Plant Hormones Lecture 6 PDF

Lecture 6 Regulation of Developmental and Differentiation Hormones by Assoc. Prof. Dr. Normi Mohd Yahaya Learning outcomes At the end of the lecture, students should be able to: 1. Lists the plant growth regulators found in plants and explain...

Lecture 6 Regulation of Developmental and Differentiation Hormones by Assoc. Prof. Dr. Normi Mohd Yahaya Learning outcomes At the end of the lecture, students should be able to: 1. Lists the plant growth regulators found in plants and explain their function 2. Discuss the effects of hormones on the development of plants Assoc. Prof. Dr. Normi Mohd Yahaya (FBSB, UPM)  Plant hormones also known as plant growth regulators, phytohormones are chemicals produced by plants that alter growth patterns and/or maintenance of the plant Substance produced in a very low concentration and are mobile, both over short (diffusive) and long (mass-flow) distances. The hormones are regulated in different tissues during the different plant development stages. In general, plant hormones can be divided into 2 groups: a) Endogenous hormones Substance produced by a plant that affects the pattern of growth and development. Production by the plant is regulated by the environment. b) Exogenous hormones Substance applied to the plant that alters growth and development in the same way that endogenous substances do Assoc. Prof. Dr. Normi Mohd Yahaya (FBSB, UPM)  Plant hormones Most hormones have multiple effects in plants, i.e.: control cell activities by sending chemical signals or messages to cells to do something or to not do something, including activating the genes that code for specific enzymes or blocking gene transcription inhibit as well as promote cellular activities most often regulate division, elongation and differentiation of cells help in regulation of the plant body by responding to the various signals from the plant and environment. Assoc. Prof. Dr. Normi Mohd Yahaya (FBSB, UPM)  Auxin Chemically, auxin is indoleacetic acid (IAA), which is synthesized from indole or tryptophan. There are a number of synthetic “auxins” too, e.g. NAA, 2,4- D Auxins move mainly from apex (top) to down Influences plant growth and development Functions: a) Stimulation of cell elongation; cell division in cambium, differentiation of phloem and xylem, root initiation on stem cuttings, lateral root development in tissue culture b) Delaying leaf senescence Assoc. Prof. Dr. Normi Mohd Yahaya (FBSB, UPM)  Callus formation Plant growth regulators in the tissue culture medium can be manipulated to induce callus formation. And subsequently changed to induce embryos to form from the callus. Assoc. Prof. Dr. Normi Mohd Yahaya (FBSB, UPM)  Auxin c) Suppression of lateral bud growth when supplied from apical buds – apical dominance Auxin produced in apical buds tends to inhibit the activation of buds lower on the stems. Auxin promotes synthesis of strigolactones, growth regulators that repress lateral bud growth. If shoot tip is removed, the auxin level behind the lateral buds is reduced and the lateral buds begin to grow d) Inhibition or promotion of fruit and leaf abscission through ethylene stimulation Fruit setting and growth is induced through auxin in some plants Development of fruits without fertilization is known as parthenogenesis and the fruit is said to be parthenocarpic fruit The auxin induced fruits, besides seedless, they are larger in size and sweeter. Auxin can delay fruit ripening Assoc. Prof. Dr. Normi Mohd Yahaya (FBSB, UPM)  Auxin e) In Bromeliads, the auxin hormone promotes flowering Stimulation of flower parts, femaleness of dioecious flowers and production of high concentration of ethylene in flowering plants f) Phototropism Wellcome Trust https://www.youtube.com/ watch?v=HR9KHW-e0pY Assoc. Prof. Dr. Normi Mohd Yahaya (FBSB, UPM)  Cytokinin are derivatives of the purine adenine When auxin is added to a tissue culture medium, are found in actively dividing tissues of seeds, fruits, cells in the absence of cytokinins enlarge, but leaves and root tips, and wound tissue sites division does not take place. Promotes cell division When kinetin and IAA are both added to a tissue Roots supply cytokinins upward to the culture medium, cells divide rapidly producing shoots. Cytokinins migrate from roots to the shoot many small cells. systems of plants in xylem tissue (acripetal) The ratio of cytokinins to auxin controls what If present during the cell cycle, cytokinins promote tissues differentiate in the tissue culture. cell division by speeding up the progression from the G2 phase to the mitosis phase. Functions: a) The effects of cytokinins are often studied in tissue culture. Normally, cytokinins work in conjunction with auxins, which promote elongation and cell expansion. Assoc. Prof. Dr. Normi Mohd Yahaya (FBSB, UPM)  Cytokinin Functions: b) Can promote axillary (lateral) bud growth by over-riding the apparent inhibiting effect of auxin. This is one of the ways in which plants balance root and shoot growth. c) retard leaf senescence, probably by stimulating RNA and protein synthesis and delaying degradation of chlorophyll. d) Can promote germination in some seeds in the absence of light, when the seeds normally require light for germination Assoc. Prof. Dr. Normi Mohd Yahaya (FBSB, UPM)  Gibberellin Named after the fungus that produced it (Gibberella fujikuroi) Most GA produced by plants are inactive, apparently functioning as precursors to active forms. Are a family of compounds Biosynthesis of GA takes place in proplastids or plastids or both. Accelerates the rate of cellular metabolic pathways such as respiration, protein synthesis etc. Functions: a) Gibberellins signal germination activities - important in breaking dormancy of the seed after