Plant Hormones and Sensory Systems

what does a plant's sensory response rely on?

chemical messengers (hormones)

how do hormones travel in plants?

vascular tissue (xylem and phloem) and cell-to-cell via plasmodesmata

how is the production of plant hormones different form animals?

potentially every cell produces plant hormones so they do not have specialized glands

auxin

master growth regulator; responsible for cell elongation in phototropism and gravotropism

phototropism

movement in response to light

gravitotropism

movement in response to gravity

apical dominance

inhibition of lateral bud formation; triggered by auxin in apical meristem; auxin inhibits later buds and cytokinins promote bushier growth

what are some things besides cell elongation and growth does auxin play a role in?

flowering, fruit ripening, inhibition of abscission (leaf falling)

how is auxin related to light?

it is a relay for the effects of blue light and red/far red responses

synthetic auxin

used as rooting hormone to promote growth of roots on cuttings and detached leaves

cytokinins

promote cytokinesis, delay senescence in leaf tissues, promote mitosis, stimulate differentiation of meristem and shoots and roots

where are cytokinins abundant?

growing tissues like roots, embryos, and fruits where cell division is occurring

gibberellins

stem, fruit, and seed growth; shoot elongation, seed germination, and fruit/flower maturation

senscence

aging

where is giberellins produced?

root and stem apical meristems, young leaves, seed embryos

what plant hormone delays senescence in leaves/fruits and helps to break dormancy?

gibberellins

Abscisic Acid

causes abscission of leaves, inhibits stem elongation, induces dormancy in lateral buds/seeds, closes stomata

what causes ABA to accumulate?

response to stressful environmental conditions like cold temp., dehydration, shorter days

how does ABA cause dormancy?

blocks germination and promoting synthesis of storage proteins

why do plants require a long period of cold temperature before they germinate?

protects young plants from sprouting too early during warm weather in winter

what happens to ABA concentration during the spring?

it declines so that the seed is released from dormancy and can germinate

what does ABA do for short-term drought response?

it lowers soil moisture and cases an increase in ABA which causes the stomata to close and reduces water loss

ethylene

aging; promotes fruit ripening, flower wilting, and leaf fall

why is ethylene unusual?

it is a volatile gas

what on the plant produces ethylene?

aging tissues and nodes of stems

how does ethylene induce fruit ripening?

it converts starch and acids into simple sugars

how is ethylene used in agriculture?

used to control timing of ripening of fruit

photoreceptors

protein covalently bonded to a light-absorbing pigment called a chromophore

what behaviors are regulated by light stimuli?

phototropism, stem elongation, germination, photoperiodism

phototropins

chromoproteins responsible for mediating the phototropic response; leaf opening and closing, chloroplast movement, opening stomata to permit gas exchange

phot1 and phot2

phototropins that detect blue light in apical meristem

how does auxin stimulate cell elongation on plants in the shade?

acid growth hypothesis

acid growth hypothesis

Auxin causes cells to activate proton pumps, which then pump protons out of the cells and into the space between the plasma membrane and the cell wall

what does the movement of protons during the acid growth hypothesis do?

- lowers pH which activates expansive (breaks links between cellulose fibers in cell walls to make them more flexible)

- high concentration of protons causes sugars to move into cell which creates an osmotic gradient where water moves into cell causing it to expand

phototropic response steps

1. phot1 and phot2 present in plant apical meristem

2. blue light causes phot1 and phot2 to accumulate auxin in the shaded side

3. auxin promotes cell elongation due to weakening of cell wall and stretching of cell through water

4. plant bends away from shade and toward the light

what does red light promote?

cell elongation/growth bc full sun is on plant

far-red light

indicates that a plant is being shaded out by another plant

does full sunlight have more red or far-red light?

red light so chlorophyll absorbs strongly on red region

do plants on the forest floor experience more red or far-red light?

far red light

phytochromes

chromoproteins responsible for red/far-red light detection; Pr (phytochrome red) and Pfr (phytochrome far-red)

what happens when Pr absorbs red light? what happens when Pfr absorbs far-red light?

it becomes Pfr; it becomes Pr

how does the absorption of red/far-red light affect the chromophore?

the conformation and activity of the phytochrome protein to which is is bounded changes

is Pfr or Pr active?

