Concepts: Cytokinins, abscisic acid and ethylene

4.1 Cytokinins

Cytokinins are adenine derivatives and they are named after cytokinesis, because their first recognised effect was the promotion of cell division. Kinetin was the first member, discovered as a modified form of adenine obtained from autoclaved herring sperm DNA; it does not occur naturally in plants. The search for natural cytokinins led to zeatin, isolated from corn kernels and from coconut milk. Besides cell division, cytokinins promote the growth of lateral buds and delay the senescence of leaves. A detached leaf kept in a cytokinin solution stays green longer than a leaf in water, and this effect is used in tissue culture, where cytokinin is supplied along with auxin to produce and maintain callus.

4.2 Abscisic acid

Abscisic acid is a carotenoid derivative and is called the stress hormone because its level rises when a plant faces drought, salinity or cold. Under drought it closes the stomata, which reduces water loss, and it also promotes seed dormancy so that germination waits for better conditions. In tissue culture it can inhibit growth, which is why it is classed mainly as an inhibitor. The name comes from its once-suspected role in leaf abscission, though ethylene is the main regulator of that process.

4.3 Ethylene

Ethylene is the only gaseous regulator in the group. It promotes the ripening of fruits, so unripe fruit kept with a ripe fruit ripens faster, and it is used for the artificial ripening of bananas and tomatoes. It also causes epinasty, the downward bending of a leaf, and it promotes abscission and senescence. In some plants it breaks seed and bud dormancy. Because it is a gas, ethylene can move from one fruit to another, which is why a rotten fruit in a basket speeds up the ripening of the fruit around it.

4.4 The table of uses

In practice the five groups are used in different industries: auxin for propagation and weed control, gibberellin in malting and on sugarcane, cytokinin in tissue culture and for keeping cut leaves green, abscisic acid in stress research, and ethylene for the artificial ripening of bananas and tomatoes. The regulators rarely work alone. Auxin and cytokinin together decide whether a cultured tissue forms roots or shoots, and the balance between them is the practical tool of plant tissue culture. The same principle explains why a single hormone treatment can give different results in different tissues. In the laboratory the auxin level is usually held fixed while the cytokinin level is varied to switch a culture between root and shoot formation.

4.4.1 Why the balance matters

The regulators work as a system rather than as five separate switches. Auxin from the apex keeps the lateral buds quiet while cytokinin from the roots can release them, so the shape of the shoot depends on the ratio of the two signals that reach a bud. Gibberellin lengthens the stem while abscisic acid slows growth and closes the stomata when water is short. Ethylene, being a gas, can move between organs and between neighbouring fruits, which is why a single ripe fruit changes the behaviour of a whole basket. The practical uses in the table follow directly from these roles: the same regulators that shape a plant in the field are used in tissue culture, in orchards and in the malting industry.

4.4.2 Practice from the NCERT exercises, questions 5, 8 and 10

Exercise 5. Abscisic acid is called the stress hormone because its concentration rises under drought, salinity and cold, and it then closes the stomata, promotes seed dormancy and slows growth so that the plant conserves water and survives the stress period.

Exercise 8. (c) cytokinin or gibberellin delays leaf senescence, (d) cytokinin induces growth of axillary buds, (e) gibberellin induces bolting of a rosette plant, (f) abscisic acid induces immediate stomatal closure.

Exercise 10(b). If dividing cells stop differentiating, the tissue keeps producing an undifferentiated mass of cells, as in a callus, and no organised organs form. Exercise 10(c). A rotten fruit mixed with unripe fruits releases ethylene, which speeds up the ripening of the whole lot. Exercise 10(d). Without cytokinin in the culture medium, the tissue fails to divide properly and a good callus does not form.