37. Plant Sensation and Response

Tropisms and Hormones

Negative Gravitropism | Demonstration

No. Stem thickening is part of the triple response.

No. During the triple response, asymmetric growth results in stem curvature.

No. The triple response results in the inactivation of kinase. This allows synthesis of the proteins required for the triple response.

No. The triple response is promoted by ethylene.

How does ethylene affect growth? .

The triple response to mechanical stress results in __________.

No. Gibberellins and auxin are responsible for cell elongation.

No. As an inhibitor of both primary and secondary growth, abscisic acid may play a role in the onset of dormancy.

No. Strigolactones help establish mycorrhizal associations.

Review the functions of plant hormones. .

A graduate student growing plant cells in a laboratory dish wants them to __________; therefore, the student treats them with cytokinins.

Thigmotropism and Cytokinins

Animation:  Water-Soluble Hormone Pathway

Short Video: Prokaryotic Flagella

Plant hormones act by affecting the activities of
a. genes.
b. membranes.
c. enzymes.
d. genes, membranes, and enzymes.

Plant Hormones - Tropisms & Auxins  #52

Plant Senescence II: Signals Affecting Senescence

To explore how hormones function, researchers have begun to transform plants with particular genes. In one experiment, a gene involved in cytokinin synthesis was introduced into tobacco plants. Which one of the following results would be expected?
a. Individuals produced more lateral branches.
b. Stems grew extremely tall and slender.
c. Roots were incapable of responding to gravity.
d. Stomata were closed most of the time.

Leaflets of Mimosa pudica (common names: sensitive plant, touch-me-not) have a remarkable ability to close up in response to being touched or physically moved. How fast can the leaflets close? How does this occur? And more importantly, what benefit could this unusual response provide to the plant?

If just the leaflets located toward the end of a leaf are touched, the adjacent leaflets close in fairly rapid succession until all leaflets on a leaf close up. Explain how electrical signaling may be involved in this response.

Why was it logical to predict that amyloplasts function as statoliths?
a. They are dense and settle to the bottom of gravity-sensing cells.
b. They are present only in gravity-sensing cells.
c. They make a direct physical connection with membrane proteins that have been shown to be gravity-receptor molecules.
d. Their density changes in response to gravity.

Leaflets of Mimosa pudica (common names: sensitive plant, touch-me-not) have a remarkable ability to close up in response to being touched or physically moved. How fast can the leaflets close? How does this occur? And more importantly, what benefit could this unusual response provide to the plant?

The mechanism of leaflet closure is similar to what happens during the shrinking of guard cells and closing of stomata (see Figure 37.22). When in the open position, special cells on the upper surface of Mimosa leaflets are filled with water and are under pressure. Explain how osmosis and flow of ions into and out of these special cells may be involved in leaflet closure and reopening. <img class="fr-dib" src="https://lightcat-files.s3.amazonaws.com/problem_images/224d429176878671-1677307130584.jpg" style="width: 300px;">

Test your knowledge of the five major classes of plant hormones (auxins, cytokinins, gibberellins, abscisic acid, ethylene) by matching one hormone to each lettered box. (Note that some hormones will match up to more than one box.) <img src="https://lightcat-files.s3.amazonaws.com/problem_images/ba01d0c3d3df4350-1672854731376.jpg" alt="" /><img class="fr-dib" src="https://lightcat-files.s3.amazonaws.com/problem_images/2c03df4946469372-1677808961599.jpg" style="width: 300px;">

Animation: Reproductive Cycles of the Human Female

No. Gibberellins stimulate growth in length but do not by themselves inhibit branching. Ethylene may be a growth inhibitor.

No. Gibberellins stimulate growth in length but do not by themselves inhibit branching. Abscisic acid inhibits plant growth.

No. Auxin is the correct choice. Abscisic acid, however, inhibits plant growth.

The interaction of various hormones results in specific responses in the plant.

In shoots, branching is inhibited by __________ from the tip of a growing shoot, but this effect is countered by __________ from the roots.

Leaflets of Mimosa pudica (common names: sensitive plant, touch-me-not) have a remarkable ability to close up in response to being touched or physically moved. How fast can the leaflets close? How does this occur? And more importantly, what benefit could this unusual response provide to the plant?

