discovery of cytokinins-biosynthesis of zeatin-mechanism of cytokinin action-function

1. PRESENTED BY
Solanki Anita D
M.Sc sem -3
Paper CBO -501
Department of life
sciences
H.N.G.U Patan
Cytokinins : Plant growth
Regulators

2. Contents
 Discovery of cytokinins
 Structure of cytokinins
 Biosynthesis of zeatin
 Mechanism of cytokinin action
 Function

3. Discovery of cytokinins
o cytokinins are plant growth hormones which are
basic in nature , either amino purine or phenyl
urea derivatives , that promote cytokinesis (=
cell division) either alone or conjugation with
auxin .
o Skoog and co-workers found that callus from
intermodal segments of tobacco proliferate only
when in addition to auxin .
o They were analysed to find out the growth
promoting chemical .
o The first cytokinin was discovered from degarded
from autoclaved herring sperm DNA by miller
1955.

4. Cont..
 It is called kinetin. Kinetin does not occur
naturally . It is synthetic hormone.
 The first natural cytokinin was obtained from
unripe maize grains or kernels by letham
(1964).
 It is known as zeatin. It is believed to be
widely distributed in the plant kingdom.
 Yet another naturally occuring cytokinin has
been found to be present in the endosperm
of coconut , cocos nucifera.
 A number of workers like steward , caplin ,
kuraishi , okuwara ,have worked for years
to isolate the active ingredients of coconut

5. Cont ..
 Some of cytokinins are constituents of
transfers RNAs .
 Roots seem to be the major source of cytikinin
synthesis. From roots the cytokinins pass
upwadly through xylem.
 Coconut milk is rich source of cytokinin.

6. Structure of cytokinins
Kinetin has been found to be a deravative of
the purine base adenine which bears furfuryl
substituent at the 9 position which migrated to
the 6 position of the adenine during
autoclaving of DNA.

7. Cont …
 All the cytokinins have purine ring with a side
chain at N⁶ position (amino substituted
adenine).
 Several workers found certain substituted
purine derivative to be more effective than
kinetin .
 One of the most active substances yet
synthesized is N⁶ – isopentenyl adenine.
 Another important cytokinin is zeatin. In the
zeatin structure adenine substituted in the 6
amino group with an unsaturated side chain
having at least one –OH component. zeatin is

9. Biosynthesis of zeatin & other natural
cytokinins

10. Cont..
 Biosynthesis of free cytokinins has been shown
schematically in the figure.
 Cytokinins are synthesized from adenosine
monophosphate (AMP) and isopentenyl -
pyrophosphate (∆²-ipp) by condension reaction
that is catalysed by the enzyme isopentyl
transferase.
 The product of this condensation is N⁶-(∆²-
isopentyle)-adenosine 5’-monophosphate(9R -5’-
P)ip) which is supposed to be precursor to all
other natural cytokinis.

11. Con..
 (the isopentyle pyrophosphate is infact trasferred
from dimethylallyl pyrophosphate (DMAPP) to
AMP. The enzyme isopentyle transferase is
specific to nucleotide only.
 It will not bind with either adenine or
adenosine.this enzyme has been isolated from
slime mold dictyostelium discoideum, tobacco
callus tissue and crown gall tissue.)
 The [9R-5’-P]ip is readily dephosphorylated to
yield N⁶-(∆²-isopentyl)-adenosine.
 Ribose sugar is now removed from N⁶-(∆²-
isopentyl) –adenosine, so that N⁶-(∆²-isopentyl)-
adenine is formed.

12. Con …
 Isopentenyl side chain of ip is now
hydroxylated to from free zeatin.
 Alternatively.[9R-5’-P] ip may be hydroxylated
directly to give 9-ribosyl-zeatin-5’-
phosphate.
 The phosphate group and then the ribose
sugar are removed from [9R-5’P]Z in
sequence to from free zeatin.
 Reduction of the double bond in isopentyl side
chain of zeatin would give rise to
dihydrozeatin (diHZ).

13. Con ..
 As mentioned earlier, the cytokinins are also
found in t-RNA in wide variety of organisms
from bacteria to man.
 However free cytokinins in cells do not arise
simply by hydrolysis of cytokinins containing
t-RNA.
 Experimental evidence have suggested and it
is generally believed that there is denovo
synthesis of free cytokinins in cells .

