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.

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Cytokinins, Ethylene and ABA

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NEET UG

Version 1Updated 22 Mar 2026

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Plant growth regulators (PGRs), also known as plant hormones or phytohormones, are small, simple molecules of diverse chemical composition, which regulate plant growth and development. They act as chemical messengers, influencing physiological processes at very low concentrations. Cytokinins, Ethylene, and Abscisic Acid (ABA) represent three crucial classes of these endogenous regulators. Cytokini…

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Plant growth regulators (PGRs) are chemical messengers that control plant development. Cytokinins, Ethylene, and Abscisic Acid (ABA) are three crucial types. Cytokinins, derivatives of adenine, primarily promote cell division, break apical dominance by stimulating lateral bud growth, and delay leaf senescence.

They are vital in tissue culture for shoot differentiation. Ethylene, a gaseous hormone, is renowned for its role in fruit ripening, accelerating senescence, and promoting abscission of leaves and fruits.

It also induces the 'triple response' in dark-grown seedlings, aiding their emergence from soil. Abscisic Acid (ABA), often called the 'stress hormone,' is a carotenoid derivative that helps plants cope with adverse conditions.

Its key functions include inducing stomatal closure to conserve water during drought, promoting seed and bud dormancy, and contributing to abscission. These hormones rarely act alone; their effects are often a result of complex synergistic and antagonistic interactions, forming an intricate regulatory network essential for plant survival and adaptation.

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Key Concepts

Cytokinin's Role in Tissue Culture

Cytokinins are indispensable in plant tissue culture, a technique used to grow plant cells, tissues, or…

Ethylene's Triple Response

The 'triple response' is a classic demonstration of ethylene's effect on dark-grown dicot seedlings. When…

ABA's Role in Seed Dormancy

Abscisic Acid (ABA) is a critical regulator of seed dormancy, a state where a viable seed will not germinate…

Cytokinins — Adenine derivatives. Promote cell division, break apical dominance, delay senescence, promote lateral bud growth. Used in tissue culture (high cytokinin:auxin for shoots).

Ethylene — Gaseous hormone (

C_2H_4

). Promotes fruit ripening (climacteric), senescence, abscission. Induces 'triple response' in seedlings.

Abscisic Acid (ABA) — Carotenoid derivative. 'Stress hormone'. Induces stomatal closure (drought), promotes seed dormancy, bud dormancy, abscission. Antagonistic to gibberellins.

Cell Keeps Always Branching, Delaying Senescence. (Cytokinin: Cell division, Apical dominance breaking, Delay Senescence)

Every Tree Ripens Slowly And Thickens. (Ethylene: Triple response, Ripening, Senescence, Abscission, Thickening)

Always Be Alert, Stress Shuts Down Growth. (ABA: Stress hormone, Stomatal closure, Seed dormancy, Growth inhibition)

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Biology

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Auxins and Gibberellins