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.”
References:
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).