The Plant Hormone Cytokinin

Cytokinins were first discovered in 1955 as promoters of cell division in the DNA of Herring sperm. Since that discovery many different compound with cytokinin activity have been discovered, both natural (trans-zeatin; benzladenine) and synthetic (diphenylurea).

The natural occurring active cytokinins tend to be derived from adenine and have either an aromatic or isoprene derived side chain on the N⁶ terminus. This enables cytokinins to be split into two main classes:

1. Aromatic cytokinin
2. Isoprenoid cytokinin

What do Cytokinins do?

There are many growth and developmental processes that cytokinins influence in addition to it being one of the major regulators of plant cell proliferation and differentiation. Some of the processes in which the plant hormones cytokinins are involved are:

1. Cell proliferation
2. Cell Differentiation
3. Nutritional signal transduction
4. Delay of senescence
5. Controlling the balance of roots and shoot

Recently many genes have been discovered that control important key steps of cytokinin biosynthesis; this has allowed the biosynthesis and signal transduction capabilities of cytokinins to be better understood. This has led to the discovery that cytokinins themselves are under fine control by internal factors such as phytohormones and exogenous substances such as inorganic nitrogen supply.

Cytokinin as a plant hormone

Cytokinin has long been known to be involved in long distance messaging in plants. The hormone had long been thought to be produced only in the apices of roots and tips of plants; however recently this theory has been renewed; this is because Cytokinins are now known to be created in many different parts of a plant. Once synthesized cytokinins are able to play a role in both long distance signalling (acting as a long distance hormone) and local signalling.

The structure of natural cytokinins

Natural cytokinins are derived from adenine and carry either a isoprene or aromatic side chain on their N6 terminal. Some of the common types of cytokinins are as follows:

1. Isoprenoid cytokinin:
a. cis-zeatin (cZ)
b. trans-zeatin (tZ)
c. N⁶-(D2-isopentenyl)adenine (iP)
d. dihydrozeatin (DZ)

2. Aromatic cytokinin:
a. ortho-topolin (oT)
b. meta-topolin (mT)
c. ortho-methoxytopolin (MeoT)
d. meta-methoxytopolin (MemT)
e. benzladenine (BA)

The major forms of cytokinins differ in different plant species. For example in rice the isoprenoid cytokinin cis-zeatin is the major form, whereas in Arabidopsis it is the trans-zeatin and iP forms that predominate. Differences between aromatic cytokinins are also seen between plants. It is thought that the structural variation of cytokinin isoprenoid and aromatic side chains affect how cytokinins interact with their receptors, and therefore play are involved in both the specificity and activity of cytokinin function.