Which of the following terms best describes the leaflet movement?
a. thigmonastic movements

b. thigmotropism

c. thigmomorphogenesis

d. apical dominance

No. Abscisic acid is an inhibitor of plant growth.

No. Cytokinins stimulate lateral branching.

No. Ethylene is an inhibitor of cell elongation.

No. Brassinosteroids induce cell elongation and division in stem segments and seedlings. The effects of brassinosteroids are quite similar to the effects of auxin.

Consider the hormone responsible for "foolish seedling disease."

As leaf lettuce matures, a tall flowering shoot extends beyond the basal, edible leaves. After the plant bolts like this, it no longer produces broad, tasty leaves. Suppose you want to prevent bolting so that you can harvest lettuce longer. You may want to prevent the plant from synthesizing __________.

PLANT HORMONES - Auxin Gibberellin Cytokinin Ethylene Abscisic Acid

"Match each definition with the correct term.
Bending of a shoot toward light
Growth response to touch
Cycle with a period of about 24 hours
Pigment that helps control flowering
Relative lengths of night and day
Growth response to gravity
Folding of plant leaves at night
a. phytochrome
b. photoperiod
c. sleep movement
d. circadian rhythm
e. thigmotropism
f. phototropism
g. gravitropism"<img src="https://lightcat-files.s3.amazonaws.com/problem_images/62530f6b77970be6-1672943655275.jpg" alt="" />

If apples are to be stored for long periods, it is best to keep them in a place with good air circulation. Explain why.

Which of the following statements about hormones is correct?
a. They tend to be large molecules.
b. They exert their effects only on the same cells that produce them.
c. They can exert strong effects only when they are present in high concentrations.
d. They trigger a response by binding to target-cell receptors.

No. Heat-shock proteins do not promote water loss.

They regulate stomatal opening and closing.

No. This is not the function of heat-shock proteins.

No. This is not how heat-shock proteins work.

No. Water potential can be affected by the production of organic compounds retained within the cell.

They help other proteins retain their functional shapes.

Consider the mechanisms by which excessive heat exerts its effects.

How are heat-shock proteins thought to work?

Animation: Binding of Hormones

Leaflets of Mimosa pudica (common names: sensitive plant, touch-me-not) have a remarkable ability to close up in response to being touched or physically moved. How fast can the leaflets close? How does this occur? And more importantly, what benefit could this unusual response provide to the plant?

Researchers have hypothesized that rapid leaf movements in Mimosa serve as a defense mechanism (e.g., closing leaflets may deter plant-eating insects). Propose an experiment to test this hypothesis.

A plant mutant that shows normal gravitropic bending but does not store starch in its plastids would require a reevaluation of the role of            in gravitropism.
a. auxin
b. calcium
c. statoliths
d. differential growth

Buds and sprouts often form on tree stumps. Which hormone would stimulate their formation?
a. auxin
b. cytokinin
c. abscisic acid
d. ethylene

Plant Hormones and Senescence

Leaflets of Mimosa pudica (common names: sensitive plant, touch-me-not) have a remarkable ability to close up in response to being touched or physically moved. How fast can the leaflets close? How does this occur? And more importantly, what benefit could this unusual response provide to the plant?

Researchers studied the rate of closing and reopening of leaflets by measuring changes in the distance between opposing leaflets after being touched. Their results are shown in the accompanying graph (d/dmax is actual leaflet distance relative to maximal leaflet distance). How long does it take for leaflets to close? How long to reopen?

In autumn, the amount of _________ increases and the amount of _________ decreases in fruits and leaf stalks, causing a plant to drop fruit and leaves.
a. ethylene … auxin
b. gibberellin … abscisic acid
c. cytokinin … abscisic acid
d. auxin … ethylene

DRAW IT Indicate the response to each condition by drawing a straight seedling or one with the triple response.

What evidence suggests that ABA from roots can signal guard cells to close?
a. If roots are given sufficient water, guard cells close anyway.
b. If roots are dry, guard cells begin to close—even though leaves may not be experiencing water stress.
c. Applying ABA on guard cells directly causes them to close.
d. If roots are dry, the ABA concentration in leaf cells drops dramatically.