14. Mechanism of cytokinin action

15. Cont …
 In Arabidopsis , a cytokinin receptor has
been identified which is a transmembrane
protein called as CRE1 and is similar to
bacterial two component sensor histidine
kinases.
 CRE1 is probably a dimer, each polypeptide
of which consists of three domains,
I. An extracellular CHASE domain towards the
amino terminus,
II. A middle histidine kinase domain and
III. A receiver domain towads the carboxyl
terminus of the polypeptide .

16. Cont..
 Besides this, two other hybrid sensor histidine
kinases in Arabidopsis called AHK2 and AHK3
with CHASE domains may also be cytokinin
receptors.
 Some important events of cytokinin signalling are
shown in figure and are as follows ;
I. Cytokinin binds to extracellular CHASE
domain of the receptor
(CRE1/AHK2/AHK3Present on plasa-
membrane.
II. The middle histidine kinase domain of the
receptor is activated by ATP and the phosphate
is transferred to the aspartate residue (d) on

17. Cont..
iii. From receiver domain, the phosphate (p) is
transferred to a conserved histidine residue
located in an AHP protein( i.e., Arabidopsis
histidine phosphater protein ) the
phosphorylated AHP protein move to nucleus.
iv. In nucleus, the phosphorylated AHP protein
transfers its phosphate group to an aspartate
residue present in receiver domain of type B
ARR ( Arabidopsis response regulator) that
consists of a receiver domain and an additionl
transcription factor domain or output domain .

18. Cont..
 Type B ARRS may interact with other
effectors leading to cytokinin responses
and/or ,
v. The phosphorylated B type ARR activates
is own output domain (i.e; transcription factor
domain) and induces trascription of genes
which encode type A ARRs ( ARRs which
consist of only a receiver domain).
vi. Type A ARRs may also be phosphorylated
by phosphorylated AHP.
vii. Phosphorylated type A ARRs may now
interact with other effectors leading to

19. Function
 Cell division
 Cell enlargement
 Initiation of interfascicular cambium
 Inflorescence growth
 Morphogenesis
 Delay senescense
 Dormancy of seeds

20. Cont ..
1) Cell divison :-
-One of the most important biological
effects of kinetin on plants is to
induce cell division in the
presence of sufficient amount
of auxin (IAA) especially in tobacco
pith callus , carrot root tissue ,
soyabean cotyledon , pea callus etc.

21. Cont…
2) Cell enlargement :-
- Like auxin and gibberellins , the kinetin may
also induce cell enlargement. Significant cell
enlargement has been observed after kinetin
treatment in leaf discs cut from etiolated
leaves of pumpkin cotyledons , tobacco pith
cultures , cortical cells of tobacco roots etc.
3) Initiation of interfascicular cambium :-
-kinetin can induce formation of intrafascicular
cambium. This has in fact been shown by
sorokin et al (1962) in pea stem sections .

22. Cont ..
4) Inflorescence growth :
 Cytokinins induce division in inflorescence
tips. leads to more flowering which yields
more fruit
 ‘Cytokinin oxidase regulates rice grain
production’ ( Ashikari et al.2005)
-

23. Cont ..
5) Morphogenesis:-
-kinetin also has ability to cause morphogenetic
changes in an otherwise undifferentiated callus.
For instance, the tobacco pith callus can be
made to develop either buds or roots by
changing the concentration of
kinetin and auxin:

24. Cont ..
6) Delay senescence : The Richmond Lang Effect
- The ageing process of the leaves usaually
accompaines with loss of chlorophyll and rapid
breakdown of proteins. This is called as
senescence.

25. Cont ..
- In 1957, Richmond and Lang showed that
this senescence could be postponed to
several days in dethached xanthium leaves by
kinetin treatment.
- This effect of kinetin in delaying the
senescence is called as Richmond – Long
effect.

26. Cont ..
7) Dormancy of seeds :-
-Like gibberallins , the dormancy of certain light
sensetive seeds such as lettuce and tobacco
can also be broken by kinetin treatment in
dark . Furthermore , the inhibitory effect of
farred light treatment on the germination of
the above seeds is also overcome by kinetin
treatment.

27. References
 Fundamentals of plant physiology
 -V.k.jain
 A Text book of plant physiology
 -V.verma
 www.slideshare.net
 www.wikipedia.com