The major group of cytokinins are N^6 -isoprenoid adenine analogues and can be divided into three
groups. The first subgroup consists of zeatin and its derivatives—ribosides, glucosides, and nucleotides—
whose N^6 -isoprenoid side chain is either 4-hydroxy-3-methyl-2-trans-butenylaminopurine or its cisiso-
mer. The second subgroup of N^6 -isoprenoid analogues consists of dihydrozeatin (diH)Z and its ribosides
and glucosides. The third subgroup includes 6-(3-methyl-2-butenylamino)purine and N^6 -(^2 -isopen-
tenyl)adenine (2iP) and its ring substitution products.
The second and minor group of cytokinins (i.e., N^6 -benzyladenine analogues) was first synthesized
as 6-benzylaminopurine (BAP) with high biological activity. Later it was found to exist in a number of
plant species.
Certain nonpurine compounds, such as 8-azakinetin, benzimidazole, N,N -diphenylurea, and 2-ben-
zthiozolyloxyacetic acid, have also been reported to have cytokininlike activity. Of these, three are syn-
thetic and only N,N -diphenylurea occurs naturally in plants. It has been suggested by some workers that
these so-called urea cytokinins may also be considered true cytokinins, but most hormone physiologists
do not agree [72,73]. These compounds may not be active as such, but they may serve as precursors or
inducers for the commonly accepted cytokinins.
B. Metabolism
Miura and Miller [74] have suggested that all plant cells are capable of synthesizing cytokinins provided
that the mechanisms to do so are “switched on.” However, this does not mean that cytokinins are biosyn-
thesized in the entire plant. Evidence suggests that actively dividing regions of plants are the sites of cy-
tokinin biosynthesis. Because the root system possesses the most actively dividing regions, these regions
are considered to be the major sites of cytokinin production.
Compared with other aspects of cytokinin physiology, little is known about their biosynthesis, which
is comparatively quite complicated. The circumstances of its discovery and its effect on cell division and
protein synthesis have somehow closely associated free cytokinins with RNA and DNA. The production
in plants can be accounted for either by the turnover of cytokinin-containing transfer RNA (tRNA), by de
novo biosynthesis, or by both mechanisms. It has also been reported to be present in ribosomal RNA
(rRNA) [75].
The major cytokinin-active base in tRNA, [9R]iP, is formed by the condensation of adenine with an
appropriate donor of the N^6 substituent during posttranscriptional processing. The ^2 -isopentenyl py-
rophosphate (IPP) is the immediate precursor (donor) of the ^2 -isopentenyl side chain of N^6 -(^2 -isopen-
tenyl)adenosine in tRNA. A cell-free enzyme system, isopentenyl AMP synthase, has been isolated from
cultured autotrophic tobacco tissue which forms cytokinin from adenosine monophosphate (AMP) and
IPP as substrate [76]. The ^2 -isopentenyl pyrophosphate is a product of mevalonic acid (MVA) (an im-
portant precursor of carotenoids, abscisic acid, gibberellins, sterols, and other isoprenoid compounds) via
^3 -IPP.
Besides the formation of free cytokinins from tRNA, there is strong evidence that they are also
formed by de novo biosynthesis. Beutelmann [77] supplied labeled adenine to moss callus cells and ob-
tained labeled cytokinin that cochromatographed with 2iP, but no labeled cytokinin was detectable from
tRNA. Similar results were obtained from the cytokinin-autotroph tobacco callus tissues, Vinca rosea
crown gall tissues, and synchronously dividing tobacco callus cells [78].
The amount of cytokinin present in the tissue is regulated by conversion to a diversity of metabolites
by the following reactions: (1) trans-hydroxylation of the terminal methyl group on the side chain, (2)
side-chain reduction, (3) isoprenoid side-chain cleavage, (4) O-glucosylation, (5) N-glucosylation, (6)
ring substitution by alanine moiety, and (7) base-ribonucleoside-ribonucleotide interconversion. These
types of reactions have been observed in a number of plant species as well as in crown gall tissues. These
reactions have also been obtained from tissues exogenously supplied with the hormone. Three enzymes,
cytokinin oxidase [molecular weight (MW) 88,000], cytokinin 7-glucosyltransferase (MW 46,500), and
(9-cytokinin)alanine synthase (MW 64,500), have been purified and characterized [78]. The free base, nu-
cleotide, and nucleoside forms of cytokinins appear to be easily interconverted in plant tissues. Incorpo-
ration of labeled cytokinin bases into ribosides (ribonucleoside) and ribotides (riboside 5 -phosphates)
have been observed in a number of plant species. Five enzyme systems, purified from wheat germ, may
be responsible for this interconversion. These are (1) adenosine phosphorylase, (2) adenosine kinase, (3)
adenine phosphoribosyltransferase, (4) (5 -ribonucleotide phosphohydrolase) 5 -nucleotidase, and (5)
512 NAQVI