A root derived factor required for shoot growth in culture. Cytokinins: regulators of cell division (that function in conjunction with auxin) Discovered as: A root derived factor required for shoot growth in culture. A factor in endosperm that promotes cell division in culture (in the presence of auxin). A DNA breakdown product that causes shoot cell division (in the presence of auxin).

Ch. 21 In-Text Art, p. 623 Kinetin PP5e-ITA-21-p623-0.jpg

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 PP5e-Fig-21-04-1.jpg

Figure 21.17 Map of the T-DNA from an Agrobacterium Ti plasmid PP5e-Fig-21-17-1.jpg

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 PP5e-Fig-21-12-0.jpg Triple mutant Cytokinin receptors are a multi-gene family (3 receptor genses in Arabidopsis).

Cytokinin receptor mutants are insensitive to cytokinen Figure 21.7 Phenotypes of Arabidopsis plants harboring mutations in the cytokinin receptors PP5e-Fig-21-07-0.jpg 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 PP5e-Fig-21-10-0.jpg

Wild type: shoot apical meristem Figure 21.11 Cytokinin is required for normal growth of the shoot apical meristem Wild type: shoot apical meristem PP5e-Fig-21-11-0.jpg 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.

How is cytokinin perceived in signalling? Figure 21.6 Simple versus phosphorelay types of two-component signaling systems How is cytokinin perceived in signalling? PP5e-Fig-21-06-0.jpg

Figure 21.9 Model of cytokinin signaling PP5e-Fig-21-09-0.jpg

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). PP5e-Fig-21-13-0.jpg

Cytokinin suppresses the size and cell division activity of roots. Figure 21.14 Cytokinin suppresses the size and cell division activity of roots PP5e-Fig-21-14-0.jpg Cytokinin suppresses the size and cell division activity of roots. 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 PP5e-Fig-21-16-0.jpg

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 PP5e-Fig-21-17-0.jpg

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). PP5e-Fig-21-19-0.jpg

Figure 21.18 Interaction of auxin and cytokinin in the regulation of shoot branching PP5e-Fig-21-18-0.jpg

The END.

Figure 21.18 Interaction of auxin and cytokinin in the regulation of shoot branching PP5e-Fig-21-18-1.jpg

Figure 21.21 Cytokinin influence on the development of wild-type Arabidopsis PP5e-Fig-21-21-0.jpg

Figure 21.23 Cytokinin regulates grain yield in rice (A) PP5e-Fig-21-23-1.jpg

Figure 21.23 Cytokinin regulates grain yield in rice (B) PP5e-Fig-21-23-2.jpg

Ch. 21 In-Text Art, p. 624 trans-zeatin, cis-zeatin, benzyladenine, and thidiazuron PP5e-ITA-21-p624-0.jpg

Figure 21.1 Tumor that formed on a tomato stem infected with the crown gall bacterium PP5e-Fig-21-01-0.jpg

Figure 21.5 Biosynthetic pathway for cytokinin biosynthesis PP5e-Fig-21-05-0.jpg

Figure 21.2 Structures of other aminopurines that are active as cytokinins PP5e-Fig-21-02-0.jpg

Figure 21.3 Witches’ broom on a fir tree PP5e-Fig-21-03-0.jpg

Figure 21.5 Biosynthetic pathway for cytokinin biosynthesis (Part 1) PP5e-Fig-21-05-1.jpg

Figure 21.5 Biosynthetic pathway for cytokinin biosynthesis (Part 2) PP5e-Fig-21-05-2.jpg

Figure 21.4 Tumor induction by Agrobacterium tumefaciens PP5e-Fig-21-04-0.jpg

Figure 21.5 Biosynthetic pathway for cytokinin biosynthesis (Part 3) PP5e-Fig-21-05-3.jpg

Figure 21.8 Comparison of the structures of the type-A and type-B ARRs PP5e-Fig-21-08-0.jpg

Ch. 21 In-Text Art, p. 629 Level of active cytokinin in a particular cell PP5e-ITA-21-p629-0.jpg

Figure 21.9 Model of cytokinin signaling PP5e-Fig-21-09-1.jpg

Figure 21.15 CYCD3-expressing callus cells can divide in the absence of cytokinin PP5e-Fig-21-15-0.jpg

Figure 21.20 Effect of cytokinin on the movement of an amino acid in cucumber seedlings PP5e-Fig-21-20-0.jpg

Figure 21.20 Effect of cytokinin on the movement of an amino acid in cucumber seedlings PP5e-Fig-21-20-1.jpg

Figure 21.22 Leaf senescence is retarded in transgenic lettuce plants expressing ipt PP5e-Fig-21-22-0.jpg