Cytokinins: Definition, Functions, Uses, Structure, Topics

Edited By Irshad Anwar | Updated on Jul 02, 2025 07:05 PM IST

What are cytokinins?

This group of plant hormones induces cell division and differentiation and, consequently, greatly impacts many areas of plant growth and development. Cytokinins were initially identified in the 1950s as a class of molecules that promoted the division of plant cells. The first to be identified was extracted from coconut milk.

They were then chemically synthesised. They are involved in controlling the growth of shoots and roots, senescence of leaves, and mobilizing nutrients to develop seeds or other storage organs and promoting chloroplast formation. For agriculture and horticulture, cytokinins play a vital role in enhancing crop yield, improving the health of plants, and extending the shelf life by delaying leaf senescence and hence facilitating stress tolerance

Chemical Structure and Types of Cytokinins

The structure of cytokinins is described below-

Chemical Structure

Cytokinins share a basic structure of purine and adenine with a bicyclic ring system. The structural variation among the different types of cytokinin is the variation in the side chain attached to the purine ring; they determine the bioactivity of the molecule and its stability.

Types of Cytokinins

Natural cytokinins

Natural cytokinins include Zeatin, which was isolated from corn kernels, and Kinetin, which was also isolated from herring sperm DNA.

Synthetic cytokinins (e.g., 6-Benzylaminopurine)

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Synthetic cytokinins are those that have been engineered to be more stable and more beneficial concerning their intended use, such as 6-Benzylaminopurine.

Functions of Cytokinins in Plants

The function of cytokinin is described below-

Cell Division and Growth

Cytokinins promote cell division and cell expansion, thus playing a crucial role in shoot and root growth. Through the induction of mitosis, cytokinins allow for an overall increase in plant growth and the growth of new tissues.

Leaf Senescence

These hormones enhance delayed leaf senescence keeping the content of the chlorophyll and also preventing the falling of the leaves at an early stage. This extends the period of leaf functioning and allows better performance of the plant's photosynthesis.

Nutrient Mobilization

Cytokinins move nutrients around the plant. This hormone enables the re-radiation of nutrients from the older tissues to the growing points. This function is significant in the development and productivity of the plant.

Regulation of Apical Dominance

Cytokinins often act with auxins in controlling apical dominance, thereby affecting the development of lateral buds. Cytokinins can increase the strength of apical dominance in stems, possibly by reducing the export of auxin from these buds. This leads to increased branching and thus a more robust plant structure and yield.

Applications of Cytokinins

The applications of cytokinins is described below-

Agricultural Applications

Cytokinin is used in agriculture to increase crop production and plant growth. It forms an integral part of tissue culture and micro-propagation because, through it, tissue culture can develop into a young plant.

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Frequently Asked Questions (FAQs)

1. What are cytokinins and their function?

Cytokinins are classic plant hormones that regulate cell division, growth, and development. They appear to have a broad influence on multiple processes to optimize plant growth, from shoot and root development through leaf senescence and nutrient mobilization.

2. How do cytokinins affect plant growth?

Cytokinins encourage cell division and cell elongation, which, in turn, equals shoot and root growth. Cytokinins inhibit leaf senescence and are involved in the mobilization of nutrients, which helps the plant remain healthy and productive.

3. What is the role of cytokinins in agriculture?

Cytokinins are used in agriculture to increase yields and improve quality by promoting growth and development. They are used heavily in tissue culture and micropropagation, aiding the mass production of plants from tissue samples.

4. How are cytokinins synthesized in plants?

Cytokinins are produced in different tissues of the plant; the major site of production is the roots, where they are synthesized as precursors and transported to other parts of the plant. The complex biochemical pathways include adenine derivatives and enzymatic reactions that form the active compounds of cytokinins.

5. What is the difference between natural and synthetic cytokinins?

Natural cytokinins include Zeatin and Kinetin, which occur physiologically in plants and perform normal hormonal functions. 6-Benzylaminopurine belongs to synthetic cytokinins that have been chemically modified to increase their stability and effectiveness in agricultural and horticultural applications.

6. What is the difference between natural and synthetic cytokinins?

Natural cytokinins are produced by plants and typically have an adenine-based structure with an isoprenoid side chain. Synthetic cytokinins, while designed to mimic the effects of natural cytokinins, often have different chemical structures. For example, benzylaminopurine (BAP) is a common synthetic cytokinin that lacks the isoprenoid side chain. Synthetic cytokinins are often more stable and potent than natural ones, making them useful in agricultural and research applications.

7. How are synthetic cytokinins used in agriculture and horticulture?

Synthetic cytokinins are used in various ways in agriculture and horticulture. They can be applied to promote branching in ornamental plants, delay leaf senescence in cut flowers, stimulate fruit set in certain crops, and enhance shoot proliferation in tissue culture. Some synthetic cytokinins are also used to break bud dormancy or to promote uniform budding in fruit trees.

