Cytokinins: regulators of cell division
(that function in conjunction with auxin)
Discovered as:
1. A root derived factor required for shoot
growth in culture.
2. A factor in endosperm that promotes cell
division in culture (in the presence of auxin).
3. A DNA breakdown product that causes shoot
cell division (in the presence of auxin).
Ch. 21 In-Text Art, p. 623 Kinetin
• Some interacting insects, fungi, and
bacteria supply cytokinin to promote gall
formations or other growths.
Agrobacterium: transform plant
plant cells with DNA coding for
cytokin/IAA production.
R. fascians: secretes cytokin/IAA
to promote ‘witches broom’ in conifers
Figure 21.4 Tumor induction by Agrobacterium tumefaciens
Figure 21.17 Map of the T-DNA from an Agrobacterium Ti plasmid
• Understanding the physiological roles of
cytokinins (and their relationship with
auxin) come from mutant analysis.
Cytokine perception (receptors and signaling pathway mutants)
Cytokine levels (express Agro ipt gene to level or express oxidase to )
Figure 21.12 Comparison of the rosettes of wild-type Arabidopsis and the mutant
Triple mutant
Cytokinin receptors are a multi-gene family (3 receptor
genses in Arabidopsis).
Figure 21.7 Phenotypes of Arabidopsis plants harboring mutations in the cytokinin receptors
Cytokinin receptor mutants are insensitive to cytokinen
(and see affect of cytokinin oxidase over-expression).
Figure 21.10 Tobacco plants overexpressing genes for cytokinin oxidase
Figure 21.11 Cytokinin is required for normal growth of the shoot apical meristem
Wild type: shoot apical meristem
Cytokinin oxidase expression
reduces size of shoot meristem
and number of dividing cells.
Ctyokin increases expression of KNOX transcription factors.
KNOX proteins down regulate Gibberellic acid (GA) synthesis enzyme
and upregulate cytokinin cytokinin synthesis enzymes.
Result is high ratio of cytokinin:GA in shoot apical meristem,
which signals cells to divide rather than differentiate
into leaf primordia.
Figure 21.6 Simple versus phosphorelay types of two-component signaling systems
How is cytokinin perceived in signalling?
Figure 21.9 Model of cytokinin signaling
Figure 21.13 Cytokinin suppresses the growth of roots
Cytokinin suppresses root growth:
Cytokinin oxidase over-expression
reduces cytokinin levels (right) relative
to wild type (left).
Figure 21.14 Cytokinin suppresses the size and cell division activity of roots
Cytokinin suppresses the size and cell division activity of roots.
(A) Wild type (B) Plants over-expressing cytokinin oxidase to reduce
the level of cytokinin. Root meristem is indicated in bright blue
(nuclear DNA staining).
• High cytokinin levels increases rate of root
meristem cell differentiation into vascular
tissue.
• The result is fewer cells remaining in root
apical meristem (few dividing cells remain to
support root growth).
• In roots, auxin promotes cell division in
meristem; cytokinin promotes cell
differentiation.
Figure 21.16 Regulation of growth and organ formation in cultured tobacco callus
• High auxin:low cytokinin levels promote
roots development.
• Low auxin:high cytokinin levels promote
bud/shoot development.
Intermediate to high concentrations of both
hormones promote undiferentiated callus
growth.
Figure 21.17 Map of the T-DNA from an Agrobacterium Ti plasmid
Figure 21.19 Leaf senescence is retarded in a transgenic tobacco plant containing ipt
Agricultural applications of
cytokinins: Increased levels of cytokin
prevent leaf senescence (programmed
cell death).
Figure 21.18 Interaction of auxin and cytokinin in the regulation of shoot branching
• The END.
Figure 21.18 Interaction of auxin and cytokinin in the regulation of shoot branching
Figure 21.21 Cytokinin influence on the development of wild-type Arabidopsis
Figure 21.23 Cytokinin regulates grain yield in rice (A)
Figure 21.23 Cytokinin regulates grain yield in rice (B)
Ch. 21 In-Text Art, p. 624 trans-zeatin, cis-zeatin, benzyladenine, and thidiazuron
Figure 21.1 Tumor that formed on a tomato stem infected with the crown gall bacterium
Figure 21.5 Biosynthetic pathway for cytokinin biosynthesis
Figure 21.2 Structures of other aminopurines that are active as cytokinins
Figure 21.3 Witches’ broom on a fir tree
Figure 21.5 Biosynthetic pathway for cytokinin biosynthesis (Part 1)
Figure 21.5 Biosynthetic pathway for cytokinin biosynthesis (Part 2)
Figure 21.4 Tumor induction by Agrobacterium tumefaciens
Figure 21.5 Biosynthetic pathway for cytokinin biosynthesis (Part 3)
Figure 21.8 Comparison of the structures of the type-A and type-B ARRs
Ch. 21 In-Text Art, p. 629 Level of active cytokinin in a particular cell
Figure 21.9 Model of cytokinin signaling
Figure 21.15 CYCD3-expressing callus cells can divide in the absence of cytokinin
Figure 21.20 Effect of cytokinin on the movement of an amino acid in cucumber seedlings
Figure 21.20 Effect of cytokinin on the movement of an amino acid in cucumber seedlings
Figure 21.22 Leaf senescence is retarded in transgenic lettuce plants expressing ipt