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Unformatted text preview: 1. Mature plant cells cease dividing,
but many can be induced to begin dividing again by wounding or abscission.
[1913, the Austrian plant physiologist G. Haberlandt
demonstrated that vascular tissue contained a water-soluble substance(s) that
stimulated the cell division of wounded potato tuber tissue] 2. Wounding-induced cell division is self-limiting
The derivative cells stop dividing and re-differentiate
3. Agrobacteria cells invade a wound, cells keep dividing
leading to a neoplastic (tumor forming) disease known as crown call
A. Heat tumor to 42OC, bacteria die, callus continues to grow!
B. Infected cells can produce cell-division stimulatory substance(s)
C. Cells can grow on a simple nutrient medium as a mass of
unorganized, undifferentiated cells called callus tissue. Plant tissue culture without hormones
1. 1930s, Philip White demonstrated that tomato roots can be grown indefinitely
in a simple nutrient medium containing only sucrose, mineral salts, and a few
vitamins, with no added hormones
2. Isolated stem tissues exhibit very little growth in such medium
limited growth may occur when auxin is added,
but such an auxin-stimulated growth is due to cell enlargement
3. but, the cultured stem can grow into a whole new plant
when adventitious roots form on an auxin-containing medium
A. There is a difference in the regulation of cell division in root and shoot meristem
B. Root may synthesize some factors that promote shoot growth
B. Search for substances that promote shoot growth in culture
1. In 1941, Johannes van Overbeek
discovered that the milky endosperm from coconut also had this ability.
2. 1940-1950 Skoog and his coworkers at University of Wisconsin
found that autoclaved herring sperm DNA had a powerful cell-division promoting effect
3. The first cytokinin was isolated from herring sperm by Miller in 1955
This compound was named kinetin because of its ability to promote cytokinesis. A by-product of
degradation of DNA
degradation 4. The first naturally occurring cytokinin was
isolated from corn in 1961 by Miller and Letham.
It was later called zeatin.
5. In higher plants, zeatin occurs in both the cis and
trans configurations. Trans-zeatin is much more active
6. Zeatin is the most prevalent cytokinin in higher plants. Cytokinin are defined as compounds that have
biological activities similar to those of trans-zeatin
1. Induce cell division in callus cells
in the presence of auxin
2. Promote root formation from callus cultures
when in the appropriate molar ratios to auxin
3. Delay senescence of leaves
4. Promote expansion of dicot cyteldons Nearly all compounds active as cytokinins
are N6-substituted aminopurines. Biosynthesis of cytokinin
Adding a 5-C side chain to
a free adenine base
1. 1st major precursor is
AMP (adenosine monophosphate)
2. The side chains of the cytokinins
are made by the terpene pathway
(IPP---isoprene) and added to the
AMP by isopentenyl
3. The plant and bacterial enzymes differ
in adenosine substrate and the side chain
Donor 4. Addition of a -OH group by a
cytochrome P450 monooxygenase
5. Removal of the ribose group to become
6. Cis-trans isomerase
8. Cytokinin oxidase (inactivation)
8. Cytokinins are synthesized primarily in the meristematic region of the roots.
Cytokinins are also produced in other cells/tissues
They enter the shoot organs via the xylem (transpiration stream).
the zeatin ribosides are the main transport form;
converted to the free base or glucosides in the leaves
some cytokinin also moves in the phloem.
Free cytokinins are readily converted to their respective
nucleoside and nucleotide forms 1a. Cytokinins Regulate Cell Division in Shoots
CKX-OE 1b. Cytokinin suppresses the size and cell division activity of roots
1b. CKX-OE CKX-OE Increasing cytokinin concentration 2. The auxin/cytokinin ratio regulates morphogenesis
in cultured tissues
Increasing auxin concentration
Increasing Low auxin/cytokinin ratio: shoot development
High auxin/cytokinin ratio: root growth
Intermediate ratio: callus tissue (disorganized cell mass) 3. Cytokinin modify apical dominance and promote lateral bud growth
Auxin suppresses the growth pf axillary buds
(via inhibiting AtIPT genes)
cytokinin stimulates cell division/outgrowth
(cytokinin-overproducing mutants are bushy)
(cytokinin-overproducing 5. Delay leaf senescence
6. Promote chloroplast development
(even in the dark!)
(even 7. Promote cell expansion in leaves and cotyledons
8. Cytokinin promotes movement of nutrients
DARK LIGHT Screen for cytokinin-independent mutants.
Mutated Arabidopsis calli were cultured in the absence of phytohormones for 3 weeks.
Note that a portion of one callus is green, whereas most of the transformed calli are a yellowish color.
This green region proliferated rapidly and regenerated shoots in the absence of exogenous cytokinin. Identification of the 1st cytokinin receptor, CKI1 for cytokinin-independent
Identification Identification of the second cytokinin receptor in Arabidopsis
Screening for Arabidopsis mutants that were
impaired in cytokinin responses,
including rapid cell proliferation and shoot
formation in tissue culture.
cytokinin response 1-1 (cre1-1)
CRE1 is a homologue of CKI1
CRE1 is identified by a loss-of-function approach
CKI1 is discovered by a gain-of-function method A yeast genetic experiment proving that CRE1 is a cytokinin receptor
yeast (In the absense of
galactose) Cytokinin receptors
are histidine kinase
which, when bound by
hormone, can activate
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