“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 auxin cytokinin gibberellin abscisic acid ethylene Endogenous Hormone indoleacetic acid zeatin, zeatin riboside GA x...125

abscisic acid (ABA) ethylene Growth Regulators IBA, NAA, 2,4-D, others kinetin, BA, 2iP, TDZ GA 3 , GA 4+7 Ethephon, Ethrel

Frits Went’s experiments

Auxin (indoleacetic acid)

Produced in apical and root meristems, young leaves, seeds in developing fruits

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cell elongation and expansion

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suppression of lateral bud growth

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initiation of adventitious roots

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stimulation of abscission (young fruits) or delay of abscission

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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

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indolebutyric acid (IBA)

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2, 4 dichlorphenoxyacetic acid (2,4D)

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2, 4, 5 trichlorophenoxyacetic acid (2, 4, 5 T)

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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

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stimulation of adventitious rooting

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2,4-D as a herbicide for dicots

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sprout prevention in pruned trees

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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)

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cell division factor

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stimulates adventitious bud formation

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delays senescence

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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 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 b.

Commercial uses - cytokinins

Applied as kinetin, benzyladenine, or zeatin conjugates

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axillary bud growth in orchids, daylilies

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antioxidant (browning preventer) in cut salads

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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

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height control in flowering pot plants (lilies, orchids)

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height control in bedding plants

Commercial uses - GA application

Apply as GA 3 , or GA 4+7

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increase flower size on certain ornamentals (eg, “gibbing” camellias)

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increase berry separation and size in bunch grapes

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overcome shallow dormancies in vegetative buds

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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

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autocatalytic (stimulates its own production)

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volatile gas

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production stimulated during ripening, flooding, stress, senescence, mechanical damage, infection

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product of combustion of petrochemicals

Commercial uses - ethylene application

Applied as ethylene gas or Ethephon or Ethrel sprays

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flower initiation (bromelliads, pineapples)

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stimulation of ripening (bananas, tomatoes)

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degreening of citrus

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abscission induction prior to mechanical harvest (cherries)

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increased color development in once-over harvested processor type tomatoes

Commercial uses - ethylene inhibition

Removed by chemical “scrubbing” or low atmosphere

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long term storage of apples in CA storages

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treatment of cut flowers with silver thiosulfate

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long-keeper (delayed ripening) mutants (or rDNA) of tomato

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hypobaric storage of many fruits, vegetables, flowers

Abscisic acid (ABA)

Found in stressed leaves, dormant seeds, dormant buds

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stomatal closure

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inhibits germination of some seeds

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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