Table of Contents
Overview of Plant Defense
Plants cannot run away from enemies, so they rely on a wide range of structural and chemical defenses to protect themselves from herbivores (animals that eat plants) and pathogens (viruses, bacteria, fungi, protists, and parasitic plants). These defenses can be:
- Constitutive (always present) – built into the plant’s structure or metabolism.
- Induced (activated on attack) – triggered only when the plant is damaged or infected.
Defense mechanisms can act at different stages:
- Before contact (e.g., repellent chemicals on the surface),
- During attack (e.g., toxins that harm herbivores),
- After damage/infection (e.g., sealing wounds, killing infected cells).
Below, the individual types of plant defenses are described in more detail.
Structural (Physical) Defenses
External Barriers
These defenses make it difficult for attackers to reach plant tissues.
- Cuticle
The outer surface of leaves and young stems is covered by a waxy layer, the cuticle.
Functions: - Prevents water loss.
- Acts as a physical barrier to many fungi and bacteria.
- Waxy surfaces can also make it harder for insects to gain a foothold.
- Epidermis and Bark
- The epidermis (outermost cell layer) of leaves and young organs forms a first line of defense.
- Bark on older stems and trunks is a thick, multi-layered barrier that protects against:
- Mechanical damage,
- Insect feeding,
- Entry of pathogens.
- Stomatal Defense
Many pathogens enter through stomata (small pores for gas exchange). Plants can: - Close stomata in response to certain signals (e.g., pathogen molecules),
- Alter stomatal density and distribution during development.
Surface Structures
Specialized structures further complicate attack.
- Trichomes (Plant Hairs)
Trichomes can: - Form a mechanical barrier,
- Trap or impale small insects (e.g., sticky hairs in some species),
- Secrete substances such as:
- Glandular secretions with toxins or repellents,
- Sticky resins that immobilize small herbivores.
- Spines, Thorns, and Prickles
These modified organs act mainly against larger herbivores: - Spines – modified leaves or leaf parts (e.g., cacti),
- Thorns – modified branches (e.g., hawthorn),
- Prickles – outgrowths of the epidermis or cortex (e.g., roses).
They can: - Cause pain and injury,
- Discourage grazing,
- Force animals to feed more carefully and slowly.
Mechanical Tissue Properties
- Sclerified Cells and Lignification
Some cells develop very thick, lignified walls (e.g., sclerenchyma fibers, stone cells in pears). These: - Increase tissue hardness,
- Wear down insect mouthparts,
- Reduce digestibility.
- Silica Deposition
Certain grasses and other plants deposit silica crystals in their tissues: - Makes leaves rough and abrasive,
- Damages herbivore teeth and mouthparts,
- Reduces feeding efficiency.
Chemical (Biochemical) Defenses
Plants produce a huge variety of organic compounds. For defense, secondary metabolites are particularly important. They are not directly required for growth or energy, but serve ecological roles like defense and communication.
Main Classes of Defensive Secondary Metabolites
Alkaloids
- Nitrogen-containing compounds; often physiologically active in animals.
- Examples:
- Nicotine (tobacco),
- Caffeine (coffee, tea),
- Morphine (opium poppy),
- Quinine (cinchona).
- Effects on herbivores:
- Interfere with nervous system function,
- Disrupt signaling at synapses,
- Can cause paralysis or death at high doses.
- Many alkaloids are bitter, warning herbivores that the plant is unpalatable.
Terpenoids (Isoprenoids)
- Derived from isoprene units; very diverse.
- Include:
- Essential oils (e.g., menthol, pinene),
- Resins (e.g., conifer resins),
- Latex components (e.g., in dandelion, rubber tree),
- Some phytohormones (e.g., abscisic acid, gibberellins – though their hormonal role is not the focus here).
- Defensive roles:
- Toxic or repellent to insects and vertebrates,
- Glue-like resins trap insects and seal wounds,
- Volatile terpenoids can attract natural enemies of herbivores (see below under indirect defenses).
Phenolic Compounds
- Compounds with one or more aromatic rings and hydroxyl groups.
- Tannins
- Found in leaves, bark, unripe fruits.
