How do plants know where they should defend?

If you’re a plant what is the best way to defend yourself, given that you can’t run from attackers? I’ve had a think and I went for being massive, with armour-planting, spikes, thorns and stings, along with poisonous leaves and roots. If this is such a good idea (and it’s not), why aren’t plants doing it? Defence on demand: mechanisms behind optimal defence patterns by Stefan Meldau, Matthias Erb and Ian Baldwin examines the optimal defence hypothesis which explains why.

Value of plant tissues.
Assumptions of the optimal defence hypothesis (ODH). Meldau et al 2012.

The optimal defence hypothesis makes a few assumptions. First, a plant has limited resources so defences have a cost. Cover yourself in toxins if you like, but all that effort creating complex chemicals will detract from efforts you could have used elsewhere, like reproduction. Next, not all parts of a plant are equally valuable. Meldau et al. have this diagram showing a gradient of value for plants. Young leaves, they argue, are more valuable to a plant than old leaves. They also argue that the different values of the plant parts make them more or less attractive to attack. Given that there’s such a difference in value, there won’t be random attacks on the plant. They’ll be more concentrated in certain areas, so it makes more sense for a plant to allocate its limited defences to the more important organs.

By itself that suggests that a quick audit of a plant’s defences would reveal how important the various parts are to it, but there’s more to it than that.

Some parts you will want to keep defended. It makes sense to have a woody stem if you think that’s likely to be attacked. It’s a very high value part, because it supports most of the rest of the plant. On the other hand the youngest leaves might not be attacked. Meldau et al. point out young tissues are more easy stimulated to produce induced defences. Induced defences could well be cheaper in the long run as some plants are not going to get eaten in any specific year and so could spend their energy on reproduction instead. That’s fine as far as it goes, but it also means that plants will need to be able to signal when organs are under attack, and this is a key factor for Meldau et al.

The probably lies with how plants use Jasmonic acid. Botany would be so much simpler if you could simply point at chemical and say this leads to that. Jasmonic acid is an example of why this is never going to happen in botany. Plant defences cannot be neatly divided from plant development, so it makes sense for the same hormone to be involved with both.

Our understanding of the underlying mechanisms mediating ontogenic regulation of JA biosynthesis and signalling is still very fragmentary.

It’s the multiple and complex use of jasmonates that comes across very strongly in this paper. Jasmonates don’t seem to work simply as jasmonates but also interact with cytokinins, brassinosteroids, gibberellins and auxin. The optimal defence hypothesis is an idea that works at the level of the whole plant. You can’t alter one part in isolation from the rest. For this reason I think that Meldau et al. make an excellent case that reducing plants to simple single hormone paths is not a reliable way of looking at plants as a systemic whole. While hormone pathways are important, they argue that for analysing plant defence you need network analysis tools.

What strikes me overall is the sheer potential for research in plant defence. Meldau et al. show that there’s so much work needed to piece together various lines of research, but at the same time this isn’t simply working in the dark. If you’re looking for a guide to other papers on the subject, this is a very helpful paper to start from.

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