Plant Hormones: Auxins

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Plant Hormones: Auxins

• Auxins derive their name from the Greek word "auxein," meaning "to grow."
• They were initially isolated from human urine.

Types of Auxins

• Synthetic: Examples include NAA (Naphthalene Acetic Acid), 2,4-D (2,4-Dichlorophenoxyacetic acid), and 2,4,5-T.
• Natural: Includes IAA (Indole Acetic Acid) and IBA (Indole Butyric Acid).
• IBA is naturally occuring and synthetic.
• Tryptophan is a precursor of auxin.

Uses of Auxins

• Initiates rooting in stem cuttings while inhibiting root growth.
• Prevents abscission (shedding) of young flowers, leaves, and fruits but promotes abscission of mature leaves, flowers, and fruits.
• Auxins promote flowering, such as in pineapples.
• It promotes apical dominance that favors the growth of the apical bud while inhibiting the activity of axillary buds.

Auxins as Weedkillers

• Auxins have been used as Agent Orange in the US - Vietnam War resulting in killing dicots (broadleaf weeds).
• Applied in tea plantations and for hedge making.
• Promotes parthenocarpy, for example, in tomatoes.
• Used as a weedicide or herbicide that kills dicots.
• Auxins, such as 2,4-Dichlorophenoxyacetic acid, are ineffective against monocots (grasses and cereals).
• Promotes xylem differentiation.

Gibberellins

• There are over a hundred types of gibberellins that have been discovered.
• GA3 is one of the most studied & most common forms.
• GA3 was one of the first to be discovered.

Functions of Gibberellins

• Increases the length of the axis of a plant, such as a stem.
• Increases the length of the internode of sugarcane.
• Speeds up the malting process in the brewing industry.
• Promotes seed germination by promoting the release of α-amylase.
• Delays senescence (aging).
• Causes fruits to elongate and improves its shape.
• Promotes maturity period in juvenile conifers allowing early seed production.
• Causes bolting in rosette plants like beet and cabbage, which is the stem elongation.
• Breaks seed dormancy.

Cytokinins Discovery

• Cytokinins were discovered as kinetin from autoclaved herring fish sperm DNA.
• Kinetin is not naturally present in plants.
• In plants, kinetin-like substances are isolated from corn kernels and coconut milk.

Cytokinin Functions

• Cytokinins promote cytokinesis, which is cell division.
• They initiate shoot formation during callus culture in morphogenesis.
• In culture, cytokinins promote shoot growth first.
• In seeds, cytokinins promote root growth first.
• Cytokinins help in the development of new leaves and chloroplasts in new leaves.
• They promote lateral shoot growth and adventitious shoot formation.
• Cytokinins overcome apical dominance and act as antagonists to auxin.
• Delays leaf senescence by promoting nutrient mobilization (Richmond-Lang effect).

Abscisic Acid (ABA)

• A general plant growth inhibitor and an inhibitor of plant metabolism.
• ABA promotes seed and bud dormancy.

Ethylene

• Stem elongation in deep water rice plants is enhanced by Ethylene.
• Causes closure of stomata.
• Promotes abscission (shedding of leaves, fruits, and flowers).
• ABA acts as an antagonist to GA (Gibberellic Acid).

Ethylene functions

• It promotes the ripening of fruits by increasing the rate of respiration known as climacteric respiration, resulting in climacteric fruits.
• Promotes horizontal growth of seedlings, swelling of the axis, and apical hook formation in dicot seedlings.
• Promotes abscission and senescence of leaves and flowers.
• Promotes seed germination and sprouting of potato tubers by breaking seed and bud dormancy.
• Causes stem elongation and elongation of internodes and petioles in deep-water rice plants.
• Promotes root growth and root hair formation which is antagonistic to auxin's role.
• Initiates flowering and synchronizes fruit set up.
• Induces flowering in mangoes.
• Promotes the number of female flowers in cucumbers.
• Promotes abscission and causes thinning of cotton, cherry, and walnut.
• Ethylene is one of the most used plant hormones in agriculture, available in the form of ethephon.