How do plants make tannins when attacked by caterpillers?

27 July 2008

Question

How do plants make tannins when attacked by caterpillers?

Answer

Helen - I guess first you want to know what tannins are and why do plants have them. It's something called a secondary metabolite which basically means it's something that plants make which isn't absolutely essential for plant life. It's not part of growing or repairing itself or existing. It's something that's 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 they've got themselves growing and they're up doing quite well they'll 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 - they're 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 that's 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 that's 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. It's plants communicating to each other and protecting themselves from creatures that are trying to eat them.

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