Pfr

how does the phytochrome promote stem elongation?

it stimulates plant growth toward red light via cytokinin which promotes cell division and gibberellin which promotes stem elongation

what hormone must be there for cytokinin to work?

auxin

how does the phytochrome system regulate seed germination?

red light provides the signal that the seed is ready to grow

what happens with the phytochrome system when the seed is in the shade?

phytochrome is in the Pr (inactive form) so the seed will not germinate

what happens with the phytochrome system when the seed is in the dark but then goes to the light?

Pfr signaling causes transcription of amylase which breaks down starches and seeds stored to allow germination to begin

what hormone regulates a light-independent germination process?

gibberellin

what does gravitropism ensure?

the root grows into the soil and the shoots grow toward the light

negative gravitropism

growth of the shoot apical tip upward

positive gravitropism

growth of the roots downward

amyloplasts

specialized cellular compartments that contain starch granules that move in response to gravity

where are amyloplasts found?

shoots and specialized cells of root cap

what happens when a plant is titled?

statoliths drop to the new bottom cell wall which causes auxin to redistribute to the new bottom of the root

what happens when there is a high concentration of auxin in roots?

inhibits cell elongation, slowing growth on the lower side of the root, while cells develop normally on the upper side and causing the root to bend toward the high concentration of auxin and thus causing the root to grown down

what happens when there is a high concentration of auxin in shoots?

higher concentration stimulates cell expansion, causing the shoot to bend away from the higher concentration of auxin

what happens when a root begins to grow vertically?

amyloplasts return to their normal position and auxin is equally distributed to both sides of the root tip

what do cytokinins need to promote cell division?

auxin

abscisic acid in lateral buds

inhibits production of auxin, and removal of the apical bud will release this inhibition of auxin, allowing the lateral buds to begin growing

leaf abscission

some plants drop leaves in response to changing seasons (based on temperatures, photoperiod, water, or other environmental conditions)

what regulates leaf abscission?

auxin and ethylene

how do leaf hormones change based on the seasons?

uring the growing season, the leaf produces high levels of auxin which blocks activity of ethylene; however, as the seasons change, the leaf produces lower levels of auxin which allow ethylene to initiate aging and programmed cell death

fruit growth

growth of fruits in size is promoted by gibberellins

fruit ripening

once fruits have grown to the appropriate size, they begin ripening; this process is stimulated by ethylene

if seeds do not rely on the phytochrome system for germination, what do they do instead?

the inhibition of water; gibberellin is activated when water is consumed which triggers amylase to break down starches stored in the seed which causes germination

what happens when water is absent in germination (not thinking about phytochromes)?

germination is blocked by abscisic acid which inhibit gibberellins

stomatal closing

abscisic acid causes the stomata to close preventing gas exchange and halting photosynthesis even if blue light is present

local cell death

plants die if stomata is closed for too long; when drought persists, plants sacrifice parts of itself using ethylene

thigmotropism

movement in response to touch

Slow thigmotropism

a plant response to a touch stimulus that affects direction of growth, such as vines that wrap around or grow along structure

what regulates slow thigmotropism?

auxin; it redistributes in the elongating stem in response to touch which causes differential cell elongation

fast thigmotropism

only occurs in a few plant species, and describes a rapid plant response to touch such the way the venus flytrap snaps shut to trap an insect, or the way mimosa plants clamp their leaves closed in response to touch

what causes fast thigmotropism?

electrical signals which cause rapid changes in cell turgor pressure and thus rapid movement of structures associated with those cells

If a plant failed to grow taller, which of the following hormones might you artificially add to stimulate growth?

auxins

gibberellins

abscisic acid

ethylene

cytokinins

blue light

auxins

gibberellins

cytokinins

Blue light induces the phototropic response. It does so through which hormone?

