Ethylene and abscisic acid

From Stimulation to Restriction: The Two Poles of Hormonal Regulation

In previous lectures, we got acquainted with three groups of hormones commonly referred to as growth stimulators. Auxins, gibberellins, and cytokinins — each in its own way — all work towards the same task: ensuring plant growth and development. Auxin triggers cell elongation, gibberellin stimulates division and elongation, and cytokinin supports the mitotic activity of meristems (Schopfer & Brennicke, 2016; Tretyakov et al., 2000).

A student just beginning to learn about hormonal regulation might naturally form the impression: hormones are what make the plant grow. This impression would be correct until we ask ourselves a simple but fundamental question:

Why can growth not continue indefinitely?

Indeed, if auxins, gibberellins, and cytokinins are so effective at stimulating cell division and elongation, why doesn't a tree grow to the sky? Why does an annual plant, even with all necessary resources, not continue to vegetate indefinitely? And why, at a certain point, does the plant not merely slow down growth but actively initiate senescence and cell death?

The short answer: because infinite growth is evolutionarily disadvantageous. A plant that grows forever will ultimately lose to one that knows when to stop in time and switch to other programs. However, behind this simple answer lies a complex physiological reality, which will be the subject of our discussion over the next two lectures.

Why Can't Growth Be Infinite?

Let's imagine a hypothetical plant that truly grows forever. What would happen to it? First, it would face a problem of resource supply. Every new cell requires building material — carbon, nitrogen, mineral elements, water. Even the most powerful plant cannot sustain continuous biomass accumulation without exhausting its own reserves and environmental resources (Connor et al., 2011).

Second, growth has its cost. As shown in the chapter on respiration (Connor et al., 2011), a significant portion of assimilates — up to 30-50% of total photosynthesis — is spent on respiration, primarily on maintaining existing structures (so-called maintenance respiration). The greater the biomass, the more energy is required just to sustain it. At some point, maintenance costs may equal photosynthetic income, and further growth becomes energetically inefficient.

Third, growth makes the plant vulnerable. Thin, fast-growing shoots are easily damaged by wind, grazed by herbivores, and attacked by pathogens. It is no coincidence that fast-growth strategies are typical of species living in conditions where the main priority is to occupy space as quickly as possible, rather than strength and longevity (Lambers & Oliveira, 2019).

But most importantly — from a biological point of view, the meaning of a plant's life is not to reach maximum size, but to leave offspring. This is a fundamental difference between multicellular plants and, say, bacterial colonies. Unlimited growth is possible for bacterial populations in ideal laboratory conditions, but in nature it is always limited by resources. For a plant, the task of growth is to accumulate enough resources to form reproductive organs: flowers, fruits, seeds. Once this task is accomplished, continued vegetative growth becomes not just superfluous but harmful — it diverts resources away from seed maturation (Taiz et al., 2023; Tretyakov et al., 2000).

Two Strategies: To Grow or Not to Grow

Thus, at every moment, the plant faces a strategic choice: continue growing or switch to something else. This "something else" can be various:

• Defense against unfavorable conditions (cold, drought, pathogens)
• Completion of the life cycle (flowering, seed maturation)
• Transition into dormancy — as a way to survive an unfavorable season

This is precisely where the two groups of hormones covered in this lecture come into play: ethylene and abscisic acid (ABA).

If auxins, gibberellins, and cytokinins are "builder" hormones, then ethylene and ABA are "regulator" hormones that can halt construction and initiate entirely different programs. It is important to understand: they are not simply "brakes" operating on a "stop-signal" principle. These hormones initiate complex, often irreversible changes that require active work of the genetic apparatus and the involvement of multiple metabolic pathways (Hopkins & Hüner, 2009; Schopfer & Brennicke, 2016).

Ethylene and ABA: Two Approaches to Growth Cessation

Although both ethylene and ABA can inhibit growth processes, the mechanisms of their action and physiological context differ significantly. The following analogy can be made.

Ethylene is a hormone that is activated when the plant needs to quickly change its developmental program. It acts like an emergency switch, triggering a cascade of events that can lead to fruit ripening, leaf senescence, or even cell death. Importantly, ethylene often operates on the principle of positive feedback: the more it is produced, the more its own synthesis is stimulated. This creates a "chain reaction" effect — the transition from one state to another becomes rapid and irreversible (Hopkins & Hüner, 2009; Schopfer & Brennicke, 2016).

