The Biosynthesis of Cytokinin
Since their discovery in 1955 much work has been carried out on cytokinins. They are known to be formed from adenine, and there are two main classes, one containing aromatic side chains has members such as meta-topolin; the other group has isoprene side chains and includes cis-zeatin. Cytokinins have many roles to play in plants; they are involved in cell proliferation, differentiation, and many processes that are under signal transduction control. The following section takes a look into how cytokinin is biosynthesized.
Isoprenoid Cytokinin Biosynthesis
The first step of the biosynthesis of isoprenoid cytokinins involves the enzyme adenosine phosphate-isopentenyltransferase. This enzyme catalyses the N-prenylation of adenosine 5'-phosphates such as ATP, ADP and AMP with either hydroxymethylbutenyl diphosphate or dimethylallyl diphosphate.
There are two main pathways involved in cytokinin biosynthesis, these are the MEP (methylerythritol phosphate) and the MVA (mevalonate) pathway. The majority of iP and trans-zeatin cyttokinins are derived from the MEP pathway, whereas the MVA pathway is largely responsible for the creation of cis-zeatins. For further information on the biosynthesis of isoprenoid cytokinins it is recommended to read the papers by Takei and Sakakibara (see references).
Aromatic Cytokinin Biosynthesis
Little is know about how aromatic cytokinins are biosynthesized in plants. It is thought that the pathways involved in the creation of aromatic biosynthesis may be shared with those of isoprenoid cytokinin biosynthesis. This was concluded because many of the receptors and enzymes involved in isoprenoid cytokinin synthesis are able to recognise aromatic cytokinins too. In addition to this it is thought that P450 enzymes may be involved in the biosynthesis of the topolins that are found in many aromatic cytokinin side chains.
Cytokinin Biosynthesis Regulation
Cytokinins are known to play very important roles in the regulation of plant development. They are involved in processes ranging from cell differentiation through to the control of senescence. Since their discovery in 1955 much work has been done looking into the form and function of cytokinins.
There are known to be two main classes of cytokinins; those with isoprenoid side chains and those with aromatic side chains. This section of the cytokinin section of plant biology advice takes a look at how the biosynthesis of cytokinins are regulated.
Nitrogen Supply and Cytokinin Biosynthesis
It is well known that inorganic nitrogen has a huge impact upon the growth and development of plants. The cytokinin hormone is a key signalling factor used in analysing, and responding to, the nitrogen nutrient levels available to the plant. Cytokinins are able to travel via the xylem from the root to the shoots and acts as a long range signal of nitrogen availability. In Arabidopsis there is a molecular mechanism in place that controls nitrogen dependent biosynthesis; similar mechanism are expected across the plant kingdom. In Arabidopsis the regulation of the genes IPT3 and IPT5 are dependent on the availability of nitrogen. These genes code for adenosine phosphate-isopentenyltransferases and are located in the plastids. This strongly suggests that nitrogen levels have a major role to play in the biosynthesis of cytokinin in Arabidopsis.
Plant Hormones and Cytokinin Biosynthesis Regulation
Many of the genes that are known to be involved in the biosynthesis of cytokinins are themselves under the influence of plant hormones; these hormones include auxin, abscisic acid and even cytokinins themselves. Some of the main genes that are involved in cytokinin biosynthesis are IPT (encodes adenosine phosphate-isopentenyltransferase) and CKX (encodes cytokinin oxidase/dehydrogenase). it has been shown in Arabidopsis that the transcripts of some IPT genes accumulate in the presence of auxin in the roots, and some IPTs are down regulated by the existence of cytokinins. The CKX genes have been shown to be unregulated in maize roots in the presence of abscisic acids and cytokinins.
The Transport of Cytokinins in the Plant (Translocation)
The cytokinins were first isolated in herring sperm DNA in 1955. Since then much research has been done on them in plants leading to their classification as a plant hormone. There are two distinct classes of cytokinins, these are dependent upon the type of adenine side chain that they have: either aromatic or isoprenoid.
Cytokinins play many roles in the growth and regulation of plants and play active roles in cell proliferation and differentiation, in addition to acting in both long and short distance signalling mechanisms. The genes that encode for key steps of cytokinin biosynthesis are themselves under the control of plant hormones, these include abscisic acid, auxin and cytokinin itself. This section of plant biology advice takes a look into the transport of cytokinins in plants.
Cytokinin Transloacation
Cytokinins are biosynthesized in specific cells and tissues (such as the plastid), therefore to be involved in translocation the cytokinin must be able to move first move from the cells in which they are created. This movement of cytokinins can occur by the use of transport systems specific to cytokinins and by diffusion. The evidence for selective transport mechanisms comes from the ability of trans-zeatin to build up in the xylem and iP to build up in the phloem. In vivo experiments have shown that cytokinins are able to use the same transport mechanisms that are used to transport purines and nucleosides in the plant. It is presently unknown as to the exact mechanism that transports cytokinins from the plastids to cytosol.
Control of Cytokinin Transport
Nucleosides are thought to be major form of cytokinins involved in translocation. It is known that some equilibrative nucleoside transporters in plants such the rice OsENT2 gene are able to regulate the uptake of cytokinin nucleosides. the transportation system of cytokinin is not as elaborate of transport systems used by many other plant hormones; it is thought that the loading of either the xylem or the phloem tissue is enough to mediate the systemic transport of cytokinins. The biosynthesis and homoeostasis of cytokinins is highly dependent upon nitrogen and other hormone levels.
Kasahara et al. (2004). Distinct isoprenoid origins of cis- and trans-zeatin biosyntheses in Arabidopsis. J. Biol. Chem. 279:14049–54
Sakakibara (2006) Cytokinins: Activity, Biosynthesis, and translocation. Annu. Rev. Plant Bio. 57: 431 to 449
Takei et al. (2001). Identification of genes encoding adenylate isopentenyltransferase, a cytokinin biosynthesis enzyme, in Arabidopsis thaliana. J. Biol. Chem. 276: 26405–10
Takei et al. (2004). Arabidopsis CYP735A1 and CYP735A2 encode cytokinin hydroxylases that catalyze the biosynthesis of trans-zeatin. J. Biol. Chem. 279:41866–72