Long-distance communication

Signal about mineral nutrition: cytokinins

Cytokinins are a group of hormones that are primarily synthesised in the roots. Their synthesis and transport to the shoot directly depend on the plant’s nitrogen supply.

When there is enough nitrate in the soil, the root actively synthesises cytokinins. These hormones travel via the xylem to the shoot and act as a signal: “There is enough nitrogen in the soil, you can actively grow.” In the shoot, cytokinins stimulate cell division, leaf growth, delay senescence, and activate the use of amino acids for protein synthesis (Medvedev, 2012; Engels et al., 2012).

When nitrogen becomes scarce, cytokinin synthesis in the root sharply decreases. Less “growth signal” reaches the shoot. This leads to a slowdown in above‑ground development, yellowing and senescence of lower leaves. The plant, in effect, tells itself: “There is little nitrogen, it is not profitable to increase green mass; better direct resources to root development in search of nitrogen.”

Interestingly, not only nitrogen but also phosphorus and potassium affect cytokinin synthesis. Deficiency of any macronutrient reduces the level of cytokinins in the root and their delivery to the shoot (Marschner, 2012).

Key cytokinin signal: “Nitrogen availability and general mineral nutrition are normal/deficient.”

Signal about water stress: abscisic acid

Abscisic acid (ABA) is a stress hormone. When the root begins to experience water shortage (due to soil drying, salinity, or chilling), ABA synthesis in root tips sharply increases (Medvedev, 2012; Schopfer & Brennicke, 2016).

ABA travels via the xylem to the leaves and acts as an alarm signal: “Water in the soil is becoming scarce, close the stomata immediately!” In leaves, ABA causes rapid stomatal closure, reducing transpiration and preventing further water loss.

In addition, ABA affects shoot growth: it inhibits cell division and elongation, especially in young leaves. Thus, it “slows down” above‑ground growth under water deficit (Schopfer & Brennicke, 2016).

Important: ABA acts not only as a signal to the shoot. Its increase in the root also affects the root itself—stimulating root hair growth and branching in search of water.

Key ABA signal: “Water is becoming less—reduce transpiration and slow growth.”

Signal about salinity and osmotic stress

Under soil salinisation (increased salt concentration), the root experiences osmotic stress—water enters cells less readily. This also activates ABA synthesis in the root, and the mechanism described above comes into play.

In addition, under salinity, specific signalling molecules may be synthesised in the root that affect ion distribution in the shoot and ion channel function.

Key signal: “Osmotic stress—reduce transpiration.”

Signal about mechanical resistance: ethylene

In dense soil or when obstacles are present, the root experiences mechanical resistance. This stimulates ethylene synthesis in the root system. Ethylene is a gaseous hormone that can spread not only through vessels but also diffuse through intercellular spaces.

Ethylene from the root can be transmitted to the shoot. In the shoot, it causes a complex of reactions known as the “triple response”—slowing of elongation growth, thickening of the stem, and change in growth orientation. This helps the plant overcome obstacles (Medvedev, 2012).

Key ethylene signal: “There is a mechanical obstacle—change growth.”

Signal about pathogens: jasmonates and salicylic acid

When attacked by soil pathogens (fungi, bacteria, nematodes), the root activates defence responses. In response, jasmonic and salicylic acids are synthesised—signalling molecules that trigger expression of defence genes.

These signals can be transmitted to the shoot via the vessels, preparing above‑ground organs for possible attack by the same pathogens (Medvedev, 2012). This is called systemic acquired resistance.

Key signal: “There is a pathogen—prepare defence.”

What is a hydraulic signal?

Water moves through xylem vessels under negative pressure (tension) created by leaf transpiration. This tension (“suction force”) is transmitted from leaves to roots along the entire xylem.

When the root begins to absorb less water (due to soil drying, salinity, or chilling), the tension in the xylem increases—pressure falls. This pressure drop is instantly transmitted throughout the xylem, reaching the most distant leaves within seconds (Schopfer & Brennicke, 2016).