Abscisic acid acts differently. It is a hormone that helps the plant survive stress, especially stress related to water deficiency. ABA does not simply inhibit growth — it switches the plant into a resource-saving mode. It closes stomata, slows cell division and elongation, and promotes entry into dormancy. Unlike ethylene, ABA often operates on the principle of negative feedback: the signal for its synthesis is the stress condition itself, and the action of ABA is aimed at reducing the consequences of that stress (Kuznetsov & Dmitrieva, 2006; Taiz et al., 2023).

Key Question

Thus, the central question we will address is:

How does the plant "know" that it is time to stop growth and switch to defense or completion of the life cycle?

The answer will come in two parts. First, we will examine how ethylene works — the hormone that switches the plant from vegetative development to ripening, senescence, and stress defense. Then we will move to abscisic acid — the hormone that allows the plant to survive drought and prepare for unfavorable conditions by entering a state of dormancy.

Together, these two hormones reveal the remarkable ability of plants not merely to react to environmental changes but to actively manage their own destiny, choosing the right moment to stop growth and transition to subsequent life stages.

2.1. Ethylene: The Simplest and Most Unusual Hormone

Ethylene is a gaseous hydrocarbon (C₂H₄) with a very simple molecular structure (Hopkins & Hüner, 2009). But it is precisely this simplicity that makes it unique among phytohormones. Unlike all others, ethylene is not stored in cells in conjugated form and does not require complex transport systems. It simply diffuses through membranes and intercellular spaces, and its excess easily volatilizes into the atmosphere. This means that ethylene concentration in tissue is determined primarily by its synthesis rate and removal rate (diffusion and oxidation). Once synthesis stops, the hormone quickly disappears.

This feature makes ethylene an ideal "switch signal": its appearance means that a synthesis process has been activated in the cell, and this process often amplifies itself (autocatalysis). The disappearance of ethylene allows the system to quickly return to its original state if conditions change (Schopfer & Brennicke, 2016).

Ethylene is synthesized in almost all plant tissues, but especially actively in senescing leaves, ripening fruits, and under stress (mechanical damage, flooding, drought). Its biosynthesis proceeds from the amino acid methionine through three stages: methionine → S-adenosylmethionine (SAM) → 1-aminocyclopropane-1-carboxylic acid (ACC) → ethylene (Hopkins & Hüner, 2009; Kuznetsov & Dmitrieva, 2006).

The key enzyme that determines the rate of the entire process is ACC synthase. Its activity sharply increases in response to signals: auxin, wounding, senescence. The second enzyme — ACC oxidase — converts ACC into ethylene, and this step requires oxygen.

The most remarkable aspect of ethylene biosynthesis is autocatalysis. Ethylene can stimulate its own ACC synthase and, consequently, enhance its own synthesis (Taiz et al., 2023). This creates a positive feedback loop: a small initial impulse triggers an avalanche-like increase in hormone concentration, making the transition from one state to another rapid and irreversible. This is why fruit ripening, once begun, cannot be stopped.

2.2. One Strategy, Many Manifestations

All the diverse effects of ethylene can be reduced to one common task: to reorganize the developmental program in response to a change in internal or external signals. Let's examine the main manifestations of this strategy.

2.3. How Does a Cell "Hear" Ethylene? Perception and Signal Transduction

Ethylene is perceived by a family of receptors — membrane proteins similar to two-component sensory systems of bacteria. In Arabidopsis, there are five such receptors (ETR1, ETR2, ERS1, ERS2, EIN4). They are localized in the endoplasmic reticulum and contain a domain that binds ethylene, requiring a copper ion as a cofactor (Taiz et al., 2023; Hopkins & Hüner, 2009).

A crucial feature of this system: the receptors are constitutively active — that is, in the absence of ethylene, they transmit a signal that suppresses the ethylene response. When ethylene binds to the receptor, it turns off this suppressive signal, allowing the ethylene response to occur. This is so-called "negative regulation" (Taiz et al., 2023; Schopfer & Brennicke, 2016).

