Introduction
Cytokinins are a bunch of plant growth regulators which are primarily involved in performing cellular division in plant roots and shoot systems. This hormone helps in increasing the cell’s growth, development, and differentiation, affecting apical dominance, leaf senescence, and axillary bud growth.
These hormones are mostly found altogether in all the complex plants, bacteria, mosses, and fungi. There are some 200 different synthetic and natural cytokinins and most of them are usually developed within the meristem of the roots.
Discovery of Cytokinin
F. Skoog and co-workers observed that from the internodal segments of tobacco stems the callus (a mass of undifferentiated cells) proliferated only if, in addition to auxins the nutrients medium was supplemented with one of the following: extracts of vascular tissues, yeast extract, coconut milk or DNA.
Chemical Nature
They are Adenine derived and basic in nature.
They are derivatives of either aminopurine or phenyl urea derivatives.
First synthetic cytokinin was discovered by Miller et al, 1955. It is called kinetin (6-Furfuruyl amino-purine). It is a synthetic cytokinin that was obtained from autoclave Herring Sperm DNA.
The first natural cytokinin discovered was by Lethan et al (1964). It is known as Zeatin (6-Hydroxy, 3-methyl, trans-2-butenyl amino purine). It is obtained from unripe maize grains and coconut milk.
6-benzyl amino purine or BAP is the first generation synthetic cytokinin. Other examples of synthetic cytokinin are diphenylurea and thidiazuron (TDZ).
Sources of Cytokinin
The major source of cytokinin synthesis is roots. Cytokinin is transported from roots to other regions of the plant through the xylem.
It is also synthesized in other regions where cell division takes place like endosperm of seeds, growing embryos, developing seeds, young fruits, and developing shoot buds.
What are the Functions of Cytokinins?
Cytokinins help in increasing cellular division by maintaining the protein production that’s important for mitosis. The term Mitosis refers to a non-sexual division of cells that sometimes occurs in all living things by developing additional cells for the growth of the body. Mitosis occurs a day by replacing the damaged cells by allowing them to grow.
In plants, mitosis develops some more cells that make the plant grow. You must have played with building blocks in your childhood days, from this you will consider them the same as plant cells. With the occurrence of this process, mitosis, a replacement cell is made and transported to the top end of the plant by increasing its length. This is a bit like cementing a building block to your original structure.
The functions are mentioned below:
Cell division- it is essential for cytokinesis though chromosome doubling can occur in its absence. Cell division in callus requires both hormones auxin as well as cytokinin.
It also causes cell elongation.
They are responsible for lignification and differentiation of Interfasiculuar cambium
It induces the formation of young leaves, chloroplasts, lateral shoot, and adventitious shoot formation.
For morphogenesis or differentiation of tissue and organs, plants require both auxin and cytokinin hormones. Bud develops when cytokinin is present in excess as compared to auxin while roots are formed when this ratio is reversed.
High cytokinin/auxin ratios favor the formation of shoots
Low cytokinin/auxin ratios favor the formation of roots
Intermediate cytokinin/auxin ratios- callus formation
Cytokinin act antagonistically to auxin. Auxin promotes apical dominance whereas it suppresses apical dominance. Thus when cytokinin is applied to lateral buds, it helps in their growth despite the presence of apical bud.
It shows the Richmond-Lang effect which means it delays the senescence of leaves and other organs by mobilization of nutrients towards them.
Cytokinin increases disease resistance and high and low-temperature resistance in plants.
They overcome seed dormancy.
They induce femaleness in flowers and induce salt accumulation in cells.
They can induce parthenocarpy in plants. It was reported by Crane in 1965.
What are the Uses of Cytokinins?
Plays an important role in tissue culture, because it is involved in cell division as well as in morphogenesis.
It increases the shelf life of flowers, shoots, and vegetables by keeping them fresh for several days. Therefore, farmers can sell them fresh for longer periods of time.
It delays the senescences of intact plant parts.
It plays an important role in plant pathogenesis by inducing resistance against certain disease-causing bacteria.
Conclusion
Cytokinin plays an important role in both cell division and differentiation. It also plays a major role in increasing the yield of flowers and fruits which provides ecological benefits to farmers.
FAQs on Cytokinins
1. What are cytokinins and where are they primarily produced in plants?
Cytokinins are a class of plant growth-regulating hormones primarily involved in promoting cell division (cytokinesis). They are synthesised mainly in areas of active cell growth, such as the root meristems, and are then transported to other parts of the plant through the xylem.
2. What are the main functions of cytokinins in a plant's life cycle?
Cytokinins play a vital role in several aspects of plant growth and development. Their key functions include:
- Promoting cell division and cell enlargement.
- Inducing the formation of new leaves, chloroplasts, and lateral shoots.
- Helping to overcome apical dominance by promoting the growth of lateral buds.
- Delaying leaf senescence (aging) by mobilising nutrients to the leaves.
- Influencing morphogenesis, where the ratio of cytokinin to auxin determines if roots or shoots will develop.
3. Can you provide examples of natural and synthetic cytokinins?
Yes, cytokinins can be both natural and synthetic.
- Natural Cytokinin: The most well-known example is Zeatin, which was first isolated from unripe maize grains.
- Synthetic Cytokinin: A common example is Kinetin, which was derived from autoclaved herring sperm DNA.
4. Who is credited with the discovery of the first cytokinin?
The first cytokinin, Kinetin, was discovered by Miller and his colleagues in 1955. It is a synthetic cytokinin. The first naturally occurring cytokinin, Zeatin, was later discovered by Letham et al. in 1964 from maize grains and coconut milk.
5. What are some practical applications of cytokinins in agriculture and horticulture?
Cytokinins have several important commercial uses. They are used to:
- Increase the shelf life of vegetables, cut flowers, and fruits by delaying senescence.
- Play a crucial role in plant tissue culture for regenerating whole plants from a callus.
- Promote lateral branching to create bushier, more compact plants.
- Increase resistance to certain diseases and temperature stresses.
6. How do cytokinins and auxins interact to control plant development?
Cytokinins and auxins often have an antagonistic (opposing) relationship that is crucial for plant morphogenesis. While auxin promotes apical dominance (growth of the main stem), cytokinin promotes the growth of lateral buds. The ratio of cytokinin to auxin directs development in tissue culture:
- A high cytokinin-to-auxin ratio promotes shoot formation.
- A low cytokinin-to-auxin ratio promotes root formation.
- An intermediate ratio results in the growth of an undifferentiated cell mass called a callus.
7. What is the chemical nature of cytokinins?
Chemically, cytokinins are derivatives of the nitrogenous base adenine, which is a type of purine. They are therefore classified as aminopurine derivatives. For instance, the chemical name for Kinetin is 6-Furfuruyl amino-purine. Due to their chemical structure, they are basic in nature.
8. How do cytokinins delay the aging of leaves, a phenomenon known as the Richmond-Lang effect?
The Richmond-Lang effect describes the ability of cytokinins to delay senescence (aging) in plant organs, especially leaves. They achieve this by promoting the mobilisation of essential nutrients into the leaf tissues. This influx of nutrients helps maintain chlorophyll content, protein synthesis, and overall cellular function, keeping the leaf green and photosynthetically active for a longer period.
9. Is there any function or presence of cytokinins in the human body?
No, cytokinins are plant-specific hormones and have no known natural presence or function in the human body. Human growth, development, and cellular processes are regulated by an entirely different set of hormones, such as insulin, growth hormone, and testosterone. The biological pathways in plants and animals are distinct.