Information carried by the hydraulic signal

The hydraulic signal is a general‑purpose signal: “Less water is coming in!” It does not say why (drought, salinity, or cold), but it tells what is happening. And it does not require synthesis of new molecules.

A classic example: with sudden soil drying, leaves begin to lose turgor and wilt within minutes. This is a response to the hydraulic signal. Stomata close even if the hormonal signal (ABA) has not yet reached the leaves (Schopfer & Brennicke, 2016).

Why is the hydraulic signal the fastest?

The speed of propagation of the hydraulic signal is determined by the speed of water movement in the vessels—from 10 to 150 metres per hour. This is thousands of times faster than hormone transport with water flow (where speed is measured in centimetres per hour) (Brodribb et al., 2015).

Hydraulic signal and cavitation

Under very strong water stress, the tension in the xylem can become so high that air bubbles form in the vessels—this phenomenon is called cavitation. Bubbles block water flow. The occurrence of cavitation is another hydraulic signal that reports a critical level of stress. Irreversible damage to the conducting system may occur.

Plants can “hear” cavitation—it is accompanied by acoustic signals (ultrasound) that have been recorded in experiments (Brodribb et al., 2015).

Hydraulic signals and local decisions

The hydraulic signal acts on the whole plant as a whole. However, local pressure changes can also occur in individual root segments, allowing local decisions (see Section 4).

Carbohydrates from leaves—a signal for the root

The flow of sucrose from leaves to the root is not just nutrition. It is information about how intensively photosynthesis is proceeding. The more sugars reach the root, the better the shoot provides the root with energy.

The root “evaluates” the sugar flow as a signal: “Photosynthesis is active, there is plenty of energy—we can actively absorb minerals and grow” (Engels et al., 2012).

When photosynthesis decreases (cloudy weather, drought, leaf ageing), the sugar flow to the root diminishes. The root receives a signal: “There is little energy—need to economise, reduce uptake.” This leads to a decrease in ion uptake rate and root growth (Medvedev, 2012).

Ions as trophic signals

The concentration and ratio of ions in the xylem sap carry information about the mineral composition of the soil. An increased concentration of nitrates in the xylem is a signal to the shoot that nitrogen is sufficient and can be actively used for amino acid and protein synthesis (Medvedev, 2012).

A change in the potassium‑to‑sodium ratio in the xylem sap signals salinity. This can affect the function of ion channels in leaves and the regulation of osmotic pressure.

Metabolites as signals

Some products of root metabolism (e.g., organic acids, amino acids) can perform signalling functions when they enter the shoot. They inform about the metabolic status of the root system.

Difference between trophic signals and hormonal/hydraulic ones

• Hydraulic signal — fastest but carries minimal information (only the fact: “pressure dropped”).
• Hormonal signal — slower but carries specific information about the type of stress (drought, nitrogen deficiency, pathogen).
• Trophic signal — slowest but most informative. It shows the scale of the problem or well‑being (exactly how many nutrients are available, how active photosynthesis is).

Signal about mineral nutrition: cytokinins

What happens in the root. Cytokinins are a group of hormones (adenine derivatives) synthesised primarily in root tips, especially in zones of active cell division (meristems). The intensity of their synthesis directly depends on the plant’s nitrogen supply (Medvedev, 2012; Engels et al., 2012).

When there is enough nitrate (\mathrm{NO_3^-}) in the soil, the root actively absorbs it, and cytokinin synthesis is initiated in root cells. These hormones enter the xylem sap and are carried with the ascending water flow to the shoot.

What information the signal carries. Cytokinins tell the shoot: “There is enough nitrogen in the soil. You can actively build green mass, develop leaves, divide cells. Growth conditions are favourable.”

In the shoot, cytokinins act as a “permissive signal” for many processes:

• stimulate cell division in meristems;
• activate leaf and shoot growth;
• delay leaf senescence (suppress chlorophyll and protein degradation);
• enhance the use of amino acids for protein synthesis;
• promote the conversion of etiolated seedlings into normally developed ones (photomorphogenesis).

