Complex chemical signals are triggered when water lands on a plant to help it prepare for the dangers of rain, according to a new study published in the Proceedings of the National Academy of Sciences.

In contrast to humans, plants cannot feel pain. However, so-called mechanical stimulation — rain, wind and physical impact from humans and animals — contributes to the activation of a plant’s defense system at a biochemical level. This in turn triggers a stress hormone that, among other things, can lead to the strengthening of a plant’s immune system.

“As to why plants would need to panic when it rains, strange as it sounds, rain is actually the leading cause of disease spreading between plants,” said University of Western Australia’s Professor Harvey Millar, co-author of the study.

“When a raindrop splashes across a leaf, tiny droplets of water ricochet in all directions. These droplets can contain bacteria, viruses, or fungal spores.”

“The sick leaves can act as a catapult and in turn spread smaller droplets with pathogens to plants several feet away. It is possible that the healthy plants close by want to protect themselves,” added study lead author Dr. Olivier Van Aken, a biologist at Lund University.

In lab experiments, Dr. Van Aken, Professor Millar and their colleagues used a common plant spray bottle set on a soft spray.

Arabidopsis thaliana plants were showered once from a distance of 6 inches (15 cm) after which the researchers noticed a chain reaction in the plant caused by a protein called Myc2.

“When Myc2 is activated, thousands of genes spring into action preparing the plant’s defenses,” Professor Millar explained.

“These warning signals travel from leaf to leaf and induce a range of protective effects.”

“Our results show that plants are very sensitive and do not need heavy rain to be affected and alerted at a biochemical level,” Dr. Van Aken said.

The findings also suggest that when it rains, the same signals spreading across leaves are transmitted to nearby plants through the air.

“One of the chemicals produced is a hormone called jasmonic acid that is used to send signals between plants,” Professor Millar said.

“If a plant’s neighbors have their defense mechanisms turned on, they are less likely to spread disease, so it’s in their best interest for plants to spread the warning to nearby plants.”

“When danger occurs, plants are not able to move out of the way so instead they rely on complex signaling systems to protect themselves.”

“It was clear plants had an intriguing relationship with water, with rain a major carrier of disease but also vital for a plant’s survival,” Professor Millar concluded.

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Alex Van Moerkercke et al. A MYC2/MYC3/MYC4-dependent transcription factor network regulates water spray-responsive gene expression and jasmonate levels. PNAS, published online October 29, 2019; doi: 10.1073/pnas.1911758116