Plant hormones are regulators of almost all aspects of plant development and plant responses to their environment. Active at very low concentrations, with tight spatial regulation of synthesis and response, many plant hormones have key roles in the interactions between plants and beneficial microbes. These hormones are essential regulators of plant development beginning with seed germination and culminating in whole-plant senescence. Until recently it was generally believed that the five classes of compounds comprising abscisic acid, auxin, cytokinins, ethylene, and gibberellins could account for most or all of the growth regulatory effects of plant hormones. Plant functioning under stress is affected by plant hormones, which can help the plant to tolerate the environmental stresses. A wide range of actions has been illustrated for plant hormones.

Let’s take a look at five major types of plant hormones: auxins, gibberellins, abscisic acid, cytokinins, and ethylene.

Auxins:Auxins promote stem elongation, inhibit the growth of lateral buds (maintains apical dominance). They are produced in the stem, buds, and root tips. Auxin moves to the darker side of the plant, causing the cells there to grow larger than corresponding cells on the lighter side of the plant. This produces a curving of the plant stem tip toward the light, a plant movement known as phototropism.

Gibberellins:Gibberellins are the plant growth regulators involved in regulating the growth and influencing different developmental processes which include stem elongation, germination, flowering, enzyme induction, etc. There are more than 70 gibberellins isolated. They are GA1, GA2, GA3, and so on. Gibberellic acid is the most widely studied plant growth regulator. Some seeds that are sensitive to light such as tobacco and lettuce exhibit poor germination in the absence of sunlight. Germination begins rapidly if the seeds are exposed to sunlight. If the seeds are treated with gibberellic acid, the light requirement can be overcome.

Abscisic acid:Abscisic acid ABA) is the hormone that is usually associated with major plant responses to stress. It is an important phytohormone regulating plant growth, development, and stress responses. It has an essential role in multiple physiological processes of plants, such as stomatal closure, cuticular wax accumulation, leaf senescence, bud dormancy, seed germination, osmotic regulation, and growth inhibition among many others. Abscisic acid controls downstream responses to abiotic and biotic environmental changes through both transcriptional and posttranscriptional mechanisms.

Cytokinins:Cytokinins (CK) are a class of plant growth substances (phytohormones) that promote cell division, or cytokinesis, in plant roots and shoots. They are involved primarily in cell growth and differentiation, but also affect apical dominance, axillary bud growth, and leaf senescence. Cytokinins are not only produced by plants but are also by other prokaryotic and eukaryotic organisms such as bacteria, fungi, microalgae, and insects.

Ethylene:Ethylene is a group of plant growth regulators which are widely used for ripening fruits and for the production of more flowers and fruits. It is also used in agricultural practices to ripen fruits, germinate the seed, etc. This plant hormone is essentially produced in all parts of grown plants including roots, stems, tubers, leaves, flowers, fruits, and seeds.

Plant hormones as signal molecules regulate cellular processes in targeted cells locally and then moved to other locations of the plant. They also determine the formation of the root, stem, leaf, and flower and facilitate the shedding of leaves and the development and ripening of fruits.