Cytokinins, Ethylene and ABA — Explained

On this page

1. I. Cytokinins
2. A. Conceptual Foundation and Discovery:
3. B. Chemical Nature:
4. C. Key Physiological Effects:
5. D. Mechanism of Action:
6. II. Ethylene
7. A. Conceptual Foundation and Discovery:
8. B. Chemical Nature:
9. C. Key Physiological Effects:
10. D. Mechanism of Action:
11. III. Abscisic Acid (ABA)
12. A. Conceptual Foundation and Discovery:
13. B. Chemical Nature:
14. C. Key Physiological Effects:
15. D. Mechanism of Action:
16. IV. Interactions and NEET-Specific Angle
17. Common Misconceptions:

Detailed Explanation

Plant growth regulators (PGRs) are intrinsic chemical messengers that orchestrate the intricate processes of plant growth, development, and responses to environmental cues. Among the diverse array of PGRs, Cytokinins, Ethylene, and Abscisic Acid (ABA) stand out for their distinct chemical natures and profound physiological impacts. Understanding their individual roles and interactions is fundamental for comprehending plant biology and is a frequently tested area in NEET UG.

I. Cytokinins

A. Conceptual Foundation and Discovery:

Cytokinins are a class of plant hormones primarily known for their role in promoting cell division (cytokinesis). Their discovery stemmed from efforts to culture plant tissues. In the 1940s, Johannes van Overbeek observed that coconut milk, a liquid endosperm, stimulated cell division in plant embryos.

Later, in the 1950s, Skoog and Miller isolated kinetin (6-furfurylaminopurine) from degraded herring sperm DNA, which was highly effective in promoting cell division in tobacco pith callus cultures. Kinetin is an artificial cytokinin.

The first naturally occurring cytokinin, zeatin, was isolated from immature corn kernels by Letham in 1963. Zeatin is considered one of the most active natural cytokinins.

B. Chemical Nature:

Most natural cytokinins are derivatives of adenine (a purine base), specifically N6-substituted adenines. Their structure typically involves an adenine ring with an isopentenyl or aromatic side chain attached to the N6 position. This structural similarity to a component of nucleic acids hints at their fundamental role in cellular processes.

C. Key Physiological Effects:

1
Cell Division and Differentiation: — This is their hallmark function. Cytokinins, in conjunction with auxins, regulate the cell cycle, particularly promoting the transition from G2 to M phase. In tissue culture, the ratio of auxin to cytokinin dictates whether a callus proliferates, differentiates into roots (high auxin:cytokinin), or differentiates into shoots (high cytokinin:auxin).
2
Breaking Apical Dominance: — Apical dominance, where the apical bud inhibits the growth of lateral buds, is primarily maintained by auxins. Cytokinins counteract this by promoting the growth of lateral buds, leading to a bushier plant habit. This antagonistic interaction is crucial for plant architecture.
3
Delay of Senescence (Richmond-Lang Effect): — Cytokinins delay the aging process in leaves by promoting nutrient mobilization to cytokinin-rich areas and maintaining protein and chlorophyll synthesis. This keeps leaves green and metabolically active for longer.
4
Chloroplast Development: — They promote the development of chloroplasts in leaves, enhancing photosynthetic capacity.
5
Lateral Shoot Growth: — By releasing lateral buds from apical dominance, cytokinins directly stimulate the growth of side shoots.
6
Nutrient Mobilization: — Cytokinins can act as 'sink' attractors, drawing nutrients to the areas where they are present, further supporting growth and development.

D. Mechanism of Action:

Cytokinins are perceived by specific receptor proteins located in the plasma membrane. Upon binding, these receptors initiate a signal transduction pathway, often involving a two-component system (histidine kinase receptor and response regulator), leading to changes in gene expression that promote cell division and other cytokinin-specific responses.

