Plant communication
Plants are exposed to many stress factors such as disease, temperature changes, herbivory, injury and more. Therefore, in order to respond or be ready for any kind of physiological state, they need to develop some sort of system for their survival in the moment and/or for the future. Plant communication encompasses communication using volatile organic compounds, electrical signaling, and common mycorrhizal networks between plants and a host of other organisms such as soil microbes, other plants, animals, insects, and fungi.
Plants communicate through a host of volatile organic compounds that can be separated into four broad categories, each the product of distinct chemical pathways: fatty acid derivatives, phenylpropanoids/benzenoids, amino acid derivatives, and terpenoids. Due to the physical/chemical constraints most VOCs are of low molecular mass, are hydrophobic, and have high vapor pressures. The responses of organisms to plant emitted VOCs varies from attracting the predator of a specific herbivore to reduce mechanical damage inflicted on the plant to the induction of chemical defenses of a neighboring plant before it is being attacked. In addition, the host of VOCs emitted varies from plant to plant, where for example, the Venus Fly Trap can emit VOCs to specifically target and attract starved prey.
While these VOCs typically lead to increased resistance to herbivory in neighboring plants, there is no clear benefit to the emitting plant in helping nearby plants. As such, whether neighboring plants have evolved the capability to "eavesdrop" or whether there is an unknown tradeoff occurring is subject to much scientific debate.
As related to the aspect of meaning-making, the field is also identified as phytosemiotics.
Volatile communication
In Runyon et al. 2006, the researchers demonstrate how the parasitic plant, Cuscuta pentagona, uses VOCs to interact with various hosts and determine locations. Dodder seedlings show direct growth toward tomato plants and, specifically, tomato plant volatile organic compounds. This was tested by growing a dodder weed seedling in a contained environment, connected to two different chambers. One chamber contained tomato VOCs while the other had artificial tomato plants. After 4 days of growth, the dodder weed seedling showed a significant growth towards the direction of the chamber with tomato VOC's. Their experiments also showed that the dodder weed seedlings could distinguish between wheat VOCs and tomato plant volatiles. As when one chamber was filled with each of the two different VOCs, dodder weeds grew towards tomato plants as one of the wheat VOC's is repellent. These findings show evidence that volatile organic compounds determine ecological interactions between plant species and show statistical significance that the dodder weed can distinguish between different plant species by sensing their VOCs.Tomato plant to plant communication is further examined in Zebelo et al. 2012, which studies tomato plant response to herbivory. Upon herbivory by Spodoptera littoralis, tomato plants emit VOCs that are released into the atmosphere and induce responses in neighboring tomato plants. When the herbivory-induced VOCs bind to receptors on other nearby tomato plants, responses occur within seconds. The neighboring plants experience a rapid depolarization in cell potential and increase in cytosolic calcium. Plant receptors are most commonly found on plasma membranes as well as within the cytosol, endoplasmic reticulum, nucleus, and other cellular compartments. VOCs that bind to plant receptors often induce signal amplification by action of secondary messengers including calcium influx as seen in response to neighboring herbivory. These emitted volatiles were measured by GC-MS and the most notable were 2-hexenal and 3-hexenal acetate. It was found that depolarization increased with increasing green leaf volatile concentrations. These results indicate that tomato plants communicate with one another via airborne volatile cues, and when these VOC's are perceived by receptor plants, responses such as depolarization and calcium influx occur within seconds.
Terpenoids
Terpenoids facilitate communication between plants and insects, mammals, fungi, microorganisms, and other plants. Terpenoids may act as both attractants and repellants for various insects. For example, pine shoot beetles are attracted to certain monoterpenes -a-pinene, produced by Scots pines '', while being repelled by others.Terpenoids are a large family of biological molecules with over 22,000 compounds. Terpenoids are similar to terpenes in their carbon skeleton but unlike terpenes contain functional groups. The structure of terpenoids is described by the biogenetic isoprene rule which states that terpenoids can be thought of being made of isoprenoid subunits, arranged either regularly or irregularly. The biosynthesis of terpenoids occurs via the methylerythritol phosphate and mevalonic acid pathways both of which include isopentenyl diphosphate and dimethylallyl diphosphate as key components. The MEP pathway produces hemiterpenes, monoterpenes, diterpenes, and volatile carotenoid derivatives while the MVA pathway produces sesquiterpenes.
