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.

Translocation of cytokinins

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.