BPSC104 Class 12

Plant hormones are also called plant ____?

growth regulators

Plant hormones can either ____ or _____ a process

promote; inhibit

Depends on:

• Process

• Hormone

  • if same plant hormone, can produce a different response in different tissues or at different times

Plant hormones rare act ____

alone

Hormones:

• Hormones that activate the synthesis or other hormones

• Hormones that oppose the activity of another hormone

Most hormones:

• Usually a combination of specific hormones produces a response

3 Major steps to a plant/animal to stimulus

1. Perception

2. Transduction

3. Response

Perception

Step 1 of plant or animal responding to stimulus

• Sensing the stimulus

Transduction

Step 2 to a plant or animal responding to a stimulus

• Changing the stimulus to a form that can be communicated to other cells and parts of the plant

  • A hormone or other signaling molecule moves through vasculature or from cell to cell

Response

Step 3 to plant or animal responding to stimulus

• Change in:

  • Growth

  • Morphology

  • Physiology (resulting in gene expression)

The 3 steps to plants and animals reacting to a stimulus (at the cellular level)

1. Perception / Reception

2. Transduction

3. Response/Induction

Signals are transmitted across ____

• Cells

• Tissues

• Organs

Perception/ Reception

Step 1 to plant or animal responding to stimulus at cell lvl

• Receptor molecule detects stimulus in the cell membrane

Transduction

Step 2 to plant or animal responding to stimulus at cell lvl

• Receptor transfers the signal to a secondary messenger within the cell

Response/induction

Step 3 to plant or animal responding to stimulus at cell lvl

• Secondary messengers direct a change in gene expression that will then produce a change in cell function

(Major) Plant hormones

• Auxin

• Cytokinin

• Ethylene

• Abscisic acid

• Gibberellins

• And others

Auxin (IAA)

• Apical dominance

• Adventitious rooting

• Tropic responses

Cytokinin

• cell division

• Shoot production in tissue culture

• Delay or leaf senescence

Gibberellin (GA)

• Stem elongation

• Release from seed dormancy

• Stimulation of flowering

Abscisic acid

• Stomatal closure

• Stress resistance

• Induction of dormancy of seeds and buds in certain gas

Ethylene (gas)

• Abscission (leaves and fruits)

• Fruit ripening

Phototropism (plant response)

• growth of the shoots toward light (bending toward light)

• Influence is produced in the shoot tip and transmitted down the stem

(darwin decided that the influence was in the shoot tip)

If light comes from the top, there is no influence

If light comes on the shady side: it will promote these cells to elongate,

• longitudinal elongation (leads to bending to light)

• differential cell elongation - these cell are elongating these cells are not

Gravitropism

shoot will elongate and respond to gravity and shoot will go upwards

Phototropism

on the shaded side

Negative gravitropism

cell elongation and allow shoot to move upward

Think:

• hormone can have different responses depending on different tissues

Auxin

• Synthesized in baby leaves, young primordia

  • Developing seeds and roots

Auxin functions in ____

• Tropisms

• Apical dominance

• Vascular tissue differentation

• Inhibittion of leaf and fruit abscission

• Phyllotaxis (does it attach opp or same, 2 leaves per node etc.)

• Lateral root development

• Many developmental processes in roots and shoots

What happens to the plant if the apical bud is removed?

• The source of auxin is removed, axillary buds are no longer repressed

(If removed, axillary buds are now giving rise to new shoots)

Auxin promotes ___ development

Fleshy fruit

Baspiteal

Polar transport is always this

can mean both directions

• away from shoot tip (down)

• away from root tip (upward)

Nonpolar or acropetal (base of tip) transport

• can also occur

Cytokinin

• work together to promote plant growth in tissue culture

• Callus - forms when a piece of plant tissue is placed on culture media (mass of undifferenitated cells)

Callus

groups of undifferentiated cells

• ready to be differentiated

• Can be induced to form roots and/or shoots

• New plants are clones of the original

Auxin promotes cell ___ and __ formation in tissue culture

Elongation; root;

Cytokinins promote ___ and ___ formation in tissue culture

cell division; shoot

Ratio of auxin:cytokinin (determine fate of callus)

