Question

Describe the roles of auxin in plants.

Answer 1

Auxin is a plant growth regulator that plays a vital role in the processes which are very important for the normal growth and development of a plant.

Complete answer:
- At cellular level, auxin affects different cellular processes. It affects cytoplasmic streaming i.e., the movement of fluids within a cell, and the activity of various enzymes. Auxin participates in various physiological processes like phototropism and geotropism (tropic growth) and various developmental processes as given below:

- Auxin affects the processes of phototropism: When unilateral light is distributed unevenly in stem tips, the concentrations of auxin increases on the shaded side that stimulates the cells of that side to elongate more rapidly which results in bending of stem tip towards the unilateral light.

- Auxin promotes cell elongation: In plants, auxin stimulates cell elongation by inducing loosening of the cell wall.

- Auxin induces apical dominance: In higher plants, the growing apical bud inhibits the growth of lateral (axillary) buds - a phenomenon called apical dominance. Removal of the shoot apex (decapitation) usually results in the growth of one or more of the lateral buds. After the discovery of auxin, it was found that auxin could substitute for the apical bud in maintaining the inhibition of lateral buds and thus, stimulating apical dominance.

- Auxin promotes root initiation: In horticulture and plant tissue culture, auxins are used to stimulate initiation of rooting in stem cuttings.

- Auxin promotes the formation of Lateral and adventitious roots: Although elongation of the primary root is inhibited by auxin, initiation of lateral roots and adventitious roots is stimulated by high auxin levels. Lateral roots are found above the elongation and root hair zone and they originate from the cells in the pericycle. Auxin stimulates these cells of the pericycle to divide. The dividing cells gradually develop into, first, a root apex, and finally, the lateral root.

- Auxin delays the beginning of abscission: The detaching of leaves, flowers, and fruits from the living plant is known as abscission. The abscission of these parts takes place in a region called the abscission zone, which is situated near the base of the petiole of leaves. Auxin inhibits abscission. Thus, it delays the senescence of leaves, flowers and fruits.

- Auxin promotes flower and fruit development: Auxin transport plays an important role in initiation and development of floral bud. There is much evidence which suggests that auxin is required for the growth and development of fruits.

- Auxin induces vascular differentiation: Auxins are involved in the regulation of cambium activation and differentiation of vascular tissues. There is evidence that the differentiation of vessels and sieve tubes is induced by IAA (indole-3-acetic acid) which is an auxin.

- Auxin induces formation of parthenocarpic fruits: The commercial uses of auxin include prevention of fruit and leaf drop, promotion of flowering in fruit trees, induction of parthenocarpic fruit and rooting of cuttings for plant propagation. In some plant species, the unpollinated flowers are treated with auxin to induce the formation of seedless fruits. Such fruits are called parthenocarpic fruits.

Note: Auxin was the first growth hormone to be discovered in plants. It was discovered by a Dutch botanist F.W. Went in 1928.