8. How do cytokinins affect plant tissue culture techniques?

Cytokinins are extensively used in plant tissue culture. They are crucial for inducing shoot proliferation from explants and promoting the formation of adventitious buds. In tissue culture media, cytokinins are often used in combination with auxins to control morphogenesis. The ratio of cytokinins to auxins can determine whether cultured tissues form shoots, roots, or undifferentiated callus. This makes cytokinins essential tools in plant biotechnology and propagation.

9. How do cytokinins affect leaf senescence?

Cytokinins delay leaf senescence by promoting the maintenance of chlorophyll and proteins in leaf tissues. They help preserve the photosynthetic capacity of leaves and slow down the degradation processes associated with aging. This effect is why external application of cytokinins can sometimes be used to extend the shelf life of cut flowers and vegetables.

10. What role do cytokinins play in fruit development?

Cytokinins are important in fruit development, particularly in the early stages. They promote cell division in developing fruits, contributing to fruit size and shape. Cytokinins also influence fruit set (the initial formation of fruit after pollination) and can delay fruit senescence. In some fruits, cytokinins contribute to the development of seeds within the fruit.

11. What is the relationship between cytokinins and plant stress responses?

Cytokinins play a complex role in plant stress responses. In some cases, they can help plants tolerate stress by promoting antioxidant activity and maintaining photosynthetic capacity. However, stress conditions often lead to a decrease in cytokinin levels, which can trigger adaptive responses like leaf senescence. The exact role of cytokinins in stress response can vary depending on the type of stress and the plant species involved.

12. What is the chemical structure of natural cytokinins?

Natural cytokinins are typically adenine derivatives. The most common natural cytokinin is zeatin, which has an adenine ring with a side chain attached at the N6 position. This side chain often contains an isoprenoid group. The specific structure of the side chain can vary, giving rise to different types of cytokinins, but the adenine core remains constant in most natural cytokinins.

13. How do plants regulate cytokinin levels?

Plants regulate cytokinin levels through several mechanisms:

14. What is the significance of cytokinin receptors in plant cells?

Cytokinin receptors are crucial for plants to sense and respond to cytokinin signals. These receptors are typically membrane-bound histidine kinases that initiate a signaling cascade when they bind to cytokinins. The presence and sensitivity of these receptors determine how a cell or tissue will respond to cytokinins. Understanding cytokinin receptors is key to comprehending how plants regulate growth and development at the cellular level.

15. What is the significance of cytokinin oxidase in plant development?

Cytokinin oxidase is an enzyme that degrades cytokinins, playing a crucial role in regulating cytokinin levels in plants. It helps maintain the balance of cytokinins by breaking them down when levels are too high. This enzyme is particularly important in controlling cytokinin-mediated processes like shoot branching and leaf senescence. The activity of cytokinin oxidase can be regulated by various factors, allowing plants to fine-tune their cytokinin levels in response to developmental and environmental cues.

16. What is the main site of cytokinin production in plants?

Cytokinins are primarily produced in the root tips of plants. However, they can also be synthesized in other actively growing tissues such as young leaves, developing fruits, and seeds. The production site is significant because cytokinins are then transported throughout the plant to regulate various growth processes.

17. How do cytokinins influence apical dominance in plants?

Cytokinins play a role in counteracting apical dominance, which is the suppression of lateral bud growth by the main shoot tip. While auxins from the shoot tip promote apical dominance, cytokinins can stimulate the growth of lateral buds. When the ratio of cytokinins to auxins increases in lateral buds, it can break apical dominance and promote branching.

18. What are cytokinins and why are they important for plants?

Cytokinins are a class of plant hormones that promote cell division and growth. They are crucial for plant development, playing key roles in processes like shoot formation, leaf expansion, and delaying senescence (aging) in plant tissues. Cytokinins work in balance with other plant hormones to regulate overall plant growth and development.

19. How do cytokinins differ from other plant hormones?

Cytokinins are unique among plant hormones because they primarily stimulate cell division and differentiation. While other hormones like auxins also influence growth, cytokinins specifically promote the formation of new cells and tissues. They also have distinct chemical structures, typically being adenine derivatives, which sets them apart from other plant hormone classes.

20. What is the relationship between cytokinins and cell cycle regulation?

Cytokinins are crucial regulators of the plant cell cycle. They promote the transition from the G1 to S phase and from the G2 to M phase of the cell cycle. This stimulation of cell division is one of the primary reasons why cytokinins are essential for plant growth and development, particularly in meristematic tissues.

21. What is the "cytokinin fountain" hypothesis?

The "cytokinin fountain" hypothesis suggests that cytokinins are primarily synthesized in root tips and then transported upward through the xylem to aerial parts of the plant. This upward flow of cytokinins is thought to play a crucial role in coordinating root and shoot growth, as well as influencing various developmental processes throughout the plant.

