Plant Hormone Notes

Plant Hormones

Animal Biology

• Next week, Andrew will discuss animal biology.

• He is nearing the end of his Ph.D. with successful experiments.

• He specializes in livestock research.

• Andrew is an excellent resource for those interested in agricultural science.

Crop Agronomy Research

• For crop agronomy research, consult the instructor.

Animal Research

• For animal research, speak with Andrew.

Plant Hormones Overview

• Plant hormones are similar to animal hormones with a few differences.

• The lecture will cover:

  • The classic five plant hormones

  • Plant behavior, focusing on red light sensing and flowering time.

Signaling Systems

• Plants respond to the world around them.

• Any signaling system has three parts:

  • Perception: Receiving an environmental signal.

  • Transduction: Transmitting and amplifying the signal.

  • Response: A developmental or behavioral adjustment to the signal.

Human Example

• Perception: Seeing a bear (visual signal).

• Transduction: Signal travels through the central nervous system/brain.

• Response: Brain tells muscles to run away.

Plant Example

• Perception: A herbivore (giraffe) eating leaves, causing a wounding signal.

• Transduction: Leaves create a volatile enzyme that travels through the air to surrounding leaf tissue.

• Response: Leaves produce tannins that make them taste unpleasant to the giraffe.

  • Tannins are also found in tea and red wine.

Plant Signaling

• Plants lack specialized organs for signal perception, unlike animals.

• Every plant cell is prepared to receive and respond to signals.

• Plants respond to:

  • Internal cues (e.g., age, triggering flowering).

  • External cues (e.g., cold, sunshine).

• Plant signaling follows the same three stages as other organisms.

  • Perception: leaf perceiving light.

  • Transmission: signal travels from the leaf to the growing tip/shoot apical meristem.

  • Growth regulation: shoot apical meristem changes growth pattern to grow towards the light.

• In plants:

  • Perception is usually done through pigments (e.g., chlorophyll).

  • Transmission is often through hormones.

  • Growth regulation is known as tropisms.

Plant Hormones - Classic Five

• Small, simple molecules that are diverse.

• Not produced in specialized glands (unlike animal hormones).

• Generally produced throughout the plant, though some are localized.

• Have protein receptors on target cells.

• Small amounts can have a large effect.

• Two broad groups:

  • Growth promoting (involved in growth direction and encouragement).

  • Stress response (stops growth under stress).

The Classic Five

• Auxins

• Ethylene

• Cytokinins

• Gibberellins

• Abscisic acid

Auxins

• Charles Darwin identified something in the tip of a plant that enabled it to respond to light.

  • Seedlings grow towards the light.

  • If the top of the coleoptile is cut off, it doesn't curve towards the light.

  • If a cap is put on top, it also doesn't curve.

• If the tip is moved to one side without changing the light, curvature occurs.

• There is something happening in the tip that causes it to grow in a particular direction, moving from the tip down the stem.

• Phototropism: Plants grow in the direction of the light source.

• Plants also respond to gravity.

Tropisms

• Phototropism: growing in the direction of a light source.

• Gravitropism: Plants responding to gravity.

  • Auxin is involved in both.

• The key is that auxin molecules are not evenly distributed across the tip.

  • Auxin is made in the shoot apical meristem.

  • If light is perfectly overhead, auxin is evenly distributed.

  • If light is to one side, auxin is actively transported to the shady side.

  • This causes cells in the shade to elongate faster, pushing the growing tip towards the light.

• Auxin accumulates in the shady side.

Gravitropism

• Plants perceive gravity using statoliths, which are dense starch-filled plastids in the root cap cells.

• Statoliths sink to the bottom of the cell, indicating the direction of gravity.

• Roots exhibit positive gravitropism (grow towards gravity).

• Shoots exhibit negative gravitropism (grow away from gravity).

• Auxin stimulates tissue growth in the desired direction.

Apical Dominance

• Trees tend to grow from the apex.

• Auxin produced at the shoot apex travels down the stem and inhibits the growth of lateral buds/axial buds.

• Chopping off the apical bud eliminates the auxin production, allowing lateral buds to grow.

• This is why pruning works.

• The further away from the apex, the more lateral growth you have.

Auxin Functions

• Regulates numerous processes:

  • Embryo development

  • Root development and lateral root growth

  • Fruit growth development

  • Gravitropism and phototropism

• Associated with rapidly dividing and growing tissue (shoot apical meristem, new leaves, developing fruits).

• Synthetic auxins are used as herbicides (e.g., 2,4-D, dicamba).

  • More active against broadleaves because grasses can inactivate it.

Ethylene

• Very simple molecule.

• History:

  • Ancient Chinese farmers used incense in closed rooms with pears to ripen them (incense gives off ethylene).

  • Ancient Egyptians slashed figs to promote ripening of remaining figs.