II. Ethylene

A. Conceptual Foundation and Discovery:

Ethylene is unique among PGRs as it is a simple gaseous hydrocarbon ($C_2H_4$). Its role in plant physiology was first observed in the late 19th century when street lights using illuminating gas caused premature defoliation of trees. Later, in 1901, Dimitry Neljubow identified ethylene as the active component in illuminating gas responsible for inhibiting pea seedling elongation. Subsequent research confirmed its endogenous production by plants and its widespread effects.

B. Chemical Nature:

Ethylene is the simplest alkene, a gaseous molecule at physiological temperatures. Its gaseous nature allows it to diffuse rapidly through plant tissues and even into the atmosphere, influencing neighboring plants.

C. Key Physiological Effects:

1
Fruit Ripening: — This is perhaps its most famous role. Ethylene triggers a climacteric rise in respiration and a cascade of biochemical changes that lead to the softening of fruit walls, degradation of chlorophyll (color change), synthesis of pigments (e.g., carotenoids), production of volatile compounds (aroma), and conversion of starches to sugars (sweetness). Examples include bananas, mangoes, apples, and tomatoes.
2
Senescence and Abscission: — Ethylene accelerates the aging process (senescence) in leaves and flowers and promotes the shedding (abscission) of leaves, flowers, and fruits. It stimulates the formation of an abscission layer at the base of the petiole or pedicel, leading to separation.
3
Triple Response in Seedlings: — In germinating dicot seedlings growing in the dark, ethylene induces a characteristic 'triple response':

Inhibition of hypocotyl elongation (stunted growth). Swelling of the hypocotyl (thickening). * Exaggeration of the apical hook (maintaining the hook to protect the meristem as it pushes through soil). This response helps seedlings navigate through soil obstacles.

1
Root Growth and Root Hair Formation: — Ethylene can inhibit root elongation but significantly promotes the formation of root hairs, increasing the root's surface area for water and nutrient absorption.
2
Flowering: — While generally inhibiting flowering in most dicots, ethylene can promote flowering in some plants, like pineapples.

D. Mechanism of Action:

Ethylene perception involves a family of receptor proteins located in the endoplasmic reticulum. These receptors are constitutively active in the absence of ethylene, repressing the ethylene response pathway. Ethylene binding inactivates these receptors, leading to the activation of a signal transduction cascade that ultimately alters gene expression, mediating the various ethylene responses.

III. Abscisic Acid (ABA)

A. Conceptual Foundation and Discovery:

Abscisic Acid (ABA) was independently discovered by several groups in the 1960s while investigating substances that promote abscission (Addicott et al., 'abscisin II') and induce dormancy (Wareing et al., 'dormin'). It was later recognized as a single compound and named Abscisic Acid. ABA is often referred to as the 'stress hormone' due to its critical role in mediating plant responses to environmental adversities.

B. Chemical Nature:

ABA is a 15-carbon sesquiterpenoid, derived from the carotenoid pathway. It is a relatively stable molecule and can be transported over long distances within the plant, acting as a systemic signal.

C. Key Physiological Effects:

1
Stomatal Closure: — Under drought stress, ABA levels rapidly increase, signaling guard cells to close stomata. This reduces transpiration and conserves water, preventing dehydration. This is a rapid and crucial response for plant survival in dry conditions.
2
Seed Dormancy: — ABA is a primary regulator of seed dormancy, preventing premature germination even under otherwise favorable conditions. It ensures that seeds germinate only when environmental conditions are optimal for seedling survival. The ratio of ABA to gibberellins often determines the state of dormancy or germination.
3
Bud Dormancy: — Similar to seeds, ABA induces and maintains dormancy in buds, particularly in perennial plants during unfavorable seasons (e.g., winter), protecting delicate meristematic tissues.
4
Abscission: — While ethylene is the primary promoter of abscission, ABA can also promote the shedding of leaves, flowers, and fruits, especially under stress conditions, by sensitizing the abscission zone to ethylene.
5
Inhibition of Growth: — ABA generally acts as a growth inhibitor, counteracting the effects of growth-promoting hormones like auxins, gibberellins, and cytokinins. It inhibits shoot growth and cell division in many contexts.