What happens under nitrogen deficiency. When soil nitrogen is low, its uptake by the root decreases, and cytokinin synthesis drops sharply. Less “growth signal” reaches the shoot. This leads to:

• slowed shoot and leaf growth;
• yellowing and senescence of lower leaves (nitrogen is remobilised from old organs to young ones);
• redistribution of resources in favour of the root system—the plant begins to grow roots more actively to find nitrogen.

Thus, the cytokinin signal is an indicator of the nitrogen status of the soil. It allows the shoot to “know” whether to invest energy in above‑ground growth (Engels et al., 2012).

Important: Nitrogen is not the only factor. Cytokinin synthesis is also stimulated by phosphorus and potassium, so overall cytokinins reflect the general level of mineral nutrition (Marschner, 2012).

Signal about water stress: abscisic acid (ABA)

What happens in the root. Abscisic acid is a classic stress hormone. When the root experiences water deficit (due to soil drying, salinity, or chilling), ABA synthesis in root tips sharply increases (Medvedev, 2012; Schopfer & Brennicke, 2016).

The reason is simple: under water shortage, root cells lose turgor, their membranes become destabilised. This triggers enzymatic cascades leading to ABA formation from carotenoid precursors. ABA is actively loaded into the xylem and quickly reaches the leaves.

What information the signal carries. ABA delivers a clear message to the shoot: “Water in the soil is becoming scarce. Close stomata immediately, reduce transpiration, slow growth. Water is a critical resource.”

In leaves, ABA causes a set of rapid responses:

• Stomatal closure — the main effect. ABA activates anion channels in the plasma membrane of guard cells, causing efflux of \mathrm{K^{+} and \mathrm{Cl}-}, loss of turgor, and closure of the stomatal pore. Transpiration drops sharply (Medvedev, 2012).
• Growth inhibition — ABA suppresses cell division and elongation in young leaves and stems, especially under drought.
• Activation of protective protein synthesis (LEA proteins)—these protect cells from dehydration.
• Stimulation of root growth — in the root system, ABA can paradoxically stimulate growth (so the plant “throws resources” into finding water).

An important nuance: ABA from the root is not the only source. In the shoot itself, ABA is also synthesised under water stress. But the root signal often precedes local synthesis and serves as a “warning” that prepares the shoot for impending danger (Schopfer & Brennicke, 2016).

Signal about mechanical resistance: ethylene

What happens in the root. Ethylene is a gaseous hormone synthesised in many tissues, but in the root its synthesis is activated under mechanical resistance (compacted soil, stones) or under anaerobic conditions (flooding) (Medvedev, 2012).

What information the signal carries. Ethylene from the root can spread through intercellular spaces and via vessels to the shoot, signalling: “There is a mechanical obstacle or oxygen deficiency. Change the pattern of growth.”

In the shoot, ethylene causes a characteristic “triple response” (especially in dicots):

• slowing of elongation growth;
• stem thickening (radial growth);
• change in growth orientation (horizontal growth or epinasty—leaf drooping).

Thus, the ethylene signal is a signal for growth reorganisation in response to physical barriers or lack of air.

In addition, ethylene coming from the root can influence leaf senescence and fruit abscission, though these effects are more often associated with local synthesis.

Signal about pathogens: jasmonic and salicylic acids

What happens in the root. When the root is attacked by soil pathogens (fungi, bacteria, nematodes), defence responses are triggered in infected cells, accompanied by synthesis of jasmonic and salicylic acids—key signalling molecules of systemic acquired resistance (Medvedev, 2012).

What information the signal carries. These substances, entering the xylem and reaching the shoot, report: “There is a pathogen in the soil. Activate the general defence programme.”

In the shoot, jasmonate and salicylate activate expression of genes encoding:

• protease inhibitors (protection against herbivorous insects);
• phytoalexins (antimicrobial substances);
• enzymes strengthening cell walls (lignin, suberin);
• enzymes involved in the hypersensitive response (local cell death around the pathogen).