• determines root vs shoot formation in tissue culture (determine fate of callus - undiff cells)

• High auxin/cytokinin = root formation

• High cytokinin/auxin - shoot fomration

• Balanced = undifferentiated (callus) proliferation

Ethylene background information

• Promotes growth of most tissues

• Inhibits cell elongation

• Promotes fruit maturation

• Promotes senescence and abscission (elongation)

• Can inhibit OR promote cell division

• Synthesized in response to stress or damage

Ethylene (inhibits vertical cell elongation) causes

Triple response - seedlings grown in the dark show this in response to ethylene

Causes:

• Decreased vertical growth (shorter)

• Increased lateral growth (wider)

• Horizontal growth (curve)

Takeaway: Short, thicker, curved helps to protect shoots pushing through the soil to get light after germination

Ethylene in promoting fruit ripening

• Climacteric fruits ripen rapidly

  • Cellular respiration - increases dramatically

  • Banana, tomato, avocado

• Spike in ethylene production - precedes ripening and induces the increase of respiration

• Continue to ripen after harvest (climatic fruits)

Ethylene promotes ripening of some fruits

• Important for agriculture and economics

• these fruits can continue to produce ethylene as they ripen

Reduced ethylene production or sensitivity

• facilitates food storage and transport

• Inhibiting ethylene synthesis can prolong the storage life of fruits and vegetables

Abscisic acid (ABA)

slowing things down

• Involved in prventing seed germination

• root to shoot signaling

• Response to drought stress, stomatal closure

• No direct role in abscission

• (Synthesized in mature leaves roots)

Dormin - growth inhibitor found in dormant buds

Abscisin - an accelerator of leaf and fruit abscission

ABA levels

• correlated to seed quiescence and dormancy

• Aba required to maintain dormancy/quiescence

  • Decreasing ABA levels = breaking dormancy

  • Mutant seeds - insensitive to ABA germinate within the fruit

Turogor pressure

• mediated by the vacuole

Gibberellins

• Induces seed germination (opposing ABA)

• promotes stem elongation

• Stimulates cell division and cell elongation

• Can stimulate flowering

Over 136 giberellins

• Produced in seeds and young tissues

• Found throughout the plant body

How are giberellins important in seed germination

• (giberellins important in seeds accessing the endosperm and starch in that endosperm)

Giberellins have ___ and ___ uses

Agricultural; commercial

Mutants that cannot produce of perceive Giberellins are

Dwarfed

How do hormones work?

• Absisic acid is a down regulat or of the genes expression those genes are involved in

• Giberellins are up regulators

Photo

light, daylength

Morphogenic

changing growth pattern of entire plant

Morphogenesis — response to stimulus by changing the growth pattern

Nastic (nasty)

• Response to stimulus

• Not specifically towards or away from stimulus

Tropic (tropism)

• Response to stimulus by growing towards or away from the stimulus

Gravi

gravity, weight

Thigmo

touch (harmful or helpful)

Thermo

temperature (cold or hot)

Chemo

Chemicals

Hydro

water (prescence or absence)

thigmomorphogenesis

thigmo - touch

morphogenic - morphogenesis (plant growth)

Phototropisms

can be positive or negative

• Positive - toward light

• negative - away from

towards or away from light

Gravitropic responses

Starch-stratolith hypothesis*

• Statoliths

Statoliths

amyloplasts (starch-filled plastids) within gravity-sensing cells

In stems:

• In cells surrounding the vascular tissues

In roots:

• In columella cells (Stratocytes) of the root cap

Sedimentation of stratoliths

perceives gravity

Stratolith sedimentation induces differential auxin levels in the root elongation zone

• Auxin moves cell to cell (polar auxin transport)

• Higher auxin levels on lower side inhibits elongation

• Response in roots is opposite to that inshoots

Thigmotropic responses

• shown in both tendrils and roots

• Cells on the side opposite the touch stimulus elongate

  • Tendrils wraps - around the object it has touched

  • Roots grow around rocks and other obstacles

Likely at least partially auxin mediated, but unclear for now