Their main difference lies in their two most important respective functions. Cytokinin is required for cell division – in other words, the production of new cells. They are also responsible for cell differentiation. This produces different parts of the plant. Auxin is required for the elongation of plant cells – in other words, they help the cells produced and differentiated by cytokinin to grow.

Auxins and cytokinins work in concert. Many of their functions can only be carried out if the other plant hormone is present. What effect their interactions will have depends on the exact ratio between auxin and cytokinin in a particular area of the plant. Further, the effects of different concentrations and relative ratios of auxin or cytokinin will vary depending on the plant species.

Auxins

Auxin is essential for root growth Sakartvelo, commons.wikimedia.org/wiki/File:Roots_of_Hyacinthus_orientalis.JPG

What do auxins do?

Auxin is involved in:

• cell elongation and vertical growth in the plant by increasing cell wall plasticity, enhancing water uptake. These effects allow the stem to bend and grow towards light
• enzyme synthesis at the gene level for cell wall and cytoplasm
• root growth and development
• leaf senescence (when living tissues in the plant ages and dies)
• Growth inhibition of plant shoots, if highly concentrated

Auxins are found everywhere in the plant but are concentrated in areas of active growth. Auxins are made in growing stems and roots, and move through the plant to reach stems and roots.

One visible effect of auxin that many gardeners would be familiar with is its role in suppressing lateral bud growth. If you look at the end of a shoot, you will see a bud right at the tip. This is called the apical bud. When the apical bud is pinched off – a practice recommended to encourage bushier plant growth - the lateral buds which sit right below the apical bud will start growing. Apical buds release auxin to inhibit the growth of lateral buds.

Another observable effect of auxin is the way plants curve toward the strongest light source. When light hits a plant at an angle, auxin will accumulate on the side that receives least light. This auxin build up in the shaded part of the plant will cause cells to elongate on that side only, causing the stem to bend towards the light.

The ratio of auxin to other plant hormones will determine whether new root, buds, or existing cells will develop. For example, a high auxin-to-cytokinin ratio will prompt root formation, while a high cytokinin-to-auxin ratio will prompt new shoot formation. 

Indole-3-acetic acid (IAA) is the best known natural auxin that is present in all plants. Chemical synthetic auxins are indole—3-butyric acid (IBA), 2-methyl-4-cholorophenoxy aectic (MCPA), Indole-3-princpionic acid (IPA).

How were auxins discovered?

The idea that plant growth is regulated by chemical substances – what we now call hormones – was essential for the discovery of auxin and cytokinnins. The concept that plant growth and plant organ development are controlled by chemical substances was first put forward by Julius von Sachs in 1880.

Evidence for the existence of these plant chemicals came in 1909. Scientist Hans Fitting extracted substances from orchid pollen which caused ovary swelling – a phenomenon seen in fruit-setting. Fitting suggested these substances were hormones. A. Paal in 1919 suggested that similar substances were responsible for the way plants bend towards the light (phototropism).  

1928, F. W. Went published his ground-breaking experiment on the role of auxin in phototropism. Realizing that agar attracts auxin, he would infuse blocks of agar with auxins by placing the agar on a plant wound. He then attached this auxin-loaded agar on one side of a plant stem. After a while, the plant would bend away from the stem part in contact with the auxin-loaded agar. This occurred even without light, showing auxins were actually what drove plants to bend. Previously, scientists had mistakenly believed that sunlight made plants produce growth hormones. The fact that light wasn’t needed to produce the bending growth effect in plants showed that plants generate their own hormones independently of sunlight.

Went’s experiments explained the mechanics of phototropism: when a plant is lit at an angle, auxin moves away from the stem part exposed to light and accumulates instead in the shaded part of the stem. This higher auxin concentration in the shaded part of the stem elongates the cells there. At the same time, the lower auxin concentration in the lighter parts slows cell growth. This results in overall stem bending. Went also showed that auxin stimulated regular vertical growth in plants as well.

Went also discovered that auxins stimulate the formation of adventitious roots – any new root tissues that emerge from the stems or leaves of a mature plant, rather than from an existing root system or within a plant embryo.

How are auxins used?

Rooting hormones are a major application of auxins. These speed up rooting from cut parts of plants.

Auxins are useful in fruit growing. Because auxin controls the senescence process in plants, auxins like naphtaleneacetic acid and 4-Dichlorophenoxyacetic acid are sprayed to prevent fruits from dropping off plants prematurely.  

Because auxins in high concentrations will actually inhibit growth, they can be sprayed to prevent stored potatoes to prevent them from sprouting. Their growth inhibiting functions are also exploited to kill and control weeds.

Cytokinins

Cytokinins are essential in cell division Rickfrombaltimore commons.wikimedia.org/wiki/File:Roma_Tomato_Skin_150x.jpg

What do cytokinins do in plants?

Cytokinins are involved in:

• cell division,
• shoot growth,
• regulating seed germination,
• new bud formation,
• leaf expansion
• reproductive development.
• The delay of senescence (plant aging and die off).
• The uptake of minerals in the root
• the movement of nutrients into the leaves.

The main function of cytokinin is to produce new cells through cell division. These cells are differentiated to become either callus cells (the cells that grow over a plant wound), roots, or shoots. The type of cell that will be produced, and hence the plant organ that will emerge, will depend on the exact ratio of cytokinin to auxin present.

Cytokinin is mainly produced in roots. From here, they are transported to different parts of the plant. They are mainly found in areas of active growth in roots, young leaves, developing fruits, and seeds. The functions above usually involve auxins.

There are over 200 natural and synthetic types of cytokinin.  Many have unpronounceable names based on their molecular make up: some common ones are N⁶, N⁶-dimethylallyIaminopurine or N⁶ (Δ²– isopentenyl) adenine (i⁶ADE) and its ribosyl derivative N⁶ (Δ²-isopentenyl) adenosine (i⁶A).

How were cytokinin discovered?

Before the discovery of cytokinin, scientists had speculated chemical substances in plants that induce cell division. In 1941 Johannes van Overbeek discovered such a substance in coconut milk. When mixed with auxin, it encouraged plant cell division.

The first cytokinins to be isolated and extracted were not from a plant source. In 1954, Miller and Skoog discovered a form of cytokinin in herring sperm DNA. It was only in 1963, a natural cytokinin was isolated from a plant (maize kernals) for the first time. This cytokinin was called 6-(4-hydroxy-3-methylbut-trans-2-enylamino), termed ‘zeatin’.

How are cytokinins used?

The growth-inducing properties of cytokinins have been proven to increase yield in wheat, barley, oat, soybean, lentils and chickpeas. They have also been used to improve the production and quality of fruits.  Cytokinins also give vegetables and cut flowers a longer shelf-life.

Cytokinin (6-BenzylAminoPurine) is the active hormone inside Keiki cloning paste. Applied to a wound on a plant node, it will encourage new shoots and leaves to form.

Cytokinin is used in a chemical method for thinning out apple fruits early in their development. Thinning is when you remove flowers or young immature fruit early in the growing season to ensure the plant produces fewer but more sizeable fruit.

Cytokinin has been used in the genetic engineering of plants. For example, plants resistant to herbicide can be produced. When the callus cells of a plant are exposed to the herbicide, most will die but some survive. The surviving callus cells with the herbicide resistant genes are selected and transferred to an auxin and cytokinin mixture. The hormones will encourage the new plant with the desired resistance to develop.