HORMONE PHYSIOLOGY AND SIGNAL TRANSDUCTION Huseyin Tombuloglu, Phd. GBE 304 Spring 2015

CYTOKININS Regulators of Cell Division

Cytokinins promote cell division….cytokinesis. Discovered in the 1950s during the “golden age” of tissue culture of plants. Were discovered in the search for factors that stimulate plant cells to divide (i.e., undergo cytokinesis) Most are purines…refers to organic chemical structure. Kinetin as an example.

There are many forms of this hormone

Figure 21.2 Structures of other aminopurines that are active as cytokinins

Functions of Cytokinins Close relationship w/ auxins in plants. Promotes mitosis…cell divisions. Cell expansion of leaves, stems, roots. Formation of bud tissues. Involved in light responses. Regulate protein synthesis.

Cytokinins and Auxins Concentration of cytokinins relative to auxins are very important In tissue culture…auxin : cytokinin ratios affect growth of tissues.  auxin :  cytokinins = Increased ROOT growth.  auxin :  cytokinins = Increased SHOOT growth.

Some Plant Pathogenic Bacteria, Insects, and Nematodes Secrete Free Cytokinins Many of these microorganisms produce and secrete substantial amounts of cytokinins and/or cause the plant cells to synthesize plant hormones, including cytokinins Infection of plant tissues with these microorganisms can induce the tissues to divide and, in some cases, to form special structures, such as mycorrhizae, in which the microorganism can reside in a mutualistic relationship with the plant.

Mycorrhizae

Witches’broom on balsam fir (Abies balsamea) Balsam köknarı

Figure 21.3 Witches’ broom on a fir tree

Agrobacterium rhizogenes infection

Tobacco plants overexpressing the gene for cytokinin oxidase. The plant on the left is wild type. The two plants on the right are overexpressing two different constructs of the Arabidopsis gene for cytokinin oxidase: AtCKX1and AtCKX2. Shoot growth is strongly inhibited in the transgenic plants This is evident that without cytokinin plant growth is strongly inhibited.

Cytokinins can also control branching in some species…. Cytokinins promote lateral buds. Auxins suppress lateral buds (apical dominance).

The regulation of growth and organ formation in cultured tobacco callus at different concentrations of auxin and kinetin. Buds development Root development No differentiation

Figure Cytokinin suppresses the growth of roots The cytokinin-deficient AtCKX1 tobacco-right WT- left

Figure Cytokinin suppresses the size and cell division activity of roots

Figure Cytokinin regulates grain yield in rice (A)

Figure Cytokinin regulates grain yield in rice (B)

Figure 21.7 Phenotypes of Arabidopsis plants harboring mutations in the cytokinin receptors