Biological functions of Cytokinin

Morphogenesis High auxin: cytokinin ratio stimulates the formation of roots whereas low ratio leads to the formation of shoots.

Lateral Bud Formation: 

Cytokinins are known to regulate axillary bud growth and apical dominance. It is postulated that auxin from apical buds travels through shoots to inhibit axillary bud growth. This promotes shoot growth and restricts lateral branching. On the other hand, cytokinins move from root to the shoot and stimulate lateral bud growth.

Delay of Leaf Senescence: Senescence is the process of getting old, it is a degenerative process which involves breakdown of plant cells. 

Cytokinins slows aging of plant organs by preventing cellular breakdown, activating protein synthesis, and assembling nutrients from nearby tissues.

 

Movement of Nutrients:

Cytokinins  creates source-sink relationship and helps in promoting the movement of nutrients across the plant. Cytokinin-treated tissue acts as strong sink and thereby nutrients are channelized toward it. If a plant requires excessive nutrients, cytokinins accumulate in the root zone to stimulate growth.

Chloroplast Development:

Etiolated leaves (dark growing), when treated with cytokinin, develop chloroplasts with more extensive grana, and photosynthetic enzymes are synthesized at a greater rate upon illumination. This suggests that cytokinins along with other factors, like light and nutrition, regulate the biosynthesis of photosynthetic pigments and proteins.

Mechanical Extensibility of Cell Wall:

Cotyledons of dicots, like mustard and sunflower, expand on treatment with cytokinin.

Cytokinins modify apical dominance and promote lateral bud growth:

One of the primary determinants of plant form is the degree of apical dominance. Plants with strong apical dominance, such as maize, have a single growing axis with few lateral branches. In contrast, many lateral buds initiate growth in shrubby plants. Branching patterns are normally determined by light, nutrients, and genotype. Physiologically, branching is regulated by a complex interplay of hormones, including auxin, cytokinin, and a recently identified root-derived signal. Auxin transported polarly from the apical bud suppresses the growth of axillary buds. In contrast, cytokinin stimulates cell division activity and outgrowth when applied directly to the axillary buds of many species, and cytokinin-overproducing mutants tend to be bushy.

Cytokinin inhibit root growth by promoting the exit of cells from the root apical meristem:

Cytokinin plays a very different role in the root apical meristem than it does in the shoot apical meristem. In contrast to its effect on the shoot, overexpression of cytokinin oxidase in tobacco increases root growth, primarily by increasing the size of the root apical meristem.

 

Cytokinins promote shoot growth by increasing cell proliferation in the shoot apical meristem:

Several lines of evidence suggest that cytokinins also play key roles in the regulation of cell division in vivo. Much of the cell division in an adult plant occurs in the meristems. Cytokinin plays a positive role in the proliferation of cells in the shoot apical meristem.

Regulation of Cell Cycle:

Cytokinin also plays important role in regulation of specific components of cell cycle.

Cell Division: 

When plants are cultured on auxin-containing medium without cytokinin, they increase in size but cells do not divide. Addition of cytokinin to the medium leads to cell division and differentiation. Presence of both auxin and cytokinin in equal concentration leads to formation of undifferentiated callus. 

QUESTION PRACTICE:

Which of the following statements are correct for “Cytokinins”? (ICAR PG Agronomy 2022)

 A. Cytokinin stimulates cell division 

 B. One of the potent cytokinin is 6‐Benzyladenine 

 C. Cytokinin hastens senescence 

 D. Cytokinin breaks dormancy of seeds and buds 

 Choose the correct answer from the options given below:

1. B, C and D only 

2. A, B and C only

3. A, B and D only 

4. B and D only