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.
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
These chemicals, called
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
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
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
chemicals toxic to insects.
Mint produce menthol - an insect toxin, and witch hazel produce
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
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
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,
be used as drugs to treat human diseases or in medicines to relieve symptoms
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
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
The pharmaceutical industry and genetic
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
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!!!
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