Plant hormones Plant Hormones –Organic compounds produced in low concentrations –Produced in one part of plant (i.e. source) –Transported to another part of plant (i.e. target) –Cause physiological or developmental responses (stimulatory or inhibitory) Also called plant growth regulators or phytohormones

Plant Hormones Major types of plant hormones: –Auxins- most common is IAA or indole –acetic acid –Gibberellins –Abscisic acid ABA –Cytokinins –Ethylene

Plant Hormones Auxins –Types of auxins used in horticulture Natural auxin is indole-3-acetic acid (IAA) –IAA is broken down in sunlight Synthetic auxins used in horticulture: 2, 4-dichlorophenoxyacetic acid (2, 4-D) –Site of auxin production in plants: Shoot and root tips (apical meristems) Young, expanding leaves Young seeds

Plant Hormones Auxins –Action of auxins in plants: Stimulate cell elongation –Auxins increase the length of most plant cells and thereby contribute to the growth and elongation of the plant. Also cause a bend toward light - positive phototropism- because more auxin remains in cells on opposite side of light source Promotes apical dominance –Pinching bud removes source of auxin and releases axillary buds from apical dominance Promotes growth of adventitious roots- roots in uncommon place – Eg. geranium cuttings, transplanting.

Plant Hormones Auxins –Actions of auxins in plants Promotes or inhibits abscission of leaves, flowers and fruit –NAA (synthetic auxin) applied shortly after bloom is used to thin apples –NAA applied during fruit development inhibits abscission of apples Auxin concentration determines if action is stimulatory or inhibitory –Low concentration promotes adventitious root growth, but high concentration inhibits root growth of cuttings –2, 4-D is used as herbicide by applying at high concentrations

Plant Hormones Gibberellins Several different gibberellins (GA) produced by plants –Large, complicated molecules not synthesized –Commercial gibberellins produced by fungus –Site of gibberellin production in plants: Shoot and root tips (apical meristems) Young, expanding leaves Embryos Fruits Tubers

Plant Hormones Gibberellins –Action of gibberellins in plants: Stimulate cell elongation –Dwarf plants treated with gibberellins produce normal growth –Applied to grapes to elongate the peduncle (stem of flower cluster) and pedicels (stem of single flower), making looser cluster Promotes cell division in vascular cambium Promotes seed germination –causes production of amylase enzyme that breaks down starch into energy needed for growth –Used by beer brewers to stimulate sugar production in barley malt (sugar is converted into alcohol during fermentation) Influences flower and fruit development

Plant Hormones Abscisic Acid (ABA) –Similar structure as gibberellins –Site of abscisic acid production in plants: All organs (e.g. roots, leaves, stems, fruits) –Actions of abscisic acid in plants Counteracts effects of auxins and gibberellins Maintains dormancy in seeds and buds Stimulates guard cells to close stomates (to conserve water) source of ABA are the spongy mesophyll cells

Plant Hormones Ethylene has been used in practice since the ancient Egyptians, who would gas figs in order to stimulate ripening. –Site of ethylene production in plants: Throughout plants –Actions of ethylene in plants Inhibits root and shoot elongation by blocking transport of auxins from apical meristems In addition to causing fruit to ripen, it can cause plants to die. It can be produced when plants are injured, either mechanically or by disease. Ethylene will cause a wide range of effects in plants, depending on the age of the plant and how sensitive the plant is to ethylene.

Plant Hormones Cytokinins –Types of cytokinins used in horticulture Several natural forms Synthetic cytokinins used in horticulture: –Benzyladenine (BA) –Site of cytokinin production in plants: Embryos Young leaves and fruit Apical meristems of roots

Plant Hormones Cytokinins –Actions of cytokinins in plants Promotes cell division (cytokinesis) Contributes to cell enlargement (in leaves) Stimulates differentiation of cells (with auxins) –High cytokinin and low auxin promotes shoot initiation in tissue culture, whereas reverse combination promotes root formation –Moderate levels of both hormones promotes callus growth Delays senescence =changing of colour in leaves (maintains and promotes synthesis of chlorophyll)

Plant Hormones Ethylene –Actions of ethylene in plants Induces adventitious root formation by blocking (and accumulating) auxin at tip of stem cutting Enhances flow of latex in rubber trees Stimulates abscission of leaves and fruit –Used as a harvest aid for cherries Promotes fruit ripening (apple, tomato, citrus, coffee) Promotes senescence (aging) of flowers Enhances flowering in pineapples