So I know that “decide” isn’t really the right word, but plants very clearly respond to their environment in a way that is dynamic. They can determine which side of the tree is getting more sun, and put more leaves there. they can determine where there tends to be more water in the soil, and grow more roots in that direction.
Since they don’t have what we would call a nervous system, how is this information processed? There seems to be clear research into the electrical signals moving through the Phloem, but where do these get “processed” in a way that the organism as a whole can make “decisions”?
In: Biology
Living cells are full of molecular machinery. They use hormones and amino acids and chemical concentration gradients to communicate and think. Even the source code is stored as a molecule.
A bark cell will produce a certain chemical when its getting light. The chemical takes several days to break down, so the light levels are averaged accross the whole day. If enough cells in an area produce enough of this hormone, one of them will start making a new branch. The new branch eats up all the hormone, to prevent the tree from growing too many branches close together. A new branch has a certain pattern that it follows, which includes leaves. The leaves block light from reaching the bark, preventing it from making more branch hormone.
When a new branch starts to populate itself with leaves, it starts using more water. The extra demand for water puts more stress on the roots, which causes more rooting hormone to be produced. Roots can sense water – they grow lots of little tendrils in every direction, and whichever tendril finds the most water will grow the most. This makes the roots seem like they’re intelligently growing towards the water, but really they’re just following the smell of water.
I would say it’s less of a decision and more of a response. Like if sun is shining on one side and not the other, the side with the sun shining will enact a response at a cellular level while the other side wouldn’t.
I also think that there can be an evolution aspect to it. Like the plants that don’t respond in a proper way will die and this leave all the ones that are having an appropriate response. Eventually, the traits that help the plant survive more (such as maybe having leaves grow in a certain way or as a reaction to certain conditions), will be passed down, while the other ones won’t be.
When a plant receives a signal, such as light, water, or an attack, this signal is first detected by specific receptors or sensors located on the surface of cells or within cells. Light is detected by photoreceptors, such as phytochromes and phototropins, which absorb specific wavelengths of light and undergo a structural change. This change activates the photoreceptor, triggering a cascade of intracellular events that often start with the photoreceptor phosphorylating itself or other proteins, initiating a signaling pathway.
For water detection, root cells have sensors that detect changes in soil water concentration. These sensors can be proteins that change conformation in response to moisture levels. When these sensors detect higher moisture levels, they often trigger ion channels to open or close, altering the flow of ions like calcium (Ca2+), potassium (K+), and chloride (Cl−) across the cell membrane.
In the case of herbivore attack, damage from herbivores is detected through mechanical stress sensors and chemical signals released from damaged cells. This leads to the production and release of signaling molecules like jasmonates, which then initiate defense responses.
Once a signal is received, it needs to be encoded into a form that can be transmitted and interpreted by the plant’s cells. Plants generate electrical signals through changes in membrane potential, similar to nerve impulses in animals. When a receptor is activated, it can cause ion channels to open, leading to an influx or efflux of ions. These electrical signals can travel along the plant’s vascular system, particularly the phloem, allowing rapid communication between distant parts of the plant.
Calcium signaling plays a significant role, with changes in calcium ion concentration (Ca2+ spikes) acting as a secondary messenger within cells. Specific patterns of calcium spikes encode different types of information. Proteins such as calmodulins bind to Ca2+ and change conformation, activating downstream signaling pathways that lead to specific cellular responses.
Hormonal signals also contribute to this process. Hormones like auxins, ABA, and jasmonates are synthesized in response to the initial signal. These hormones are then transported to target cells through the plant’s vascular system or via cell-to-cell transport. Target cells have specific receptors for these hormones. When a hormone binds to its receptor, it triggers a signal transduction pathway that often involves changes in gene expression, leading to the production of proteins that cause growth, stress responses, or defense mechanisms.
The final step is the integration and interpretation of these signals, leading to a coordinated response. The activation of signaling pathways often leads to the activation or repression of transcription factors, which are proteins that control the expression of specific genes. These genes might encode enzymes, structural proteins, or other factors that contribute to the plant’s response. For example, genes involved in cell wall loosening might be activated to allow cell elongation in response to auxin during phototropism.
Some responses require the activation of enzymes that modify other proteins or cellular structures. For instance, in response to herbivory, enzymes that produce defensive chemicals are activated. Proteins involved in the construction or modification of cellular structures, such as cell walls, might be activated to strengthen the plant’s defenses.
At the cellular and tissue levels, plants exhibit various changes. In phototropism, cells on the shaded side elongate more than those on the light-exposed side, causing the plant to bend towards the light. In response to herbivory, plants may produce toxic compounds or reinforce their cell walls to deter further attacks.
The decentralized nature of plant signaling means that each part of the plant can process information locally but also communicate with other parts to ensure a coordinated response. Individual cells or tissues respond to local signals, such as a leaf adjusting its angle to optimize light capture based on local light intensity. Hormones and electrical signals integrate these local responses into a cohesive action plan, ensuring that resources are allocated efficiently and the plant can respond to multiple stimuli simultaneously.
Plant cells have sensors! For example, they can sense up-and-down by growing a tiny, dense lump of minerals inside the cell. Because it’s denser than the rest of the cell, the lump will sink in the cell’s liquid — pulled in the direction of gravity. Then the cell can do different things on the side with the lump than the side without it. When coordinated with the other cells around it, this lets the plant grow in different ways depending on gravity.
Not coincidentally, the earth, water, and nutrients are usually in the direction of gravity; while air and sun are usually in the direction away from gravity. So a new sprout will grow roots in the direction of gravity, and shoots and leaves in the opposite direction.
How did you decide where to grow feet?
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