Cytokinins: Metabolism, Assay and Responses | Plant Hormones | Botany

Here is an elaborated discussion on cytokins, highlighting:- 1. Meaning of Cytokinins 2. Natural Occurrence of Kinins 3. Metabolism 4. Assay 5. Responses 6. Agricultural Uses.

Meaning of Cytokinins:

Cytokinin (kinin) is the generic name for growth substances that typically stimulate cell division (cytokinesis). Cytokinin was discovered in the 1950s from observations in the Skoog laboratory on cell division in callus grown from tobacco pith or carrot root phloem. This work clearly demonstrated there was little growth of parenchyma cells of excised tissues unless a growth factor found in coconut milk or yeast extract was added to the culture medium.

Using only IAA, which was formerly considered to be the growth hormone, only cell elongation occurred. However, rapid callus growth from cell division and elongation occurred in the presence of IAA plus coconut milk, yeast extract, or the purine base adenine, 6-aminopurine.

In 1961 a com­pound that stimulated cell division was isolated from a degraded autoclaved sample of DNA. The active substance, an artifact present only in autoclaved samples, was identified as 6-furfurylaminopurine and named kinetin.

Cytokinin was first isolated from higher plants in the milky endosperm of young maize seeds in 1964. It was named zeatin.

Natural Occurrence of Kinins:

Cytokinins required for cell division also regulate and correlate a wide range of activities in morphogenesis. Young roots, immature fruits and seeds, and the nurse tissues (e.g., liquid endosperm) are rich in kinins. Immature fruits of maize, horse chestnut, banana, apple, and coconut (milky endosperm) are especially rich sources. Since kinins are evidently not translo­cated to these tissues, they are probably the sites of synthesis.

In general, kinins occur naturally as conjugates of sugar and phosphate ions. Zeatin riboside is the principal kinin in the roots of cocklebur (Xanthium), while the principal one found in leaves of disbudded bean plants was zeatin glucoside. Eight dif­ferent kininlike substances have been isolated from maize kernel extract; zeatin was biologically the most active. The purine base is the common chemical moiety in both natural and synthetic kinins.

Kinin Metabolism:

The natural kinins appeared to be synthesized by fixing a side chain, usually a 5-carbon chain, to an adenine molecule. The 5-carbon chain is be­lieved to be derived from isoprene, the basic unit in GA, chlorophyll, xanthophyll, and abscisic acid.

There is evidence that the kinins in leaf and bud tissue are immobile. However, it is well known that kinins produced in roots are translocated throughout the plant via the transpiration stream. Buds were stronger sinks for kinins than leaves.

Kinin Assay:

Due the difficulty of chemical analysis of the minute concentrations that occur naturally, quantitative analysis of kinins has, until recently, been re­stricted to biological methods. Presently gas chromatography can be used effectively to separate kinins.

Five bioassays, which reflect kinin activities by increasing parenchymous tissue cell mass, have been described; probably the one most commonly used is the tobacco tissue culture assay. This test has advanced the state of knowledge of cytokinins equivalent to the knowledge of auxins re­vealed by the Avena curvature test.

Responses to Kinins:

Kinins elicit a broad range of responses, but they act synergistically with auxins and usually with other hormones. Adventi­tious bud formation was believed to be triggered by the synergism of auxin and cytokinin. Cytokinin movement from the roots probably has a marked influence on stimulation of new growth from quiescent axillary buds subject to apical dominance.

Evidently lateral buds are deficient in kinins, since an exogenous source stimulated their growth. The requirement of kinins for the formation of adven­titious buds was manifested on root pieces of field bindweed (Convolvulus arvensis). On stem cuttings kinetin, even more than GA, severely inhibited formation of root initials, whereas auxin stimulated it.

GA and kinin promoted germination of seeds of certain species, such as lettuce and white clover. Seeds of witchweed, a parasitic weed, germinate only if a stimulus is received from the host plant. The stimulus was reported to be kinin.

The role of kinin in retention of chlorophyll, incorporation of amino acids, and protein retention in leaves, all of which indicate delayed senescence, is of particular interest to physiologists. Searching for more effective means of supplying exogenous kinin sources or, alternatively, of improvement of root delivery to prolong senescence and photosynthetic output could become a fertile area for future research.

Agricultural Uses of Kinins:

Weaver (1972) suggested numerous potential uses of kinins, including increasing fruit-set in grapes and improving size and shape of the ‘Delicious’ apple. Lettuce seeds may acquire a secondary dormancy in soils with high temperatures (25°C); germination is increased if treated with kinetin, which is more effective than benzyladenine (BA). In general, kinin use in crop produc­tion is still in the potential stage.

The discovery of cytokinin has added a new dimension to plant breeding. Cloning of haploid plants from pollen, diploid plants from somatic cells, embryos from fertilized eggs, and plants from excised tissues provides new tools for achieving breeding objectives that were previously unobtainable.

The finding that varying kinin concentration in the culture medium at certain critical stages in the explant growth cycle can induce nonnuclear, transmittable genetic changes opens the way for exciting genetic-engineering possibilities. Using this technique, new potato genotypes have been isolated that produced a significant yield advantage in field trials.