Hormone cytokinin

1. Cytokinin are a class of plant growth
substances (phytohormones) that promote
cell division, or cytokinesis, in plant roots
and shoots. They are involved primarily in
cell growth and differentiation.

2. First cytokinin discovered -- kinetin.
3
N
N
N
N
H
Not reported in plants
Reported in human urine.
H
N CH2
O

3. *Coconut endosperm (coconut milk) – supported continued
division of mature, differentiated cells
*1940s-50s – Adenine had some effect
*Aged herring sperm … . .
*Kinetin
Corn endosperm (1973) – zeatin
*Notice the double bond … . .
4

4. 5

5. *Primary site of synthesis -- root tips.
*High concentrations -- immature seeds and
developing fruits
*synthesis *OR* transport?
*Evidence indicates that locally produced cytokinins
required to release buds from dormancy
6

6.  Adenosine phosphate-isopentenyltransferase (IPT)
catalyses the first reaction in the biosynthesis of isoprene
cytokinins.
 It may use ATP, ADP, or AMP as substrates and may use
dimethylallyl pyrophosphate (DMAPP) or
hydroxymethylbutenyl pyrophosphate (HMBPP) as prenyl
donors.
 This reaction is the rate-limiting step in cytokinin
biosynthesis.
 DMADP and HMBDP used in cytokinin biosynthesis are
produced by the methylerythritol phosphate pathway
(MEP).

7.  Cytokinins can also be produced by recycled tRNAs in
plants and bacteria.
 tRNAs with anticodons that start with a uridine and
carrying an already-prenylated adenosine adjacent to
the anticodon release on degradation the adenosine as
a cytokinin.
 The prenylation of these adenines is carried out by
tRNA-isopentenyltransferase.
 Auxin is known to regulate the biosynthesis of
cytokinin.

9. Naturally-occurring
zeatin
isopentenyl adenine
Synthetic
kinetin
N
N
H
CH2 NH
N
N
benzyl amino purine

10. *Cytokinins extracted from coconut milk, Tomato juice
*Flowers and fruits of pear, plum
*Cambium tissues of Eucalyptus, Nicotina
*Immature fruits of Zea Mays, Musa sp.
*Root exudates of Sunflower
Cytokinins found in plants
1. Ribosylzeatin,
2. Zeatin,
3. Dihydrozeatin.

11. 1. Isoprenoid
a. cis-zeatin
b. trans-zeatin
c. N6-(D2-isopentenyl)adenine
d. dihydrozeatin
cytokinin:
(cZ)
(tZ)
(iP)
(DZ)
2. Aromatic
a. ortho-topolin
b. meta-topolin
c. ortho-methoxytopolin
d. meta-methoxytopolin
e. benzladenine
cytokinin:
(oT)
(mT)
(MeoT)
(MemT)
(BA)
The major forms of cytokinins differ in different plant
species. For example in rice the isoprenoid cytokinin cis-
zeatin is the major form, whereas in Arabidopsis it is the
trans-zeatin and iP forms that predominate.

12. Synthesis
Root Apex
Transport
Upward in Xylem

13. Transport of cytokinins from roots to shoot via
xylem
cytokinins in xylem exudates are mainly in form
of zeatin ribosides via xylem (transpiration
stream) in peas, a signal from the leaves may
signal/regulate transport of cytokinins from the
roots zeatin ribosides are the main transport
form; converted to the free base or glucosides in
the leaves some cytokinin also moves in the
phloem.

14. Activity of cytokinin oxidase induced by high cytokinin concentrations.

15. Initially the cytokinin signal binds to a receptor's
CHASE domain. This triggers a cascade of
phosphorylations of proteins, ultimately ending in
phosphorylation of a shuttle protein, AHP.
The phosphorylated AHP protein enters the nucleus,
phosphorylates type B ARR proteins, that turn on the
synthesis of type A ARR proteins. When these gene
products are, in turn, phosporylated, they influence
other effectors that result in cytokinin responses.
There is also a negative feedback loop here to shut
down the system when enough phosphorylated ARR is
present.

16. 1. Promote cell division in target cells.
2. Promote axillary bud out growth.
3.Balance root/shoot growth so vigorous roots can support
greater shoot system.
4. Important in seed development - find in endosperm
(including coconut milk) and in cotyledons.
5. May delay senescence in leaves – lab experiment!
6.May play role in differentiation of vascular cambium in
spring in conjunction with auxin Stimulates.

17. 7.Morphogenesis (shoot initiation/bud formation) in tissue
culture.
8.Stimulates the growth of lateral buds-release of apical
dominance.
9. Stimulates leaf expansion resulting from cell enlargement.
10. May enhance stomatal opening in some species.
11.Promotes the conversion of etioplasts into chloroplasts via
stimulation of chlorophyll synthesis.
12. Resistance against extreme temperature.

18. Cytokinins: Activity, Biosynthesis, and Translocation
Hitoshi Sakakibara
Abstract
Cytokinins (CKs) play a crucial role in various phases of plant growth and
development, but the basic molecular mechanisms of their biosynthesis and
signal transduction only recently became clear. The progress was achieved by
identifying a series of key genes en- coding enzymes and proteins controlling
critical steps in biosynthesis, translocation, and signaling. Basic schemes for
CK homeostasis and root/shoot communication at the whole-plant level can
now be devised. This review summarizes recent findings on the relationship
between CK structural variation and activity, distinct features in CK
biosynthesis between higher plants and Agrobacterium infected plants, CK
translocation at whole-plant and cellular levels, and CKs as signaling
molecules for nutrient status via root-shoot communication
Case study

19. Reference's
Sakakibara H (2006). "Cytokinins: activity, biosynthesis, and translocation".
Annu Rev Plant Biol 57 (1): 431–49.
doi:10.1146/annurev.arplant.57.032905.105231. PMID 16669769.
Chen CM, Ertl JR, Leisner SM, Chang CC (July 1985). "Localization of
cytokinin biosynthetic sites in pea plants and carrot roots". Plant Physiol.
78 (3): 510–3. doi:10.1104/pp.78.3.510. PMC 1064767. PMID 16664274.

20. Organo-genesis