The signal is then transmitted via the protein CTR1 (a kinase) and EIN2 (a membrane protein) to the nucleus, where transcription factors EIN3/EIL1 are activated, initiating the expression of hundreds of genes responsible for ethylene effects (Taiz et al., 2023). Such a mechanism allows for very rapid and finely tuned regulation: as soon as ethylene disappears, the receptors are reactivated, and suppression is restored. This makes the system easily reversible — except in cases where autocatalysis is triggered.

2.4. Ethylene as a Catalyst for Life Transitions

Thus, ethylene is not simply a "senescence hormone" or "ripening hormone." It is a program-switching hormone. Its appearance means that the plant must either rapidly change the nature of growth, initiate the final developmental stage of an organ (ripening), start a separation process (abscission), or activate defense mechanisms (injury or flooding). In all cases, ethylene ensures speed, irreversibility (when needed), and coordination of these changes at the tissue and organ levels.

Its uniqueness lies in its autocatalytic synthesis, which turns a slowly building signal into an avalanche-like process. This makes ethylene an ideal mediator for transitions that must happen quickly and once and for all (e.g., fruit ripening). In other cases, when the transition must be reversible or dose-dependent (e.g., under drought), ethylene works in concert with other hormones, particularly abscisic acid, which we will discuss in the next part.

Thus, we see how ethylene answers the question "when to stop growth?": it does so not by gradual braking, but by switching to alternative developmental programs. Now we turn to abscisic acid — a hormone that acts differently but also helps the plant stop growth and switch to a survival mode.

3.1. Why Does a Fruit Need to Ripen? The Physiological Meaning of the Process

The fruit is not merely an organ in which seeds mature. It is a "bait" for animal dispersers. From an evolutionary perspective, the success of a seed plant is determined not only by seed quality but also by the ability to deliver them to a suitable place for germination. Fleshy, brightly colored, and sweet fruits attract animals that eat them and then carry the seeds over considerable distances (Taiz et al., 2023).

However, such fruits become attractive only at the final stage of development — when the seeds have already matured and are ready for dispersal. Until that time, the fruit must remain green, hard, tasteless, and inconspicuous — so as not to attract animals prematurely. The plant's task is to precisely synchronize the appearance of the fruit's attractive properties with the completion of seed maturation.

This synchronization requires a special mechanism — one that would allow the fruit to "know" that the seeds have matured, and then quickly and coordinately change all its properties. That mechanism is autocatalytic ethylene synthesis.

3.2. Autocatalytic Ethylene Synthesis: How Does the "Chain Reaction" Work?

As we have already mentioned, ethylene synthesis follows the pathway: methionine → SAM → ACC → ethylene. The key enzymes are ACC synthase and ACC oxidase (Hopkins & Hüner, 2009; Schopfer & Brennicke, 2016).

In young, non-ripening fruits, these enzymes operate at minimal levels. Ethylene concentration is low, and the fruit remains in a "waiting" state. However, as seeds mature within the fruit, signals accumulate that trigger the expression of ACC synthase genes (Taiz et al., 2023). The initial, very small amount of ethylene that begins to be released binds to its receptors and triggers a cascade in which one of the effects is additional activation of transcription of ACC synthase and ACC oxidase genes.

This is autocatalysis: ethylene stimulates the synthesis of its own enzymes, leading to even more intense ethylene production. Hormone concentration rises exponentially, reaching levels hundreds of times higher than the initial level within hours or days. This process is often termed positive feedback, in contrast to negative feedback (where a signal inhibits its own synthesis), which is typical of most other hormonal systems (Schopfer & Brennicke, 2016).

It is important to emphasize that autocatalysis is not just signal amplification. It is a physiological state switch. Until autocatalysis is triggered, the fruit can remain in the green state indefinitely, even if some ethylene is present. But once the trigger mechanism fires, the process becomes irreversible.

3.3. Climacteric Rise in Respiration: Energy Support for the Restructuring

Associated with autocatalytic ethylene synthesis is the so-called climacteric rise in respiration — a sharp increase in respiration intensity observed in climacteric fruits at the onset of ripening (Taiz et al., 2023; Schopfer & Brennicke, 2016).

