Cytokinins are a class of plant growth hormones (phytohormones) that promote cell division and cell differentiation. Cytokinins are adenine derivatives that carry an isoprene-derived or an aromatic side chain at the N6 position of the purine (Fig. 3.1). In the 1950s, Folke Skoog and colleagues isolated the first cytokinesis-promoting factor, kinetin, from autoclaved herring sperm DNA (Miller et al., 1955a, 1955b). Since then several other growth-promoting factors similar to kinetin have been identified from plants. The first naturally occurring cytokinin, trans-zeatin (tZ), was isolated from maize (Zea mays L.) endosperm (Miller, 1961). In the following years, researchers isolated compounds with cytokinin activities from many plant species (Mok and Mok, 2001). Currently, the most prevalent and most studied natural cytokinins are isopentenyladenine (iP) and tZ.
The discovery of cytokinins facilitated the development of methods for plant tissue culture and also had a profound effect on studies of plant biology. Cytokinins participate in regulating various processes in plant growth and development, including female gamete and embryo development, seed germination, vascular development, shoot apical meristem development, photomorphogenesis, leaf senescence, and floral development, as well as regulating adaptive responses to environmental stresses (Schmülling, 2002). In the past decades, the discovery of many enzymes that control the modification and activity of cytokinins has shed new light on the basic molecular mechanisms of cytokinin biosynthesis (Sakakibara, 2006; Werner and Schmülling, 2009; Zürcher and Müller, 2016). Cytokinins occur in plants as free bases, nucleosides (ribosides), glycosides (O- and N-glycosides) and nucleotides (Fig. 1B). The free bases represent the active forms, whereas the ribosides have low activities (Lomin et al., 2015; Yamada et al., 2001). The occurrence, distribution and variation of individual cytokinins depend on plant species, tissue, and developmental stage (Sakakibara, 2006). Interestingly, the inactive cytokinins are much more abundant compared to the free bases (Kiba et al., 2013; Miyawaki et al., 2006; Svačinová et al., 2012; Takei et al., 2004b), which indicates that the concentration of active cytokinins is tightly controlled to prevent unregulated signaling. This is achieved by coordination of the enzymes involved in biosynthesis, modification, and degradation of cytokinins.
Besides kinetin, other structurally related compounds with cytokinin activity have been identified (Fig. 1A). Naturally occurring cytokinins are all comprised of adenine derivatives but differ in the side chain attached to the N6-position of the purine (Mok and Mok, 2001). Two classes of side chains can be distinguished; the isoprenoid cytokinins and the aromatic cytokinins (Mok and Mok, 2001; Strnad, 1997). Little is known about the biosynthesis of the latter, and they have been identified in only a limited number of plant species so far. The best-studied isoprenoid cytokinins are N6-(Δ2-isopentenyl)-adenine (iP), zeatin, and dihydrozeatin. Zeatin's side chain contains a double bond and its hydroxy group can therefore be oriented in a trans- or cis-configuration representing trans-zeatin (tZ) or cis-zeatin (cZ), respectively. The side chain of dihydrozeatin is saturated, whereas the functional group of the iP side chain is a methyl compared to a hydroxymethyl in zeatin.
In Arabidopsis, tZ and iP are the prevalent cytokinins, whereas in maize, chickpea, and rice, cZ is common as well (Mok and Mok, 2001; Sakakibara, 2006). In order to determine the structural requirements for cytokinin activity and because of the strong agricultural interest in cytokinins’ potential to increase growth and resistance to abiotic stress, several chemicals were synthesized and assayed for activity (Mok et al., 1978; Skoog et al., 1967). The most commonly known synthesized cytokinins are benzyladenine (BA), an aromatic cytokinin that was later found to occur naturally in some plant species, and thidiazuron, which has been used as a defoliant in cotton fields (Mok et al., 1982).
