Cytokinins are compounds with a structure resembling adenine which
promote cell division and have other similar functions to kinetin.
Kinetin was the first cytokinin discovered and so named because of the
compounds ability to promote cytokinesis (cell division). Though it is a
natural compound, It is not made in plants, and is therefore usually
considered a "synthetic" cytokinin (meaning that the hormone
is synthesized somewhere other than in a plant). The most common form of
naturally occurring cytokinin in plants today is called zeatin which was
isolated from corn (Zea mays). Cytokinins have been found in almost all
higher plants as well as mosses, fungi, bacteria, and also in tRNA of
many prokaryotes and eukaryotes. Today there are more than 200 natural
and synthetic cytokinins combined. Cytokinin concentrations are highest
in meristematic regions and areas of continuous growth potential such as
roots, young leaves, developing fruits, and seeds (Arteca, 1996; Mauseth,
1991; Raven, 1992; Salisbury and Ross, 1992).
CK activity has been detected in extracts of almost all plants
organs and in many organisms (Letham, 1978). Although root apices have
been recognized as a major site of CK biosynthesis in plants, there is
circumstantial evidence to indicate that developing fruits and seeds,
developing/growing buds and shoots apices and leaves are additional
sites of CK biosynthesis. Furthermore, cambium and embryonic axis have
also been suggested as possible sites of biosynthesis.
The
view that roots are a major site of CK synthesis, and that
root-produced CKs move in the xylem to the shoot to participate in
control of development and senescence appears to be widely accepted
(Letham and Palni, 1983). Since CKs have been detected in extracts of
all plant parts, the major question that remains to be clarified is
under what conditions the observed CK activity in various plant parts
is derived solely from the roots, and when and to what extent it is
derived by synthesis in situ.
Roots
The first experimental demonstration that roots could be a site of
CK biosynthesis was provided by Mothes (1960) using detached and
senescing tobacco leaves. It was shown that leaf senescence could be
delayed by the application of kinetin and a similar effect was
observed if roots were formed on the petioles. Later, Kulaeva (1962)
discovered that xylem exudate of tobacco plants could substitute for
kinetin in retarding leaf senescence. Since then compounds with the
CK-like activity have been detected in xylem sap of a number of
plants. Weiss and Vaadia (1965) analyzed various regions of sunflower
and pea roots and found that the apices were particularly rich in CK
activity. The terminal mm segments of pea roots were reported to
contain about 40 times more free CK than the segments 1-5 mm from the
apex ; CK activity could not be detected in segments further from the
apex (Short and Torrey, 1972).
Additional evidence that roots
produce CKs was provided by Engelbrecht (1972) who reported CK
accumulation in the leaf blades of excised bean leaves following
development of roots on the petiole. Moreover, the total CK production
by roots of Xanthium strumarium measured in the root exudate,
was higher than the amount of CK present in the roots at the time of
decapitation (Henson and Wareing, 1976). Removal of roots from plants
resulted in considerable loss of CK activity in detached leaves and
buds. A more direct evidence that CKs are synthesized by roots was
provided by Van Staden and Smith (1978) who demonstrated CK
accumulation by excised maize and tomato roots grown in aseptic
culture. The developing roots were also found to release CK into the
culture medium. This is an agreement with the findings by Koda and
Okazana (1978) who showed that cultured tomato root tips released CK.
The root tips were subcultured eight times and the level of CK
accumulation in the medium after each passage was reported to be
nearly same. Finally, Chen et al. (1985) showed adenine
incorporation into CKs by pea and carrot roots grown on CK and
auxin-free medium. The evidence presented above collectively argues
that roots (root tips in particular) are an active site of CK
biosynthesis.
Seeds and fruits
Letham (1963) was the first to suggest that CK biosynthesis occurs
in seeds. The growth of cultured apple frui-tlet explants was more
dependent on exogenous CKs if the developing seeds were removed from
explants before culture (Letham and Bollard, 1961). The CK activity
was also reported to be higher in developing apple and avocado seed
than in the receptacle and mesocarp tissues, respectively. in
addition, the cotyledonary tissue from avocado seed could be grown in
vitro without added CKs (Blumenfeld and Gazit, 1971). The reported
lower endogenous CK levels in genetically parthenocarpic fruits
compared to seeded fruits also suggest that seeds produce CKs. Further
evidence that seeds and fruits synthesize CKs comes from experiments
in which fruits were shown to develop even when roots were excised at
flowering, and the formation of root primordia was prevented by
periodic excision of stem bases (Paterson and Fletcher,
1973).
Hahn et al. (1974) were able to culture excised
pea pods and showed that the CK contents of developing seeds continued
to increase with time. However, others could not confirm this result
when young pea pods were cultured in vitro. The pod walls in lupin
contain high CK activity (Davey and Van Staden, 1977) and it is
possible that in experiments by Hahn et al the observed increase in CK
levels in developing pea seeds could have originated from the pod
walls (Summons et al., 1979). However, Summons et al., (1981) have
demonstrated CK biosynthesis in immature lupin seeds supplied with
adenosine.
Although developing seeds and fruits seem to have
the capacity for CK biosynthesis, the high CK levels in these organs
may also be due to transport from other sites of synthesis. CKs can be
transported into seeds and/or fruits via the phloem. It has been
suggested that seeds and fruits may compete for root derived CKs like
vegetative parts, and that they act as strong physiological sinks (Van
Staden and Davey, 1979; Van Staden et al., 1982). As for example, the
removal of grape berries have been shown to result in increased CK
levels in the leaves (Hoad et al. 1977). However, Nooden and Letham
(1984) have demonstrated that radioactively labeled [9R]Z and Z,
previously shown to be endogenous CKs in the root derived xylem sap,
when introduced into the xylem of soybean explants, do not readily
enter the developing seeds; a major proportion of the supplied CK was
associated with the leaf blade and the stem. Thus, developing pods
(and seeds) do not appear to act as metabolic sinks. Although
transport of root derived CKS to developing seed is negligible
(Summons et al., 1981; Nooden and Letham, 1984), adenoside, a
precursor of CKs, readily enters the embryo (Nooden and Letham, 1984).
This observation is particularly important since developing seeds of
lupin have previously been shown to synthesize CKs when supplied with
[H]adenoside (Summons et al., 1981).
Leaves
Developing leaves have been shown to contain high CK activity, and
the level of active CKs declines markedly upon maturation and
senescence which accumulate storage CKs, e.g., glucosides. Thus, it is
possible that younger leaves could be an additional site of CK
biosynthesis. Salama and Wareing (1979) reported an increase in the
level of active CKs in detached sunflower leaves when supplied with
nitrate-containing nutrient solution. However, the ability of leaves
to synthesize CK was never tested recently. More recently, CK
biosynthesis has also been demonstrated in young excised tobacco
leaves following incubation with [14C]adenie (Palni et al., 1988).
