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 growth factors 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: The Role 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.
Since their first discovery in 1955, much work has been carried out on cytokinins. This has led to a better understanding of the structure and function of these plant hormones. Cytokinins play many roles in plants ranging from long distance signalling, cell differentiation through to regulation of senescence. This section of plant biology advice takes a look into the metabolism of cytokinins.
Classifications of Cytokinin Metabolism
There are two major classifications of cytokinin metabolism:
1. Modification of the moiety of adenine
2. Modification of cytokinin side chains
It is thought that many of the steps of cytokinin metabolism are shared with those of the purine salvage pathway, and that the flow of cytokinin nucleotides to the active nucleobases has a circular nature. This is supported by the fact that applying cytokinins exogenously to plants results in their incorporation into into plant nucleotides and nucleosides. This is thought to be because many isoenzymes that are encoded by genes in the purine salvage pathway are able to recognise and interact with cytokinins in addition to adenine.
Cytokinin Glycosylation and the Control of Cytokinin Homoeostasis
Alterations of cytokinin moiety and side chains by glycosylation play a major role in the metabolism of cytokinins. Glycosylation has been shown to affect the moiety of cytokinins by interacting at the N3, N7 and N9 positions. With regards to the side chains glycosylation has been demonstrated to affect the hydroxyl groups of trans-zeatin , cis-zeatin and dihydrozeatin.
It is known that the levels of cytokinins in plants are kept at homoeostasis by control of the release rate of cytokinin conjugates and the rate of the degradation of cytokinins. The interactions of glycosides on cytokinins enable the inactivation of cytokinins (The interactions that are catalysed by b-glucosidease lead to either reversible o-glucosylation or a non-reversible cleaving N-glucosylation). Cytokinin oxidases and dehydrogenase are involved in the degradation of cytokinin side chains. Another way in which the quantity of cytokinin in the plant system is by phosphoribosylation.
Amasino (2005). 1955: Kinetin arrives. The 50th anniversary of a new plant hormone. Plant Physiology 138: 1177 to 1184
Kakimoto (2003). Perception and signal transduction of cytokinins. Annu. Rev. Plant Biol. 54: 605 to 27
Mok and Mok (2001). Cytokinin metabolism and action. Annu. Rev. Plant Physiol. 52: 89–118
Sakakibara (2006) Cytokinins: Activity, Biosynthesis, and translocation. Annu. Rev. Plant Bio. 57: 431 to 449