This rise has a complex nature. Partly, it is related to ethylene activating the alternative respiratory pathway (cyanide-resistant respiration), where energy is dissipated as heat (Schopfer & Brennicke, 2016). But more importantly, the sharp increase in respiration provides the energy and reducing power (ATP, NADPH) for the intense biosynthetic processes occurring during ripening: synthesis of carotenoids and anthocyanins, sugar accumulation, formation of aromatic compounds, and synthesis of cell-wall-degrading enzymes (Taiz et al., 2023; Tretyakov et al., 2000).

Interestingly, in non-climacteric fruits (grapes, strawberries, citrus), ethylene autocatalysis is not triggered. They ripen gradually, without a sharp rise, and show no apparent positive feedback (Taiz et al., 2023). This shows that ethylene autocatalysis and climacteric ripening are not universal but rather a particular, yet highly successful, evolutionary pathway typical of many important agricultural crops.

3.4. Why Does the Fruit "Wait": The Role of Auxin as an Inhibitor of Ripening

Now we know the mechanism, but the question remains: what prevents the fruit from prematurely triggering autocatalysis? Why does a young, immature fruit not start ripening, even though all the enzymes for ethylene synthesis are potentially present?

The answer lies in the antagonism between ethylene and auxin. In young, actively growing fruits, high auxin levels suppress the expression of ACC synthase and ACC oxidase genes (Schopfer & Brennicke, 2016; Tretyakov et al., 2000). The fruit remains in a state of "braking." As seeds mature, auxin concentration in the fruit decreases, and ethylene accumulation gradually begins to rise.

When a threshold level of ethylene is reached, autocatalysis is triggered. Interestingly, ethylene, in turn, can suppress auxin synthesis and transport, further enhancing the effect (Taiz et al., 2023; Schopfer & Brennicke, 2016). This creates a situation where the two hormones act as a switch: while auxin dominates, the fruit vegetates; once ethylene dominates, ripening is launched. The transition is very fast, almost like a trigger.

3.5. Ripening as a Coordinated Transition

It is important to understand that fruit ripening is not simply the "turning on" of some individual processes. It is a coordinated reorganization of entire metabolism, during which hundreds of genes change their expression (Taiz et al., 2023). Ethylene acts as the master regulator of this process, ensuring:

• Synchronization — all fruit cells begin to ripen simultaneously.
• Coordination — chlorophyll breakdown, sugar accumulation, aroma synthesis, and softening occur in the correct sequence.
• Irreversibility — once started, the process cannot be stopped.

This is why one ripening fruit can "infect" neighboring ones: the ethylene it emits acts on them as an external signal, triggering their own autocatalytic synthesis (Taiz et al., 2023). This effect is widely used in fruit storage and ripening practices.

3.6. Autocatalysis as an Answer to an Evolutionary Challenge

Returning to the original question: why do fruits start to ripen by themselves? Because only an autocatalytic mechanism provides the rapid, coordinated, and irreversible change necessary for successful seed dispersal.

Imagine an alternative: if ripening were regulated by gradual accumulation of some substance on a "sand-timer" principle, fluctuations in external conditions could delay it or cause it to start too early. Autocatalysis, however, guarantees that as soon as seeds are mature and the auxin block is removed, the fruit enters a new state very quickly, leaving no room for random factors to disrupt the process. This is an "all-or-nothing" strategy (Schopfer & Brennicke, 2016) that minimizes the risk of error.

Thus, the ability of a fruit to start ripening "by itself" is not an accident but an evolutionarily refined mechanism that allows the plant to make maximum use of its dispersers and ensure that seeds are ready for dispersal at the most opportune moment.

In the next part, we will turn to abscisic acid and learn how the plant "decides" to close its stomata even before wilting begins. We will see that ABA is not just a "stress hormone" but a remarkable mechanism of foresight, allowing the plant to avoid danger rather than merely respond to it.

4.1. The Paradox: How Does the Plant Know About Drought Before the Leaves Lose Water?

Imagine: the soil gradually dries out. One might think the plant should respond when leaves begin to lose water and turgor drops. However, numerous experiments show that stomatal conductance decreases long before leaves lose turgor (Hopkins & Hüner, 2009; Lambers & Oliveira, 2019). The plant seems to "know" about impending drought in advance.

How is this possible? The answer came from classical experiments with split root systems. If the roots of one plant are placed in two containers — one with moist soil, the other with drying soil — stomata will close even if the leaves receive enough water from the moist side (Kuznetsov & Dmitrieva, 2006; Lambers & Oliveira, 2019). This means the signal originates not from the leaves but from the roots that are in contact with the drying soil.