This allows the shoot to “prepare” for possible attack by the same pathogens that may spread through the vessels.

Summary: what do hormonal signals carry?

Important note: multiplicity of signals and cross‑talk

We have considered the main hormonal signals separately, but in a real plant they do not act in isolation. For example, during drought, ABA synthesis increases and cytokinin synthesis may decrease. It is the balance of hormones that determines the final response of the shoot. Not the absolute content, but the ratio (e.g., ABA/cytokinins) is an informative signal (Medvedev, 2012).

Moreover, hormonal signals interact with hydraulic and trophic ones, creating an integrated control system (this will be discussed in the next lecture).

Key conclusion: Hormonal signals from the root to the shoot are chemical messages containing specific information about the soil condition: nitrogen availability, water stress, mechanical obstacles, presence of pathogens. Each hormone is a separate communication channel, and their interaction creates a complex signalling network that ensures adequate adaptation of the shoot to root conditions.

What is a hydraulic signal?

Water moves through xylem vessels from root to leaves under negative pressure (tension). This tension is created by transpiration—evaporation of water from leaves into the atmosphere. The more intense the transpiration, the stronger the “suction force” that pulls water from the root (Schopfer & Brennicke, 2016).

Imagine a continuous water column filling the xylem vessels from roots to the most distant leaves. This column is under tension—like a rubber band that is stretched and about to snap. Any change at one end of this system is instantly transmitted to the other end.

When the root begins to absorb less water (due to soil drying, salinity, chilling), the balance between supply and evaporation is disturbed. The tension in the water column increases—pressure in the xylem drops. This pressure change propagates at enormous speed along the entire xylem, from root to the farthest leaves (Schopfer & Brennicke, 2016; Brodribb et al., 2015).

This is a hydraulic signal—a change in hydrostatic pressure in the water‑conducting system that carries information about water supply disruption.

Information carried by the hydraulic signal

The hydraulic signal is an alarm signal of the most general nature. It tells the shoot only one fact:

“Less water is coming in!”

This signal does not contain information about the cause of the disturbance (drought, salinity, root cooling, mechanical damage to vessels). It does not indicate the scale of the problem. It simply states the fact: the “supply‑evaporation” balance is disturbed.

However, precisely because of this “poverty” of content, the hydraulic signal becomes the fastest. It needs no time for molecule synthesis, loading into vessels, or transport. It is the change in the physical state of water itself.

Example: sudden wilting

A classic example of the hydraulic signal in action is the plant’s response to sudden soil drying. If, on a bright sunny day, the roots suddenly stop receiving water, the leaves lose turgor and begin to wilt within minutes. Stomata close almost instantaneously (Schopfer & Brennicke, 2016).

In this case, the hormonal signal (ABA) has not yet been synthesised in the root and reached the leaves. The hydraulic signal—the pressure drop in the xylem—is at work. The water supply disruption is “felt” by the leaves directly, without intermediaries.

Why is the hydraulic signal the fastest?

The speed of propagation of the hydraulic signal is determined by the speed of water movement in xylem vessels. This speed varies among plants and conditions but typically ranges from 10 to 150 metres per hour (Brodribb et al., 2015). For comparison:

Why is the hydraulic signal so much faster than the hormonal one? Because:

1. It is a physical, not chemical, phenomenon. Pressure change is a change in the state of water, not movement of messenger molecules. It requires no synthesis time.

2. Transmission occurs along a continuous water column. Xylem is a continuous system of capillaries from root to leaf. A pressure change at one point is instantly (within hydrodynamics) transmitted throughout the system.

3. No active transport is needed. The signal moves passively, along with water, without energy expenditure.

That is why the hydraulic signal acts in seconds, while the hormonal one requires minutes or even hours.