Electrical signaling
Many researchers have shown that plants have the ability to use electrical signaling to communicate from leaves to stem to roots. Starting in the late 1800s scientists, such as Charles Darwin, examined ferns and Venus fly traps because they showed excitation patterns similar to animal nerves. However, the mechanisms behind this electrical signaling are not well known and are a current topic of ongoing research. A plant may produce electrical signaling in response to wounding, temperature extremes, high salt conditions, drought conditions, and other various stimuli.There are two types of electrical signals that a plant uses. The first is the action potential and the second is the variation potential.
Similar to action potentials in animals, action potentials in plants are characterized as "all or nothing." This is the understood mechanism for how plant action potentials are initiated:
- A stimulus transitorily and reversibly activates calcium ion channels
- A short burst of calcium ions into the cell through the open calcium channels
- Calcium ions reversibly inactivate H+-ATPase activity
- Depolarization activates voltage gated chloride channels causing chloride ions to leave the cell and cause further depolarization
- Calcium-ATPases decreases intracellular calcium concentration by pumping calcium ions to the outside of the cell
- Repolarization occurs when the activated H+-ATPase pumps H+ out of the cell and the open K+ channels allow for the flow of K+ to the outside of the cell
Calcium's interaction with the H+-ATPase is through a kinase. Therefore, calcium's influx causes the activation of a kinase that phosphorylates and deactivates the H+-ATPase so that the cell can depolarize. It is unclear whether all of the heightened calcium ion intracellular concentration is solely due to calcium channel activation. It is possible that the transitory activation of calcium channels causes an influx of calcium ions into the cell which activates intracellular stores of calcium ions to be released and subsequently causes depolarization.
Variation potentials have proven hard to study and their mechanism is less well known than action potentials. Variation potentials are slower than action potentials, are not considered "all or nothing," and they themselves can trigger several action potentials. The current understanding is that upon wounding or other stressful events, a plant's turgor pressure changes which releases a hydraulic wave throughout the plant that is transmitted through the xylem. This hydraulic wave may activate pressure gated channels due to the sudden change in pressure. Their ionic mechanism is very different from action potentials and is thought to involve the inactivation of the P-type H+-ATPase.
Long distance electrical signaling in plants is characterized by electrical signaling that occurs over distances greater than the span of a single cell. In 1873, Sir John Burdon-Sanderson described action potentials and their long-distance propagation throughout plants. Action potentials in plants are carried out through a plants vascular network, a network of tissues that connects all of the various plant organs, transporting signaling molecules throughout the plant. Increasing the frequency of action potentials causes the phloem to become increasingly cross linked. In the phloem, the propagation of action potentials is dictated by the fluxes of chloride, potassium, and calcium ions, but the exact mechanism for propagation is not well understood. Alternatively, the transport of action potentials over short, local distances is distributed throughout the plant via plasmodesmatal connections between cells.
When a plant responds to stimuli, sometimes the response time is nearly instantaneous which is much faster than chemical signals are able to travel. Current research suggests that electrical signaling may be responsible. In particular, the response of a plant to a wound is triphasic. Phase 1 is an immediate great increase in expression of target genes. Phase 2 is a period of dormancy. Phase 3 is a weakened and delayed upregulation of the same target genes as phase 1. In phase 1, the speed of upregulation is nearly instantaneous which has led researchers to theorize that the initial response from a plant is through action potentials and variation potentials as opposed to chemical or hormonal signaling which is most likely responsible for the phase 3 response.
Upon stressful events, there is variation in a plant's response. That is to say, it is not always the case that a plant responds with an action potential or variation potential. However, when a plant does generate either an action potential or variation potential, one of the direct effects can be an upregulation of a certain gene's expression. In particular, protease inhibitors and calmodulin exhibit rapid upregulated gene expression. Additionally, ethylene has shown quick upregulation in the fruit of a plant as well as jasmonate in neighboring leaves to a wound. Aside from gene expression, action potentials and variation potentials also can result in stomatal and leaf movement.
In summary, electric signaling in plants is a powerful tool of communication and controls a plant's response to dangerous stimuli, helping to maintain homeostasis.