Scientists in Japan appear to be intent on solving Zen riddles. A tree that falls in the forest when no one is around to hear it does indeed make a sound—or rather, a smell.
By probing deeper into the hidden ways that plants communicate, researchers at Saitama University have visually witnessed the transmission and reception of “warning” signals from stressed and injured plants to their neighbors. Within seconds of being wounded, they emit a fine mist of airborne compounds, signaling for other plants to raise their defenses.
Since the early 1980s, scientists have known about interplant interaction, called “plant eavesdropping.” Plants either damaged manually or by herbivores, such as insects, give off volatile organic compounds (VOCs) known to be perceived by other plants nearby. Sitka willow and poplar trees displayed antiherbivore properties on exposure to VOCs. So have 30 other species of plant, such as lima bean, tobacco, tomato, sagebrush, and Arabidopsis.
It wasn’t a natural setup. Captured in a bottle, caterpillars were fed leaves cut from tomato plants, and the gas contents were examined. The resultant concentration was then air-pumped to the recipient plant, bathing it in volatile compounds.
“In addition to insect attacks, VOCs released from manually smashed leaves induced [calcium ion] signals in undamaged neighboring plants.”
The Arabidopsis plants were genetically altered so that certain cells contained fluorescent protein sensors. This allowed the researchers to observe bursts of fluorescent green rippling across the plant’s spread leaves as its defensive mechanism was triggered by exposure to the volatile compound.
The fluorescent sensors would detect a stress reaction that has long been known about from other experiments. In particular, calcium ion signaling has been linked to stress perception not only in plant cells, but in humans too. Now these calcium signals—warning signs to fellow plants—would be witnessed in real time.
Exclusively engineered in the guard cells, mesophyll cells, and epidermal cells of the plant, the protein sensors would offer clues as to which cells responded first.
On the plant’s surface, guard cells are the bean-shaped cells that form stoma—the tiny pores that open the insides of a plant to the atmosphere, allowing it to “breathe.” The mesophyll cells comprise the inner tissue of a plant, while the epidermal cells are like its skin cells, forming the exterior.
So, using a fluorescence microscope, researchers Yuri Aratani and Takuya Uemura observed “warning” signals coming from the guard cells within about a minute of being exposed to danger cues. The mesophyll cells responded later.
Further testing isolated the “nostrils” of the plant as being the stoma; pre-treating the plant with abscisic acid, a phytohormone, effectively shut the plant’s stoma, which showed greatly reduced calcium signaling.
“Plants do not possess a ‘nose,’ but stomata serve as a plant gateway mediating rapid GLV entry into interspaces in leaf tissues,” Mr. Toyota said.
The same pre-treatment of abscisic acid was then applied to mutants that had impaired stomatal closures and resulted in a normal green glow. The mutant “nostrils” remained open, and warning signals appeared despite the phytohormone.
“We have finally unveiled the intricate story of when, where, and how plants respond to airborne ‘warning messages’ from their threatened neighbors,” Mr. Toyota said.
“This ethereal communication network, hidden from our view, plays a pivotal role in safeguarding neighboring plants from imminent threats in a timely manner.”