Auxins

Gibberellins

Cytokinins

Ethylene

auxins

Which of the following signals indicates shade to a plant?

light quantity

far-red light

blue light

red light

far red light

Red light induces germination. Which hormone also contributes to germination (through a different mechanism)?

Auxins

Gibberellins

Cytokinins

Ethylene

Jasmonic acids

Abscisic acid

gibberellins

Gravitropism in roots is mediated by:

Redistribution of auxin to the top side of a growing root in response to gravity

Redistribution of auxin to the bottom side of a growing root in response to gravity

The movement of statoliths altering the distribution of auxin the roots

A and C

B and C

Redistribution of auxin to the bottom side of a growing root in response to gravity

The movement of statoliths altering the distribution of auxin the roots

Plants bend towards light in a process called phototropism. Which of the following is true of this process?

Plants use phototropins to receive a blue-light signal

Red and far-red light trigger bending in different directions

Auxin is produced in cells in response to light and causes bending

Asymmetric auxin distribution causes differential cell elongation that causes bending

Plants use phototropins to receive a blue-light signal

Asymmetric auxin distribution causes differential cell elongation that causes bending

What conformation will the phytochrome be for seeds exposed to red, then blue, then red, then far-red light?

Pr

Pfr

Pb

Some will be Pr and some will be Pfr

Some will be Pfr and some will be Pb

Some will be Pr and some will be Pb

Some will be Pr, some will be Pfr, and some will be Pb

Pr

Seeds exposed to red, then blue, then red, then far-red light will:

Germinate because red light was the first light they were exposed to

Not germinate because far-red light was the last light they were exposed to

Germinate because blue light induces germination

Not germinate because blue light inhibits germination

Germinate because far-red light was the last light they were exposed to

Not germinate because red light was they first light they were exposed to

Not germinate because far-red light was the last light they were exposed to

As a vinyard manager, which of the following should you apply to your grapes in order to maximize your wine production?

Cytokinins then auxin

Auxin then cytokinins

Auxin and cytokinins together

Gibberellins then ethylene

Ethylene then gibberellins

Ethylene and gibberellins together

None of these

Gibberellins then ethylene

What is a probable response for a plant facing a short-term (days) drought?

Increase tracheid diameter

Localized cell death

More daytime photosynthesis

Keep stomata closed

Keep stomata closed

What hormone(s) would mediate the previous response to a short term drought?

high auxin levels

low auxin levels

gibberellins

abscisic acid

ethylene

cytokinins

abscisic acid

What is a probable response for a plant facing a long-term (months) drought?

Increase tracheid diameter to enhance transport of water

Localized cell death to sacrifice some leaves/stems

More daytime photosynthesis to increase sugar production

Keep stomata closed permanently to avoid water loss

Localized cell death to sacrifice some leaves/stems

What hormone(s) would mediate the response to a long-term drought?

high auxin levels

low auxin levels

gibberellins

abscisic acid

ethylene

cytokinins

low auxin levels

ethylene

Which of the following is true regarding thigmotropism?

Thigmotropism is movement in response to touch

Rapid thigmotropism is mediated by auxin

Slow and rapid thigmotropism are both mediated by action potentials

Thigmotropism is movement in response to touch

Given what you know about how signal processing of hormones works, how is auxin able to regulate phototropism, apical dominance, gravitropism, cell division, and senescence?

Different receptors

Different transduction pathways

Different proteins activated as a result of signal transduction

Crosstalk with other hormones

Different genes expressed as a result of signal transduction

Different receptors

Different transduction pathways

Different proteins activated as a result of signal transduction

Crosstalk with other hormones

Different genes expressed as a result of signal transduction

Prednisone is a synthetic glucocorticoid hormone, often given as an immunosuppressant to treat chronic or acute inflammation. The reason(s) prednisone only affects a small number of cells in the body is/are:

Only target cells express receptors for the hormone

Only target cells are exposed to the hormone

Nontarget cells destroy the hormone before it can produce any effect

Nontarget cells convert the hormone to a different structure to which they do respond

None of these

Only target cells express receptors for the hormone

Negative feedback is defined as ________.

the release of one hormone is inhibited by another hormone

the product of a process stimulates its production

the release of one hormone is stimulated by another hormone

the product of a process inhibits its production

the product of a process inhibits its production

Which of the following is an example of hormonal crosstalk?