22. How do cytokinins interact with auxins in plant development?

Cytokinins and auxins often work antagonistically in plant development. While auxins promote apical dominance and root growth, cytokinins stimulate lateral bud growth and shoot formation. The balance between these two hormones is crucial for determining overall plant architecture and growth patterns. Their interaction is a classic example of hormonal crosstalk in plant biology.

23. How do cytokinins influence root development?

Cytokinins have a complex relationship with root development. While they are produced in root tips, high levels of cytokinins can actually inhibit root growth and branching. This is part of the plant's mechanism for balancing above-ground and below-ground growth. However, cytokinins are essential for the formation of root meristems during lateral root development. The overall effect on roots depends on the concentration and location of cytokinins within the root system.

24. How do cytokinins affect chloroplast development?

Cytokinins have a positive effect on chloroplast development. They stimulate the formation of chloroplasts from proplastids and enhance the synthesis of chlorophyll and other photosynthetic proteins. This role is crucial in leaf development and in maintaining the photosynthetic capacity of leaves. The effect of cytokinins on chloroplasts is one reason why they can delay leaf senescence and maintain leaf greenness.

25. How do cytokinins influence apical meristem activity?

Cytokinins are crucial for maintaining the activity of apical meristems, particularly in shoots. They promote cell division in the meristematic region and help maintain the undifferentiated state of meristematic cells. In the shoot apical meristem, cytokinins work in balance with other hormones to regulate stem cell proliferation and organ formation. This role is essential for continuous growth and development of the plant's aerial parts.

26. What is the role of cytokinins in nutrient mobilization?

Cytokinins play a significant role in nutrient mobilization within plants. They promote the movement of nutrients from older to younger tissues, a process known as nutrient mobilization or translocation. This is particularly important for mobile nutrients like nitrogen. Cytokinins can enhance the sink strength of developing organs, drawing nutrients to areas of active growth and helping to maintain the vitality of new tissues.

27. What is the role of cytokinins in seed germination?

Cytokinins play a supportive role in seed germination. While they are not the primary triggers of germination (which is usually gibberellins), cytokinins can enhance the germination process. They promote cell division in the embryo and can help overcome dormancy in some seeds. Cytokinins also support the mobilization of stored nutrients from the endosperm to the developing seedling, aiding in early growth after germination.

28. How do cytokinins influence flower development?

Cytokinins influence several aspects of flower development. They can promote flower initiation in some plant species, especially when applied exogenously. During flower development, cytokinins contribute to the growth and differentiation of floral organs. They also play a role in determining flower size and can influence the number of flowers produced. In some cases, cytokinins can delay flower senescence, prolonging the life of the bloom.

29. What is the relationship between cytokinins and plant longevity?

Cytokinins have a positive relationship with plant longevity. By delaying senescence in leaves and other plant organs, cytokinins can extend the overall lifespan of plants. They maintain cellular integrity and function, particularly in leaves, which allows plants to photosynthesize and remain productive for longer periods. This effect is particularly noticeable in annual plants, where maintaining high cytokinin levels can sometimes extend their life cycle.

30. How do cytokinins affect cell differentiation in plants?

Cytokinins play a crucial role in cell differentiation in plants. They promote the transition of cells from an undifferentiated state to specialized cell types. This is particularly important in shoot development, where cytokinins influence the formation of different tissue types from meristematic cells. Cytokinins also play a role in the differentiation of chloroplasts and other organelles within cells, contributing to the overall cellular specialization in plant tissues.

31. What is the role of cytokinins in plant-pathogen interactions?

Cytokinins have complex roles in plant-pathogen interactions. In some cases, they can enhance plant defense responses by promoting the production of defense compounds and strengthening cell walls. However, some pathogens can produce cytokinins or alter plant cytokinin levels to manipulate host physiology for their benefit. For example, some gall-forming pathogens induce localized increases in cytokinin levels to promote cell division and create a nutrient-rich environment for themselves.

32. How do environmental factors affect cytokinin production and activity in plants?

Environmental factors significantly influence cytokinin production and activity:

33. How do cytokinins interact with other plant hormones besides auxins?

Cytokinins interact with several other plant hormones:

34. What are the main differences between cytokinin biosynthesis in plants and microorganisms?

While both plants and some microorganisms can produce cytokinins, there are key differences:

35. How do cytokinins influence plant responses to abiotic stresses?

Cytokinins play a complex role in plant responses to abiotic stresses:

36. What is the role of cytokinins in plant-insect interactions?

Cytokinins influence plant-insect interactions in several ways:

37. How do cytokinins affect root-to-shoot signaling in plants?

Cytokinins play a crucial role in root-to-shoot signaling: