Lecture 11 Plant Defenses

Constitutive defense

A plant defense that is always present, regardless of whether an attack is occurring

Induced defense

A plant defense that is produced/activated in reaction to the presence of a pathogen, herbivore, or threat

Cuticle

Waxy, constitutive outer-surface barrier (with thick cell walls, epidermis, bark) that physically blocks pathogens/herbivores

Trichomes

Hairs on plant surfaces that act as a constitutive physical and chemical deterrent (e.g., cannabis trichomes)

Thorns

Constitutive mechanical defense structures that are modified stems

Prickles

Constitutive mechanical defense structures that arise from the epidermis (e.g., on roses)

Spines

Constitutive mechanical defense structures that are modified leaves (e.g., on cacti like Echinocactus)

Laticifers

Latex-filled channels in a plant that serve as a constitutive defense

Phytoliths

Silica bodies (e.g., in grasses) that are abrasive and damage the mandibles and guts of insects

Epicuticular waxes

A crystalline wax layer deposited atop the cuticle; reduces insect attachment (e.g., Latania palm, Nepenthes pitcher plant "slippery zone")

Secondary metabolite

A substance not strictly essential to the plant's own biological functioning (e.g., a toxin); can be constitutive or inducible

Extrafloral nectaries

Indirect constitutive defense structures (e.g., in passion fruit) that attract and feed beneficial insects that prey on herbivores

Domatia

Plant structures that house defensive ants or carnivorous/predatory mites (indirect constitutive defense)

Beltian bodies

Nutrient-rich structures on acacia plants that recruit and feed defensive ants

Indirect defense

A defense strategy where the plant attracts, houses, or supports natural enemies (predators, parasitoids) of its herbivores

Alkaloids

Nitrogen-containing secondary metabolites that include stimulants and narcotics (e.g., caffeine, nicotine, cocaine, morphine)

Isoprenoids (terpenoids)

Diverse secondary metabolites derived from 5-carbon isoprene units; many low-MW forms are volatile components of essential oils (e.g., pinene, linalool, eucalyptol, artemisinin)

Phenolics

A class of secondary metabolites including flavonoids, anthocyanins, and tannins (e.g., eugenol in cloves, tannins in tea/wine/nuts)

Isoprene

The 5-carbon building-block unit from which isoprenoids/terpenoids are derived

Artemisinin

A terpenoid compound from Artemisia annua used as an antimalarial medicine

Diosgenin

A plant compound from wild yam (Dioscorea) converted by Russell Marker into progesterone, foundational to synthetic hormone production

Doctrine of Signatures

Historical (~1600, G. della Porta) belief that a plant's physical form reveals the ailment it treats (now discredited)

Why are secondary metabolites so chemically diverse?

They function as "chemical warfare" to repel/inhibit herbivores and pathogens; diversity makes it harder for attackers to evolve resistance, even though most are built from only a few dozen versatile central intermediates

Why are secondary metabolites important sources of toxins and medicines?

They evolved to have potent, targeted effects on animal physiology (as plant defenses), and that same potency makes them valuable as human drugs/toxins depending on context and dose

Avr (avirulence) gene

A pathogen gene that codes for an elicitor (Avr protein)

Elicitor

A distinctive molecule produced by a pathogen that triggers plant defense responses when recognized

R (resistance) gene

A plant gene whose product recognizes a specific elicitor via a "lock-and-key" mechanism, conferring resistance to that pathogen

Gene-for-gene (lock-and-key) recognition

The mechanism by which plant R gene products specifically recognize pathogen elicitors to trigger induced resistance

Local response

An immediate, localized plant defense reaction confined to the site of wounding or infection

Phytoalexins

Antibiotic compounds produced in the local response that are toxic to bacteria and fungi

Pathogenesis-related (PR) proteins

Enzymes produced in the local response that break down pathogen cell walls (e.g., chitinase destroys fungal cell walls)

Chitinase

A pathogenesis-related (PR) protein enzyme that destroys fungal cell walls

Physical isolation (local response)

Quarantining an infected plant region via programmed cell death or plugging of plasmodesmata

Systemic response

A whole-plant defense response that spreads after an attack, providing broad-spectrum protection to undamaged tissue

Jasmonate pathway

A systemic response pathway that produces proteins (e.g., protease inhibitors) that reduce insect growth by inhibiting their digestion

Phloem signaling

The route by which systemic defense signals travel through the plant to induce protection in undamaged tissues

Glutamate (as a defense signal)

Acts as a rapid, wound-induced "alarm signal" that triggers long-distance calcium (Ca2+) signaling waves throughout the plant (Toyota et al. 2018)

Local vs. systemic response — key difference

Local = immediate reaction at the wound site only; Systemic = spreads throughout the whole plant to protect undamaged tissue