(3) A plant's natural chemical defences against disease

Plants do not have specialised immune cells or antibodies like animals do.

However, if a pathogen gets past the physical defences (described above), the plant can detect it and the response is to produce antimicrobial molecules that act as a chemical defence.

So, plants can produce chemicals to help defend themselves against damage.

These chemicals can make the plants unpleasant to taste to herbivores (grazers!) or even cause death by interfering with a pathogen's metabolic pathways - poisoning!

However, some of these poisons are useful to us humans via the pharmaceutical industry.

e.g. The deadly nightshade (great name!) produces a toxin called atropine which is used to treat cardiac arrest.

 

These chemicals, called antimicrobials (act against microbes) which can kill pathogens or inhibit their growth.

Some plants increase production of these antibacterial chemicals when they are attacked by herbivores or pathogens.

Some plants produce toxic chemicals called saponins, which are believed to destroy the cell membranes of fungi and other pathogens.

Certain plants produce chemicals called phytoalexins when pathogen infection is detected. Phytoalexins disrupt the metabolism and cell structure of some species of bacteria and fungi.

 

Other plants can produce chemicals called antiseptics that kill bacteria and fungal pathogens.

The willow tree produces an antiseptic chemical.

 

Mint and witch hazel produce antibacterials and chemicals toxic to insects.

Mint produce menthol - an insect toxin, and witch hazel produce antibacterial chemicals which kill bacteria and tannin which is another insect toxin.

Spearmint plants produce an oil that acts as a natural fungicide to kill fungus.

The concentration of antibacterials increases if the plant is attacked by herbivores or pathogens.

 

Plants can produce chemicals to deter being eaten e.g. insect pests from feeding on their leaves.

Chemical defences include plants containing unpleasant tasting chemicals to deter animals from feeding off them, some chemicals are so toxic they may even kill the 'grazer'.

Plants like the tobacco plant, foxgloves and deadly nightshade produce poisons that inhibit organisms that eat plants (herbivores).

Tobacco plants contain nicotine which acts as an insecticide.

Foxgloves contain a poison called digoxin, which can be toxic if ingested by insects, but digoxin is also used in some treatments for cardiovascular disease!

 

Some of these plant produced natural chemicals, or their derivatives, can be used as drugs to treat human diseases or in medicines to relieve symptoms e.g.

The medicinal compound Aspirin is used to relieve pain and fever - aspirin is an anti-inflammatory drug.

It is synthetically derived from a chemical compound found in the bark and leaves of the willow tree.

The willow extract was known for centuries to be a pain reliever - an example of traditional medicine - now transformed into a 'modern' analgesic drug completely synthesised from a basic organic compound.

 

Quinine, an ant-fever drug, is still one of the main treatments for malaria.

Malaria is caused by a parasitic single celled organism from which you can be infected from a mosquito bite. Quinine is a very complex molecule and it is too costly to synthesise.

Therefore it is still obtained from its original natural source, the bark of the cinchona tree.

 

The pharmaceutical industry and genetic engineering applications

There are many cases where a naturally occurring chemical compound in plants (with known 'medical' effects found) is used as a starter molecule for developing new drugs ad medicines.

Some of these molecules are those produced by the plant for self-defence.

Pharmaceutical chemists can then synthesise different forms of the molecules and these derivatives then tested to see if they are potentially useful drug.

Scientists have identified the plant genes that are responsible for producing these self-defence molecules. This genetic knowledge used in genetic engineering to produce insect resistant and disease resistant crops. In other words, to get plants that don't normally produce these self-defence molecules to be genetically modified to produce them! Smart stuff!!!

• Know and understand that plants may be adapted to cope with specific features of their environment, these specialised features to deter predators include thorns, poisons and warning colours to deter predators e.g.

  • Roses have thorns, hedgehogs have needle like spikes/spines over the upper side of their body and can curl up to give all round protection.

  • Cacti have sharp spines to deter animals (herbivores) eating them, turtles, armadillos and tortoises have hard protective shells. These are examples of organisms having a sort of 'armour' for protection!

  • Plants like ivy contain poisons, some desert shrubs secrete toxic compounds into the soil to prevent other plants growing nearby.

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Key points about a plant's natural chemical defences against disease and pests

Source of information is based on textbooks and syllabus-specifications for students taking the AQA GCSE, Edexcel GCSE and OCR GCSE level biology examinations (~US grades 9-10)

Plant Chemical Defences Against Disease and Pests

Plants don’t have immune systems like humans, but they’ve evolved a powerful arsenal of chemical defences to fight off pathogens (like fungi, bacteria, protists) and deter insect pests.

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Why Plants Use Chemical Compounds

• To prevent infection by pathogens.

• To stop the spread of infection once inside the plant.

• To repel or kill insect pests that might cause damage or transmit disease.

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Types of Natural Chemical Defences

1. Antimicrobial Compounds

Substances that inhibit or destroy the growth of microorganisms, including:

• Antifungal chemicals that break down fungal cell walls or membranes.

• Antibacterial compounds that kill bacteria or prevent their reproduction.

Example: Some plants produce phytoalexins - specialised antimicrobial chemicals released after infection.

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2. Antiseptic Compounds

These prevent infection by cleaning plant tissues - similar to how antiseptics are used on human skin.

• Damage bacterial/fungal cells or disrupt enzymes.

• Often secreted around wounds or sites of pathogen entry.

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3. Toxins (Poisonous Compounds)

• Produced to deter or kill herbivorous insects.

• Can disrupt insect metabolism or cause physical damage.

Example:

• Alkaloids (e.g. nicotine, caffeine): Disrupt nervous systems of insects.

• Tannins: Bitter-tasting compounds that bind to proteins in insects’ saliva and guts, reducing digestion.

• Cyanogenic glycosides: Release cyanide when plant cells are damaged - lethal to many pests.

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Where Are These Compounds Found?

• Leaves – Often targeted by herbivores, so rich in toxins and antimicrobials.

• Roots – Protect against soil-borne pathogens.

• Stems and bark – Defensive barriers loaded with antiseptic resins.

• Wounded tissue – Produces more chemicals when damaged, as a rapid-response defence.

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Summary Table

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Keywords, phrases and learning objectives for this part on plant diseases and nutritional deficiency

• Know that a plant's natural chemical defences against disease and pests includes secretion of chemical compounds like poisonous toxins, antimicrobials, antiseptics and antibacterial organic chemical molecules.

• Be able to demonstrate an understanding that plants produce chemicals that have antibacterial effects in order to defend themselves, some of which are used by humans.

  • Plants attacked by pathogens can defend themselves by producing chemicals, often in oil secretions, that have antibacterial properties.

  • Some of these oils have medicinal properties that humans have used in traditional medicine recipes.

  • Other oils have been used as additives in products of the cosmetics industry.

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