Plants fight back!

We tend to view plants as passive but because they can’t move and hence can’t escape their enemies, they actually have to be a lot more aware than animals do.

‘Every plant is a chemical factory for complex substances which exceeds any human capability. In their poisons, antibiotic agents, prickles and foul tastes, they developed defences against attack long before human stockades and pesticides.’
– Anthony Huxley, Green Inheritance, 1984

Plants, whilst they might seem defenceless, are actually very well equipped to fight their enemies.  Some of this is down to their structure, for example trees have thick bark to protect them and some plants have spines to deter animals, and some of it is chemical warfare.

Plants produce what are called primary metabolites, which are used to keep the plant alive.  But they also produced secondary metabolites which are really interesting.  They aren’t involved in growth or keeping the plant alive but they do have a purpose.  They tend to be specialised and might be specific to species.  An example is the chemical which the eucalyptus tree produces; an oil which is antiseptic and keeps the tree’s leaves and bark healthy and protects any wounds from infection.  Whilst not directly involved in growth and food, it’s clear that this is important for the plant’s wellbeing.

Some secondary metabolites are used to attract animals eg pollinators, but the ones I find really interesting are the ones used for defence and it is these which have fascinating uses within the human world.

Plants have evolved to manipulate the behaviour of certain animals, often insects, and the mind of an insect turns out to be quite similar to our own.  So plants which produce certain chemicals with insects in mind, can be used to alter our own behaviour.  For example, caffeine and amphetamines make insects more active, cocaine makes bees dance and morphine kills the pain of insects.  The reasons for these may become clearer when you think about what a plant needs from an insect; pollination and being left alone are two key things.  So if a plant can ramp up an insects activity level, it stands a better chance of reproduction.  Similarly if a bee dances on cocaine, they might pick up more pollen.  NB. These are just my own thoughts on how the impacts might be helpful, but the chemical impact on insects comes from a research project.

Plants can sense insects and can identify what species it is and hence what response is appropriate.  They can also use this information to accurately warn neighbouring plants, this is what is happening when you can smell freshly cut grass.

The example of wild tobacco is very useful for illustrating some of the ways plants defend themselves.  When it is attacked, it increases the amount of nicotine produced and nicotine is poisonous to most animals.  If they’re being attacked by one of the animals which isn’t affected by the nicotine, the plant sends out an SOS call.  This attracts predators of the animal which is attacking the plant.  They can send out different SOS calls to different species based on what is eating it, which they know by the saliva of the herbivore.  Wild tobacco also has a chemical which enters the caterpillars eating it and makes them smell and again attracts predators.  Moths pollinate the plant but also lay their eggs on it.  But the plant is so adaptable that if too many caterpillar eggs are being laid, it changes when it blooms from dusk (when moths are around) to dawn.  If they bloom at dawn, they use different scents to attract different pollinators such as birds.  This is a very adaptable, complex plant, intelligent in it’s own way and definitely not passive and defenceless!

It’s not just animals which are under attack from the plants, they also attack competing plants. For example knapweed, a highly invasive species, releases chemicals into the soil which hard other plants and allow the knapweed to hold it’s territory.

Plants are dynamic agents which are so underestimated.

But it’s even more amazing than that.  These chemicals which the plants produce for protection are the very chemicals which make up so many of our medicines today.  We’ve already seen how nicotine is really a plant defence and whilst not medicinal, I hope it illustrates how useful these chemicals are to us.

There are three major categories of secondary metabolite: alkaloids, phenols and terpenoids.  I’m not going to get scientific here but it’s a good way of illustrating the different ways we use plant chemicals.

  • Alkaloids – many are extremely poisonous to humans but these also include morphine, atropine and cocaine.  Nicotine is an example of an alkaloid as well, as is coffee.
  • Phenols – these include tannins which we use in processing animal hides.  Phenols are also used in pesticides and are apparently found in chillis and cannabis.  Again, please note, I’m not a scientist so any scientific knowledge, whilst carefully looked into, may be slightly off the mark…
  • Terpenoids – like alkaloids, these can be useful in medicine.  For example, they include steroids as well as essential oils which often have antiseptic, antibacterial and antibiotic properties.

Today, almost 30,000 plants are known to have medicinal properties from blood thinners to antimalarials to anti cancer drugs to pain relief and more.  And all down to the active role plants play in keeping themselves safe!

A bit more reading:

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