Roots, sensing water deficiency, synthesize ABA and send it via the xylem (transpiration stream) to the leaves, where it induces stomatal closure (Marschner, 2012; Taiz et al., 2023). This is a root signal that precedes changes in leaf water status. The plant does not wait until dehydration becomes critical — it prevents it.

4.2. ABA as a "Warning Signal": From Roots to Stomata

The scheme of this mechanism is as follows (Hopkins & Hüner, 2009; Kuznetsov & Dmitrieva, 2006):

1. In drying soil, water potential decreases, and roots begin synthesizing ABA de novo (as opposed to mobilizing from stores). Synthesis proceeds via the carotenoid pathway: from violaxanthin through xanthoxin and abscisic aldehyde to ABA (Taiz et al., 2023; Schopfer & Brennicke, 2016).

2. ABA is loaded into the xylem and transported with the transpiration stream to the leaves. ABA concentration in xylem sap can increase several tens of times within the first hours of soil drying (Schopfer & Brennicke, 2016).

3. In the leaves, ABA reaches the guard cells of stomata, where it binds to specific receptors — PYR/PYL/RCAR proteins (Taiz et al., 2023; Hopkins & Hüner, 2009).

4. This triggers a signal transduction cascade leading to stomatal closure: efflux of potassium ions (K⁺) and anions (Cl⁻) from guard cells, water efflux by osmosis, turgor loss, and closure of the stomatal pore (Schopfer & Brennicke, 2016; Taiz et al., 2023).

Thus, ABA acts as a preemptive signal that allows the plant to minimize water loss before it becomes inevitable. This is especially important under conditions where drought develops gradually and the plant has time to prepare.

4.3. Why Is "In Advance" Advantageous?

Closing stomata before turgor loss provides the plant with several benefits:

• Water savings: if stomata close at the first signs of soil drying, a significant amount of water that would otherwise be lost to transpiration can be conserved.
• Prevention of damage: sudden water loss can damage membranes, denature proteins, and disrupt photosynthesis. Preemptive stomatal closure helps avoid these consequences (Lambers & Oliveira, 2019; Taiz et al., 2023).
• Maintenance of photosynthesis: although stomatal closure reduces CO₂ uptake, it also prevents photoinhibition, which can occur when high light intensity coincides with water deficit (Marschner, 2012).

It is important to emphasize that ABA does not act in isolation. Other hormones are also involved in the regulation of the stomatal apparatus: cytokinins, which can attenuate ABA action, and ethylene, which under certain conditions may enhance closure (Taiz et al., 2023; Tretyakov et al., 2000). However, ABA is the primary mediator in transmitting the signal from roots to leaves.

4.4. ABA — Not Only a "Drought Signal"

ABA is a hormone with a broad spectrum of action, and its functions are not limited to water regime regulation. It participates in:

• Winter preparation: ABA synthesis in autumn initiates dormancy in buds and seeds (Schopfer & Brennicke, 2016; Tretyakov et al., 2000).
• Root growth regulation: under water stress, ABA promotes continued root growth while shoot growth is inhibited (Lambers & Oliveira, 2019). This allows the plant to increase the absorbing root surface in search of water.
• Seed maturation: high ABA levels in developing seeds promote storage compound accumulation and entry into dormancy (Taiz et al., 2023; Schopfer & Brennicke, 2016).
• Response to other stresses: ABA participates in responses to salt stress, cold, and certain pathogens (Hopkins & Hüner, 2009).

4.5. ABA and Ethylene: Two Approaches to Growth Cessation

Now we can compare ethylene and ABA as two tools for stopping growth.

Thus, ethylene and ABA complement each other. Ethylene triggers processes that must happen once and for all (fruit ripening, leaf senescence). ABA manages reversible changes that help the plant survive temporary unfavorable conditions.

4.6. Molecular Mechanism of ABA Action: How the Hormone "Turns Off" Growth

We have already mentioned ABA receptors — PYR/PYL/RCAR proteins. How do they work? In the absence of ABA, protein phosphatases (PP2C) suppress the activity of SnRK2 protein kinases. When ABA binds to the receptor, it inhibits these phosphatases, and SnRK2 kinases are activated. They, in turn, phosphorylate ion channels in the guard cell membrane, leading to ion efflux and stomatal closure (Taiz et al., 2023; Hopkins & Hüner, 2009). This mechanism, as you can see, is very fast and does not require new protein synthesis.

