Plant Growth Hormones Lecture 4 PDF

Lecture 4 a Plant Growth Hormones Prepared and presented by : Dr. Abdelaziz Mohamed Nasr Lesson Objectives Understand the various types of plant phytohormones. Explore the role of phytohormones in plant tissue culture. Hormones in higher plants The form and funct...

Lecture 4 a Plant Growth Hormones Prepared and presented by : Dr. Abdelaziz Mohamed Nasr Lesson Objectives Understand the various types of plant phytohormones. Explore the role of phytohormones in plant tissue culture. Hormones in higher plants The form and function of multicellular organisms would be impossible without efficient communication among cells, tissues, and organs. In higher plants, regulation and coordination of metabolism, growth, and morphogenesis often depend on chemical signals from one part of the plant to Hormones in higher plants German botanist Julius von Sachs proposed that chemical messengers are responsible for the formation and growth of different plant organs. He also suggested that external factors such as gravity could affect the distribution of these substances within a plant. Hormones in higher plants Plants produce signaling molecules, called hormones, that have profound effects on development at vanishingly low concentrations. Plants produce auxins, gibberellins, cytokinins, ethylene, abscisic acid, brassinosteroids, jasmonic acid, and salicylic acid. Auxin First studied by Charles Darwin and his son. Indole-3-acetic acid (IAA) is the most common type of auxin found in higher plants. Laboratories synthesized a wide array of molecules with auxin activity like 2,4-Dichlorophenoxyacetic acid (2,4-D), and 2- Methoxy-3,6-dichlorobenzoic acid (dicamba) are synthetic auxins. Auxin Auxin Auxins have multiple roles in tissue culture, according to their chemical structure, their concentration, and the plant tissue being affected. Auxins cause the production of callus and roots as well as the extension growth of stems. Auxins generally stimulate cell elongation by stimulating gibberllins, cell division in cambium tissue, and, together with Auxin Additionally, a high concentration of exogenous auxin can induce somatic embryogenesis. The essential function of auxins and cytokinins is to reprogram somatic cells that had previously been in a determined state of differentiation. Reprogramming causes dedifferentiation and then redifferentiation into a new developmental pathway. High concentrations of exogenous auxin can be toxic, in large part because they stimulate the production of concentrations of ethylene, which can cause growth inhibition. Role of auxin in rooting medium In many plant species (e.g., Nicotiana species, cucumber, squash, Acacia species, and some cultivars of rose), shoots generated in vitro root easily after transfer from the regeneration medium, which usually contains a high level of cytokinin, to a medium without PGRs. Cytokinins inhibit rooting and this treatment effectively lowers their level. Role of auxin in rooting medium However, many economically important species lack the ability to root easily. The reluctance to root could be due to a high level of cytokinin persisting in stem tissues from previous treatments. In such a case, rooting can be induced by transfer of the shoots once or twice to medium without PGRs to allow cytokinin levels to drop. Alternatively, shoots may be reprogrammed by treatment with auxin to produce roots if they have too much endogenous cytokinin. Auxin appears to induce root initiation within a few hours of application. Role of Auxin in rooting medium Most commonly IAA (0.6–60 µM), IBA (2.5–15 µM) and/or NAA (0.25–6 µM) are used to promote rooting in vitro. It is important to note that the concentration of auxin that gives the greatest number of roots may not be the concentration that produces the greatest survival rate on subsequent hardening, as some roots induced directly by exogenous auxin are of low quality. A greater than optimum concentration of auxin often causes callus production and a reduction in root growth and quality. Occasionally mixtures of auxins, usually at lower concentrations than those used singly, can promote root initiation whereas individual auxins have no activity, as shown Cytokinin The cytokinins were discovered in the search for factors that stimulate plant cells to divide. Since their discovery, cytokinins have been shown to have effects on many other physiological and developmental processes, including leaf senescence, nutrient mobilization, apical dominance, the formation and activity of shoot apical meristems, floral development, the breaking of bud dormancy, and seed Cytokinin The first cytokinin isolated was kinetin. shortly after its discovery, adenine was identified as possessing considerable cytokinin activity, as were many adenine derivatives. Zeatin and N6-(2-isopentyl) adenine (2-iP), two other naturally occurring cytokinins, are often used in tissue culture. The synthetic cytokinin PGRs 6-benzyladenine (BA) and Cytokinin Generally, a high concentration of cytokinin will block root development. Cytokinins can cause release of shoot apical dominance, thereby stimulating growth of lateral buds and resulting in multiple shoot formation. Cytokinin in micropropagation Tobacco shoots grow and root easily on Murashige and Skoog (1962) basal medium (left). Cytokinin (5 µM) enhances branching and multiple shoot growth and callus development at the stem base, and blocks rooting (center). Cytokinin in micropropagation Micropropagation is the mass vegetative production of plants in vitro for the purpose of commercial plant production. Micropropagation is performed by shoot multiplication followed by rooting or more rarely by somatic embryogenesis. It is best to avoid unstable forms of multiplication such as through callus, due to the possibility of somaclonal variation. Therefore, the major regeneration pathway utilized is axillary shoot multiplication, noted for its genetic stability. Cytokinin in micropropagation Shoot multiplication is induced by the application of exogenous cytokinins in the growth medium. The applied cytokinin breaks shoot apical dominance (in dicotyledonous plants), stimulating the growth of axillary buds into shoots, and often induces the growth of additional subsidiary buds adjacent to the new axillary shoots. Too high a concentration of the applied cytokinin can cause many small shoots to grow, which fail to develop. Gibberellins Gibberellins are another class of PGRs with nearly 100 different variants identified from various sources, although all are based on the same gibbane structure. The activity of exogenously applied gibberellin varies with the gibberellin type and the plant species treated. The gibberellin commonly applied in plant tissue culture is GA3, also known as gibberellic acid. Gibberellins In tissue culture GA3 has generally been used to stimulate either shoot elongation or the conversion of buds into shoots. GA3 interferes with bud initiation at very early stages of meristem formation, and thereby may reduce shoot production in vitro if given to plant tissue cultures at the shoot bud initiation stage. GA3 generally reduces root formation and embryogenesis in vitro. For practical usage, it is necessary to optimize the stage-specific effects of GA3. Gibberellins Stimulation of shoot elongation by gibberellin can be observed easily with tobacco shoots. In some varieties of melon and squash, following induction of shoots on medium with BA (5 µM), shoot elongation is best stimulated by a medium with a reduced cytokinin concentration (0.5 µM) and the addition of GA3 (1.5–3 µM). The reduction of the cytokinin concentration permits buds to elongate, aided by the cell elongating effect What is the take-home message from today’s lecture. Let’s discuss. Any more questions?