- Bind to proteins in the herbivore’s gut:
- Reduce protein digestibility,
- Cause astringent taste (dry, bitter),
- Deter feeding.
- Flavonoids and Related Compounds
- Some are pigments (e.g., anthocyanins) with additional roles in UV protection.
- Others act as:
- Antimicrobials (inhibiting fungal/bacterial growth),
- Signals in plant–microbe and plant–insect interactions.
- Lignin
- Structural polymer in cell walls.
- Raises mechanical strength and reduces digestibility:
- Herbivores get less energy from lignified tissue,
- Microbial decomposers and pathogens struggle to penetrate.
Sulfur- and Nitrogen-Rich Compounds
- Glucosinolates (in cabbage family, e.g., mustard, broccoli):
- In intact cells, glucosinolates are stored separately from activating enzymes (myrosinases).
- When tissue is damaged, enzymes mix with glucosinolates and produce isothiocyanates and related compounds:
- Pungent and bitter (mustard taste),
- Toxic or repellent to many generalist herbivores.
- This is a classic “two-component” defense system.
- Cyanogenic Glycosides (in many plants, e.g., cassava, some clovers, stone fruits):
- When tissue is crushed, enzymes release hydrogen cyanide (HCN):
- Cyanide blocks cellular respiration in herbivores.
- Plants must carefully compartmentalize these compounds to avoid self-poisoning.
Toxin and Anti-Digestive Strategies
- Acute Toxins:
- Cause rapid poisoning (e.g., cardiac glycosides, some alkaloids).
- Typically deter all but specialized herbivores.
- Chronic Toxins and Anti-Nutritional Factors:
- Do not kill quickly but:
- Reduce growth,
- Alter reproduction,
- Interfere with hormone signaling (e.g., phytoestrogen-like compounds),
- Damage the liver or kidneys over time.
- Protease Inhibitors:
- Block digestive enzymes (proteases) in insect guts or mammalian intestines.
- Herbivores must eat more plant material to obtain the same amount of protein.
Inducible Defenses
Inducible defenses save resources by being activated only when needed. They rely on signal perception and signaling molecules inside the plant.
Local Induction at the Site of Attack
When cells are damaged or infected, they can:
- Increase production of toxins or anti-digestive compounds near the wound.
- Reinforce cell walls (e.g., deposit more lignin or other wall materials).
- Secrete antimicrobial compounds (phytoalexins).
This local response:
- Limits the spread of pathogens,
- Makes feeding less profitable for the herbivore in that area.
Systemic Induction (Whole-Plant Responses)
Signals formed at the attack site can move through the plant and trigger defense in distant parts.
Important features:
- Systemic resistance:
After initial infection or herbivory, other tissues become: - More capable of producing defensive compounds quickly,
- Less attractive or suitable for future attackers.
- Priming:
The plant “remembers” an attack state. Later stress leads to: - Faster,
- Stronger,
- Or more targeted defense responses.
This systemic coordination allows the whole plant to behave as an integrated defense unit.
Recognition of Attackers
To mount targeted defenses, plants must distinguish different types of attackers.
Recognition of Pathogens
- Pathogens carry characteristic molecular patterns on their surfaces, often called pathogen-associated molecular patterns (PAMPs) (for example, specific parts of fungal cell walls or bacterial flagella).
- Plant cells have receptors in their outer membrane that can detect these patterns.
- Result:
- Activation of early defense responses (e.g., production of reactive oxygen species, strengthening of cell walls),
- Induction of antimicrobial compounds.
More specific recognition mechanisms, including gene–for–gene interactions, are described in detail in other chapters that treat plant–pathogen relationships.
Recognition of Herbivores
- Plants do not only respond to mechanical damage. They can distinguish between:
- Pure mechanical injury (e.g., wind damage),
- Herbivore feeding.
- How?
- Herbivore saliva and oral secretions contain specific molecules (e.g., proteins, fatty acid derivatives) that act as herbivore-associated molecular patterns.
- These are perceived by plant receptors and lead to:
- Tailored production of toxins or anti-digestive compounds,
- Emission of specific volatile substances that attract natural enemies of that herbivore.