However, ABA also acts at the transcriptional level: it activates genes encoding proteins that help cells survive stress (e.g., heat shock proteins, osmoprotectant synthesis enzymes). This is a slower but important component of adaptation (Schopfer & Brennicke, 2016).

Summary

ABA is a hormone that allows the plant to anticipate danger and respond to it in advance. It is synthesized in roots as the soil dries and transported to leaves, where it induces stomatal closure before leaves lose turgor. This mechanism conserves water and prevents damage. ABA is not just a "stress hormone" — it is a key component of the early warning system that enables the plant to adapt to changing conditions.

In the next, concluding part, we will examine how ABA and other hormones participate in the plant's transition to dormancy — another example of how growth cessation can be an active, controlled process rather than passive decline.

5.1. Why Does the Plant Enter Dormancy? An Evolutionary Strategy

Dormancy is a state in which visible growth is absent, but all vital processes continue, albeit at a reduced level. Seeds, buds, tubers, bulbs, and rhizomes can remain dormant for long periods, awaiting favorable conditions for growth resumption (Schopfer & Brennicke, 2016; Kuznetsov & Dmitrieva, 2006). This strategy allows the plant to survive unfavorable seasons: winter, drought, cold.

However, dormancy is not simply "switching off" metabolism. It is active inhibition, requiring the synthesis of specific inhibitors, changes in gene expression, membrane remodeling, and accumulation of protective substances. Without these active processes, dormancy would not occur, and if it did, the plant could not maintain it.

5.2. ABA — The Primary Initiator of Dormancy

A key role in dormancy induction is played by abscisic acid. Its concentration sharply increases in seeds, buds, and tubers during the transition to dormancy (Taiz et al., 2023; Schopfer & Brennicke, 2016). ABA acts as the "master brake," which:

• suppresses the synthesis of nucleic acids and proteins required for growth;
• inhibits the activity of hydrolytic enzymes that could mobilize storage reserves (e.g., α-amylase in cereals);
• promotes the accumulation of storage proteins and lipids, which will be used during germination.

But most importantly, ABA causes reprogramming of gene expression, switching cells into a stress-resistant state. In response to ABA, proteins that protect cells from dehydration, cold, and oxidative stress (so-called LEA proteins, late embryogenesis abundant proteins) are synthesized (Taiz et al., 2023; Lambers & Oliveira, 2019). This is why dormant organs can withstand very low temperatures and deep dehydration.

5.3. Deep and Forced Dormancy: The Role of External Factors

Two types of dormancy are distinguished: forced and deep (organic) (Kuznetsov & Dmitrieva, 2006; Tretyakov et al., 2000).

Forced dormancy is caused by unfavorable external conditions (low temperature, water shortage, poor aeration). As soon as conditions become favorable, dormancy ceases. This type of dormancy does not require special endogenous mechanisms — it is simply a growth stop due to resource deficiency.

Deep dormancy is a genetically programmed state into which the plant enters under the control of internal signals, including ABA. Even under optimal external conditions, such a plant will not grow until it has passed a certain period of cooling (stratification) or receives a specific signal (e.g., light or gibberellin). Deep dormancy is characteristic of seeds of many woody species, winter cereals, potato tubers, and buds of fruit trees.

5.4. Breaking Dormancy: The Role of Gibberellins and Cytokinins

If ABA is the "entry" into dormancy, then gibberellins and cytokinins are the "exit" from it. Their ratio determines whether the plant will germinate or remain dormant (Schopfer & Brennicke, 2016; Tretyakov et al., 2000).

When a dormant seed or bud receives the necessary signals (e.g., sufficient moisture, temperature alternation, long day), gibberellin synthesis begins in the embryo or meristem. Gibberellins activate hydrolytic enzymes (especially α-amylase), which break down storage reserves, providing energy and building materials for growth resumption. Gibberellins also stimulate cell division and elongation, leading to the rupture of seed coats and seedling emergence.

