Plant Hormones and Their Effect

24.3

Hormones

• A hormone is a chemical signal that affects growth, activity or development in a living organism

• A plant hormones control development, but also coordinate responses to environment.

How do hormones work?

• Specific target cell that have a receptor for the hormone– Response depends on

the receptors

Auxin

• Auxin– Stimulate cell elongation– Promote growth of roots– Regulate division in

meristem

• Apical Dominance– Prevent growth of buds

closer to the stem’s tip

• Also play a role in phototropism– Bending towards a light

source

Cytokinins

• Produced in growing roots and developing seeds and fruits

• Stimulate cell division• Regeneration of

damaged tissue• Delay aging of leaves

• Usually have the opposite affect of Auxin– Auxin: made in shoots,

sent to roots– Cytokinins: made in

roots, sent to shoots– Helps to restore lost

organs

More Hormones

• Gibberellins– Stimulate growth

• Dramatic increase is size, specially in stems and fruits

• Abscisic Acid– Controls seed dormancy until a shift in hormones

brought on by environmental conditions– Inhibits the cellular division

• There opposite affect helps to control homeostasis

Ethylene

• In the form of a gas

• Stimulates fruits to ripen

• Helps to “seal off” and drop organs that are no longer useful for the plant

Tropisms

• Tropism – a response to the environment– Phototropism– Gravitropism– Thigmotoropism

Phototropism

• Tendency to grow towards light

• Auxin in responsible– Migrate towards shaded

area

• Seedlings can quickly change to grow in opposite directions

Gravitropism

• Shoots grow upward, roots grow downward

• Auxin is also responsible for this

• Still not understood why auxin migrates to the areas that cause this

Thigmotoropism

• Response to touch

• Plant will wrap around an object to help support its growth

• Vines and climbing plants– Tomato plants and wind

Rapid Response

• Some responses are extremely quick– Ex: Mimosa pudica• “sensitive plant” – fold together in seconds when

touched

– Ex: Venus flytrap• Close when an organism lands inside• Triggers sensory to close the leaves

Response to Seasons

• Plants flower according to photoperiod– Length of daylight– Photoperiodism

• Major factors in the timing of seasonal activities– Flowering and growth

• Phytochrome – absorb red light and different signaling pathways within the plant– Not completely undestood

Photoperiodism

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Winter Dormancy

• Deciduous trees stop photosynthesis, transport important material to roots and seal leaves off from rest of plant– This is why leaves drop

• Phytochrome absorbs less due to day length less auxin produced ethylene production increases

Winter Dormancy

• Because of the change in hormone levels– Chlorophyll breaks down and pigments that were

there but were not displayed become more visible• Yellow and red pigments

• This is why leaves change color. What hormone helps them fall off?– Ethylene

Winter Dormancy

• More effects:– Meristems produce protecting scale over buds• Waxy coating

– Xylem and Phloem are occupied by many ions and organic compounds• Anti-freeze• Prevents sap from freezing