Auxin in Plants

Growth is one of the most fundamental and conspicuous characteristics of living organisms. It is the sum total of various processes that combines to cause an irreversible increase in mass, weight or volume. Growth is often referred to as increase in size or weight (fresh or dry) of a cell, organ or organism

Plant growth regulators are small, simple molecules of diverse chemical composition, which in low concentration regulate growth, differentiation and development by promoting or inhibiting the same. They are variously described as plant growth substances, plant hormones or phytohormones in literature. Plant growth regulators could be indole compounds (Indole-3 acetic acid or IAA), adenine derivatives (N6 furfurylamino purine or kinetin), derivatives of carotenoids and fatty acids (abscisic acid or ABA), terpenes (gibberellic acid or GA3) or gases (ethylene, C2H4). We will discuss auxin.

Discovery of auxins: The existence of the first plant growth hormone came from the observation of Charles Darwin and his son Francis Darwin (1880). They observed that the coleoptiles of canary grass (Phalaris canariensis) responded to unilateral illumination by growing towards the light source (phototropism). After a series of experiments, it was concluded that the tip coleoptile was the site of transmittable influence that caused the bending of the entire coleoptile. Later, the experiments conducted in different laboratories, resulted in the discovery of the first PGR called auxin (Gk. Auxin = to grow), by F.W. went (1928) in complete tip Avena sativa seedlings.Auxins, isolated initially from human urine were the first PGR to be discovered. Kogl and Haagen Smith (1931) isolated three chemicals from human urine. They were named as auxin a, auxin b and  heteroauxin. Kogl et.al (1934) found that heteroauxin is the real plant auxin and is chemically indole 3- acetic acid or IAA.

TYPES OF AUXINS

There are two major categories of auxins- natural auxins and synthetic auxins.

1. Natural auxins. These are naturally occurring auxins in plants and therefore, regarded as phytohormones. Indole 3-acetic acid (IAA) is the best known and universal auxin. It is found in all plants and fungi. Besides IAA, indole 3-acetaldehyde, indole 3-acetonitrile, indole 3-butyric acid (IBA), phenyl acetic acid 4- chloro- indole acetic acid are some other natural auxins.

  2.Synthetic auxin. Many synthetic auxins are also being manufactured. The important ones are 2,4- D (2, 4- dichlorophenoxy acetic acid) , 2, 4, 5-T (2, 4, 5 – trichlorophenoxy acetic acid),  and NAA (naphthalene acetic acid), IBA (indole butyric acids) etc. IBA is both natural and synthetic. Synthetic auxins move in all directions inside the plant.

FUNCTIONS OF AUXINS

1.Cell enlargement. Auxins promote elongation and growth of stems and roots and enlargement of many fruits by stimulating cell enlargement. However, this effect is not observed when the treatment of exogenous auxins is given to intact plants. Probably it is because the required amount of auxin is already present in plants.The auxins cause cell enlargement by solubilisation of carbohydrates, loosening of micro fibrils, synthesis of more wall materials. Increased membrane permeability and respiration.

2.Cell division in cambium. Auxin is responsible for initiation and promotion of cell division in cambium. The reactivation of cambium in the growing season is apparently triggered by IAA moving from the developing shoots buds. The degree of cambial activity is directly proportional to auxin concentration.

3.Cell division and tissue culture. In Tissue culture auxins initiate and promote cell division,and result in the formation of callus (i.e., undifferentiated mass of cells). However differentiation of callus occurs in the presence of both auxin and cytokinin.             

4.Cell growth and root initiation. Auxins promote root growth only at extremely low concentration (i.e., 0.0001 ppm). At higher concentration of auxin which is suitable for shoot elongation is always inhibitory to root growth. It is remarkable that the concentration of auxin which is inhibitory to root growth causes imitation of roots.

5.Apical Dominance. Apical dominance is the phenomenon by which the presence of apical buds does not allow the nearby lateral buds to grow. Lateral buds start developing into branches when the apical bud is removed. However, if a paste of auxin is applied on the decapitated shoot, the growth of lateral buds remain inhibited as if the apical bud is present.                    

6.Parthenocarpic fruits. Auxins are known to induce parthenocarpy. It has been observed that the carpels producing parthenocarpic or seedless fruit (e.g, Banana, grapes, etc.) have a higher internal production of auxins.

USES OF AUXINS

1. Eradication of weeds. Some synthesis auxins are used as weedicides (i.e.,  

chemicals which kill weeds growing with the crops). Application of 2, 4-D (2, 4- dichlorophenoxy acetic acid) and 2, 4, 5-T (2, 4, 5 – trichlorophenoxy acetic acid) removes broad leaved weeds in cereals crops, while dalapon (2, 2-dichloropropionic acid) kills grasses in broad leaved crops.

Synthetic auxins have also been misused by people. Large scale aerial application of weedicides or defoliants used in Vietnam to expose the forests have exterminated the wild relatives of Citrus.   

2. Rooting.  Auxins stimulate root formation on stem cuttings, particularly the woody ones. The common auxins used for inducing rooting include NAA (naphthalene acetic acid), IBA (indole butyric acids) etc.

Conclusion 

To summarise, auxins are hormones that help the plant grow effectively. Auxin promotes shoot growth, cell elongation and cell division. It inhibits the growth of roots. Auxin is most of the time produced on the tip of roots and shoots. It promotes the growth of shoots. Higher concentration of auxin results in more growth of shoots. In a graph, the relationship of the concentration of the auxin to the shoot length will be a positive correlation.  It inhibits the growth of roots in the plant.  Lower the concentration of auxin, the higher the growth of roots. In a graph, the relationship of concentration of auxin will be inversely proportional to the length of the roots.  When a root is growing, the auxin gathers around the lower half of the root. This results in the upper part growing more as auxin inhibits the growth in lower regions. As a result, the root is tilted downwards, it is also known as positive gravitropism, which means growing according to gravity.