Cytokinins, synthesized in roots, also promote dormancy breaking: they activate protein and nucleic acid synthesis, stimulate cell division in meristems, and attenuate the inhibitory action of ABA (Kuznetsov & Dmitrieva, 2006; Taiz et al., 2023). Thus, breaking dormancy is not a passive "release of the brakes" but an active process requiring the synthesis of new enzymes and resumption of growth-related gene expression.

5.5. The Role of Ethylene in Dormancy Regulation: A Dual Nature

Ethylene participates in dormancy regulation in an ambiguous way. In some cases, it promotes dormancy breaking (e.g., in peanut seeds and some weeds), while in others, it deepens dormancy (e.g., in potato tubers) (Schopfer & Brennicke, 2016; Tretyakov et al., 2000). This is related to ethylene's ability to influence ABA and gibberellin synthesis and metabolism.

For instance, during potato storage, excess ethylene can inhibit sprouting by enhancing ABA action. This underlies some technologies for extending tuber dormancy. Conversely, ethylene treatment of lettuce seeds can stimulate germination, especially in combination with light. Thus, ethylene acts as a modulator that, depending on context and interactions with other hormones, can influence the transition from dormancy to growth or vice versa.

5.6. Physiological Processes in the Dormant Organism: Maintaining Life

It is important to emphasize that during dormancy, the following processes continue in cells:

• Respiration — although reduced, sufficient to maintain membrane potential and ATP synthesis (Marschner, 2012).
• Protein synthesis — renewal of enzymes and structural proteins, especially those protecting cells (e.g., antioxidant enzymes, chaperones) (Taiz et al., 2023).
• Maintenance of ion homeostasis — operation of membrane pumps to retain potassium and other important ions (Marschner, 2012).
• Storage compound turnover — though slowed, continued transformation of complex compounds into more accessible forms as needed (Kuznetsov & Dmitrieva, 2006).

Thus, dormancy is a dynamic equilibrium, not a static state. If the plant simply "switched off," it would die from starvation, membrane damage, or accumulation of toxic metabolic products. Maintaining dormancy requires energy and regulation, making it as active a process as growth.

5.7. Agricultural Significance of Dormancy Management

Understanding dormancy physiology has enormous practical importance for agriculture:

• Storage of seeds, tubers, and fruits — maintaining dormancy allows keeping produce for extended periods without quality loss (Tretyakov et al., 2000; Marschner, 2012). Controlling temperature, humidity, and gas composition (e.g., reducing oxygen and increasing CO₂) can prolong forced dormancy.
• Stratification — artificial cooling of seeds to overcome deep dormancy, necessary for cultivating many fruit and forest species (Schopfer & Brennicke, 2016).
• Flowering regulation — in winter crops, a dormancy period (vernalization) is required for the transition to generative development (Taiz et al., 2023).
• Prevention of pre-harvest sprouting — preventing premature germination of grain in the ear under wet conditions (related to the ABA-gibberellin balance).

5.8. Summary: Dormancy as Part of the Hormonal Regulatory System

Now we can tie everything together. Ethylene and ABA are two hormones that, in different situations, ensure growth cessation. Ethylene does this by triggering irreversible processes (ripening, senescence), while ABA acts more flexibly, allowing the plant to enter a reversible dormancy state or defend against stress.

Dormancy is an active state maintained by ABA, which allows the plant to survive unfavorable conditions. Breaking dormancy is initiated by gibberellins and cytokinins, which suppress ABA action and resume growth programs. Such a dynamic balance between inhibitors and stimulators of growth is the basis of plant ontogenesis.

Thus, in answering the main question of the lecture — "how does the plant know when to stop growing?" — we see that this knowledge is not a single signal. It is a complex system in which ethylene and ABA play different roles, but both are aimed at one goal: ensuring the survival and reproduction of the species in constantly changing environmental conditions.

Conclusion

In this lecture, we examined two hormones often called "stress hormones" or "inhibitory hormones." However, we have seen that their role is much deeper: they are key regulators of life transitions. Ethylene triggers ripening, senescence, and defense, acting as an irreversible switch. ABA allows the plant to anticipate danger (drought) and enter dormancy, which is actively maintained and reversible. Together, these hormones enable the plant not only to grow but also to stop in time, defend itself, and complete its life cycle with maximum efficiency. Understanding these mechanisms underlies many agricultural practices: from managing fruit ripening to extending storage life and regulating flowering.