No. Gibberellins do not help establish mycorrhizal associations.

No. Brassinosteroids do not help establish mycorrhizal associations.

No. Although cytokinins do interact with auxin in the modification of apical dominance, they do not participate in the establishment of mycorrhizal associations.

No. One of the choices is a correct answer.

In addition to interacting with auxin in the control of apical dominance, newly discovered plant hormones help to establish mycorrhizal associations by attracting mycorrhizal fungi to the plant roots. What are these hormones called?

Plant Senescence Basics

No. Water deficits stimulate an increased synthesis and release of abscisic acid from mesophyll cells, which keeps stomata closed by acting on guard cell membranes.

No. A response to oxygen deprivation in waterlogged soils is the formation of air tubes that provide oxygen to submerged roots.

No. The closing of stomata and the production of heat-shock proteins are responses to heat stress.

No. Some plants respond to herbivory by producing compounds that deter herbivores by a variety of means.

How might the tonicity be equalized? .

The production of organic solutes by plants is a response to __________.

Short Video: Gravitropism

Cytokinins

The hormone that helps plants respond to drought is
a. auxin.
b. abscisic acid.
c. cytokinin.
d. ethylene.

Plant Responses to the Environment: Tropisms and Defenses

increase the hydrostatic pressure in cells, stretching their walls

No. Although the increased uptake of water does occur, another of the choices is a more immediate and direct effect of auxin.

shift the pH and thus activate enzymes that break the hydrogen bonds

No. Although the acidification of the cell wall activates expansins that break hydrogen bonds, another of the choices describes a more immediate and direct effect of auxin.

No. Although ion uptake by the cell increases the movement of water into the cell, another of the choices describes a more immediate and direct effect of auxin.

No. The synthesis of cytoplasm occurs after the initial elongation.

activate ATP-driven proton (H+) pumps

This mechanism is based on ion transport. .

The immediate, direct effect of auxin in cell elongation, according to the acid growth hypothesis, is to __________.

Plant Tropism

Short Video: Mimosa Leaf

No. Gravitropism is movement in response to gravity. This is an example of a tropism, but there is a better response.

No. Phototropism is movement in response to light. This is an example of a tropism, but there is a better response.

No. Thigmotropism is movement in response to touch. This is an example of a tropism, but there is a better response.

No. Heliotropism is movement in response to solar position. This is an example of a tropism, but there is a better response.

Consider the meanings of the suffixes and prefixes of the choices.

What is growth that results in curvatures of whole plant organs toward or away from stimuli called?

No. Phytoalexins are antimicrobial compounds produced in response to infection.

No. Gibberellins stimulate plant growth but not plant orientation.

No. Cytokinins stimulate plant growth but not plant orientation.

No. Phytochromes play a role in a plant's response to light and photoperiod.

A type of plastid is involved in the response to gravity.

__________ appear to allow a plant cell to sense its orientation with respect to gravity.

Plant Growth Regulators: What are Plant Hormones [Horticulture 101 Series]

Mechanisms of Plant Growth

Animation: Steroid Hormone Pathway

No. A plant in salty water is in a hypertonic solution.

No. The formation of air tubes is a response to the oxygen deprivation that occurs when roots are submerged.

No. This is a response to a deficit of water.

Consider the effects on tonicity and water potential of increased soil salinity. .

When a plant that is not adapted to salty water is exposed to salty water, it will __________.

No. Equal concentrations of auxin and cytokinin allow the cells of the callus to increase in number, but the cells remain undifferentiated.

No. A low concentration of auxin relative to cytokinin would stimulate the development of shoot buds from the callus.

No. Different ratios of the two hormones have different effects on the callus.

The interaction of the two hormones results in specific responses. .

A callus will develop roots if you use a __________ auxin concentration and a __________ cytokinin concentration in the medium.

No. A florist would most likely use a cytokinin spray because cytokinins slow aging and deterioration.

Which hormone would a florist most likely spray on cut flowers to keep them fresh?

No. These are the correct hormones, but it is a decrease in auxin that results in changes in the abscission layer.

No. Neither of these hormones plays a role in leaf abscission.

No. Only one of these hormones plays a role in leaf abscission.

The ratio of which two hormones affects the interface between the stem and petiole? .

