Auxin : The Plant Growth Regulator
Auxin (fr. G. auxein, to increase) was the first plant hormone to be discovered. Auxins are a class of plant hormones that play a fundamental role in regulating various aspects of plant growth and development. They exert a wide range of effects on plant cells and tissues, influencing processes such as cell elongation, root development, fruit ripening, apical dominance, phototropism, and more.
1.0Discovery of Auxin
The discovery of auxin, the first identified plant hormone, traces back to Charles Darwin's pioneering studies on plant movements in the late 19th and early 20th centuries. He observed the bending of canary grass seedlings towards light (phototropism), concluding that a signal causing bending was perceived at the tip of the coleoptile. Later experiments by Boysen-Jensen and Paal supported the idea of a chemical signaling substance.
The active substance, later named "auxin," was successfully isolated in 1928 by F. W. Went, a graduate student working in his father's laboratory in Holland. Went's experiments involved the removal of oat (Avena sativa) coleoptile apices, which were then placed on small agar blocks. The substance responsible for the growth response diffused from the block into the coleoptile, stimulating differential cell elongation, resulting in coleoptile curvature.
2.0Biosynthesis of Auxin
Tryptophan as a Precursor: Tryptophan, an essential amino acid in plants, serves as a key precursor for auxin biosynthesis.
Indole-3-Pyruvic Acid (IPA) Formation: Tryptophan is converted to indole-3-pyruvic acid (IPA) through a series of enzymatic reactions. The IPA undergoes further conversion to form the most common naturally occurring auxin, Indole-3-Acetic Acid (IAA). This transformation often involves several enzymatic pathways and can occur in multiple steps, including amino transferases and oxidative decarboxylation processes.
3.0Types of Auxin
Natural Auxins:
- Indole-3-Acetic Acid (IAA):
- Indole-3-Butyric Acid (IBA):
Synthetic Auxins:
- Naphthaleneacetic Acid (NAA):
- 2,4-Dichlorophenoxyacetic Acid (2,4-D):
4.0Physiological Effects
- Cell elongation : auxin stimulates the elongation of cells which is present in the shoot, the higher concentration of auxin on the shaded side stimulates the elongation of cell results in the bending of the shoot tip towards the unilateral light.
- Apical dominance : Apical dominance in plants, inhibiting lateral bud growth, is attributed to the actively growing apical bud. With the discovery of auxin, specifically IAA, it was revealed that IAA substitutes the apical bud's role in suppressing lateral buds. Removing the shoot tip (decapitation) prompts lateral bud growth, overriding apical dominance. This method is commonly used in tea plantations and hedge-making to stimulate lateral growth and achieve bushier plant formations.
- Cell division and cell enlargement : Auxin influences both cell division and cell enlargement in plants. It regulates cell division by initiating and controlling the cell cycle in actively growing tissues, facilitating gene expression that triggers division. Simultaneously, auxin regulates cell enlargement by modifying cell wall plasticity, adjusting the wall's pH to allow increased water uptake, leading to elongation or enlargement of cells.
- Root initiation : In horticulture, auxins, especially naphthalene acetic acid (NAA)and Indole butyric acid (IBA) are commonly applied to stimulate root initiation in cuttings of plants. However, high concentrations of auxin inhibit root elongation and instead enhance adventitious root formation.
- Parthenocarpic : auxin can induce the formation of parthenocarpic fruits. In many cases the concentration of auxin in the ovaries has been found to be higher than the ovaries of plants which produce fruits after fertilization.
- As herbicides : 2, 4-D and 2, 4, 5-trichlorophenoxyacetic acid (2, 4, 5-T) were marketed as herbicides. 2, 4-D was the first widely used herbicide, and it is still so. It is easy and inexpensive to manufacture.
- Sex Expression: Auxins change the sex ratio in some plants by increasing the number of female flowers and decreasing the number of male flowers.
- Abscission : The falling of leaves, flowers and fruits is called abscission. Auxin promotes the abscission of older leaves and fruits. The green lamina of young leaves produce auxin and the old leaves cannot produce sufficient auxin. The cell of the abscission layer secretes hydrolytic enzymes like cellulase and pectinase. These enzymes dissolve the cell wall. The organ detaches at this point and falls.