Hydraulic signal and cavitation

Under very severe water stress, the tension in the xylem can become critical. The water in the vessels is in a metastable state—highly stretched and ready to break. If the tension exceeds a certain threshold, air bubbles form in the water column—this phenomenon is called cavitation (Schopfer & Brennicke, 2016; Brodribb et al., 2015).

Cavitation is not just a hydraulic signal; it is a rupture of the water column. Bubbles block water flow, and the vessel becomes non‑conducting. This can lead to irreversible damage to the conducting system, especially if cavitation affects many vessels.

Cavitation as a signal of critical stress

Cavitation is an alarm of the highest level. It tells the shoot: “Water supply is critically disrupted—tissue death may occur.”

Plants can “hear” cavitation. When bubbles form, microscopic acoustic vibrations (ultrasound) occur, which have been recorded in experiments. These signals can serve as an additional indicator of stress.

However, it is important to understand: cavitation is damage, not just a signal. It can lead to dieback of entire branches, especially in trees. Therefore, many plants have strategies to prevent cavitation:

• Early stomatal closure—the hydraulic signal acts before tension becomes critical.
• Narrow vessels—in narrow vessels, cavitation occurs at higher tension (capillary effect).
• Presence of alternative conducting paths—if one vessel cavitates, water can bypass it through neighbouring ones.

Hydraulic signal and local decisions

The hydraulic signal acts on the whole plant, but it can be locally modified. For example, pressure change in an individual root tip can affect local water and ion uptake. This transitions to the topic of local decisions (see Section 4), which the plant makes at the level of individual organs.

Link between hydraulic and hormonal signals

Hydraulic and hormonal signals under stress often act sequentially:

1. Hydraulic signal—instantaneous stomatal closure upon pressure drop in xylem (seconds).

2. Hormonal signal (ABA)—synthesised in the root in response to stress, reaches leaves and keeps stomata closed for a longer time (minutes–hours).

Thus, the hydraulic signal provides a rapid, and the hormonal a sustained, response. This is a reliable system that does not allow the plant to “miss” water deficit.

Key conclusion:

• The hydraulic signal is the fastest way to transmit information from root to shoot. It is a change in pressure in the water column of the xylem.
• It carries minimum information (“water supply disrupted”) but acts almost instantly.
• The speed of the hydraulic signal is thousands of times greater than that of the hormonal signal, making it indispensable for emergency responses (e.g., stomatal closure during sudden drought).
• The critical level of the hydraulic signal—cavitation (rupture of the water column)—is a signal of irreversible damage to the conducting system.

What is a trophic signal?

A trophic (from Greek trophē — nourishment) signal is a change in concentration, composition, or ratio of nutrients in conducting tissues, which is perceived by target cells as information about the resource status of the organism.

Unlike hormonal signals, trophic signals do not require specific receptors for each substance. Cells “sense” the concentration of metabolites (sugars, amino acids, ions) through already existing systems of transport and metabolism. This is an economical and reliable way to transmit information.

Sugars as a trophic signal: the “energy code”

The flow of sucrose from leaves to the root. The main flow of organic substances in the plant moves through the phloem—from leaves (sources of photosynthates) to roots and other heterotrophic organs (sinks). Sucrose is the main transport form of carbohydrates in most plants (Schopfer & Brennicke, 2016; Taiz et al., 2023).

This flow is not just nutrition for the root. It is information about the state of the shoot. The more intense the photosynthesis, the more sucrose is synthesised in the leaves and the more reaches the root.

What information does the sugar flow carry? The root “evaluates” the concentration of sucrose arriving via the phloem and receives the signal:

“Photosynthesis is active. There is plenty of energy. We can intensify mineral uptake, develop the root system, store carbohydrates” (Engels et al., 2012).

When the sugar flow decreases (due to cloudy weather, drought, leaf ageing, damage to leaf surface), the root receives the opposite signal:

“There is little energy. Need to economise. Reduce uptake activity, slow growth” (Medvedev, 2012).

