You’ll never teach your house plants to do tricks like your dog. They’ll never answer “what’s wrong?” when looking puny. But you see them close their petals a night. Venus fly traps snap shut on their prey. Studies show they even respond to music! So, what gives?
Although your household fern lacks the animal-like brain we’re used to, that doesn’t mean it’s senseless. In fact, plants are keenly aware of their surroundings. Through proprioception; specific cells, genes and hormones; the familiar circadian rhythm and other specializations, plants can sense and adapt to their environment.
Proprioception: it’s not just for animals!
Just like humans have unconscious body awareness, plants sense themselves and their spatial orientation. Humans have this ability due to muscle spindles in the muscles and tendons, whereas plants use their own distinct cells to trigger gravitropism, or growth in response to gravity. In other words, gravity tells plants where to direct their roots and their shoots.
According to ScienceDaily, there is controversy over the chemical changes that promote the proper orientation of root growth:
To date, gravity sensing in plants has been explained by the starch-statolith hypothesis. For example, in roots, gravity-sensing cells at the tip of the root contain dense, starch-filled organelles known as amyloplasts. Amyloplasts settle to the bottom of the cells in response to gravity, which then triggers the hormone auxin to move to another, distinct, area of cells and causes them to elongate and bend toward gravity. However, the molecular details of exactly how the physical movement and settling of amyloplasts in one set of cells triggers the accumulation of auxin in another, physically distant, set of cells in a plant remains a mystery.
The most prevalent current hypothesis is that the cytoskeleton, or cellular scaffolding, plays a major role in this gravity-sensing, intercellular communication; the cytoskeleton is made up of filaments, consisting of the proteins actin or tubulin, that allow movement of materials along strands, such as is seen in meiosis or mitosis. However, there is a major controversy in the field regarding the role of actin in gravitropism primarily due to contradictory outcomes in studies where actin was inhibited — the most interesting ones, according to Blancaflor, being those where actin disruption actually led to enhanced gravitropism.
So, how do they do it? One thing is sure: There are specific gravity-sensing cells that tell the plant, ‘Hey! This way is down!’
About face! The sun’s the other way!
At a young age, we learn that photosynthesis is the method plants use to make their own food. Using sunlight, water and carbon dioxide, plants synthesize glucose, which is the fuel that keeps their chemical processes going.
But what if they’re not getting enough sunlight? They’ll sense it and take action, triggering phototropism.
Defined as the orientation of an organism toward light, phototropism sends chemical signals the stem, telling it to bend. From the tip of the stem, D6PK protein kinase signals the release of the phytohormone auxin. D6PK signals to PINs, or export proteins, which guide auxin from cell to cell, and eventually to its destination. This triggers cells to elongate, causing the plant to bend in the appropriate direction to receive more light.
Growth of plants isn’t only dependent on light and gravity, however.
DYK? Plants sense temperature changes
A 2016 study published in Science showed that phytochrome B, a photoreceptor, responded not only to changes in light but also changes in temperature.
Under varying light and temperature conditions, the scientists grew Arabidopsis seedlings and studied how the differences affected growth. The results showed that even with plenty of light, higher temperatures caused phytochrome B to inactivate, which in turn led to a spurt of growth. Researchers thought the opposite would happen, but instead, the plants reacted as if they needed more sunlight.
Such a surprising outcome supports the hypothesis that phytochrome B is also a temperature sensor.
They can’t ‘talk,’ but plants can communicate!
You’ve smelled the skunk’s defense mechanism. Read of the poison dart frog’s deadly, toxic armor. Heard monkeys going ape to warn of a predator. But did you know plants warn of danger, too?
Volatile organic compounds are the plant’s communication system. These airborne, odorous chemicals warn neighboring plants about dangers such as plant-eating insects. Other plants sense these chemicals, in turn releasing their own to ward off the incoming threat, whatever it may be. But airborne signals aren’t the only method.
A 2009 study from the University of Aberdeen in Scotland showed that plants can send messages through other organisms, such as fungi:
Five weeks earlier, Babikova filled eight 30 cm–diameter pots with soil containing Glomus intraradices, a mycorrhizal fungus that connects the roots of plants with its hyphae, the branching filaments that make up the fungal mycelium. Like a subterranean swap meet, these hyphal networks facilitate the trade of nutrients between fungi and plants. In each pot, Babikova planted five broad bean plants: a “donor” plant surrounded by four “receiver” plants. One of the receivers was allowed to form root and mycorrhizal contact with the donor; another formed mycorrhizal contact only, and two more had neither root nor mycorrhizal contact. Once the mycorrhizal networks were well established, Babikova infested the donor plants with aphids and sealed each plant in a separate plastic bag that allowed for the passage of carbon dioxide, water, and water vapor but blocked larger molecules, such as the VOCs used for airborne communication.
Four days later, Babikova placed individual aphids or parasitoid wasps in spherical choice chambers to see how they reacted to the VOC bouquets collected from receiver plants. Sure enough, only plants that had mycorrhizal connections to the infested plant were repellent to aphids and attractive to wasps, an indication that the plants were in fact using their fungal symbionts to send warnings.
Not only can plants warn of attacks from herbivores and pathogens, but they can also warn of drought and adapt to information received from plants around them.
What else can plants sense?
Check out the video below to find out!