In insects, ecdysone promotes metamorphosis when juvenile hormone is absent; molting without metamorphosis occurs when juvenile hormone is present

In plants, ethylene promotes leave abscission (falling off) when auxin levels are low; leaf abscission does not occur when auxin is present

In plants, cytokinins promote cell division when auxin is also present, cell division does not occur when auxin is absent

In mammals, combinations of estradiol and progesterone inhibit follicle maturation; high levels of estradiol alone stimulates follicle maturation

None of these are examples of crosstalk.

In insects, ecdysone promotes metamorphosis when juvenile hormone is absent; molting without metamorphosis occurs when juvenile hormone is present

In plants, ethylene promotes leave abscission (falling off) when auxin levels are low; leaf abscission does not occur when auxin is present

In plants, cytokinins promote cell division when auxin is also present, cell division does not occur when auxin is absent

In mammals, combinations of estradiol and progesterone inhibit follicle maturation; high levels of estradiol alone stimulates follicle maturation

Which of the following BEST describes the mechanism of quorum sensing?

At very high abundances, physical contact among cells causes a change in morphology that promotes biofilm formation.

When abundant, cells secrete a factor that inhibits the growth of neighbors, preventing overpopulation.

When abundant, cells secrete a factor that communicates their presence.

Cells secrete a factor whose concentration in the environment reflects the density of individuals present.

None of the above applies to quorum sensing

Cells secrete a factor whose concentration in the environment reflects the density of individuals present.

A drug which inhibited quorum sensing would be LEAST likely to disrupt

biofilm behaviors

any behavior exhibited only by individual bacteria

any behavior exhibited only by groups of bacteria

infectious /virulent behaviors

all of the above are likely to be disrupted by a drug that inhibits quorum sensing

any behavior exhibited only by individual bacteria

Glucocorticoids do which of the following?

promote the release and breakdown of fatty acids

increase blood glucose levels

promote the immune response

increase insulin production

promote the release and breakdown of fatty acids

Which of the following is an example of a positive feedback loop?

juvenile hormone suppresses metamorphosis

oxytocin stimulates uterine contractions during birth; increased uterine contractions leads to release of more oxytocin

high leptin levels in the blood cause a mouse to eat less, reducing fat stores and lowering the levels of leptin in the blood

adrenocorticotropin hormone from the anterior pituitary promotes release of cortisol from the adrenal cortex

none of the above are examples of a positive feedback loop

oxytocin stimulates uterine contractions during birth; increased uterine contractions leads to release of more oxytocin

Which of the following would likely be true of an individual who lacks functional leptin receptors?

They would likely be underweight

They would likely be overweight

They would not be able to remove glucose from the bloodstream

A and C

B and C

They would likely be overweight

Which of the following is a DIFFERENCE between plant and animal hormones?

Animal hormones are produced in dedicated endocrine glands; plant hormones are not

Plant hormones have different effects in different tissues; animal hormones do not

Plant hormones exhibit crosstalk; animal hormones do not

Animal hormones are transported throughout the organism's body; plant hormones are not

None of these are differences between plant and animal hormones

Animal hormones are produced in dedicated endocrine glands; plant hormones are not

Auxin is a major regulator of plant growth and function, but it does NOT play a role in:

Stem elongation

Slow thigmotropism

Phototropism

Gravitropism

Rapid thigmotropism

Rapid thigmotropism

Auxin regulates many different processes in plants. In the case of gravitropism and phototropism:

Auxin may use different receptors and/or different signal transduction pathways in these two organs

Auxin causes bending of roots and shoots via different cellular mechanisms

Auxin causes bending of roots and shoots via the same cellular mechanism

Auxin only regulates one of these processes

A and B

A and B