Plants need light, water, oxygen, minerals, and other nutrients to grow and flourish. Aside from these external requirements, plants rely on organic chemicals to signal, regulate, and control their development. These are known as plant growth regulators or plant growth hormones.

Plant growth regulators are organic compounds that are generated in minute quantities in one region of the plant body and transferred to another section of the plant body where they regulate certain physiological processes. They are signal molecules generated by plants in extremely low quantities. 

Plant hormones regulate many aspects of plant growth and development, including embryogenesis, organ size regulation, pathogen defence, stress and tolerance, and reproductive development.

Plant Growth Regulators: What Are They?

Plant growth regulators can include a wide range of chemical compositions, including gases (ethylene), terpenes (gibberellic acid) and carotenoid derivatives (abscisic acid).

Plant growth regulators are categorised into two primary classes based on their actions:

1. Plant Growth Promoters
2. Plant Growth Inhibitors

Plant growth promoters include auxins, gibberellins, and cytokinins, whereas plant growth inhibitors include abscisic acid and ethylene.

Ethylene can be classified as either plant promoter or inhibitor.

Auxins

The word auxin comes from the Greek word auxein, which means “to grow.” Auxins are all the growth-regulating organic substances that are formed at the tips of roots and stems as a consequence of metabolism and transferred to the region of elongation, inducing cell elongation. Auxins, both natural and synthetic, are recognised to have comparable effects on plant growth and development. Thimann (1948) described auxin as “an organic substance that promotes development along the longitudinal axis when administered in low concentrations to shoots of plants that have been liberated as much as possible from their own natural growth stimulating chemicals.”

Auxins go from the shoot tip to the area of elongation, and their movement is basipetal (from the apex to the base) in the stem but acropetal (from the base to the apex) in the roots. Auxin promotes the growth of both shoots and roots. However, the optimal for the two is very different (10 ppm for stem and 0.0001ppm for root). In the presence of Zn++ ion, auxin production occurs in shoot apices, leaf primordial and developing seeds from the amino acid tryptophan. The most significant member of the auxin family is indoleacetic acid (IAA), which is the most powerful natural auxin and causes the bulk of auxin actions in intact plants.

Types of auxins

These are of two types:

1. Natural Auxins
2. Synthetic Auxins

Indole-3-acetic acid, 4-chloroindole-3-acetic acid, phenylacetic acid, indole-3-butyric acid, and indole-3-propionic acid are all naturally occurring (endogenous) auxins in plants. Auxins synthesised include 1-naphthaleneacetic acid, 2,4-D(2,4- dichlorophenoxy acetic acid), and a variety of others.

Natural Auxins

Indole 3-acetic acid (IAA) is a naturally occurring auxin in plants and is thus considered a phytohormone. It is the most well-known and ubiquitous auxin. It may be found in all types of plants and fungus. Kogl and Haagen-Smit (1931) discovered the first naturally occurring auxin from human urine.

Physiological Effects and Applications of Auxins

1. Apical Dominance (characteristic function of auxins): Apical dominance is the situation in which the apical bud controls the development of lateral buds. Pruning in the garden encourages hedge densification.
2. Cell division and enlargement/callus creation. Auxin is essential in tissue culture and grafting. It promotes intrafascicular cambium division, also present in wound healing.
3. Internode Shortening: a-NAA causes the production of dwarf shoots or spurs in apple, pear, and other fruit trees, increasing the number of fruits.
4. Lodging prevention: Auxin spray inhibits crops, immature leaves and fruits from lodging.
5. Root initiation: IEA and NAA enhance rooting on stem cuttings (auxin inhibits root development).
6. Potato dormancy: MH (Maleic-Hydrazide), a-NAA, promotes dormancy of lateral buds in potato tubers and allows potatoes to be preserved for extended periods.
7. Abscission Prevention: IAA and NAA inhibit premature abscission of plant parts.
8. Flower initiation: Auxin is a flowering inhibitor, yet it promotes uniform blooming in Pineapple and Litchi plants.
9. Parthenocarpy: The spray of IAA might yield fruits with fewer seeds. (Written by Gusteffson)
10. Selective weed killer: Dicot broad-leaf weeds can be destroyed with 2, 4-D and 2, 4, 5-T.
11. BioWare employs the usage of Agent Orange. It was deployed by the United States against Vietnam (1966-60).
12. Femaleness: Some plants have a feminising impact.
13. Flower and fruit thinning: Certain trees, such as mango, produce less fruits in alternate years. However, auxins can yield regular fruit crops year after year. This is referred to as fruit thinning.
14. Cotton balls may be easily collected when Antiauxin (TIBA-Tri-Iodo-Benzoic acid) is sprayed on a mature cotton crop.

Biosynthesis of Auxin

Indole-3-acetic acid (IAA), the most abundant natural auxin in plants, is mostly produced from the amino acid tryptophan (Trp). The amino acid tryptophan (Trp) is a precursor of IAA since it is structurally identical to it and is thought to be present in all cells. Auxin biosynthesis progress also sets the groundwork for understanding polar auxin transport and dissecting auxin signalling systems throughout plant development.

Plant IAA biosynthesis pathways:

1. The Indole-3-pyruvic acid (IPA) biosynthetic pathway: First, the amino acid tryptophan lends its amino group to a keto acid via a transamination event to form indole pyruvic acid. The enzyme indole tryptophan transaminase catalyses the process. Then, in the presence of the enzyme indole pyruvate decarboxylase, indole pyruvic acid is decarboxylated to form indole acetaldehyde. Finally, indole acetaldehyde undergoes oxidation to become indole-3-acetic acid. The enzyme indole acetaldehyde dehydrogenase catalyses this reaction. Examples include sterile pea shoots and cucumber seedlings.
2. The TAM (Tryptamine) pathway: Tryptamine is found in higher plants on an irregular basis. It was discovered in numerous additional species after being isolated from Acacia. In the presence of the enzyme tryptamine decarboxylase, tryptophan is decarboxylated to generate tryptamine, which is then deaminated to form indole acetaldehyde in the presence of the enzyme tryptamine oxidase. In the presence of the enzyme indole acetaldehyde dehydrogenase, indole acetaldehyde is then oxidised to generate indole-3-acetic acid.
3. Indole acetaldoxime (IAN) pathway: This route is unique to the Cruciferae family. Tryptophan is transformed into Indole-3-acetaldoxime, which is then turned into Indole-3-acetonitrile in the presence of Indole-3-acetaldoxime hydrolase (IAN). Nitrilase is a hidden enzyme.

Conclusion

Finally, given the importance of auxin in plant growth, understanding how auxin functions would aid in understanding how fundamental developmental processes are regulated. Aside from the intrigue of discovering how a complex living thing is wired, knowledge gathered from auxin-dependent processes may be used to design plant growth. However, this is only one of many possibilities that this interesting chemical might take us to in the future.