Example: defoliation as a strong trophic signal

Defoliation (leaf removal) is a classic experiment demonstrating trophic signalling. When we remove leaves, we stop sugar supply to the root. The root “understands” that the shoot cannot support it. In response:

• mineral ion uptake sharply decreases;
• root growth slows or stops completely;
• some roots may die (remobilisation of their resources occurs).

This is not just “starvation” of the root. It is receiving information that the photosynthetic system is damaged or insufficiently active. The root adapts to the new situation by reallocating resources.

Carbohydrates as regulators of gene expression

Modern research shows that sugars (especially glucose and sucrose) act as signalling molecules regulating the expression of thousands of genes (Taiz et al., 2023; Engels et al., 2012).

At high sugar concentrations in cells:

• photosynthetic genes are repressed (why synthesise photosynthetic proteins if sugars are already abundant?);
• genes related to storage compound synthesis (starch, proteins) and sugar utilisation for growth are activated.

At low sugar concentrations:

• photosynthetic genes are activated (more carbohydrates need to be produced);
• genes for mobilisation of reserves are activated (starch is broken down to provide energy).

Thus, sugars are not only “fuel” but also a regulatory signal linking the level of photosynthesis with the metabolic needs of the organism. And this signal is transmitted from shoot to root via the phloem sap flow.

Mineral ions as trophic signals

Not only organic substances but also mineral ions moving through the xylem from root to shoot carry information.

Nitrogen (nitrates) as a signal. The concentration of nitrates in xylem sap is a direct signal to the shoot about how much nitrogen is available in the soil. An increased nitrate concentration activates:

• synthesis of nitrate reductase and other enzymes of nitrogen metabolism;
• synthesis of amino acids and proteins;
• shoot growth and development.

At low nitrate concentration, the shoot “understands” that nitrogen is scarce and switches metabolism to an economy mode: growth slows, senescence of lower leaves accelerates, from which nitrogen is mobilised for young organs (Engels et al., 2012).

Phosphates as a signal. Phosphate signalling is also important. Under phosphorus deficiency, less phosphate reaches the shoot. This triggers adaptation programmes to phosphorus starvation in the shoot: changes in gene expression, phosphatase synthesis, mobilisation of phosphorus from old tissues.

Ion ratios. Not only absolute concentration but also the ratio of ions can be informative. For example, the potassium‑to‑sodium ratio in xylem sap indicates the degree of soil salinisation.

Amino acids and other metabolites as signals

The flow of amino acids from root to shoot and back is also a trophic signal. A high concentration of amino acids in phloem sap may indicate active protein synthesis in the root or protein breakdown in senescing tissues.

Especially important is the flow of glutamine and asparagine—the main transport forms of organic nitrogen in many plants. Their concentration is an indicator of nitrogen status and nitrogen metabolism activity (Schopfer & Brennicke, 2016).

Comparison of the three types of signals from root to shoot

Interaction of the three signal types: drought

Let us see how the three signal types work together in the example of drought:

1. Hydraulic signal (seconds): pressure in xylem drops → stomata close instantly.

2. Hormonal signal (ABA) (minutes): root synthesises ABA → ABA reaches leaves → stomata remain closed, long‑term adaptive responses are triggered (synthesis of protective proteins, changes in gene expression).

3. Trophic signal (hours–days): stomatal closure limits CO₂ uptake → photosynthesis slows → less sugars reach the root → the root “understands” that the shoot cannot supply it with energy → reduces uptake and growth.

Thus, all three channels work as a single system, providing rapid (hydraulic), sustained (hormonal), and long‑term (trophic) adaptation (Medvedev, 2012; Schopfer & Brennicke, 2016).

Trophic signals and regulation of source‑sink relationships

Understanding trophic signals is closely linked to the concept of sources and sinks. Leaves are sources of photosynthates (sucrose). Roots are typical sinks that use these sugars for growth and ion uptake.

The trophic signal (sugar flow) is the main regulator of the balance between sources and sinks. If sources produce many sugars (intense photosynthesis), sinks receive a signal “you can grow”. If sources are weakened, sinks receive a signal “save energy” (Engels et al., 2012).

