Cytokinins Cell division Delay leaf senescence Promote nutrient
mobilization Modify apical dominance The formation and activity of
shoot apical meristem Floral development
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Break bud dormancy Stimulate seed gemination Cytokinin mediate
light-regulate development Chloroplast differentiation Development
of autotrophic metabolism Leaf and cotyledon expansion
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and primary and secondary meristem( ) enlargement and
differentiation cell division ( )
Cytokinins regulate cell division in shoots and roots Several
evidence Shoot apical meristem (Werner et al. 2001) - This finding
strongly supports the notion that endogenous cytokinins regulate
cell division
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Overexpressing the gene for cytokinin oxidase
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Cytokinin oxidase enhancement of root growth (Werner et al.
2001) Root apical meristem (Werner et al. 2001) Cytokinins regulate
cell division in shoots and roots
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A small reduction in cytokinin levels would increase root
growth (data courtesy of F. J. Ferreira and J. J. Kieber.)
Cytokinins regulate cell division in shoots and roots
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Triple-receptor mutants (ahk2/ahk3/cre1) (Nishimura et al.
2004.) Mutation in maize alters leaf phyllotaxy and the size lf the
shoot apical meristem (abphyl1) (Giulini et al. 2004.) Cytokinins
regulate cell division in shoots and roots
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Cytokinins regulate specific components of the cell cycle
Cyclin-dependent protein kinases (CDKs) Cytokinin and auxin :
regulating the passage from G 2 to M phase (Zhang et al. 1996)
Overexpression of CYCD3 (Riou-Khamlichi et al. 1999)
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The auxin:cytokinin ratio regulates morphogenesis in cultured
tissues Kinetin High auxin:cytokinin ratios roots Low
auxin:cytokinin ratios shoots (Skoog and Miller 1965)
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T-DNA tms/tmr/tml v.s. tumors (Morris 1986) The auxin:cytokinin
ratio regulates morphogenesis in cultured tissues
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Cytokinins modify apical dominance and promote lateral bud
growth MAX4 and RMS1
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Cytokinins induce bud formation in a moss Funaria hygrometrica
Protonema (plural protonemata) (Courtesy of K. S. Schumaker)
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Cytokinin overproduction has been implicated in genetic tumors
Nicotiana Genetic tumors
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Cytokinins delay leaf senescence Delay senescence Limited
expression of the ipt gene
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Cytokinins promote movement of nutrients Cytokinin-treated
cotyledon has become a nutrient sink
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Cytokinins promote chloroplast development Dark-grown and
light-grown (Chory et al. 1994)
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Cytokinins promote cell expansion in leaves and cotyledons The
effect of CK on the expansion of radish cotyledons
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Cytokinin in plant physiology and development Shoot apical
meristems v.s. leaves Leaves have higher chlorophyll levels
Adventitious shoots may form from unwounded leaf veins and
petioles
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Cytokinin in plant physiology and development Leaf senescence
is retarded Apical dominance is greatly reduced More extreme CK-
overproducing plants Root
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Cellular and Molecular Modes of Cytokinin Action
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A cytokinin receptor related to bacterial two-component
receptors has been identified Simple tow-component systems
Phosphorelay-type two- component signaling system
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Cytokinins increase expression of the type-A response regulator
genes via activation of the type-B ARR genes ARR (Arabidopsis
response regulator) Type A ARR and type B ARR
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Histidine phosphotransferases are also involved in cytokinin
signaling Histidine phosphotransfer protein (Hpt)