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Cytokinin
1. CYTOKININS
DR. R. RENUKA, M.SC.,M.PHIL.,PH.D.
ASSOCIATE PROFESSOR OF BIOCHEMISTRY
V.V.VANNIAPERUMAL COLLEGE FOR WOMEN
VIRUDHUNAGAR, TAMILNAD, INDIA.
2. CYTOKININ
Cytokinins are a group of plant growth hormones that
promote cell division or cytokinesis especially
controlling the transition from G2 to mitosis in plant
roots and shoots.
They are primarily involved in cell growth and
differentiation.
3. DISCOVERY OF CYTOKININS
In 1954, when Miller and others working on tobacco pith culture ,
they found certain stimulation of cell division when it was treated
with DNA of Herring sperm which was kept in laboratory for long
time.
It was believed that it was the DNA degradation product which
caused cell division stimulation in tobacco pith culture and not
DNA itself.
They called this product as kinetin as it is caused cytokinesis.
6. OCCURRENCE OF CYTOKININ
CYTOKININS OCCUR IN EVERY DEVELOPING PART OF
PLANTS SUCH AS APICAL MERISTEMS, DEVELOPING SEEDS,
ROOT NODULES, BACTERIAL TUMORS AND ALL REGIONS
WHICH SHOW ACTIVE CELL DIVISION.
7. CHEMICAL NATURE OF CYTOKININ
WHETHER THEY ARE NATUALLY OCCURRING OR SYNTHETIC, ALL
CYTOKININS HAVE SOME STRUCTURAL SIMILARITIES.
ALL ARE DERIVATIVES OF ADENINE – WHICH IS A PURINE BASE OF
NUCLEIC ACIDS.
HYDROGEN ATOM ON N6 OF ADENINE RING IS SUBSTITUTED BY EITHER
BENZYL GROUP RESULTING IN BENZYL ADENINE OR BY FURFURAL
GROUP FORMING KINETIN OR BY ISOPENTENYL GROUP FORMING
ZEATIN.
9. PHYSIOLOGICAL ROLES OF CYTOKININS
1. CELL DIVISION
-One of the most important biologic effects of kinetin on
plants is to induce cell division in tobacco pith callus,
carrot root tissue, soy bean cotyledon, pea callus etc. in
the presence of sufficient amount of auxin.
- Cell division occurs when both IAA and cytokinin is
applied jointly rather than individually.
- Secondary growth occurs in cambium cells due to cell
division.
10. 2.CELL ELONGATION
Like auxin and gibberellins, cytokinin also promotes
cell enlargement.
Significant cell enlargement has been observed after
kinetin treatment in cortical cells of tobacco roots,
tobacco pith cultures, leaf discs cut from etiolated
leaves of pumpkin etc.
11. 3. INITIATION OF INTERFASCICULAR CAMBIUM
Kinetin can induce formation of intrafascicular cambium
and there by secondary growth.
(This is actually shown by Sorokin et al in 1962 in pea stem
sections).
12. 4.MORPHOGENESIS
KINETIN ALSO HAS THE ABILITY TO CAUSE MORPHOGENETIC CHANGES
IN AN UNDIFFERENTIATED CALLUS.
RELATIVE CONCENTRATIONS OF IAA AND CYTOKININ DETERMINES THE
FORMATION OF PARTICULAR ORGANS EITHER SHOOT
OR ROOT FROM CALLUS.
MORE AUXIN CONC. ROOT FORMATION
MORE CYTOKININ CONC. SHOOT FORMATION
APPROPRIATE CONC. OF BOTH ROOT AND SHOOT
AUXIN AND CYTOKININ FORMATION
13. 5. DELAY OF SENESCENCE
THE AGING PROCESS OF LEAVES USUALLY
ACCOMPANIES WITH LOSS OF CHLOROPHYLL AND
RAPID BREAK DOWN OF PROTEINS, CHO, NUCLEIC
ACIDS, LIPIDS AND TRANSPORTATION OF THE
PRODUCTS TO YOUNGER LEAVES OR TISSUES. THIS IS
CALLED SENESCENCE.
IN 1957, RICHMOND AND LANG SHOWED THAT THIS
SENESCENCE COULD BE POSTPONED TO SEVERAL
DAYS IN DETACHED XANTHIUM LEAVES BY KINETIN
TREATMENT.
THIS EFFECT OF KINETIN IN DELAYING THE
SENESCENCE IS CALLED AS RICHMOND-LANG EFFECT.
14. Cytokinin mediated delaying of senescence is linked with its
effect on translocation of metabolites. the cytokinin treated
leaves withdraw nutrients from adjoining leaves and maintain
their nutrition and vigour.
Action of kinetin in senescence is might be due to
maintenance of the protein synthesising machinery. At
molecular level, cytokinin may act either by depressing protein
degradation or by increasing protein synthesis and maintains
the ratio of RNA or protein to DNA.
15. 6. COUNTERACTION OF APICAL DOMINANCE
Apical dominance is caused by auxin. Cytokinins counter the action of
auxins. Here, Cytokinins are antagonist to auxin.
In the intact plants, apical bud causes a preferential withdrawal of root
manufactured cytokinin towards it. This leads to cytokinin starvation in
lateral buds. As cytokinin is essential for cell division, lateral buds are
inhibited from growing.
Exogenous application of cytokinin stimulates cell division in lateral
buds hence, growth of lateral buds are promoted even if apical buds
are intact and apical dominance is nullified.
16.
17. 7. BREAKING SEED DORMANCY
Cytokinin is effective in breaking dormancy of light
sensitive seeds like lettuce and tobacco and promote
seed germination. Thus, cytokinin substitutes the light.
Further more the inhibitory effect of far red light
treatment on the germination of above seeds is also
overcome by kinetin treatment.
Kinetin in combination with red light reverse the
influence of certain inhibitors (coumarin and
xanthyletin) of seed germination and breaks seed
germination.
18. 8.TUBER FORMATION
Cytokinins induce tuber formation and inhibit
elongation of stolons of potato.
Tuber formation in Begonia is also stimulated by
cytokinin.
19. 9. Promote cell expansion
Cytokinin promotes the expansion of excised cotyledons of
several dicots.
The mechanism is associated with increased plasticity of the
cell wall.
20. 10. INFLORESCENCE GROWTH
Cytokinins induce cell division in inflorescence tips. This
leads to more flowering
which in turn yields more fruit.
Cytokinin oxidase regulates rice
production (Ashikari et al.2005).
21. 11. Cytokinin increases anthocyanin in I. balsominia
and Betacyanin in excised cotyledons of Amaranthus
in light.
12. Cytokinin promotes chloroplast development in callus
tissues of excised cotyledons.
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