Plant tissue culture or micropropagation is an innovation to produce plants from its miniature part under an intensive care platform where all the physiological growth parameters of the plant are induced using synthetic materials such as plant hormones. Synthetic plant hormones or PGRs are used different means of plant propagation from cuttings, grafting, micropropagation, and tissue culture. There are stages for every development in plants; there is a time from leaves and branches developments, root development, flowering, and fruit settings; all these can be artificially induced during tissue culture using certain plant hormones.
Plant hormones can also be called phytohormones; they are substances or compounds are signal or control molecules, produced within plants, in extremely low concentrations. Plants rely solely on these hormones to direct the expression of DNA, the operations of the cell, and the overall physiological performance of the plant.
Plant hormones control all physiological performance of the plant such as growth and development, from embryogenesis, the generation and regulation of organ size, pathogen defense mechanism and tolerance, stress tolerance, and reproductive development. There are millions of cells within a plant, each plant cell is capable of producing hormones to regulate and control the activities within.
Phytohormones are developed by all plants in the plant kingdom, irrespective of the complexity of the plant. Some phytohormones are present in microorganisms, such as unicellular fungi and bacteria, however, these hormones do not play a hormonal role and can better be regarded as secondary metabolites.
Plant Hormones Used In Tissue Culture
The plant hormones used in tissue culture for better growth are auxins and cytokinins. The propagule is cultured on the nutrient media; it turns into a mass of undifferentiated cells called Callus. The Callus is treated with auxin to induce rooting; it is equally treated with cytokinins to stimulates cell maturation, differentiation and the development of stem to form different vegetative plant
There are 5 major hormones used in plant tissue culture, each controls various aspects of plant growth and development. They are:
#1. ABSCISIC ACID
Abscisic acid (ABA) is also referred to as plant growth inhibitors. This phytohormone possesses chemical properties called dormin and abscicin II, these two compounds are the same. This hormone was named “abscisic acid” because it was found in high concentrations in newly abscissed or freshly fallen leaves.
Abscisic acid acts as an inhibitory chemical compound that affects bud growth, and seed and bud dormancy. It initiates changes within the apical meristem, causing bud dormancy and the alteration of the last set of leaves into protective bud covers. Although Abscisic acid was found in freshly abscissed leaves, it does not initiate the processes of natural leaf drop.
ABA in plant species originating from temperate regions of the world act differently on the leaf and seed dormancy by inhibiting their growth, but, as it is dissipated from seeds or buds, growth begins. In other plants from different regions, as the level of ABA decreases, the growth of plant part commences as gibberellin levels increase. The absence of ABA makes buds and seeds grow during warm periods in winter and be killed when it froze again. Abscisic acid releases slowly from the tissues; this increases the period of its offset by other plant hormones and initiates a delay in physiological pathways that provide some protection from premature growth. Abscisic acid accumulates within seeds during fruit maturation, preventing seed germination within the fruit, or seed germination before winter.
Auxins are phytohormones discovered by Darwin’s family, Charles Darwins and his son, Francis. This plant hormone influences cell enlargement, bud formation, and root initiation. They also promote the synthesis of other hormones; Auxins and cytokinins help to control the growth of stems, roots, and fruits and convert stems into flowers.
The Auxins hormone influence cell elongation by altering cell wall plasticity; it stimulates the cambium, a subtype of meristem cells, to divide and cause secondary xylem to differentiate in stems. Auxins inhibit the growth of buds below the stems (apical dominance), and also to promote the growth and development of the lateral and adventitious root. The beginning of leaf abscission initiated by the growing point of a plant ceases the production of auxins.
Auxins present in plant seeds regulate specific protein synthesis, as they develop within the flower after successful pollination, causing the flower to develop a fruit to contain the developing seeds. Auxins are highly toxic to plants when present in large concentrations; they are most toxic to dicot plants and less so to monocots. The toxicity of Auxins when present in high concentration has been used to develop, synthetic auxin herbicides such as 2,4-D(2,4-dichlorophenoxyacetic) and 2,4,5-T; these herbicides increases the concentration of auxins in the targeted plant, thus, serve as a weed control strategy.
Auxins, especially 1-Naphthaleneacetic acid (NAA) and Indole-3-butyric acid (IBA) are also commonly applied to stimulate root growth during tissue culture of plants or when taking cuttings of plants for propagation. The most common auxin found in plants is indole-3-acetic acid or IAA.
