Science Questions

How do plants make tannins when attacked by caterpillers?

Sun, 27th Jul 2008

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Narinda, India asked:

How do plants make tannins when attacked by caterpillers?


Helen -  I guess first you want to know what tannins are and why do plants have them.  Itís something called a secondary metabolite which basically means itís something that plants make which isnít absolutely essential for plant life.  Itís not part of growing or repairing itself or existing.  Itís something thatís going to add a little extra to the way they live their lives.  These are basically things that taste bad.  They interfere with things that try to eat plants.  They do things like they bind proteins that are eaten.  Once they get inside of the caterpillar or the cow or whatever it is eating the grass it stops it from being digested.  It puts off animals from wanting to eat different types of plants.

We (humans) eat tannins.  We actually quite like tannins in tea and wine.  Some animals quite like the taste of tannins and some adapt ways of being able to deal with them.  Essentially this is a kind of plant defence mechanism to try and stop them from being munched.  Some plants will produce tannins naturally anyway.  Once theyíve got themselves growing and theyíre up doing quite well theyíll put tannins in parts of their leaves and stems just to generally keep herbivores away. 

Some plants actually do a rather clever thing which is because these are quite complex chemicals - theyíre called polyphenols - they take a bit of energy to make so it might be better to only make them when you are being attacked by something.  There are mechanisms by which a mechanical effect, whether thatís actually something eating you or even just being damaged by wind, will actually trigger a pathway that synthesises tannins.  People have shown that even by just breaking off bits of leaf it triggers genes that code for an enzyme thatís part of a pathway that generally creates this tannin.  The really clever thing is, for which I really think plants are wonderful, if one plant over here is eaten by a caterpillar and it starts producing tannins it produces another substance (a type of pheromone, a volatile hormone) that tells other plants that tells other plants nearby to start producing tannins.  Itís plants communicating to each other and protecting themselves from creatures that are trying to eat them.


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narinder asked the Naked Scientists: What is the mechanism behind the synthesis of tannins in the leaves of plants when they are attacked by caterpillars? What do you think? narinder, Sat, 26th Jul 2008

Yes, we answered this question in this show for you chris, Sat, 4th Oct 2008

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