Growth Regulators- Auxin, Gibberellin, Cytokinin, Ethylene, Abscisic Acid (ABA)

Plant growth regulators (PGRs) are the chemicals used to change plant growth such as increasing branching, suppressing shoot growth, increasing return bloom, removing excess fruit, or altering fruit maturity. Numerous factors affect the performance of PGRs including how well the chemical is absorbed by the plant.

Auxins, gibberellins, cytokinin, abscisic acid and compounds affecting the ethylene status are the five groups under which the plant growth regulators are segregated. PGRs function as chemical messengers for intercellular communication.

Auxin

Growth hormone that are produced naturally by the plants are called auxins. The Greek word ‘auxien,’ which means ‘to grow’ was first introduced by a Dutch graduate student named Frits Went in the year 1926. They provide elongation of shoots and can inhibit growth of lateral buds at high concentrations. A synthetic auxin, naphthaleneacetic acid is used in apple production that can be used to thin fruit and prevent fruit drop before harvest.

Indoleacetic acid, or simply IAA is the most widespread auxin. IAA is an auxin that is very vital for the growth and development of plant tissues. While researching about auxin molecules, scientist have been able to recreate similar structures, called synthetic growth regulators. These artificial auxins also stimulate growth in plants and are widely used in many agricultural and commercial applications.

Functions of Auxin

Auxin molecules are found in mostly all tissues of a plant. However, they are more concentrated in the meristems which are the growth centers or simply the forefront of growth. Auxin molecules are released from these centers, which are then distributed towards the roots. In this way, the size and the growth and development of different tissues of the plant can be coordinated based on the rate of the auxin concentration.

Another feature which auxin provides is apical dominance. It is formed when a single meristem is growing rapidly and more efficiently. Eventually, the auxin that gets secreted from this meristem inhibits any new shoots from budding off.  If the stem is cut off, many new shoots will break out below the stem as the auxin gradient has been disturbed and the system must create a new leading shoot.

Gibberellin

Gibberellin was introduced by Japanese scientists studying a fungal rice disease in the 1930. Gibberellin has the feature to extend the length of cells and encourage division, overcome dormancy in seeds and even has hormonal and signaling roles in the fruiting and senescence processes. It is a type of fungal and plant hormone. It is one of the major groups of plant hormones.

Functions of Gibberellin

Seed Germination: Seeds like tobacco and lettuce are sensitive to light and thereby show poor germination in the absence of sunlight. The process of germination speeds up if the seeds are exposed to the sunlight. With the use of gibberellic acid on the seeds, the requirement of light can be overcome.

Dormancy of Buds: At a higher concentration in a few plants, gibberellins show signs of growth in few plants otherwise they have almost no effect on the growth of roots.

Elongation of the Internodes: The most pronounced effects of gibberellins on plant growth is the internodes elongation. It helps to overcome genetic dwarfism in some plants like dwarf pea and maize.

Cytokinin

Cytokinin are plant-specific chemical hormones that play a major role in the regulation of the plant cell cycle and various developmental processes. Cytokinin was introduced by F Skoog, C Miller, and co-workers during the 1950 as factors that promote cell division. They are involved primarily in cell growth but also affect apical dominance, leaf aging in plants and axillary bud growth.

Functions of Cytokinin

They are known to regulate axillary bud growth and apical dominance. This promotes growth in shoot and prevents lateral branching. Cytokinin travels from the roots into the shoots thereby signaling bud growth in length.

Cytokinin has shown the slow aging of plant organs by preventing breakdown of proteins, activating synthesis of protein and assembling nutrients from tissues nearby. It was supposed to believe that cytokinin may affect enzymes that regulate protein synthesis and degradation. 

Cytokinin has no impact on parenchyma cells alone. When used with auxin but no cytokinin, they grow large in size but do not divide. When cytokinin and auxin are both used together, the cells expand and also divide. When cytokinin and auxin are present in equal levels, the parenchyma cells form an undifferentiated mass of unorganized plant parenchyma cells, generally known as callus.

Ethylene

Ethylene regulates the development of leaves, flowers, and fruits. The growth and development of plants under dynamic environmental conditions determine agricultural production. The growth, development, and senescence of a plant’s organs can influence crop production by modulating photosynthesis, nutrient remobilization efficiency, and harvest index. It promotes growth and senescence processes depending on its concentration, timing of application, and the species of plant. 

Functions of Ethylene

Germination of seeds, growth of shoot and root, development of root are some important functions of ethylene. The ripening of climacteric fruits like peach, bananas, apples, and tomatoes are also an important function of ethylene. 

Epinasty is a complicated behavior seen in plants when the roots are flooded. During floods, the top layer of the leaves grows more in comparison to the bottom ones. This induces the leaves to drop-out and become more vertical than being horizontal.

Abscisic acid  (ABA)

The abscisic acid (ABA) is one of the significant hormones in vegetables. They participate in a set of necessary physiological processes, such as seed germination and tolerance to environmental stress. ABA is generally found in all plants and can also be found in specific species of fungi. They are generally collected from the lower parts of the fruits especially from the lower region of the ovary.

Functions of Abscisic Acid

Abscisic acid has been related to a wide range of important plant processes. The development and germination of the seed are among the main functions of ABA.

Under extreme environmental conditions, such as cold, drought, and regions with high salt concentrations, they function.  ABA-mediated signaling also plays a vital part in responses by plants in environmental stress and plant pathogens. ABA is also produced by some pathogenic fungi of plants through a biosynthetic pathway different from ABA biosynthesis in plants.

Seed germination is restrained by ABA against the gibberellin. Loss of seed dormancy is also prevented by ABA.

Conclusion

In this article, we have learnt about plant growth regulators and how they are important in the growth and development in the plants. The various regulators namely- auxin, gibberellin, cytokinin, ethylene, ABA are also discussed in detail. The functions of the regulators have been discussed briefly and also their importance is described to know about the regulators of growth of the plant.