Cytokinin

Introduction

Cytokinin

A plant’s response of senses to an external stimulus depends on chemical messengers called hormones. These plant hormones play a vital role in regulating various aspects of plants life, including seeding, germination, flowering, fruiting, leaf falling etc. Every cell in a plant is capable of producing plant hormones. They can act in the cell of their origin or can be transported to other parts of the plant where required with many responses such as synergistic or antagonistic interaction of two or more hormones. Whereas in animals, the hormones are produced in specific glands followed by their transportation at the site of action and act alone. Plant hormones are a group of chemical substances that are controlled or affect the morphogenesis of plants. There are five major plant hormones, these are:

• ethylene
• gibberellins
• abscisic acid
• auxins and 
• cytokinins

Body

Cytokinins are named so because they promote cytokinesis, i.e., cell division. Cytokinins were first reported by Haberlandt in 1913 when he observed that a non-dividing potato that parenchyma cells reverts actively into dividing cells in the presence of phloem sap. This led him to suggest that there is some organic soluble material that led to the cell division. After that Folke Skoog and colleagues in the 1940s found that cultured tobacco pith tissue explants only proliferated when the media was supplemented with substances such as coconut milk. Then finally Miller in 1956 led the discovery of the first cytokinin called the kinetin in the herring sperm.

Natural cytokinin

The effect of the first cytokinin was observed when it was found that by addition of liquid coconut endosperm in plant culture for the development of embryo lead to stimulated growth. Thus stimulating growth factor was called cytokinin.

Naturally occurring cytokinin 

Cytokinin is mostly found naturally in the growing tissue of plants, such as the roots, embryos and fruits parts. Cytokinins are observed to delay aging in plants. They promote mitosis and promote cell differentiation of roots and shoots. Many effects on the plant are due to the cytokinins, either alone or in conjugation with other plant growth regulator hormones. For example, apical dominance is the result of a balance between PGR auxin and PGR cytokinin. The auxin inhibits lateral bud formation and the cytokinin promotes bushier growth. Cytokinin is generally the adenine derivative that occurs in the form of:

• free nitrogenous bases
• a nucleoside ( base + ribose )
• a nucleotide ( base + phosphate + ribose sugar)
• glycosides

The free base acts as the active form.

Zeatin was the first naturally occurring cytokinin isolated from Zea mays and is the most common form of cytokinin.

Some other naturally occurring cytokinins are Dihydrozeatin (DHZ) and Isopentyladenosine (IPA). 

Synthetic cytokinin:

Soke of the example of artificially synthesized or synthetic cytokinins are 

• Kinetin, it’s a byproduct of zeatin degradation
• Benzylaminopurine (BA) also presents the activity of a cytokinin.

Cytokinins can occur as a modified base in tRNA but rather than transform the bases exist in the cis forms. These modified bases are found in humans as well as plants. These are specifically known as tRNA cytokinins.

Cytokinins in humans 

As we know that cytokinins are phytohormones responsible for the regulation of the cell cycle and many other functions of plants. But it is experimentally demonstrated (both in vitro and in vivo) that cytokinins can have diverse effects on humans as well. They protect the cells from various stress conditions and also have some kind of effect on aging. An example is the culturing of human fibroblasts in the presence of kinetin or trans-zeatin to retain some characteristics of cells of lower passage. It is also found that cytokinin (kinetin) increases the lifespan of the invertebrates. The cytokinin (zeatin) has antioxidative properties and shows cell-protective effects against Beta-amyloid induced neurotoxicity that is similar to caffeine.

Functions of cytokinins

• Controls morphogenesis: In plant tissue culture cytokinin is required to promote the growth of callus. The ratio of cytokinin and auxin plays an important role in determining the fate of callus.
• Delay senescence: senescence can be simply defined as the process of aging determined by loss of chlorophyll, lipids and proteins in plants. It is observed that an application of cytokinin on leaves of the plant showed a reduction in the rate of chlorophyll and protein degradation and also reduced leaf drop.
• Crown gall: Crown gall is defined as the undefined growth of cells in the junction near the root and stem of the plants that are caused by Agrobacterium tumefaciens. When the infection causing plasmid of Agrobacterium is incorporated in the plant cell genome, it starts the overproduction of auxins(tms gene)  and cytokinins(tmr gene for production of zeatin). If the tms gene is removed then the cytokinin/auxin ratio in the plant is increased and the resultant crown gall is called ‘shooty’ and if the tmr genes are removed then the gall is ‘rooty’.
• Regulate cell cycle: cytokinins have a crucial role in regulating the cell cycle. This is controlled by cyclin-dependent protein kinases (CDKs).
• Promotes lateral bud development: It is observed that the application of cytokinins on dormant buds leads to their development of them. For example, Witches broom is caused by pathogens like Corynebacterium fascians or Agrobacterium tumefaciens. These produce cytokinins which lead to the development of lateral bud formation or branching. These results suggested the apical dominance may also be related to cytokinin. For example when tobacco plants are infected by Ti plasmid that has been engineered to possess a heat shock promoter, a heat treatment leads the cells to stimulate the production of cytokinins. Such plants exhibit less dominance and remain greener for long periods than non-heat treated plants.
• Greening: cytokinins promote the light-induced formation of chlorophyll and also the conversion of etioplasts to chloroplasts.
• Promotes cell expansion: Cytokinins are responsible for the stimulation of the expansion of cotyledons.

Conclusion:

A plant hormone can be simply defined as an organic substance that controls the various aspects of plant life. It is produced in one part of the plant and then transported to the other parts of the plant where it stimulates a physiological response. Plant hormones may inhibit or promote growth depending upon the type of hormone, the concentration, the time and the part of the plant involved. They are collectively known as plant growth regulators or PGRs. Some plant growth regulators work by blocking or either synthesizing the activity of hormones. They are grouped into five classes that are the auxins, cytokinins, gibberellins, abscisic acid and ethylene. Cytokinin promotes cell division and is found to be produced in roots and young fruits. Cytokinin hormones are responsible for the stimulation of bud formation, branching, apical dominance. There are many varieties of natural and synthetic cytokinin found to date. An example of synthetic cytokinin is Benzyladenine which is used for fruit thinning.