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.

R. fascians: secretes cytokin/IAAto promote ‘witches broom’ in conifers

Agrobacterium: transform plant plant cells with DNA coding for cytokin/IAA production.

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

Cytokinin receptors are a multi-gene family (3 receptor genses in Arabidopsis).

Triple mutant

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 expressionreduces size of shoot meristem and number of dividing cells.

Ctyokin increases expression of KNOX transcription factors.

KNOX proteins down regulate Gibberellic acid (GA) synthesis enzymeand 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-expressionreduces 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 reducethe 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