imbibition of water by the seed coat. During the germination of cereal grains, GA is known to promote mobilizing the food materials i.e. starch, for the growing embryo. It plays a significant role in metabolizing lipids to glucose and then to sucrose Assoc. Prof. Dr. Normi Mohd Yahaya (FBSB, UPM) Assoc. Prof. Dr. Normi Mohd Yahaya (FBSB, UPM)  Gibberellin b) Gibberellin works with auxins to promote rapid elongation and division of stem tissue. Gibberellin CAN OVERCOME GENETIC DWARFISM! Dwarf plants can be induced to grow tall by applying gibberrelins! But it is not heritable. c) Can stimulate parthenocarpic fruit In the grape industry, GA is used to grow larger grapes with longer internodes d) Gibberellin induce flowering in some species Many biennial plants (plants that grow one year, overwinter and then flower the second year) and "winter crops" such as winter wheat require a cold period to flower. An exposure to cold that stimulates flowering is called vernalization. Different plants have requirements for different vernalization temperatures and periods. Application of exogenous GA, can induce early flowering Assoc. Prof. Dr. Normi Mohd Yahaya (FBSB, UPM)  Abscisic acid (ABA) a natural plant growth retardant This involves signal transduction pathways with often serves as an antagonist to the action of calcium secondary messengers. gibberellin and auxin ABA in this case originates in roots, and detects partially produced via the mevalonic pathway in low water level in root tissues. ABA moves chloroplasts and other plastids upward into leaves and activates stomatal biosynthesis primarily occurs in the leaves closure. The transport of ABA can occur in both xylem and phloem tissues. It can also be translocated through paranchyma cells. ABA is capable of moving both up and down the stem- does not exhibit polarity like auxins Functions: a) Stimulates the closure of stomata (water stress brings about an increase in ABA synthesis) enhances drought resistance ABA is also referred to as the stress activity hormone ABA promotes stomata closure during leaf water deficit conditions by activating K+ ion transport out guard cells. Assoc. Prof. Dr. Normi Mohd Yahaya (FBSB, UPM) Assoc. Prof. Dr. Normi Mohd Yahaya (FBSB, UPM)  Abscisic acid (ABA) Functions: b) Inhibits shoot growth but will not have as much affect on roots or may even promote growth of roots. c) Induces seeds to synthesize storage proteins. Inhibits the affect of gibberellins on stimulating de novo synthesis of α-amylase. d) Has some effect on induction and maintenance of dormancy. ABA levels are high when seeds mature, promoting lowered metabolism and synthesis of proteins needed to withstand the dehydration associated with dormancy. Seeds germinate when ABA is degraded by some environmental action. Eg. Temperate area plants have ABA degraded by light- stimulated enzymes. Breaking dormancy is relative to the ratio of ABA (which keeps seeds in dormancy) and gibberellins (which promote germination) Assoc. Prof. Dr. Normi Mohd Yahaya (FBSB, UPM)  Abscisic acid (ABA) Functions: e) Low levels of ABA in maturing seeds promote premature germination. Mangrove seeds naturally have low levels of ABA and germinate while still attached to the shoot system. Certain mutations affecting ABA synthesis may result in seeds germinating prematurely f) Induces gene transcription especially for proteinase inhibitors in response to wounding which may explain an apparent role in pathogen defense Assoc. Prof. Dr. Normi Mohd Yahaya (FBSB, UPM)  Ethylene (C2H4) sole plant growth regulator known that is a gas (at room temp). Ethylene is produced in many plant organs, wounded or bruised tissues, though its effects are most studied in fruits. Ethylene is synthesized from the amino acid methionine. Ethylene synthesis High auxin concentrations promote an intermediate step in this pathway that activates an enzyme in the tonoplast (vacuole membrane) to convert the intermediate into ethylene. Toxic substances such as air pollutants also trigger the intermediate step. Assoc. Prof. Dr. Normi Mohd Yahaya (FBSB, UPM)  Ethylene (C2H4) Functions: a) stimulate fruit ripening in a number of species (banana, honeydew melon, and tomato). Ethylene activates ripening in fruits by signaling chemical reactions that degrade the pectins of the middle lamella, softening fruit, and promoting the conversion of stored starches and/or oils into sugars that attract seed dispersers. Some fruit ripening involves a significant increase in the rate of cell respiration with concurrent high O2 uptake. Using gene technology, tomatoes lacking ethylene receptor genes have fruits with a shelf life of 100 days! b) Ethylene is the direct cause of leaf and fruit abscission Ethylene synthesis is related to the decline in auxin production in the leaf. Ethylene promotes the degradation of the cell walls in the abscission zone cells. The declining levels of auxin in leaves signal the production of ethylene in the abscission zone cells as summer ends. Assoc. Prof. Dr. Normi Mohd Yahaya (FBSB, UPM)  Types of hormones and functions Ethylene (C2H4) Functions: c) Sex expression Ethylene promotes female flower production in some members of the Cucurbitaceae, whereas high gibberellins may promote formation of male flowers. d) Degreening of citrus (oranges, lemons and grapefruit) ethylene gas breaks down chlorophyll and lets colors show through Assoc. Prof. Dr. Normi Mohd Yahaya (FBSB, UPM) Neural Academy Plant Hormones https://www.youtube.com/watch?v=8Ji3g4yp4VE FuseSchool - Global Education Plant Growth: Auxins and Gibberellins https://www.youtube.com/watch?v=EZ5tU45Ti_g FuseSchool - Global Education Uses of Plant Hormones https://www.youtube.com/watch?v=TTLgTIipmA8 Assoc. Prof. Dr. Normi Mohd Yahaya (FBSB, UPM)