D. Mechanism of Action:

ABA perception involves soluble receptor proteins in the cytoplasm (PYR/PYL/RCAR family). Upon ABA binding, these receptors inhibit protein phosphatases (PP2Cs), which in turn allows activation of SnRK2 protein kinases. This cascade leads to changes in ion channel activity in guard cells (causing stomatal closure) and altered gene expression, mediating various stress responses and dormancy.

IV. Interactions and NEET-Specific Angle

Plant growth and development are not controlled by individual hormones acting in isolation but by a complex interplay of multiple PGRs, often exhibiting synergistic (cooperative) or antagonistic (opposing) effects.

Auxin-Cytokinin Ratio: — Crucial for organogenesis in tissue culture and regulating apical dominance vs. lateral bud growth.
Ethylene-Auxin Interaction: — Auxins can induce ethylene production, which then promotes abscission. This complex interaction determines the timing of fruit and leaf drop.
ABA-Gibberellin Antagonism: — ABA promotes dormancy, while gibberellins break it. This balance is vital for seed germination and bud break.
Stress Response: — ABA is central, but other hormones like ethylene can also be involved in stress signaling (e.g., flooding stress).

For NEET, focus on:

1
Specific functions: — What each hormone does (e.g., ethylene ripens fruit, ABA closes stomata, cytokinins promote cell division).
2
Chemical nature: — Gaseous (ethylene), adenine derivative (cytokinins), carotenoid derivative (ABA).
3
Antagonistic/Synergistic interactions: — Classic examples like auxin-cytokinin in apical dominance, ABA-gibberellin in dormancy.
4
Applications: — Commercial uses (e.g., ethylene for ripening, cytokinins in tissue culture).
5
Stress response: — ABA's role as a 'stress hormone'.
6
Discovery context: — Brief understanding of how they were identified.

Common Misconceptions:

ABA always causes abscission: — While ABA can promote abscission, ethylene is generally considered the primary hormone for this process. ABA's role is often more about sensitizing tissues to ethylene or promoting it under stress.
Cytokinins only promote cell division: — While primary, they have many other roles like delaying senescence and breaking apical dominance.
Ethylene only ripens fruit: — It has significant roles in senescence, abscission, and the triple response in seedlings.
Hormones act in isolation: — This is a major misconception. Plant development is a result of complex hormonal balances and interactions, not single hormone actions.

Often confused with

Side-by-side differences the NEET paper likes to test.

Cytokinins, Ethylene and ABA vs Auxins

Aspect	Cytokinins, Ethylene and ABA	Auxins
Primary Site of Production	Root tips, developing seeds, young fruits	Apical meristems of shoots, young leaves, developing seeds
Effect on Cell Division	Promotes cell division (cytokinesis) in presence of auxin	Promotes cell elongation; also promotes cell division in cambium and in tissue culture with cytokinins
Effect on Apical Dominance	Breaks apical dominance, promotes lateral bud growth	Promotes apical dominance, inhibits lateral bud growth
Effect on Senescence	Delays senescence (Richmond-Lang effect)	Generally delays senescence at low concentrations, but high concentrations can induce ethylene production, leading to senescence/abscission
Role in Tissue Culture	Promotes shoot differentiation (high cytokinin:auxin ratio)	Promotes root differentiation (high auxin:cytokinin ratio)
Chemical Nature	Adenine derivatives (e.g., Zeatin)	Indole derivatives (e.g., IAA - Indole-3-acetic acid)

Cytokinins and Auxins are two fundamental plant hormones that often interact antagonistically or synergistically to regulate plant growth and development. While auxins, primarily produced in shoot apices, promote cell elongation and maintain apical dominance, cytokinins, largely synthesized in roots, stimulate cell division and break apical dominance by promoting lateral bud growth.