Cytokinins are a group of chemicals that influence cell division and shoot development in plants. They were formerly called kinins in the past when the first cytokinins were isolated from yeast cells. Aside from their effect in cell division and shoot formation, cytokinins also help to delay senescence of tissues, create a system for the transportation of auxin within the plant, and affect leaf growth and internodal length.
Cytokinins and auxins are common hormones used in plant tissue culture, and the ratios of these two groups of plant hormones affect most major growth periods during a plant’s lifetime. Cytokinins counter the apical dominance induced by auxins; the two hormones in conjunction with ethylene promote abscission of leaves, flower parts, and fruits.
Cytokinins interact directly with the auxins; they both direct cell differentiation and various aspects of cell metabolism. The cytokinins interact with the plant’s DNA, making it express or hide synthesized proteins when needed. This action facilitates cell differentiation within the plant, allowing the plant to develop different tissues for growth and other purposes.
The cytokinins are most concentrated in the roots, and the concentration reduces towards the shoots. This is otherwise on auxins. This concentration allows the plant to develop and maintain an axis, and grow in both directions.
During tissue culture, the application of cytokinins without auxins leads to only roots development; similarly, the application of auxins alone leads to buds formation. When combined, they tend to form undifferentiated growth.
Ethylene is a single chemical, in the form of a gas formed through the breakdown of methionine in cells. This plant hormone mediates the communication between cells and other plants during tissue culture. Ethylene. This phytohormone has very limited solubility in water and does not accumulate within the plant cell but diffuses out of the cell and escapes out of the plant. The effectiveness of ethylene as a plant hormone is dependent on the rate of its production to the rate of its escape into the atmosphere.
Damage on one part of the plant initiates the presence of ethylene. When a plant stem is bent, bruised, or broken, ethylene is released. Being a gas, the hormone quickly diffuses through the fluids of the plant and can travel through the air. Plants use ethylene to communicate damage on one part to other plants, stimulating them to ripen their fruit or to develop defenses against pests.
Ethylene is usually produced at a faster rate in rapidly growing and dividing cells, especially in darkness. New shoot and newly germinated seedlings produce more ethylene than can escape from the plant, which leads to elevated amounts of ethylene, thus, inhibiting leaf expansion. As the new shoot is exposed to light, reactions mediated by phytochrome, the plant’s cells produce a signal when facilitates the decrease in the production of ethylene, thus, allowing leaf expansion.
Ethylene production increases greatly when a growing shoot or root is disturbed while underground; this prevents cell elongation and causes the stem to swell. The swollen stem becomes stronger and less likely to buckle under the pressure as it presses against the object impeding its path to the surface. If the shoot does not reach the earth’s surface and the ethylene stimulus becomes prolonged, it affects the stem’s natural geotropic response, making it grow around an object instead of its upright growth.
#5. GIBBERELLINS (GAs)
Gibberellins is one of the important hormones used in tissue culture; this phytohormone was first discovered by Japanese researchers, including Eiichi Kurosawa when they noticed a chemical produced by a fungus called Gibberella fujikuroi that produced abnormal growth in rice plants. It was later discovered that Gibberellins are also produced by the plants themselves and they control multiple aspects of metabolism and development across a plant life cycle. “
Gibberellins is responsible for cell division and overall plant growth; they are also responsible for activating several enzymes. Gibberellins present in the seed activate enzymes such as amylases, which help break down starches to glucose and in turn provide the embryo with energy. These plant hormones also activate other enzymes during tissue culture to the embryo with amino acids and lipids to grow.
Gibberellins also serve as sexual hormones in some plants as they help drive the development of male and female flowers. Gibberellin plant hormones alongside the auxins affect the senescence of plant parts. The synthesis of Gibberellins is strongly upregulated in seeds at germination and its presence is required for germination to occur. In seedlings and adults, the plant hormone strongly promotes cell elongation.
During micropropagation, Gibberellins promote the transition between vegetative and reproductive growth and are also required for pollen function during fertilization; this plant hormone ensures that all seeds sprout and become viable, also, GAs used to increase the size of grapes and other fruit, if applied at the right stage.
Functions of Plant Hormones in Micropropagation
Generally, in micropropagation, plant hormones are used to promote multiplication of cells and then rooting of new plantlets. Similarly in the tissue culture of plant cells, Phytohormones or PGRs are used to produce callus growth, multiplication, and rooting.
The propagation of plants by cuttings or other vegetative means is achieved by utilizing phytohormone called auxins as a rooting hormone applied to the cut surface; the auxins are transported into the plant and promote root initiation. In grafting of fruit trees like citrus, auxins are applied to promote callus tissue formation, which binds the surfaces of the graft together.