“Hormone” was first used to describe substances in animals: “a substance produced in a gland that circulates in the blood and has an effect far away from the site of production” In plants used to mean a compound that acts at low concentrations to affect growth and development. Produced in one part of the plant and utilized in another part.

Five plant hormones known by the mid 1960s, new compounds called plant growth regulators. The five classical hormones Auxins Cytokinins Gibberellins Ethylene Abscisic acid Other plant growth regulators Brassinosteroids Salicylic acid Jasmonic acid Systemin Florigen

Hormones Class Endogenous Hormone Growth Regulators auxin indoleacetic acid IBA, NAA, 2,4-D, others cytokinin zeatin, zeatin riboside kinetin, BA, 2iP, TDZ gibberellin GAx...125 GA3, GA4+7 abscisic acid abscisic acid (ABA) ethylene ethylene Ethephon, Ethrel

Frits Went’s experiments

Auxin (indoleacetic acid) Produced in apical and root meristems, young leaves, seeds in developing fruits cell elongation and expansion suppression of lateral bud growth initiation of adventitious roots stimulation of abscission (young fruits) or delay of abscission hormone implicated in tropisms (photo-, gravi-, thigmo-)

Auxin Indole acetic acid and related molecules Photo-and gravitropism The shoot hormone, made in the shoot apex Travels down the stem Polar Auxin Transport

Auxin promotes rooting

Auxin also: Promotes apical dominance Prevents leaf abscission Enhances fruit growth Auxin from the developing seeds results in fruit growth

Auxin-like growth regulators indolebutyric acid (IBA) 2, 4 dichlorphenoxyacetic acid (2,4D) 2, 4, 5 trichlorophenoxyacetic acid (2, 4, 5 T) picloram

Napalm, Agent Orange, dioxin

Herbicide 2,4-D – 2,4 dichlorophenoxy acetic acid Causes a plant to grow itself to death More readily absorbed by broad-leaved plants Most often the “weed” of ‘Weed and Feed’ lawn fertilizers

Commercial uses - auxins stimulation of adventitious rooting 2,4-D as a herbicide for dicots sprout prevention in pruned trees fruit thinning or fruit holding depending on stage of development

Cytokinin Cytokinin – the root hormone Discovered by Folke Skoog The last unknown compound needed to get plant cells to grow undifferentiated Cytokinins delay and even reverse senescence Release buds from apical dominance

Cytokinin (zeatin, ZR, IPA) cell division factor stimulates adventitious bud formation delays senescence promotes some stages of root development

Cytokinins Cell division and cytokinesis - Moves in xylem sap - Stimulates RNA and protein synthesis - Works in conjunction with auxin b. Apical dominance - Cytokinin and auxin are antagonistic- auxin from terminal bud causes shoot to lengthen - Cytokinins from roots stimulate axillary bud -Auxin stimulates lateral root formation cytokinins restrain it - Anti-aging - slows leaf deterioration

Commercial uses - cytokinins Applied as kinetin, benzyladenine, or zeatin conjugates axillary bud growth in orchids, daylilies antioxidant (browning preventer) in cut salads mix with GAs as fruit size stimulator

Gibberellins A large family of compounds (80) with a few biologically active members Produced in roots and young leaves Now known to be essential for stem elongation Dwarf plant varieties often lack gibberellins

Gibberellins Found as the toxin produced by some fungi that caused rice to grow too tall – “Foolish Disease” Induces parthenocarpic fruit development Delays senescence

Gibberellins are involved in bolting of rosette plants Gibberellins are used to improve grapes Gibberellins are involved in seed germination gibberellins will induce genes to make enzymes that break down starch

Commercial uses - GA inhibitors Cycocel, Bonsai, Sumagic height control in flowering pot plants (lilies, orchids) height control in bedding plants

Commercial uses - GA application Apply as GA3, or GA4+7 increase flower size on certain ornamentals (eg, “gibbing” camellias) increase berry separation and size in bunch grapes overcome shallow dormancies in vegetative buds stimulate seed germination

Ethylene The smallest hormone A gas Important in seed germination, fruit ripening, epinasty, abscision of leaves Sex expression in cucurbits

Ethylene Gaseous hormone produced in many plant tissues autocatalytic (stimulates its own production) volatile gas production stimulated during ripening, flooding, stress, senescence, mechanical damage, infection product of combustion of petrochemicals

Commercial uses - ethylene application Applied as ethylene gas or Ethephon or Ethrel sprays flower initiation (bromelliads, pineapples) stimulation of ripening (bananas, tomatoes) degreening of citrus abscission induction prior to mechanical harvest (cherries) increased color development in once-over harvested processor type tomatoes

Commercial uses - ethylene inhibition Removed by chemical “scrubbing” or low atmosphere long term storage of apples in CA storages treatment of cut flowers with silver thiosulfate long-keeper (delayed ripening) mutants (or rDNA) of tomato hypobaric storage of many fruits, vegetables, flowers

Abscisic acid (ABA) Found in stressed leaves, dormant seeds, dormant buds stomatal closure inhibits germination of some seeds inhibits active growth of axillary buds

Abscisic acid Incorrectly named, not related to abscission Important in water stress and other stresses Causes stomatal closure Prevents premature germination of seeds Changes gene expression patterns