Plants are sensitive to touch and respond according to what is touching them. The feel of a neighboring plant receives a distinct reaction from that of an herbivore or falling rain.
For a plant, rain is a mixed blessing: bringing needed moisture but also potential pathogens. Swedish botanist Olivier Van Aken: “When a raindrop splashes across a leaf, tiny droplets of water ricochet in all directions. These droplets can contain bacteria, viruses, or fungal spores. Sick leaves can act as a catapult and in turn spread smaller droplets with pathogens to plants several feet away.”
To counter the perceived threat that rain brings, plants prepare for a potential microbial onslaught. Even a few drops provoke communications among leaves of a plant and signals to those in the neighborhood. Australian botanist Harvey Millar: “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.”
Alex Van Moerkercke et al, “A MYC2/MYC3/MYC4-dependent transcription factor network regulates water spray-responsive gene expression and jasmonate levels,” PNAS (29 October 2019).
“Plants’ reaction to rain is close to panic, study shows,” Sci News (30 October 2019).