A plant cannot run away
Plant defenses fall into two broad kinds. Physical defenses are structures that block or deter attackers: the waxy cuticle, cell walls, bark, thorns, spines and hairs. Chemical defenses are substances that poison, repel or slow attackers, or kill microbes. Some defenses are always present (constitutive); others are switched on only after an attack (induced), which saves resources when there is no threat.
Physical defenses
| Structure | What it is and what it stops |
|---|---|
| Cuticle and epidermis | A waxy layer over tightly packed cells with thick walls; most microbes cannot cross it |
| Bark (cork) | Dead cells with waterproof suberin in their walls; seals stems against microbes and insects |
| Trichomes | Leaf hairs that hinder small insects; glandular ones release sticky or toxic chemicals |
| Thorns | Modified side shoots with sharp tips (hawthorn); deter large animals |
| Spines | Modified leaves (cactus); deter large animals |
| Prickles | Sharp outgrowths of the epidermis (rose); deter large animals |
Mechanical defenses such as thorns, spines and prickles work mainly against large grazing animals. Small insects can often walk straight past them, which is why plants also need chemical defenses.
Chemical defenses
Plants make thousands of secondary metabolites that are not needed for basic growth but protect them. Alkaloids such as nicotine and caffeine are toxic to many insects; tannins make leaves hard to digest; mustard and cabbage plants store harmless glucosinolates that turn into pungent, toxic compounds when a leaf is chewed.Antimicrobial compounds, including phytoalexins made only after infection, kill or slow microbes. Damaged leaves can also make proteinase inhibitors, which stop insects digesting the protein they eat.
Innate immunity in plants
Plants have no adaptive immune system like vertebrates: no antibodies, no circulating white blood cells and no immune memory built by specialised cells. Instead, every plant cell carries an innate immune system encoded in its genes. Each cell can detect danger and defend itself, and chemical signals then coordinate the response across the whole plant.
PAMPs, PRRs and PTI
Microbes carry molecules that are essential to them and shared by whole groups, calledpathogen-associated molecular patterns (PAMPs), such as bacterial flagellin and fungal chitin.Pattern recognition receptors (PRRs) on the surface of plant cells detect them and triggerpattern-triggered immunity (PTI):
- Stomata close within an hour or so, shutting a main entry route for bacteria.
- A burst of reactive oxygen species damages microbes and acts as a signal.
- Callose is deposited to reinforce cell walls, forming plugs (papillae) under fungal penetration attempts.
- Cells make antimicrobial compounds and defense proteins.
PTI stops most microbes that land on a plant, which is why most plants are resistant to most pathogens.
Effectors, R proteins and ETI
Pathogens that are adapted to a particular plant inject effector proteins into its cells. Effectors block PTI; one bacterial toxin, coronatine, even reopens closed stomata. Plants fight back withresistance (R) proteins, mostly inside the cell, that recognise specific effectors (or their effects). This triggers effector-triggered immunity (ETI), a faster and stronger version of PTI. Whether a plant variety resists a pathogen strain often depends on whether it has an R gene matching one of that strain’s effectors, the “gene-for-gene” relationship.
Hypersensitive response and SAR
ETI often ends in the hypersensitive response (HR): cells around the infection site kill themselves, leaving a small brown spot. This works best against pathogens that need living cells to feed on. The infected leaf also makes salicylic acid, and mobile signals spread through the plant. Distant leaves build up defense proteins and become primed to respond faster. This systemic acquired resistance (SAR)protects against a broad range of pathogens for weeks.
Defense against insects
When a caterpillar chews a leaf, the wounding and chemicals in its saliva trigger the jasmonic acid (jasmonate) pathway. The damaged leaf starts making toxins and proteinase inhibitors, and signals travel through the vascular system so that undamaged leaves do the same: a systemic wound response. Damaged plants also release volatile chemicals that attract predators and parasitic wasps, which lay their eggs in caterpillars. This is an indirect defense, recruiting the herbivore’s enemies. Sap-sucking aphids cause little wounding, so they trigger a weaker jasmonate response and are often stopped by sticky glandular hairs instead.
Pathogens vs herbivores
| Against pathogens | Against insect herbivores |
|---|---|
| Detected by PRRs (PAMPs) and R proteins (effectors) | Detected through wounding and chemicals in insect saliva |
| Mainly salicylic acid signalling (for pathogens that need living cells) | Mainly jasmonic acid signalling |
| Stomatal closure, callose, antimicrobials, hypersensitive cell death | Toxins, proteinase inhibitors, trichomes, volatiles that attract predators |
| Systemic acquired resistance | Systemic wound response |
Pathogens differ too: bacteria enter through stomata and wounds, many fungi break through the cuticle, and viruses need a wound or an insect vector. Pathogens that kill cells and feed on dead tissue are resisted mainly through jasmonate-related defenses, so the hypersensitive response does not help against them.
Why it matters
- Crop breeding: breeders move R genes into crops, for example to protect wheat against rust fungi; pathogens can evolve new effectors to escape them.
- Plant protection: some sprays switch on SAR without killing the pathogen directly.
- Pest control: crops with more trichomes or toxins, and the encouragement of parasitic wasps, reduce insecticide use.
- Medicines and stimulants: many drugs and flavours, from morphine to caffeine and mustard, are plant defense chemicals.
Common misconceptions
- “Plants have no immune system.” They have an innate immune system in every cell, but no adaptive immune system with antibodies.
- “Thorns, spines and prickles are the same.” They grow from a shoot, a leaf and the epidermis respectively.
- “All pathogens trigger the same response.” Entry routes, PAMPs, effectors and the best defense differ between bacteria, fungi and viruses.
- “The hypersensitive response means the plant is losing.” Sacrificing a few cells stops the pathogen spreading.
- “Plant defenses are always switched on.” Many are induced only when an attack is detected.
What the simulator simplifies
The leaf section is a cartoon with one stoma, and microbes, cells and hormone levels are tracked with simple rules. PTI, ETI, salicylic acid and jasmonate are each a single number from 0 to 100%, rather than networks of hundreds of genes. Real infections take hours to days, not seconds. In the model, pathogens only reach a plant cell’s surface, effectors simply reduce PTI to a quarter, and ETI always leads to the hypersensitive response when the R gene matches. Real fungi can also enter through stomata, and some pathogens produce several effectors that interact with several R proteins. The insect model leaves out the many other signals, such as ethylene and salicylic acid, that tune herbivore defense.
Key takeaways
- Physical defenses (cuticle, bark, thorns, spines, prickles, trichomes) block or deter attackers; chemical defenses poison, repel or kill them.
- Plants have innate immunity in every cell, but no adaptive immune system.
- PRRs detect PAMPs and trigger PTI; adapted pathogens suppress PTI with effectors; R proteins detect effectors and trigger ETI.
- The hypersensitive response sacrifices infected cells, and salicylic acid leads to systemic acquired resistance.
- Insect damage triggers jasmonate signalling, toxins in distant leaves and volatiles that attract the insects’ enemies.