# Biosynthetic Pathways of Hormones in Plants
## 9 Cytokines  
Cytokinins (CKs) regulate plant development, nutrient uptake, cell division, cell differentiation, chlorophyll senescence, apical dominance, embryonic development, and general aspects of plant growth [136]. Cytokines are \(N^{6}\)-substituted adenine derivative compounds that can be divided into two groups: (i) isoprenoid and (ii) aromatic CKs. Isoprenoid CKs more abundant than aromatic compounds [137; 138]. Isoprenoid CKs include compounds such as \(N^{6}\)-(\(\Delta^{2}\)-isopentenyl adenine (iP), dihydrozeatin, and _trans_- and _cis_-zeatin (_tZ_ and _cZ_, respectively), while \(N^{6}\)-benzyladenine and its hydroxyl derivatives, such as _ortho_- and _meta_-topolin, are representatives of aromatic CKs [139]. Cytokinin levels are spatially and temporally regulated. Cytokinesins are abundant in the apical meristems, root tips, and immature seeds. It is generally assumed that the root tip is the main site of the biosynthesis of CKs. On the other hand, literature data indicate that the shoot apex, cambium, and immature seeds are also capable of synthesizing CKs [140]. Changes in CK levels are related to the phase of cell cycle, consequences of influence of environmental factors, presence or lack of mineral nutrients, and effect of abiotic or biotic stress factors. The levels of active CKs in plants are highly regulated by the rates of production, interconversion, transport, and degradation [141]. Cytokinins can be synthesized de novo as nucleotide mono-, di-, or tri-phosphates, which are characterized by low biological activity. Moreover, tRNA is a source of CKs. The release of cytokins from this nucleic acid leads to the creation of nucleotide monophobates. The conversions of nucleotides, nucleosides, and free bases are catalyzed by enzymes involved in the metabolism of adenine [142]. In the next step, an isoprenoid moiety is added to the adenine present in the ATP and/or ADP molecule [143]. An alternative pathway, in which a hydroxylated side chain is added to the adenine moiety, has also been proposed [144]. DMAPP and HMBDP are the common isoprenoid side chain donors in biosynthetic pathway of CKs [102; 145]. In the case of formation of isopentenyladenine-type CKs via the precipitation of DMAPP, the side chain is hydroxylated by cytochrome P450 mono-oxygenase (Figure 7) [146]. The CK nucleotides that include nucleotides released from tRNA are then hydrolyzed to free bases [102]. Cytokinin biosynthesis is catalyzed by the enzyme isopentenyl transferase (IPT) (Figure 10). There are two types of the adenylate IPT, which are responsible for attachment of an isopentenyl group to the \(N^{6}\) atom present in AMP, ADP, or ATP and tRNA. IPT does not add isopentenyl group to adenosine or adenine. IPT acts in the same way on adenine present in the structure of tRNA [143]. Adenylate IPT can synthesize iP and fZ nucleotides [145; 147]. Isopentenyl transferase isolated from vascular plants preferentially acts on ADP or ATP, rather than AMP, as phenyl acceptors and almost exclusively uses DMAPP as a premyl donor, which, in turn, can synthesize iP riboside 5\({}^{\prime}\)-diphosphate or iP riboside 5\({}^{\prime}\)-triphosphate. Past kinetic studies of the activity of IPT in _A. thaliana_ showed that the Km values for AMP are much higher than those of ADP and ATP, which were used as substrates. Moreover, the cellular level of AMP is low, indicating that AMP is not an essential substrate involved in IPT activity [143]. Interestingly, IPT isolated from mulberry (_Morus alba_) can also recognize and react with dATP, dADP, CDP, and GDP as substrates. In the case of GDP, the isopentenyl moiety is added to \(N^{2}\) group (the exocyclic nitrogen), rather than \(N^{1}\) group (the endocyclic nitrogen), which is closer to \(C^{6}\)[148; 149]. However, IPT that is isolated from _Agrobacteriumium_ cells, which is called Tmr, uses AMP as an acceptor and HMBDP or  
Figure 10: The biosynthetic pathways of cytokins (phytohormes are green).