  • Whale oil lamps in 19th-century London may have caused growth abnormalities in plants due to ethylene emission.

Ethylene Properties

• Volatile, affecting both the plant and nearby plants.

• Predominantly a growth hormone, but also related to stress (associated with tissue death).

• Associated with fruit ripening, leaf abscission and flower senescence. Induced by wounding.

Fruit Ripening

• Fruits are categorized as climacteric or non-climacteric based on their ethylene response.

• Climacteric fruits: ripening is accompanied by a burst of ethylene. Example: bananas.

• Non-climacteric fruits: ethylene is still important but no burst occurs. If picked unripened they may just rot instead of ripening.

• Synthetic ethylene precursors can be used in cotton to initiate fruit ripening and defoliation.

Ripening Fruits

• To ripen fruits like avocados, mangoes, or pears, place them in a fruit bowl or paper bag with a banana or apple (which produce high levels of ethylene).

• "One bad apple spoils the bunch" is due to wounded apples producing ethylene, causing others to ripen/rot faster.

Gibberellins

• Identified as the cause of foolish seedling disease in rice, caused by the fungi Gibberella fujikuroi.

• The fungal infection causes plants to grow unusually tall.

• Gibberellins stimulate cell division and elongation, particularly in the internode (the space between nodes).

Green Revolution

• Breeding for reduced plant height in wheat and rice.

• Dwarf varieties:

  • Rice: reduced gibberellic acid biosynthesis.

  • Wheat: reduced sensitivity to gibberellic acid.

• If sprayed with gibberellic acid, rice would grow tall, but wheat would not. Therefore, the rice variety is simply not making enough of the growth hormone to reach full size.

Agricultural Importance

• Shorter plants mean less carbohydrate is wasted on the stem, leading to greater yield and reduced lodging (falling over).

Gibberellin Functions

• Stem elongation (plant height).

• Fruit development, including parthenocarpy (development of fruit without seeds) in seedless grapes.

• Stimulates seed germination.

Cytokinins

• Involved in cell division (cyto- meaning cell, -kinin from kinesis meaning division).

• Promote cell division and growth throughout the plant.

• Promote the growth of auxiliary buds.

• Inhibit the initiation of lateral roots.

• Delay leaf and flower senescence.

• Cytokinins face creams are scams.

Antagonism

• Plant hormones often act in opposition to each other.

  • The balance between hormones determines plant growth.

• Auxin and cytokinin are antagonists.

• Above ground: high auxin, low cytokinin inhibits lateral bud growth.

• Below ground: high cytokinin promotes root apex growth.

Abscisic Acid (ABA)

• Stress hormone involved in response to abiotic stress (drought and salt).

• Produced throughout the plant, highest production in roots.

• Causes the closure of stomata under water stress.

• ABA synthesized in the roots due to drought travels through the stem causing:

  • Stomatal closure

  • Inhibition of photosynthesis

  • Reduced cell division and expansion

  • Ovule abortion and pollen sterility

Dormancy

• ABA inhibits germination.

• Varieties high in ABA germinate slowly, sometimes requiring an external trigger.

• Varieties low in ABA can germinate precociously (seeds germinate while still attached to the spike) or in storage.

• Balance is important.

How Plants Sense Light and Respond Through Flowering Time

• Plants flower in response to day length.

• Short-day plants flower in winter or spring when days are short.

• Long-day plants flower in summer when days are long.

• Some plants are day-neutral.

• Interrupting the nighttime with a burst of light can trick plants into thinking that the night is shorter or longer than it really is.

• Plants perceive light in their leaves via phytochrome, a pigment.

Phytochrome

• Think of phytochrome as a light switch (on or off).

• Inactive form: switch is off.

• Red light (from the sun) turns the switch on (active form).

• Far-red light (shade light, twilight) switches it back off.

• In actual darkness, it slowly reverts back to the inactive form.

Short Day Plants Example

• Require a long nighttime to flower.

• If the day is long and the night is short, it won't flower.

• If you give it a little bit of light in the middle of the night, you trick it into thinking that it's flowering time.

  • Turning the lights off in the middle of the day will not trick a long night period.

Florigen

• Signal must get from the leaf to the shoot apex.

• Florigen is a protein that is not strictly a hormone (they are not small chemicals).

• Sunlight at the leaf activates the florigen signal.

• Florigen moves from the leaf down the stem, up to the shoot apex.

• It tells the apex to stop making leaves and start making flowers.

• It does this by directly regulating gene expression.

Summary

• Plant hormones are growth regulators, small molecules with protein receptors.

• Auxin regulates phototropism and gravitropism.

• Ethylene regulates ripening.

• Gibberellins promote stem elongation.

• Cytokinins are antagonistic with auxin.

• ABA promotes dormancy.

• Plants flower in response to light.