A CRITICAL INSIGHT INTO THE INTRICATE ROLE OF PLANT HORMONES IN GROWTH AND DEVELOPMENT PHASE

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About Authors:
Puneet Sudan*, Swati Sudan, Tapan Behl, Monika Sharma, Rashi Wanchoo Misri
Department of Pharmacognosy,
Doaba College of Pharmacy, Kharar, Punjab
*puneetsudan22@gmail.com

Abstract:
Plant hormones are specialized and very vital chemical substances produced by plants. The chemicals are each grouped together into one of these classes based on their structural similarities and on their effects on plant physiology. Plant stress hormones activate cellular responses, including cell death, to diverse stress situations in plants.

REFERENCE ID: PHARMATUTOR-ART-2128

Introduction:
Plant hormones are the main factors responsible for controlling growth and development of plant. Hormones are produced in one part of a plant and transported to others, where they are effective in very small amounts. Depending on the target tissue, a given hormone may have different effects. Hormones determine various functions in plants like formation of flowers, stems, leaves, the shedding of leaves, and the development and ripening of fruit. Plants, unlike animals, lack glands that produce and secrete hormones. Instead, each cell is capable of producing hormones. Plant hormones shape the plant, affecting seed growth, time of flowering, the sex of flowers, senescence of leaves, and fruits. They affect which tissues grow upward and which grow downward, leaf formation and stem growth, fruit development and ripening, plant longevity, and even plant death. Plant hormones are not nutrients, but chemicals that in small amounts promote and influence the growth, development, and differentiation of cells and tissues. Hormones are transported within the plant by utilizing four types of movements. For localized movement, cytoplasmic streaming within cells and slow diffusion of ions and molecules between cells are utilized. Vascular tissues are used to move hormones from one part of the plant to another; these include sieve tubes or phloem that move sugars from the leaves to the roots and flowers, and xylem that moves water and mineral solutes from the roots to the foliage.

DIFFERENT CLASSES OF PLANT GROWTH HORMONES
In general, it is accepted that there are five major classes of plant hormones, some of which are made up of many different chemicals that can vary in structure from one plant to the next. Other plant hormones and growth regulators are not easily grouped into these classes; they exist naturally or are synthesized by humans or other organisms, including chemicals that inhibit plant growth or interrupt the physiological processes within plants. Each class has positive as well as inhibitory functions, and most often work in tandem with each other, with varying ratios of one or more interplaying to affect growth regulation.

ABSCISIC ACID
Abscisic acid is one of the most important plant growth regulators. The name "abscisic acid" was given because it was found in high concentrations in newly abscissed or freshly fallen leaves. This class of PGR is composed of one chemical compound normally produced in the leaves of plants, originating from chloroplasts, especially when plants are under stress. In general, it acts as an inhibitory chemical compound that affects bud growth, and seed and bud dormancy. It mediates changes within the apical meristem, causing bud dormancy and the alteration of the last set of leaves into protective bud covers. Since it was found in freshly abscissed leaves, it was thought to play a role in the processes of natural leaf drop, but further research has disproven this. In plant species from temperate parts of the world, it plays a role in leaf and seed dormancy by inhibiting growth, but, as it is dissipated from seeds or buds, growth begins. In other plants, as ABA levels decrease, growth then commences as gibberellin levels increase. Without ABA, buds and seeds would start to grow during warm periods in winter and be killed when it froze again. Since ABA dissipates slowly from the tissues and its effects take time to be offset by other plant hormones, there is a delay in physiological pathways that provide some protection from premature growth. It accumulates within seeds during fruit maturation, preventing seed germination within the fruit, or seed germination before winter. Abscisic acid's effects are degraded within plant tissues during cold temperatures or by its removal by water washing in out of the tissues, releasing the seeds and buds from dormancy.

AUXINS
Auxins are compounds that positively influence cell enlargement, bud formation and root initiation. They also promote the production of other hormones and in conjunction with cytokinins, they control the growth of stems, roots, and fruits, and convert stems into flowers. Auxins were the first class of growth regulators discovered. They affect cell elongation by altering cell wall plasticity. They stimulate cambium, a subtype of meristem cells, to divide and in stems cause secondary xylem to differentiate.

CYTOKININS
Cytokinins or CKs are a group of chemicals that influence cell division and shoot formation. They were called kinins in the past when the first cytokinins were isolated from yeast cells. Cytokinins counter the apical dominance induced by auxins; they in conjunction with ethylene promote abscission of leaves, flower parts, and fruits.

ETHYLENE
Ethylene is a gas that forms through the breakdown of methionine, which is in all cells. Ethylene has very limited solubility in water and does not accumulate within the cell but diffuses out of the cell and escapes out of the plant. Its effectiveness as a plant hormone is dependent on its rate of production versus its rate of escaping into the atmosphere. Ethylene is produced at a faster rate in rapidly growing and dividing cells, especially in darkness. New growth and newly germinated seedlings produce more ethylene than can escape the plant, which leads to elevated amounts of ethylene, inhibiting leaf expansion.. Ethylene affects fruit-ripening: Normally, when the seeds are mature, ethylene production increases and builds-up within the fruit, resulting in a climacteric event just before seed dispersal. The nuclear protein Ethylene Insensitive2 (EIN2) is regulated by ethylene production, and, in turn, regulates other hormones including ABA and stress hormones.

GIBBERELLINS
Gibberellins or GAs, include a large range of chemicals that are produced naturally within plants and by fungi. They were first discovered when Japanese researchers, including Eiichi Kurosawa, noticed a chemical produced by a fungus called Gibberella fujikuroi that produced abnormal growth in rice plants. Gibberellins are important in seed germination, affecting enzyme production that mobilizes food production used for growth of new cells. This is done by modulating chromosomal transcription. In grain (rice, wheat, corn, etc.) seeds, a layer of cells called the aleurone layer wraps around the endosperm tissue. Gibberellins also reverse the inhibition of shoot growth and dormancy induced by ABA.

BRASSINOSTEROIDS
Brassinosteroids, playing an intricate role in cell elongation, division, gravitropism, resistance to stress, and xylem differentiation are a class of polyhydroxysteroids. Their isolation was done from rapeseeds and also promote root growth and leaf absicision.

JASMONATES
Jasmonates are believed to play a piovital role in seed germination thereby promoting the growth of plant by enhancing the protein storage.

PLANT PEPTIDE HORMONES
Plant  peptide hormonespasses all small secreted peptides that are involved in cell-to-cell signaling. These small peptide hormones play crucial roles in plant growth and development, including defense mechanisms, the control of cell division and expansion, and pollen self-incompatibility.

REFERENCES

• Tarakhovskaya, E. R.; Maslov, Yu, I.; Shishova, M. F., 200: Phytohormones in algae. Russian Journal of Plant Physiology 54(2): 163-170.
• Srivastava, L. M. (2002). Plant growth and development: hormones and environment. Academic Press. p. 140.
• Öpik, Helgi; Rolfe, Stephen A.; Willis, Arthur John; Street, Herbert Edward (2005). The physiology of flowering plants (4th ed.). Cambridge University Press. p. 191.
• Weier, Thomas Elliot; Rost, Thomas L.; Weier, T. Elliot (1979). Botany: a brief introduction to plant biology. New York: Wiley. pp. 155–170. ISBN 0-471-02114-8.
• Ren H, Gao Z, Chen L et al. (2007). "Dynamic analysis of ABA accumulation in relation to the rate of ABA catabolism in maize tissues under water deficit". J. Exp. Bot. 58 (2): 211–9. Plantarum 111 (1): 46–54.

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