Hypersensitive Response and Cell Death
One special form of induced defense is the hypersensitive response (HR), mainly directed against biotrophic pathogens (those that need living host cells).
Features:
- Localized cell death around the infection site:
- Infected and nearby cells actively destroy themselves,
- The pathogen loses access to living tissue and starves.
- Accompanied by:
- Accumulation of toxic compounds,
- Rapid production of reactive oxygen species,
- Reinforcement of cell walls (e.g., callose, lignin deposition).
The hypersensitive response is often the visible cause of small necrotic spots on leaves at early stages of infection.
Chemical Defense Against Microorganisms
Phytoalexins
- Low-molecular-weight antimicrobial compounds.
- Synthesized de novo by plants in response to infection or stress.
- Usually specific to particular plant groups (e.g., different legumes produce different phytoalexins).
- Functions:
- Inhibit fungal spore germination,
- Block fungal and bacterial growth,
- Disrupt cell membranes or key metabolic pathways in microbes.
Pathogenesis-Related (PR) Proteins
- Family of proteins whose expression increases during pathogen attack.
- Examples of functions:
- Chitinases: break down chitin in fungal cell walls,
- Glucanases: attack glucans in pathogen cell walls,
- Antimicrobial peptides: disrupt microbial membranes.
These proteins often accumulate not only at infection sites but also systemically, contributing to long-lasting resistance.
Indirect Defenses
Plants do not always confront attackers directly. They often use indirect defense strategies by recruiting other organisms.
Attraction of Natural Enemies of Herbivores
When attacked, plants can release volatile organic compounds (VOCs) into the air:
- Terpenoids, green leaf volatiles, and other scented molecules attract:
- Parasitic wasps that lay eggs into caterpillars,
- Predatory mites that feed on herbivorous mites,
- Predatory insects such as ladybirds that eat aphids.
In this way, plants:
- “Signal for help”,
- Increase mortality of their herbivores without investing directly in highly toxic chemicals.
Extrafloral Nectaries and Food Bodies
Some plants produce food rewards that support bodyguard organisms.
- Extrafloral nectaries:
- Nectar-secreting glands located on leaves or stems (not involved in pollination).
- Attract ants, wasps, and other insects.
- Food bodies (e.g., on some tropical plants):
- Nutrient-rich structures that ants or mites feed on.
In return, these animals:
- Patrol the plant surface,
- Attack or remove herbivores,
- Sometimes prune away epiphytic or parasitic plants.
This mutualism is an example of indirect plant defense through symbiotic partners.
Resource Allocation and Defense Strategies
Defensive structures and chemicals require energy and nutrients to produce and maintain. Plants must therefore balance growth, reproduction, and defense.
Key points:
- Trade-offs:
- High investment in defense can limit growth or seed production in resource-poor environments.
- Under low herbivory pressure, strong constitutive defenses may be a disadvantage.
- Defense Strategies:
- Always well-defended (constitutive defense): often seen in long-lived species or those exposed to constant attack (e.g., many woody plants).
- Induced defense: common in plants experiencing variable attack pressure or in situations where resources are limited.
- Tolerance: some plants invest less in defense and more in rapid regrowth after damage (e.g., grasses that regrow after grazing).
The mixture of structural, chemical, induced, and indirect defenses in a species reflects its evolutionary history and ecological niche.
Summary
Plants use a complex, multi-layered defense system:
- Physical barriers (cuticle, bark, trichomes, thorns, hardened and silica-rich tissues) make attack difficult.
- Chemical defenses (alkaloids, terpenoids, phenolics, glucosinolates, cyanogenic glycosides, protease inhibitors) poison, repel, or reduce the nutritional value of plant tissue.
- Inducible responses and systemic defense allow plants to respond dynamically and economically to varying threats.
- Recognition of specific attackers and the hypersensitive response help plants confine and resist pathogens.
- Phytoalexins and PR proteins directly inhibit microbial growth.
- Indirect defenses recruit bodyguard organisms through scents, nectar, and food rewards.
Together, these mechanisms enable plants to survive and reproduce in environments full of potential enemies. The economic and ecological implications of plant defenses are discussed more extensively in the subsequent chapter on economic applications.