[mario (OP)]

Do plants have an immune system? If so, how do they work?

[chris]

Yes they do, of sorts. Plants face a regular assault from bacteria, fungi and viruses. They combat them in various ways including killing infected cells (apoptosis), forming scar tissue around sites of infection to "wall off" invading organisms, and blocking up their transport systems (the phloem) that would otherwise act as a miniature motorway to help bugs translocate around a plant. By depositing the sugar callose into the sieve plates that separate the phloem vessels, infected vessels are plugged and the movement of the infecting agent, or chemical toxin, can be blocked.

Chris

[blakestyger]

As well as what Chris has described, plants have a further line of defense.

A plant chewed by a slug, say, becomes 'immunised' by the event. It somehow senses the chewing damage and sends chemical signals to the nuclei of the plant's cells which causes increased gene activity to encode proteins that accumulate - these proteins have the property of inhibiting a specific class of enzymes found in the slug's gut that digest proteins.

There are also secondary chemical signals that enter the plants vascular system and spread to all parts - root, stem, shoots... that then accumulate the inhibitors so that when one leaf is chewed all the plant becomes protected.

It is clear, however, that this enhanced defense is breachable - just look at any Hostas that slugs have been on - they're like lace! []

[opus]

Another eg.- oak galls- produced by the tree in response to wasps laying their eggs in them.....

[chris]

Thanks for these other excellent suggestions. Blakestyger raises a very good point in relation to gene expression. When plants are traumatised by herbivores they activate genes that produce toxic deterrents - like nicotine and caffeine, as well as other chemicals that make the plant taste less palatable and chemicals that make it grow faster to repair the damage.

There is also an interesting community effect whereby plants produce pheromone signals that move through the air and are picked up by their near-neighbours, which can even include plants of another species. In response to these pheromones, adjacent as yet unharmed plants also upregulate their defences.

Chris

[blakestyger]

Further to this, if you wish to find out more about the 'whys and wherefores' of the inner workings of plants, in the same vein as these answers, you could look at the very readable Reaching for the Sun by John King, Cambridge University Press, 1997.

[DoctorBeaver]

...they activate genes that produce toxic deterrents - like nicotine and caffeine,

I'm gonna start eating plants!  []

But, seriously, that's all fascinating. I didn't realise plants could do all that. That's what I love about this site - I find out about things I never even knew existed.