Cytokinin structure

Fig. 6. Cytokinin structures and receptor map for cytokinins. The model compound is the butenylaminopurine...

Cytokinin degradation via side chain cleavage is another process regulating levels of biologically active cytokinins in plant cells. Unlike other metabolic steps the cleavage of the side chain from the cytokinin molecule results in an irreversible destruction of cytokinin structure, which is of course associated with a complete loss of biological activity. 

FIGURE 1.5 Structures of auxin and abscisic acid derivatives of cytokinin. 

Kobayashi, H., N. Morisaki, Y. Tago, et al. Structural identification of a major cytokinin in coconut milk as 14-0-(3-0-[beta-D-galactopyranosyl-... 

The optimum B4 value for N4-substituents, 4.5 A coincides very well not only with that for N substituents of adenylate cytokinins, 5.2 A, shown in the previous section, but also with the value of 4.7 A observed for another class of compounds, N4-substituted 4-amino-2-methylpyrrolo[2,3-d]pyrimidines (24)47) which show either cytokinin or anticytokinin activity depending upon the structure of N4-substituents. These results in combination may help to think of the size of structure which should sterically fit well the cytokinin receptor cavity. 

The observations that cytokinins have activity in the tobacco bioassay detectable at concentrations as low as 10 H M, and that the promotion of such activity can depend on structural parameters... 

Compound 14, having the structural and growth-promoting characteristics expected of a potential anticytokinin, was tested for its ability to block the promotion of cell division and growth by other compounds having significant cytokinin activity. 

This illustrates a principle noted previously (83), namely that there would seem to be distinct cytokinin sites for individual cy-tokinin-mediated functions, the cytokinin requirements of which (in structural terms) are related but not identical. In a practical sense, it is not unreasonable to anticipate that compounds can be prepared which are capable of promoting or antagonizing single cytokinin-dependent functions. 

Structures of representative cytokinin species. Trivial names and commonly used abbreviations (parentheses)... 

Figure 11 Structures of naturally occurring cytokinin derivatives conjugated with sugars and amino acids.

Cytokinins are unique among plant hormones in that adenine compounds of identical structure occur in nucleic acids. More specifically, particular cytokinin-active ribonucleosides occur as components of certain molecular species of tRNA. Zachau et al. (19), during the determination of the base sequences of serine tRNA in yeast, first reported an "odd" base immediately adjacent to the 3 end of the anticodon. In collaboration with Biemann et al. (20), this "odd" base was identified as the natural cytokinin isopentenyl-adenosine, which is one of the most highly active cytokinins known. 

Terpenoids, which are also known as isoprenoids, constitute the most abundant and structurally diverse group of plant secondary metabolites, consisting of more than 40,000 different chemical structures. The isoprenoid biosynthetic pathway generates both primary and secondary metabolites that are of great importance to plant growth and survival. Among the primary metabolites produced by this pathway are phytohormones, such as gibberellic acid (GA), abscisic acid (ABA), and cytokinins the carotenoids, such as chlorophylls and plastoquinones involved in photosynthesis the ubiquinones required for respiration and the sterols that influence membrane stmcture (see also Steroid and Triterpene Biosynthesis) (Fig. 1). Monoterpenoids (CIO), sesquiterpenoids (Cl5), diterpenoids (C20), and... 

Mass spectrornety has been successfully used in the structure elucidation of cytokinins, the first example of which was Af -(3-methylbut-2-enyl)adenine. Its isopentenyl side chain provides the preferred site of fragmentation. The main fragmentation pathway includes also in this case the molecular ion of adenine (mje 135) which decomposes as shown. 

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