Their balanced ratio is critical in processes like organogenesis in tissue culture, where they determine whether roots or shoots develop. Both play roles in senescence, but cytokinins are known for delaying it, whereas auxins' effect can be complex due to ethylene induction.

Why it is tested: NEET relevance: Understanding the antagonistic and synergistic interactions between different plant hormones is crucial for NEET. Questions frequently test the combined effects of hormones, especially the auxin-cytokinin balance in apical dominance and tissue culture, and their roles in various developmental processes like senescence and organogenesis. This comparison highlights their distinct yet interconnected functions.

Questions students ask

6 answered on this topic.

How do cytokinins counteract apical dominance?

Apical dominance is primarily maintained by auxins produced in the apical bud, which inhibit the growth of lateral buds. Cytokinins, typically produced in the roots and transported upwards, promote cell division and growth in lateral buds.

They achieve this by altering the auxin transport stream, reducing auxin sensitivity in lateral buds, or directly stimulating cell division in these buds. The balance between auxin (inhibitory) and cytokinin (promotive) determines the extent of lateral bud outgrowth, effectively breaking apical dominance and leading to a bushier plant.

Why is ethylene called a 'gaseous hormone' and what are its implications?

Ethylene is unique among plant hormones because it exists as a gas at physiological temperatures. This gaseous nature allows it to diffuse rapidly through plant tissues and even into the surrounding atmosphere.

The implications are significant: it can influence distant parts of the same plant, and even neighboring plants. This property is crucial for its role in fruit ripening, where a small amount of ethylene produced by one fruit can trigger ripening in others (climacteric fruits).

It also means that its effects can be widespread and rapid, making it an efficient signaling molecule for processes like senescence and abscission.

What is the 'triple response' induced by ethylene in seedlings?

The triple response is a characteristic set of morphological changes observed in dark-grown dicot seedlings exposed to ethylene. It consists of three main components: 1) inhibition of hypocotyl elongation, meaning the seedling remains short and stunted; 2) swelling of the hypocotyl, making it thicker; and 3) an exaggerated apical hook, where the bend at the top of the seedling becomes more pronounced.

This response is crucial for seedlings pushing through soil, as it protects the delicate apical meristem from mechanical damage and helps the seedling navigate around obstacles.

How does Abscisic Acid (ABA) help plants cope with drought stress?

Abscisic Acid (ABA) is a key 'stress hormone' that plays a vital role in a plant's response to drought. When a plant experiences water deficit, ABA levels rapidly increase. Its primary mechanism of action in drought stress is to induce the closure of stomata.

ABA signals the guard cells surrounding the stomata to lose turgor, causing the stomatal pores to close. This significantly reduces transpiration, which is the loss of water vapor from leaves, thereby conserving precious water within the plant and preventing dehydration.

ABA also influences root growth and gene expression related to stress tolerance.

What is the role of the auxin-cytokinin ratio in plant tissue culture?

The balance between auxin and cytokinin is critical for controlling cell division and differentiation in plant tissue culture. A high auxin-to-cytokinin ratio typically promotes root formation, while a high cytokinin-to-auxin ratio favors shoot development.

An intermediate ratio often leads to the proliferation of undifferentiated callus tissue. By carefully manipulating this ratio in the culture medium, plant biotechnologists can induce specific developmental pathways, allowing for the regeneration of whole plants from small tissue explants, which is fundamental to micropropagation and genetic engineering.

Can ethylene be used to delay fruit ripening?

No, ethylene is primarily known for its role in promoting fruit ripening, especially in climacteric fruits. It initiates and accelerates the ripening process. To delay fruit ripening, strategies often involve removing ethylene (e.

g., by ventilation or ethylene scrubbers), storing fruits at low temperatures, or using inhibitors of ethylene synthesis or perception. For example, 1-methylcyclopropene (1-MCP) is a synthetic compound that blocks ethylene receptors, effectively delaying ripening and senescence in many fruits and flowers.