“I am not in danger. I am the danger.”

Plants. You might think they just stand, looking pretty, feeding the food your food eats, and are constantly in danger, unable to escape potential herbivory. Well think again! Plants are the danger, ready to defend themselves at (almost) any cost.

Plants. These organisms are one of the most ingenious to exist on this planet. Unlike animals (and uni students), they can’t simply run away (or sleep) to escape their problems. Thus the most impressive feature most plants have are the physical and chemical mechanisms used to defend themselves. So in honour of their magnificent mechanisms of defence, let us explore some of the intriguing and uncanny plant defence mechanisms discovered.

Many plants opt for chemical defences, often produced as secondary metabolite compounds (that is, something they don’t necessarily need for survival but helps a great deal). One such nauseous, nasty and potentially death-delivering methods of chemical defence are caused by alkaloids (1). These neurotoxic drugs have the ability to seriously mess with the brains and bodies of their arch-nemesis (most of the time), the invertebrates (2).

For instance, did you know that daily the drink many of us consume, that hot cup of coffee most of us crave, uses a substance that evolved as a natural insecticide? That’s right, that wonderful dose of caffeine we all love to enjoy is used to paralyse insects.  Similarly, other addictive drugs such as cocaine, morphine and nicotine are also insecticides, which all have powerful effects on an animal’s physiology, especially in large doses (3). So is it really a coincidence that humans have discovered compounds that affect pleasure centres of our brain…or is it the plant kingdom’s way of slowly taking over humanity?

“That wonderful dose of caffeine we all love to enjoy is used to paralyse insects”

Plants aren’t afraid to get a little physical with their defence too! You may have already experienced their more common weapons of spines, thorns, prickles and trichomes (little hairs) on your casual bush walks. These all aid in physically preventing herbivory, particularly from larger mammals.  There are, however, some wonderfully unusual methods some plants invest in to stop herbivory at all costs.

Buresera schlechtendalii have a wonderful squirt-gun defence to inhibit herbivory. If one touches its pressure points, it will squirt toxic resin that can travel up to 150 cm, scaring potential herbivores away. Additionally, if a very hungry caterpillar stumbles on the leaves of the Buresera, it releases copious amounts of fluids which bathe the leaf surface to trap its victims (3).

Fig. 1.

A newly discovered defence has found plants to fight back with their own bite, as seen members of the rock nettle family (4). Using materials similar to what human teeth and bones are comprised  of (calcium phosphate), these plants reinforce their stinging hairs with microscopic teeth to warn raiders, quite painfully, never to come back.

So, could plants be any more badass? Locked in an evolutionary arms race with herbivores, their battle for survival comes down to their clever defence mechanisms. From their methods of defence, to strategies of attack, many have some well thought-out plans to prolong their lives, and one day perhaps their dominion over earth too…

References

  1. In Defense of Plants. (2016). The Truth About Coffee. [online] Available at: http://www.indefenseofplants.com/blog/2016/3/23/the-truth-about-coffee?rq=defense [Accessed 30 Sep. 2016].
  2. Freeman, (2008). An Overview of Plant Defenses against Pathogens and Herbivores. The Plant Health Instructor.
  3. Becerra, J., Noge, K. and Venable, D. (2009). Macroevolutionary chemical escalation in an ancient plant-herbivore arms race. Proceedings of the National Academy of Sciences, 106(43), pp.18062-18066.
  4. Ensikat, H., Geisler, T. and Weigend, M. (2016). A first report of hydroxylated apatite as structural biomineral in Loasaceae – plants’ teeth against herbivores. Scientific Reports, 6, p.26073.

Images

  • Moles, A. (2016). [image]
  • https://mrtwohig.wikispaces.com/EspositoS
  • Becerra, J., Noge, K. and Venable, D. (2009). Macroevolutionary chemical escalation in an ancient plant-herbivore arms race. Proceedings of the National Academy of Sciences, 106(43), pp.18062-18066