Transcript Cytokinin Functions

Cytokinins
Regulators of Cell Division
Plant Physiology II
BS Botany 7th semester
Habib-ur-Rehman Athar
Institute of Pure & Applied Biology
Bahauddin Zakariya University, Multan
Cytokinins
Regulators of Cell Division
Cytokinins are substituted adenine compounds
that promote cell divisions in specific plant
tissues.
History
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1892: Weibner thought that cell division is regulated by endospermic compounds.
1941: Discovery by Johannes van Overbeek – coconut milk could sustain cell division
and prolonged growth of stem explants (excised pieces of stem) He also showed that
various other plant species had compounds which stimulated cell division (van
Overbeek, 1941).
IAA could not produce this effect.
1950s: Folke Skoog and coworkers – identified a modified purine (nucleotide) and
called Kinetin
In 1954:, Jablonski and Skoog extended the work of Haberlandt showing that
vascular tissues contained compounds which promote cell division (Jablonski and
Skoog, 1954).
1955: The first cytokinin was isolated from herring sperm in 1955 by Miller and his
associates (Miller et al., 1955). This compound was named kinetin because of its
ability to promote cytokinesis. Hall and deRopp reported that kinetin could be formed
from DNA degradation products in 1955 (Hall and deRopp, 1955).
1961: Miller isolated the first naturally occurring cytokinin from corn (Miller, 1961). It
was later called zeatin. Almost simultaneous with Miller, Letham published a report on
zeatin as a factor inducing cell division and later described its chemical properties
(Letham, 1963).
It is Miller and Letham that are credited with the simultaneous discovery of zeatin.
Since that time, many more naturally occurring cytokinins have been isolated and the
compound is ubiquitous to all plant species in one form or another (Arteca, 1996;
Salisbury and Ross, 1992).
All cytokinins have basic Adenine ring structure
There are synthetic cytokinins derived from diphenylurea (DPU) that are
structurally unrelated to the adeninetype cytokinins.
Occurrence
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Cytokinins have been found in almost all higher plants
as well as mosses, fungi, bacteria,
Cytokinins are found in actively growing tissues where
cell division takes place (root tip, shoot tip, expanding
leaf, developing endosperm – e.g. Liquid endosperm
of coconut, immature maize endosperm.
Today there are more than 200 natural and synthetic
cytokinins combined.
However it is not known whether they are synthesized
in these tissues or transported to these tissues from
other sites of synthesis. Root tips are the probable
sites of cytokinin synthesis.
The balance of cytokinins and auxins acting together
causes development of organs like shoots and roots
Site of Synthesis
• Occurs in root tips and developing seeds
Transport
• Via xylem from root to shoot
Biosynthesis
• Cytokinins occur in free form or in tRNA
• The major site of biosynthesis of free
cytokinins is root tip and distribute via
xylem – also produced in developing buds,
developing seeds.
• tRNA-cyto are formed in every living cell in
cytoplasm, chloroplast and mitochondria
Proposed biosynthetic and metabolic pathway for cytokinins. Left, The proposed biosynthesis of
zeatin tri-/diphosphate in Arabidopsis. Both ADP and ATP are likely substrates for the plant IPT
enzyme, and these and their di- and triphosphate derivatives are indicted together (e.g. ATP/ADP).
The biosynthesis of cytokinins in bacteria (e.g. A. tumefaciens) is compared next to it. Right,
Several possible modifications and the degradation of zeatin. The diagram only depicts reactions
that are described in the text; cytokinin metabolism is more complex than the pathways shown (see
Mok and Mok, 2001). See text for more details.
• There are many bioassays available for
the estimation of cytokinins activity some
of these are:
– Tobacco pith callus
– Radish cotyledon expansion
• They are directly related to the role of
cytokinins in cell division
• Specific metabolic bioassay is
– β-cyanin synthesis in Amaranthus seedlings
– Chlorophyll retention in oat leaves
Summary of perception and signal
transduction
• Binding of cytokinin to CRE1 or other Related His Kinases
• Initiation of phosphorylation
• Phosphorylation and activation of the type-B ARRs (Arabidopsis
response regulators)
• Transcription of Type-A genes which in case over-expression
negatively feedback the signaling pathway
• Type-A and Type-B ARRs interact with various molecules (effectors)
inside the cell and determine the kind of biochemical reactions in
response to cytokinin
Physiological Roles/ Mode of Action
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Cytokinins are 0.01 to 1.0 mM in plants
Most important function of cytokinins is cytokinesis. Application of
cytokinins promote cell division by increasing the change of cell
from G2 to mitosis
This is done by enhancing protein synthesis, since specific
enzymes are required for mitosis.
Cytokinins effect on translation but not on transcription. Ribosomes
frequently grouped together to form long polysomes but yet no
information about specific enzymes.
Evidence showed that cytokinins are involved in the regulation of
the cell cycle. They control the activity of cyclin-dependent kinases
(CDKs)
Cytokinis promote cell division by stimulating the expression of the
genes that gives rise to s3 cyclin, a G1-type cyclin.
Recently it has been found that cytokinins stimulate the expression
of the CYCD3 gene which encodes a D-type cyclin which plays a
role in cell cycle in G1 stage.
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D-type cyclins play a major role in regulation of cell proliferation
Cytokinin Functions
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Stimulates cell division
Promote cell expansion: only in dicot seedlings but in stem and roots they
inhibit cell inhibit cell expansion probably due to production of ethylene in
stem and roots
Promote Chloroplast maturation: Promotes the conversion of etioplasts into
chloroplasts via stimulation of chlorophyll synthesis - Etiolated leaves
treated with cytokinins develop chloroplast
Stimulates morphogenesis (shoot initiation/bud formation) in tissue culture.
Stimulates the growth of lateral buds-release of apical dominance.
Stimulates leaf expansion resulting from cell enlargement.
May enhance stomatal opening in some species.
Involved in releasing seed dormancy
Delay of senescence
Induction of enzymes and gene expression – stimulate RNA and protein
synthesis – post-transcriptional regulation
Promote nutrient metabolism in some species
Role of Cytokinins in Apical Dominance
• Measurements of cytokinin levels in axillary buds of Douglas fir
(Pseudotsuga menziesii) show a very good correlation between
endogenous cytokinin levels and bud growth (Pilate et al. 1989).
• Much of the cytokinin of the plant is synthesized in the root and
transported to the shoot.
• Studies with the 14C-labeled cytokinin benzyladenine (BA), have
shown that when the labeled compound is applied to roots, more
[14C]BA is transported to the shoot apex than to the axillary bud.
– Decapitation increases the accumulation of [14C]BA by the axillary bud,
and application of auxin to the apical stump reduces this accumulation.
– Thus auxin makes the shoot apex a sink for cytokinin from the root, and
this may be one of the factors involved in apical dominance.