This principle underlies the distribution of assimilates in the plant and largely determines its productivity.

Key conclusion:

• Trophic signals are information encoded in the concentration of nutrients (sugars, amino acids, ions) in conducting tissues.
• The flow of sucrose from leaves to the root is the main trophic signal, informing the root about the state of photosynthesis and energy availability.
• Ions (especially nitrates and phosphates) also serve as trophic signals, informing the shoot about the mineral status of the soil.
• Trophic signals are the slowest but most informative: they show the scale of a problem or well‑being.
• Trophic signals underlie the regulation of source‑sink relationships and determine resource allocation in the plant.

Classic example: defoliation

Defoliation is the most vivid experiment demonstrating the trophic signal from shoot to root. When we remove leaves (completely or partially), we sharply reduce sugar supply to the root. The root “understands” that the shoot cannot support it. In response, the following occurs:

1. Mineral ion uptake (especially nitrogen and phosphorus) sharply decreases.

2. Root growth slows or stops completely.

3. Some roots begin to die, especially old and fine ones (remobilisation of their resources—mobilisation of nitrogen, phosphorus, and carbohydrates from roots to the shoot) (Engels et al., 2012).

This is not just “starvation” of the root. It is an informational response: the root receives a signal that the shoot can no longer supply it with energy and reorganises its activity.

Reverse effect: If photosynthesis is increased (e.g., by raising light intensity or CO₂ concentration), the sugar flow to the root increases. The root receives the signal “you can grow” and enhances nutrient uptake, which may ultimately increase yield.

Senescence signal and resource redistribution

It is important to understand: leaf senescence is not just a loss. It is part of the plant’s survival strategy. When nitrogen in the soil is scarce, the plant may accelerate senescence of lower leaves to mobilise nitrogen from them and redistribute it to young leaves and roots (Engels et al., 2012).

Thus, the root receives information that resource reorganisation is occurring in the shoot. This allows the root to coordinate its uptake and distribution of mineral substances.

1. Local enhancement of growth in nutrient‑rich patches

Roots have an amazing ability to branch selectively in those soil patches where the concentration of nutrients (especially nitrogen and phosphorus) is higher. This phenomenon is called root architectural plasticity (Marschner, 2012).

How does it work? A local increase in nitrate or phosphate concentration is directly perceived by cells of the root tip. This triggers local hormone synthesis (e.g., auxin) and activation of genes responsible for pericycle cell division. As a result, lateral roots begin to form precisely in that region (Medvedev, 2012).

Important: This process does not require a signal from the shoot. It is initiated and controlled locally. The shoot may not even “know” that the root has found a rich patch, but the root already uses that advantage.

2. Local changes in uptake and exudate release

Under local phosphorus deficiency, roots can release organic acids (citric, malic, oxalic) and phosphatases into the rhizosphere. These substances solubilise insoluble soil phosphates, making phosphorus available for uptake (Marschner, 2012).

This is also a local decision. Only those root segments that encounter phosphorus deficiency activate synthesis and release of these compounds. Other segments, where phosphorus is sufficient, do not do this. This saves energy and resources.

Similarly, under local iron deficiency, roots release phytosiderophores (in grasses) or acidify the rhizosphere (in dicots) to mobilise iron. These responses are strictly localised to zones experiencing deficiency (Marschner, 2012).

3. Local response to pathogens and mechanical damage

Upon local pathogen penetration into the root, it is the affected region that triggers the hypersensitive response—programmed cell death around the infection site to isolate and destroy the pathogen. This occurs without shoot involvement (Medvedev, 2012).

Upon mechanical damage to the root (e.g., cutting), ethylene and other signals are locally activated, stimulating healing and formation of new roots.

4. Local regulation of water potential and osmosis

Roots can locally change their osmotic concentration to maintain water uptake even in saline patches. This is achieved by accumulation of osmotically active substances (proline, sugars, ions) precisely in those cells that contact saline soil (Schopfer & Brennicke, 2016).