PGR'S AND THEIR USE IN PLANT TISSUE CULTURE

Introduction:-
Plant hormones (also known as plant growth regulators (PGRs) and phytohormones) are chemicals that regulate plant growth. Plant hormones are signal molecules produced at specific locations in the plant, and occur in extremely low concentrations. The hormones cause altered processes in target cells locally and at other locations. Plants, unlike animals, lack glands that produce and secrete 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.

Auxin

They stimulate cambium cells to divide and in stems cause secondary xylem to differentiate. Auxins act to inhibit the growth of buds lower down the stems, affecting a process called apical dominance, and also promote lateral and adventitious root development and growth. Auxins promote flower initiation, converting stems into flowers

Auxins are toxic to plants in large concentrations; they are most toxic to dicots and less so to monocots. Because of this property, synthetic auxin herbicides including 2, 4-D and 2, 4, 5-T have been developed and used for weed control. Auxins, especially 1-Naphthaleneacetic acid (NAA) and Indole-3-butyric acid (IBA), are also commonly applied to stimulate root growth when taking cuttings of plants. The most common auxin found in plants is indole-acetic acid or IAA.

• Stimulates cell elongation
• Stimulates cell division in the cambium and cell swelling, in tissue culture
• Stimulates differentiation of phloem and xylem
• Stimulates root initiation on stem cuttings and lateral (adventious) root development in tissue culture
• Mediates the tropistic response of bending in response to gravity and light
• The auxin supply from the apical bud suppresses growth of lateral buds
• Induces cell division and callus formation
• Involved in assimilate movement toward auxin possibly by an effect on phloem transport
• Stimulates the production of ethylene at high concentrations
• It inhibits adventious and axillary shoot formation

Plant Cell Culture Tested Auxins are generally used in plant cell culture at a concentration range of 0.01-10.0 mg/L. When added in appropriate concentrations, they may regulate cell elongation, tissue swelling, cell division, formation of adventitious roots, and inhibition of adventitious and axillary shoot formation, callus initiation and growth, and induction of embryogenesis..

Indole-3-butyric acid (1H-Indole-3-butanoic acid, IBA) is a white to light-yellow crystalline solid, with the molecular formula : C12H13NO2

As a Plant Hormone
IBA is a plant hormone in the auxin family and is an ingredient in many commercial plant rooting horticultural products.

For use as such, it should be dissolved in about 75% (or purer) alcohol (as IBA does not dissolve in water), until a concentration from between 10,000 ppm to 50,000 ppm is achieved - this solution should then be diluted to the required concentration using distilled water. The solution should be kept in a cool, dark place for best results. This compound had been thought to be strictly synthetic; however, it was reported that the compound was isolated from leaves and seeds of maize and other species.

Abscisic acid

Abscisic acid also called ABA, was discovered and researched under two different names before its chemical properties were fully known, it was called dormin and abscicin II. Once it was determined that the two latter named compounds were the same, it was named abscisic acid. 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 effects bud growth, seed and bud dormancy

• Inhibits shoot growth but will not have as much affect on roots or may even promote growth of roots.
• Inhibits the affect of gibberellins on stimulating de novo synthesis of a-amylase.
• Induces gene transcription especially for proteinase inhibitors in response to wounding which may explain an apparent role in pathogen defense.
• Induction of embryogenesis.

Cytokinins

Cytokinins are a group of chemicals that influence cell division and shoot formation.
They help delay senescence or the aging of tissues, are responsible for mediating auxin transport throughout the plant, and affect internodal length and leaf growth.
Cytokinins counter the apical dominance induced by auxins; they in conjunction with ethylene promote abscission of leaves, flower parts and fruits.

• Stimulates cell division.
• Stimulates morphogenesis (shoot initiation/bud formation) in tissue culture.
• Stimulates the growth of lateral buds-release of apical dominance.
• Stimulates leaf expansion resulting from cell enlargement.
• Promotes the conversion of etioplasts into chloroplasts via stimulation of chlorophyll synthesis.

Plant Cell Culture Tested Cytokinins are generally used in plant cell culture at a concentration range of 0.1-10.0 mg/L. When added in appropriate concentrations, they may regulate cell division, stimulate axillary and adventitious shoot proliferation, regulate differentiation, inhibit root formation, activate RNA synthesis and stimulate protein and enzyme activity.
Stock solutions of IAA and kinetin are stored in amber bottles covered with a black paper and kept in dark since they are unstable in light

Ethylene

Ethylene is a gas that forms from 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 affects cell growth and cell shape; when a growing shoot hits an obstacle while underground, ethylene production greatly increases, preventing cell elongation and causing the stem to swell. 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. But, in tissue culture its function is unknown.

• Stimulates the release of dormancy.
• Stimulates shoot and root growth and differentiation (triple response)
• Have a role in adventitious root formation.
• Stimulates leaf and fruit abscission.

Giberallins

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 noticed a chemical produced by a fungus called Gibberella fujikuroi that produced abnormal growth in rice plants. Gibberellins play a major role in seed germination, affecting enzyme production that mobilizes food production that new cells need for growth.
During seed germination, the seedling produces GA that is transported to the aleurone layer, which responds by producing enzymes that break down stored food reserves within the endosperm, which are utilized by the growing seedling. GAs increase internodal length. They promote flowering, cellular division, and in seeds growth after germination. Gibberellins also reverse the inhibition of shoot growth and dormancy induced by ABA. In tissue culture they are mainly used for plant regeneration. They are very essential for meristem culture.

• Stimulate stem elongation by stimulating cell division and elongation and growth of meristems or buds in vitro
• Stimulates bolting/flowering in response to long days.
• Breaks seed dormancy in some plants which require stratification or light to induce germination.
• Stimulates enzyme production (a-amylase) in germinating cereal grains for mobilization of seed reserves