**2. Class of phytohormone**

Plant hormones (phytohormones) are chemicals produced by plants that regulate their growth, development, reproductive processes, longevity, and even death. These small molecules are derived from secondary metabolism and are responsible for the

*Types and Function of Phytohormone and Their Role in Stress DOI: http://dx.doi.org/10.5772/intechopen.109325*

adaptation of plants to environmental stimuli. Plants are subjected to an ever changing environment and require these phytohormones for appropriate responses. A single phytohormone can regulate many cellular and developmental processes, while at the same time multiple hormones often influence a single process [7].

Auxin, gibberellins, cytokinins, ethylene and abscisic acid are the five primary phytohormone. Other phytohormone that influences plant physiological processes include brassinosteroids, salicylates, jasmonates, strigolactones, etc (**Table 1**) [8].

#### **2.1 Auxin**

#### *2.1.1 Introduction*

The first growth hormone to be identified was auxin. They were discovered due to the observations of Charles Darwin and his son, Francis Darwin. The coleoptile (protective sheath) of canary grass develops and bends in the direction of the light source, as seen by the Darwin's. This is known as "phototropism." Additionally, their research demonstrated that the coleoptile tip was the location where the bending occurred. As a result, F. W. Went was able to isolate the first auxin from the coleoptile tip of oat seedlings [9]. The apical meristem of shoots, young leaves, and seeds is where auxin is mostly produced. From the point of production, auxin moves downward in a unidirectional or polar manner. Auxin concentration gradient produced by polar transit drives specific responses. The plasma membrane's auxin-specific transport proteins regulate how auxin leaves the call. Plant hormones work through signal transduction, triggering several cellular responses. Auxin attaches to receptors that are related to enzymes, which encourages reaction catalysis. The repressor protein for certain genes (the auxin response gene) attaches to ubiquitin when auxin binds to a receptor. This causes the repressor protein to be degraded, and the transcription of auxin response genes proceeds, promoting cellular growth and development [10].

#### *2.1.2 Function*

Auxin, plays a crucial role in regulating growth and development. Indole-3-Acetic Acid (IAA), Indole-3-Butyric Acid (IBA), and 4-chloro-indole-3-acetic acid are all members of this hormone family that are found in nature. Auxin levels vary dramatically within the plant body and throughout the life cycle of the plant, forming complex gradients that appear to be a central component of its regulatory activity for plant development. In order to control auxin levels in particular tissues in response to shifting environmental and developmental factors, plants have evolved complex networks with adaptive flexibility as well as genetic and biochemical redundancy [11]. Indole-3-acetic acid (IAA) is the main auxin in most plants (natural auxin) 2, 4-Dichlorophenoxy Acetic acid, Indole-3-Propionic Acid, alpha- Naphthalene Acetic Acid are synthetic auxins.

#### **2.2 Gibberellins**

#### *2.2.1 Introduction*

Gibberellins are plant growth regulators that control growth and have an impact on a variety of developmental processes, including stem elongation, germination, blooming, enzyme induction, and so on. The most pronounced effect of gibberellins on plant


#### **Table 1.**

*Summary of various phytohormone with their major functions.*

development is the elongation of the stem [12]. When it is administered to a shrub at low concentration, the stem begins to grow. Different dwarf kinds' genetic restrictions are overcome via Gibberellins. More than 70 gibberellins have been isolated. The numbers are GA1, GA2, GA3, and so forth. The most extensively researched plant growth regulator is GA3, or gibberellic acid. GAs are a family of plant hormones with about 135 members that are classified as diterpenoids with a gibberellin basic structure [13].
