Cytokinins: Definition, Biosynthesis, Effects, and Agricultural Uses
Unit 19: Cytokinins
Definitions
Cytokinin: A group of plant hormones that promote cell division and cell differentiation. They are derived from the purine base, adenine, and have an aromatic substituent on nitrogen in position 6 of the purine ring.
Anticytokinin: Compounds that reversibly inhibit the activity of cytokinins.
Kinetin: The first cytokinin discovered, derived from autoclavable herring sperm DNA.
Cytokinin Oxidase: An enzyme that regulates the endogenous levels of cytokinins in plants.
Bioassays for Estimating Cytokinin Content
- Growth of carrot phloem callus (fresh weight gain).
- Growth of leaves in hormone solution (increase in size or weight).
- Growth of Lemna fronds.
- Records of leaf aging (slowed yellowing).
Cytokinin Biosynthetic Pathway, Conjugation, and Oxidation
The direct synthesis of free cytokinins is the main route in plants. It begins with the formation of isopentenyladenine ribotide from AMP (adenosine 5′-phosphate) and IPP (isopentenyl pyrophosphate). The reaction is catalyzed by cytokinin synthase. This enzyme requires Mg2+ and has maximum activity at pH 7. Isopentenyl adenine riboside is formed, from which all other free cytokinins are synthesized.
Zeatin and dihydrozeatin are derived from adenine.
Zeatin is formed by hydroxylation of the isopentenyl side chain of adenine, forming trans-zeatin.
Dihydrozeatin is formed by the reduction of the zeatin side chain, catalyzed by zeatin reductase.
Oxidation
Side chain oxidation is the most common reaction that can happen to a cytokinin, giving adenine and 3-methyl-2-enal. This process is catalyzed by cytokinin oxidase. Cytokinin activity is lost, providing a mechanism for regulation in the tissue.
Conjugation
The conjugation of cytokinins with glucose takes place in both the purine ring (N-glucosides) and the hydroxyl group of the side chain (O-glucosides).
N-glucosides are biologically inactive and very stable, regulating active cytokinin levels by inactivation.
O-glucosides are very active due to the activity of β-glucosidase and can serve as storage forms.
Cytokinins can be conjugated with amino acids to form less active and very stable compounds.
Effects of Cytokinins on Cell Division in Shoot Apex and Root
Cytokinin oxidase overexpression decreases the amount of cytokinins, retarding the development of the apex by reducing the rate of cell proliferation, and increases root development by increasing the size and activity of root meristem cell division.
Effects of Cytokinins in Apical Dominance and Senescence
Apical dominance is the control exercised by the cauline apex on the growth of lateral buds. This process results from the interaction of inductive factors (auxins) and repressors (cytokinins). Auxins suppress the growth of lateral buds, while cytokinins promote regrowth of axillary buds. The cauline apex, triggered by auxin, suppresses the growth of axillary buds, acting as a sink that limits the transport of cytokinins to the lateral buds.
Cytokinins delay leaf senescence. The role of cytokinins in senescence is supported by:
- Exogenous application of cytokinins delaying senescence of leaves attached to the plant and isolated leaves incubated in the dark.
- Correlation between endogenous levels of cytokinins and degree of senescence, with hormone concentration decreasing as senescence progresses.
- Transgenic plants producing high levels of cytokinins.
Cytokinins and Nutrient Mobilization
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Main Uses of Cytokinins in Agriculture
- Induction of bud formation and branching (e.g., carnation, pink).
- Control of fruit size (e.g., apples).
- Induction of bulblets in Lilium.
- Promotion of flowering; increase the number of male cones in conifers (along with gibberellins).