Auxins and their role in plants

Auxins are a kind of growth-regulating chemical substances synthesized in plants that regulate growth and differentiation of plant body. This article explains what are auxins, what is the nature of these hormones, what is their role and significance in the plant body.

Introduction

Auxins are a kind of plant hormones (Phytohormones) or growth regulators which regulate growth and differentiation within the plant body. Auxins, the growth regulators may have a positive effect or may have a negative effect or may have an inhibitory effect on growth. Auxins are a kind of organic substance produced naturally in plants controlling growth and other functions at a site remote from its place of production in minute quantities.

Discovery of Auxins

The first person to study growth regulators or auxins was done by Darwin in 1880. He observed the tip of the coleoptile of Canary grass (Phalaris canariensis) bending towards the light. He found that light falling on the grass coleoptile causes some influence. This influence is transmitted downwards and makes the coleoptile to bend downwards. But when the coleoptile tip was removed it did not bend.

Similarly in 1910-1913, Boysen Jensen proved this fact by using agar blocs and pieces of mica. When these materials are kept below the cut end of coleoptiles he got the following results. Agar blocs allowed the growth-regulating substance diffuse through them to cause growth but when pieces of mica were used they blocked the movement of auxins from coleoptile tips and thus bending is stopped.

In 1928, Went demonstrated the influence of auxins on bending and degree of curvature of Oat coleoptile. When he cut the tips of coleoptile no growth took place since the auxin is produced at the tip of the coleoptile. Similarly when the decapitated coleoptile tips placed over agar blocs and then were placed over cut tips of coleoptile they started showing growth. The reason for this is the auxin from agar bloc diffused into cut coleoptile which caused the growth. If the agar bloc kept laterally over cut coleoptile only one side it has shown growth on one side which resulted in curvature.

Sailent features of Auxins

Auxin is a plant growth regulating substance whose name was derived from a Greek word 'auxin' which means to grow. Auxins are synthesized in the protoplasm of the cells.

Auxins were first isolated from human urine by Kogl and Smith and from Rhizopus sinus by Thimman.

Auxins are mainly synthesized at the tips of the plant body and show polar transport. Auxins show their influence while they move downwards from the tip.

The naturally occurring auxin hormone synthesized in plants is Indole-3-acetic acid or Heteroauxin. Some auxins which are artificially prepared chemicals are also available and which influence plants in a similar way as natural auxins. These artificial auxins are Indole Propionic acid (IPA), Indole Butyric acid (IBA), Naphthalene acetic acid (NAA), 2,4-Dichlorophenoxy acetic acid (2,4-D), 2,4,5 -Trichlorophenoxy acetic acid (2,4,5-T), etc.

A biological estimation test for growth hormones is called bio-assay. Bio-assay for auxins is Avena coleoptile curvature test.

The naturally occurring auxin is synthesized either from the amino acid tryptophan or from the breakdown of carbohydrates. Auxins are the plant hormones which promote cell enlargement and cell differentiation.

Functional roles and uses of auxins

1. Auxins promote cell elongation:

Auxins promote cell elongation, cell enlargement and cell differentiation. Auxin hormone controls a plant's response to light and gravity. Auxin hormone is responsible for phototropic and geotropic plant movements. Auxins are produced at the tips of stems and roots. Auxins always move away from light and move towards gravity. Auxins works the opposite way in stems and roots. It speeds up growth in stems while in roots it suppresses growth.

2. Promotes Apical dominance:

In plants, apical buds actively grow and axillary buds present below are inactive. Because of the activity of apical buds, the growth of the plant is always towards the tip. But when the terminal bud is removed, the axillary bud gets activated since the auxin is removed from the terminal bud. The property of auxins is used in agricultural practices. For fencing plants to grow in a bushy way in gardens, gardeners cut the tops of the plants which include auxins in apical buds.

3. Auxins help in producing seedless fruits (Parthenocarpy):

Parthenocarpy is a phenomenon of development of seedless fruits by spraying synthetic auxins on emasculated flower buds. Nowadays we are getting seedless grapes, watermelons, tomatoes, etc. by spraying synthetic auxins on emasculated flowers of these plants.
Seedless fruits produced by Auxins

Seedless Watermelons produced by Auxins (Courtesy:-wikipedia.org)

4. Auxins help in weed control:

Weeds are undesirable plants that compete for water, sunlight, water, and minerals. So they will affect the yield of a crop. By spraying 2,4-D the grass weeds can be destroyed without affecting the monocot matured crop.

5. Auxins influence flowering:

Artificial auxin NAA promotes flowering in Pineapple. Auxins delay flowering in Lettuce. The use of auxin on cotton crops increases cottonseed production.

6. Auxins can prevent premature dropping of fruits:

The abscission layer at the base of petiole or pedicel results in shedding of leaves, flowers or fruits. By spraying synthetic auxins like IAA, NAA, or some other auxins on flowers and fruits will prevent the premature dropping of flowers and fruits.

7. Auxins are used in tissue culture technique:

Tissue culture is a technique in which a large number of plants are produced from little tissue of any part of the plant without the aid of seeds. Few cells or little tissue of the plant will be grown on a nutrient medium under aseptic conditions in a laboratory. Auxins will be sprayed on this growing plant tissue to initiate callus (mass of plant cells), initiate roots and shoots for the young new plants.