Drought, excessive temperature, salinity, and toxic substances are all abiotic stresses that plants face. Abiotic stressors cause abnormalities in the genetic control of cellular processes, which have a deleterious impact on Plant physiology and morphology. Plants use a variety of tolerance mechanisms and pathways to counteract the impacts of stressors that arise when their metabolism is disrupted. Phytohormones are one of the most essential growth regulators; they have a significant impact on plant metabolism and are also critical in the stimulation of plant defense response systems in response to stress.

Plant hormones, also known as phytohormones, are small organic compounds or chemicals found in nature that regulate physiological processes in plants at very low concentrations. Phytohormones, in other words, are chemical messengers that help plants coordinate their cellular functions. Exogenous phytohormone supplementation has been used to aid growth and metabolism in stressed plants. Phytohormones produced by root-associated microorganisms may prove to be valuable metabolic function targets for developing host tolerance to abiotic stressors, according to new research. Auxins, cytokinins (CK), gibberellins (GA), abscisic acid (ABA), ethylene (ETH), brassinosteroids (BR), salicylates (SA), jasmonates (JA), and strigolactones (SL) are the nine categories of phytohormones that have been identified so far, from the early discovery of auxin as the first phytohormone to the recent identification of strigolactones (SL).

• Understanding Phytohormones:

Phytohormones are small organic molecules produced in minute quantities by plants in response to internal and external cues. Each hormone plays a distinct role, and their combined actions regulate virtually every aspect of plant life, from seed germination and root development to flowering, fruiting, and senescence.

Major Classes of Phytohormones:

• Auxins

Auxin is a plant growth hormone that the plant can make biologically. The natural hormone is denoted by the symbol IAA, which stands for indoleacetic acid in biological production. NAA (naphthalene acetic acid) and IBA (indole butyric acid) are chemically produced forms of this hormone. Auxins encourage plant cell development and elongation. They change the plant wall flexibility during the elongation phase, making it easier for the plant to grow upwards. They also have an impact on root development. They can also have a significant impact on plant orientation by encouraging cell division on one side of the plant in response to sunlight and gravity.

• Cytokinins

Cytokinins are phytohormones that affect not just plant growth, development, and metabolisms, such as cell division, chloroplast differentiation, and senescence delay, but also contact with other organisms, such as pathogens. Cytokinins are produced not just by plants, but also by bacteria, fungus, microalgae, and insects, among other prokaryotic and eukaryotic organisms. Cytokinins are produced by both pathogenic and helpful microorganisms, and they have been shown to increase plant tolerance to pathogen infections.

• Gibberellins

Gibberellins (GA) are phytohormones that control many aspects of plant growth, including stem elongation, germination, dormancy, flowering, flower development, and leaf & fruit senescence.

• Abscisic acid

Abscisic acid (ABA) is a plant hormone that causes seed dormancy and seed inhibition. It is considered to be the key hormone that regulates plant responses to unfavorable environmental stimuli since the amount of ABA in plants normally increases during abiotic stress conditions, and higher ABA can assist plant adaptability to diverse abiotic stresses.

• Ethylene

Ethylene is an important regulator of stress reactions and plays an important function in plant growth and development. It reduces vegetative growth by limiting cell elongation. Depending on the concentration, timing of administration, and plant types, it promotes or inhibits growth and senescence.

• Brassinosteroids

Brassinosteroids (BR) are plant steroid hormones that regulate a variety of processes in plant growth and development, including cell elongation, cell division, photomorphogenesis, xylem differentiation, reproduction, as well as abiotic and biotic stress responses.

• Salicylates

Salicylates are naturally occurring compounds produced by plants. They are found in fruits and vegetables and aid in the protection of plants against diseases and insects. Salicylic acid is essential in plant growth and development for important physiological functions such as enhancing the plant's sensitivity to stress situations (biotic and abiotic).

• Jasmonates

Jasmonate (JA) is a lipid-derived phytohormone that controls developmental processes such as pollen formation, tendril coiling, fruit ripening, and senescence, as well as biotic and abiotic stress responses.

• Strigolactones

In plants, strigolactones encourage branching and the growth of arbuscular mycorrhizal microorganisms. Aside from that, they inhibit the growth of new shoots and promote the germination of parasitic plant seeds.

• Role of Phytohormones in Plant Growth and Development:

1. Seed Germination and Seedling Growth: Phytohormones, such as gibberellins and cytokinins, are critical for breaking seed dormancy and triggering germination. They also stimulate root and shoot growth in seedlings.
2. Root and Shoot Development: Auxins and cytokinins act in tandem to control root and shoot growth. Auxins promote root elongation, while cytokinins stimulate lateral bud growth and branching.
3. Flowering and Fruit Development: The interplay of multiple hormones, including gibberellins and auxins, regulates the timing of flowering and fruit development.
4. Senescence and Abscission: Ethylene and ABA play key roles in initiating leaf senescence and promoting the abscission of leaves and fruits.
5. Responses to Environmental Stimuli: Phytohormones are pivotal in mediating plant responses to light, gravity, temperature, water availability, and various biotic and abiotic stresses.

• Environmental Responses:

Phytohormones enable plants to adapt and respond to changing environmental conditions:

1. Drought and Salinity: ABA helps plants respond to drought and salinity stress by closing stomata to reduce water loss and activating stress-responsive genes.
2. Pathogen Defense: Jasmonic acid and ethylene play crucial roles in activating plant defense responses against pathogens and herbivores.

• Applications in Agriculture and Horticulture:

Understanding phytohormones has profound implications for agriculture and horticulture:

1. Crop Improvement: Harnessing phytohormones can lead to the development of crops with desirable traits, such as improved drought tolerance, enhanced fruit set, and reduced lodging.
2. Vegetative Propagation: Manipulating hormones can facilitate vegetative propagation and tissue culture, enabling the rapid propagation of elite plant varieties.

Phytohormones are the master regulators that orchestrate the intricate dance of plant growth and development. Their multifaceted roles in plant physiology, from germination to senescence, unveil the complexity and adaptability of plants in responding to their environment. As we unravel the mysteries of phytohormones, we gain invaluable insights into the mechanisms of plant life, offering opportunities for sustainable agriculture, conservation, and ecological management. The mighty messengers of plant growth continue to captivate researchers and inspire us to appreciate the boundless diversity and resilience of the botanical world. Embracing the role of phytohormones, we embark on a journey to unlock the secrets of plant life, forging a harmonious relationship with nature and fostering a greener, more sustainable future for all.