Question

List five growth hormones. Describe their role in brief.

Answer 1

Growth hormones

1. Growth hormones or phytohormones regulate various developmental aspects in plants. They range from cell enlargement, division, and differentiation to the formation of stems, flowers, fruits, seeds, senescence, and abscission.
2. Phytohormones can either promote or inhibit growth.
3. The most important growth-promoting hormones are auxins, cytokinins, and gibberellins. The growth-inhibiting hormones are abscisic acid and ethylene.

Auxins

1. Location :Auxins are produced in the apical meristems, young leaves, buds, and developing seeds. They diffuse from the growing tips to the target sites resulting in an elongation zone.
2. Auxins enhance root formation in cuttings. They initiate the formation of the xylem in the callus (undifferentiated parenchymatous cell mass). Naphthalene acetic acid induces dormancy in bulbs and tubers for the increasing storage period.
3. Flowering, fertilization, and fruiting in litchi and pineapple is promoted by auxin. Application of synthetic IAA (Indole acetic acid) and NAA (Naphthalene acetic acid) induces parthenocarpy.
4. Auxins inhibit apical dominance. 2,4-D (2,4- dichloro phenoxy acetic acid) also prevent pre-harvest fruit drop. 2,4- D is also used against broadleaf weeds.

Gibberellins (GA)

1. Location: Synthesized in various plant parts like young leaves, seeds, and roots. GA was first obtained from the fungus Gibberella fujikuroi. More than 70 types of GA have been identified out of which GA3 is the most active.
2. GA causes elongation of the stem and expansion of the leaves. They do not act on the roots. They can cause cell enlargement or division or both.
3. GA breaks the dormancy in buds and seeds. They also facilitate early seed production.
4. GA promotes early sprouting in stem tubers. They replace the cold treatment and long day photoperiodic treatment in plants.
5. GA is used to induce parthenocarpy and increase the number of female flowers in monoecious plants (eg. Cannabis, cucurbits).

Cytokinins

1. Location: Synthesized in seed endosperm and plant roots. They are derived from adenine. Kinetin is the most active form.
2. Cytokinins promote cell growth, division, and differentiation in combination with auxins. When present in equal quantities, they lead to cell division. High cytokinin concentration leads to shoot bud development whereas high auxin concentration leads to root formation.
3. Cytokinins promote lateral bud growth.
4. Cytokinins also delay senescence by increasing the rate of protein synthesis.

Ethylene

1. Location: Ethylene is a gaseous hormone synthesized in ripe fruits and ageing tissues. High auxin concentration promotes the formation of ethylene.
2. The primary role of ethylene is to ripen the fruit.
3. Ethylene stimulates abscission in leaves, flowers, and fruits. They induce senescence in leaves (yellowing) and flowers (withering and fading).
4. Ethylene inhibits longitudinal growth but promotes transverse or isodiametric growth. It breaks dormancy in several parts.
5. Ethylene increases the number of female flowers and fruits in the cucumber plants.

Abscisic acid (ABA)

1. Location: ABA, a growth inhibitor, is synthesized in stems, leaves, seeds, buds, and fruits of the plant. They act antagonistically to cytokinins and gibberellins.
2. ABA promotes ageing in leaves and dormancy in seeds, bulbs, and tubers. It also inhibits amylase formation in germinating seeds and, RNA and protein synthesis in leaves.
3. ABA promotes stomatal closure in the leaf epidermis.