Now, imagine you're a plant that's getting browsed. Not much fun, huh? You'd want to do something about it if you could, wouldn't you? So, let's say you can detect browsing (how would you do that? Easy, as it happens, just look out for plant chemicals that should be contained within cells, in places they should be - if there's cell contents in places it shouldn't be, the chances are you'vebeen damaged - and we all know how damaged plants can smell), it would be nice to have a quick response and produce more nasty tasting chemicals straight away, and when you regrow, it would make sense to be extra thorny in this area. Even better if you could somehow warn other branches that you're been eaten and communicate with the other side of the tree, don't you think? And as it happens, plants can do this - rather than always producing lots of costly thorns and nasty chemical defenses, plants tend to just produce a minimal output, and then up the defenses if they actually come under attach. Very sensible really. And one of the key ways they have of signalling that they're under attack is through the use of a plant hormone. Now there are several plant hormones, but the simplest is called ethene (or ethylene, it's the same and I'll use the two interchangeably here) and is a colourless gas, consisting of two carbon atoms, and four hydrogen atoms:
C = C
if you're into your chemical formulae. It's a very simple organic molecule, and plants use it for almost everything you can imagine: signalling that it's time to ripen fruit, so they all ripen at the same time (fruit importers in the west use this trick - they get people to pick unripe green fruit in the tropics, stick it on a boat to Europe (it last well if it's green), then gas it with ethene so everything ripens nicely before getting into the shop. Which explains why fruit tastes much better here than in UK... You can also use ripe bananas to speed up the ripening of other fruit, if you stick them together in a paper bag.); seed germination; fertilization, etc., etc. But for us, right now, we're interested in how they use it to signal stress, and in particular herbivory.
|This avocado is ripening thanks to a ethylene signal from the ripe bananas|
To be fair, it's not actually the main signal process for herbivory - it's probably too general in function for that task – but it does play a role and the concept is the same for the other signalling processes. Once chewed, a plant rapidly (within seconds) with start to produce ethylene. Being a gas, it can drift all around the plant, and receptors in other parts of the plant pick it up, decide what it means (“help!, I'm being eaten!”) and tell the plant to get on with appropriate defences.
All well and good. But the observant among you will already have picked up on one thing – there's nothing to stop the signal moving out of the plant being eaten and into the neighbouring plant. And if that plant can pick up the signal, then all of a sudden, plants in a neighbourhood can communicate with one another. And, in fact, this is what they do. Pretty impressive for a plant, I'd say! Since most plants use the more or less the same set of signals it's quite possible for the signal to come from an Acacia, but be picked up by a Balanites – inter-specifc plant communication. And you thought plants were just sitting there and taking it! I'll save the 'what can a plant do about it' question for another time, but for now just remember that plants can signal, even to other plants, that they're being eaten and perhaps there's something here to talk about next time you watch a bush being hammered by an elephant...
|This poor Acacia xanthophloea is showing quite how many thorns it will grow when there are lots of nasty giraffe around. Arusha NP June 2011.|