeconomic and Environmental Considerations
6.1 Market Opportunities and Value Chains
Exploiting plant defense systems creates:
- New product lines (biopesticides, botanicals, nutraceuticals).
- Added value from agricultural by-products (e.g., extracting compounds from leaves, bark, or waste material).
- Income opportunities in regions rich in plant biodiversity.
However, establishing stable value chains requires:
- Standardized cultivation and processing.
- Quality control of active compound content.
- Legal frameworks for benefit-sharing and intellectual property.
6.2 Sustainability, Risks, and Ethics
While many economic applications promise environmental benefits, they also raise questions:
- Sustainability of harvesting
- Overharvesting wild plants for defensive compounds can threaten species and ecosystems.
- Cultivation and domestication are often necessary for long-term supply.
- Ecological effects
- Use of highly active plant toxins, even if “natural,” can affect non-target organisms.
- Biocontrol organisms and resistant crops may alter ecological interactions.
- Access and benefit-sharing
- Many useful defensive compounds were discovered in traditional knowledge systems.
- International agreements (such as the Convention on Biological Diversity) aim to ensure fair benefit-sharing.
Economically successful use of plant defense systems therefore depends on balancing profitability, ecological responsibility, and social fairness.
Plant defense systems thus represent a major resource for human society. From resistant crop varieties and biopesticides to pharmaceuticals and industrial materials, their economic applications are highly diverse. At the same time, they illustrate how understanding basic biological mechanisms can lead directly to innovations in agriculture, medicine, and industry.