Plant defences against pathogens Watch

Sophie.cerys
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I'm making notes on this topic but I'm a bit confused about at which point chemical defences are used? The 4 'stages' I've written down are:

- barriers (e.g. waxy cuticle, bark, cellulose cell walls)
- recognition of attack
- physical defences
- chemical defences

But I know that some of the chemicals produced repel insects/fungi/bacteria, so would they count as barriers? Basically, are chemical defences triggers once the cell has already been invaded by a pathogen, or is are these chemical defences produced before invasion to prevent invasion?
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OxFossil
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(Original post by Sophie.cerys)
I'm making notes on this topic but I'm a bit confused about at which point chemical defences are used? The 4 'stages' I've written down are:

- barriers (e.g. waxy cuticle, bark, cellulose cell walls)
- recognition of attack
- physical defences
- chemical defences

But I know that some of the chemicals produced repel insects/fungi/bacteria, so would they count as barriers? Basically, are chemical defences triggers once the cell has already been invaded by a pathogen, or is are these chemical defences produced before invasion to prevent invasion?
Your exam board might well have a particular way of classifying plant defences, so you should check this.
My - non-exam board - general answer would be to divide defences into two main types; mechanical barriers and chemical defences in case or after the mechanical barrier is breached.

Now, because all plants are living things (!), all the defences they use are usually manufactured biomolecules. And of course, all biomolecules are ultimately "chemicals". But the point is that there are some biomolecules that are designed to provide a mechanical barrier to attack/damage. That would include thorns, silica in grasses, waxy cuticles etc. These can work against both herbivores and micro-organisms

Under "chemical defences", I would have things that are designed to produce a direct toxic metabolic/physiological effect on the predator, and signalling chemicals. i.e.

- noxious smells,caustic juices and poisons. These can be poisonous to a herbivore (like the jiuces and chemicals in ragwort or foxglove) or to microbial attackers/infections (like antimicrobial enzymes)

- signalling chemicals that cause the plant to mobilise other defensive responses in itself (like those that cause it to close its stomata to limit further microbial infection, or that stimulate the production of chemicals toxic to herbivores)
- signalling chemicals that recruit other organisms to the defence of the plant (like the release of volatile chemicals that attract parasitoid wasps when a plant is attacked by caterpillars, or the release of chemicals from the roots that promote the growth of beneficial fungi)

hope that helps
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