Unlike animals, plants cannot run away from dangers like leaf-eating caterpillars or flower-chewing cows. While those are the more obvious hazards to plants, new research shows that plants also "panic" when exposed to something that is essential to their survival – rain.
It might not be obvious from looking at them, but plants have developed an intricate defence system to help them respond to and communicate to other organisms about potential danger. For example, when mechanical force receptors sense that an insect is eating their leaves, some plants release noxious chemicals with a foul taste while others release scented hormones to attract insect-eating wasps in the hope that the munching insect becomes lunch themselves.
New research published in the journal Proceedings of the National Academy of Sciences has revealed that plants can also detect when even the lightest of rain showers is upon them to help them to prepare for the very real dangers of rain.
While plants need water to survive, rain is also the leading cause of disease spread between plants, which can potentially be life-threatening.
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Absorbed through tiny hairs that cover the surface of the roots, raindrops saturating the soil surrounding a plant are crucial to keeping the plant alive. This moisture is pulled up through a pathway of tubes, which draw water upwards to the leaves through a process called capillary action.
Raindrops splashing across the surface of a leaf, however, can be much more dangerous to a plant. When a raindrop bounces off a leaf it can ricochet in different directions - if one plant is sick, these bouncing microdroplets can catapult bacteria, viruses or fungal spores from the infected leaf and transfer them to the leaves of other plants close by.
Researchers believe that healthy plants have developed a defence mechanism to protect themselves against this potential attack, triggering a complex signalling system when they detect raindrops on their leaves.
To test the theory, the scientists used a spray bottle set to a soft spray and used it to shower water on to the leaves of Arabidopsis thaliana plants. Showering the plant lightly once from a distance of 15cm away, the researchers measured how the plant responded and found that within minutes the plant produced a warning signal. This signal quickly travelled from the wet leaf to the rest of the plant through a protein called MYC2, which when activated causes thousands of genes to mobilise and prepare the plant's defences.
In an effort to protect itself the plant also communicated this rain threat to other plants in the nearby vicinity by sending a signal to them through the air. Immediately after the spray bottle shower, the researchers measured the release of a hormone called jasmonic acid from the plant. The hormone is able to be detected by other plants in the area, letting them know about the impending rain danger. The theory is that if other plants in the area also turn on their defence mechanism - such as by activating a systemic immune response - they are more likely to be able to defend against any potentially incoming pathogens and are therefore less likely to be the cause of disease spreading in the next rainstorm.
While they might not have eyes to see, noses to smell or ears to hear, plants are still incredibly aware of the environment around them. The study shows that even though they may not be able to move out of the way, by being so receptive to changes in their surroundings, plants are highly equipped to defend themselves and able to detect even the tiniest of touches.