In the autumn, the amount of __________ decreases, rendering the cells of the abscission layer more sensitive to __________.

Tropism (Types, positive & negative) | Control & Coordination | Biology | Khan Academy

No. The germination of seeds requires an uptake of water by the seeds.

No. Such a response involves either the inhibition or stimulation of cell elongation. Cell elongation involves enzymes loosening the cell wall.

stomatal opening, initiating photosynthesis

No. The loss of water from guard cells would result in the closure of stomata.

stress that results in the production of PR proteins

No. PR proteins are produced in response to infection.

Consider this question in terms of how the loss of water affects plant cells that are placed in a hypertonic solution.

A rapid loss of water in specialized cells in the sensitive plant Mimosa pudica causes __________.

No. The production of organic solutes by plants is a response to salt stress.

How would additional unsaturated fatty acids in the cell membrane affect the membrane's fluidity? .

Increasing the proportion of unsaturated fatty acids in their membranes is a response of plants to __________.

In many species native to tropical wet forests, seeds do not undergo a period of dormancy. Instead, they germinate immediately. Predict the role of ABA in these seeds. How would you test your prediction?

thigmotropism

During winter or periods of drought, which one of the following plant hormones inhibits growth and seed germination?
a. ethylene
b. abscisic acid
c. gibberellin
d. auxin

No. Roots submerged in water are not suffering from a water deficit.

No. Some plants respond to cold stress by changing the solute composition of their cells.

Consider what air tubes would bring to submerged roots.

The formation of air tubes in submerged roots is an adaptation to __________.

No. This is true, but there is a better answer. Ethylene is an organic compound.

No. This is true, but there is a better answer.

Ethylene is involved in one of the rare examples of a positive feedback loop.

No. This is true, but there is a better answer. Consider the relationship between ethylene and ripening.

A burst of ethylene is almost always associated with apoptosis.

Which of these features is/are characteristic of ethylene?

BioFlix: Mechanism of Hormone Action: Second Messenger cAMP

No. A response to oxygen deprivation in waterlogged soil is the formation of air tubes that provide oxygen to submerged roots.

No. Some plants respond to herbivory by producing chemicals that attract predators of those herbivores.

Think about the relationship between solute concentration and the movement of water across the cell membrane. .

Increasing the cytoplasmic levels of specific well-tolerated solutes, such as sugars, helps a plant to cope with __________.

Write a short paragraph explaining why a houseplant becomes bushier if you pinch off its terminal buds.

No. Hormones regulate the bending of a plant toward light.

No. Cytokinins are produced in actively growing tissues, such as roots, embryos, and fruits.

is the location of the biological clock in a plant

No. The biological clock has not been localized to such a specific region.

detects light and measures the photoperiod

No. This function is performed by phytochromes.

How does the ratio of ethylene to auxin affect the interface between the stem and petiole? .

The abscission layer __________.

No. Phytochromes play a role in photoperiodism.

No. Gibberellins stimulate stem elongation.

No. Phytoalexins are antimicrobial compounds produced by plants in response to infection.

This hormone also plays a major role in a plant's response to drought.

Seeds of many desert plants will not germinate until a heavy rain washes away their __________.

Plant Growth Regulators

Animation: Hormonal Control of the Testes

Abscisic acid is involved in seed dormancy.

The synthesis and release of abscisic acid in a plant is a response to __________.

Gravitropism

Leaflets of Mimosa pudica (common names: sensitive plant, touch-me-not) have a remarkable ability to close up in response to being touched or physically moved. How fast can the leaflets close? How does this occur? And more importantly, what benefit could this unusual response provide to the plant?

Mimosa leaflets also close on their own at dusk and reopen at dawn (this cycle develops as a circadian rhythm). What receptor molecule mentioned in this chapter is likely involved in regulating this process?

No. This statement is true, but there is a better response.

No. This response to water deficit occurs even if heat is not the cause of the water deficit.

Production of heat-shock proteins, closing of stomata, and altering the lipid composition of their membranes

No. The third part of this choice describes a plant's response to cold stress.

Production of heat-shock proteins and closing of stomata

Consider the consequences of high temperatures.

Which of the following describes a plant's response to heat stress?

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